Global Biogeochemical Cycles and the Physical Climate System

(Full 69-page Text in HTML)      

Fred T. Mackenzie
School of Ocean and Earth Science and Technology
University of Hawaii
National Oceanic and Atmospheric Administration

Environmental Law
 

Contents

Preface
Introduction
Chapter 1: Biogeochemical Processes
Chapter 2: Biogeochemical Cycles and Climate
Chapter 3: The Modern Coupled C-N-P-S-O System
Chapter 4: Carbon Cycles
Chapter 5: The Important Nutrient Nitrogen
Chapter 6: Phosphorus and Sulfur
Chapter 7: The Water Cycle
Conclusion
Study Questions and Answers
Glossary
Supplementary Reading
References
Bibliographic Information

 
Index of Items
*Please note that the following links take you to the item's instance in the text
Figures
figure 1 The ecosphere
figure 2 Periodic table of the elements
figure 3 The biogeochemical cycle of carbon prior to human interference
figure 4 The major reservoirs and fluxes in the biogeochemical cycle of carbon
figure 5 The geologic time scale - the calendar of the earth
figure 6 Model calculation of atmospheric carbon dioxide during the last 600 million years
figure 7 The biogeochemical cycle of oxygen
figure 8 Model calculation of atmospheric oxygen during the past 600 million years
figure 9 Part of the modern global biogeochemical cycle of methane, emphasizing exchanges of methane between the earth's surface and atmosphere and the fate of the gas in the atmosphere
figure 10 Part of the modern global biogeochemical cycle of carbon monoxide, emphasizing exchange of the gas between the earth's surface and its atmosphere and the fate of the gas in the atmosphere
figure 11 Part of the modern global biogeochemical cycle of carbon dioxide, emphasizing its exchanges between the earth's surface and its atmosphere
figure 12 Part of the modern global biogeochemical cycle of nitrogen, emphasizing interactions among the land, atmosphere, and ocean
figure 13 River input of N to the ocean compared to the fluxes involved with the internal recycling of N in the ocean due to biological productivity and decay
figure 14 Part of the modern global biogeochemical cycle of nitrous oxide
figure 15 Part of the modern global biogeochemical cycle of ammonia, including that of the ammonium ion (NH4+)
figure 16 Part of the modern biogeochemical cycle of the nitrogen oxides
figure 17 Part of the modern global biogeochemical cycle of the nonmethane hydrocarbons
figure 18 Part of the modern global biogeochemical cycle of phosphorus, emphasizing the exchange of P among the land, atmosphere, and ocean
figure 19 Part of the global biogeochemical cycle of reduced sulfur
figure 20 Part of the global biogeochemical cycle of oxidized sulfur
figure 21 The global biogeochemical cycle of water
figure 22 Fluxes in the global biogeochemical cycle of water
Tables
table 1 Biogeochemical reactions involving prokaryotes
table 2 Chemical formulas and names used in this module
table 3 Distribution of water in the ecosphere
Equations
equation 1 decay of plant material
equation 2 dissolving of calcium carbonate
equation 3 generalized photosynthesis
equation 4 plant photosynthesis
equation 5 respiration and decay of organic material
equation 6 reaction of dead phytoplankton with O2
equation 7 respiration and decay in an anoxic environment
equation 8 the weathering of albite
equation 9 dissolved inorganic carbon in seawater
equation 10 NO formation in the middle stratosphere (20-30 km)
equation 11 NO formation in the middle stratosphere (20-30 km)
equation 12 net reaction leading to the destruction of stratospheric ozone



Preface

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Global environmental change is a subject of considerable public and scientific interest today. Any discussion of change must involve the substances that are transported in cycles about the earth's surface - through its air, water, soil, rocks, ice, and living and dead organic matter. Thinking about these global biogeochemical cycles and their role in environmental change requires us to cross the usual boundaries between biology, ecology, oceanography, meteorology, chemistry, and geology. Because of the impact of human activities on the cycles, and consequently the climate, the subject also involves the effects and consequences of natural and human-induced change for ecosystems, humans, and human infrastructures. This leads the discussion into the fields of sociology, economics, and political science. Such a broad and interdisciplinary topic is difficult to capture completely in a module of this size. I have made no attempt to do so but have concentrated on the biogeochemical cycles of five of the major elements important to life - carbon, nitrogen, phosphorus, sulfur, and oxygen - and their role in climatic change.

Biogeochemistry is the discipline that links various aspects of biology, geology, and chemistry to investigate the surface environment of the earth. This environment, the ecosphere (see Figure 1), encompasses the biosphere (living and dead organic matter) and parts of the other large subdivisions (reservoirs) of the earth's surface of atmosphere (air), hydrosphere (water), shallow crust (soils, sediments, and crustal rocks), and cryosphere (ice). In this module, I focus on the role biogeochemistry plays in regulating and interacting with the climate system.

This module covers a great deal of material, much of which is interdisciplinary. This presents a problem for the writer of the material, the teacher, and the student. Both teacher and student generally will have more knowledge in one discipline than in another. Also, each discipline has a unique vocabulary. (Most of the interdisciplinary vocabulary in this module is defined within the text or in the extensive glossary.) Furthermore, the language of chemical equations is used to describe processes operating within the ecosphere. Therefore, it may take some additional work and perhaps reference to basic texts in chemistry, ecology, meteorology, etc., to digest the material of this module.

The text begins by introducing some important biogeochemical processes. This material is not a laundry list of processes but a selection of such processes as photosynthesis, weathering, and deposition of sediments in the ocean as examples of the nature and variety of biogeochemical processes. The next subject is the historical (geological) nature of environmental change on the earth. Emphasis is on the biogeochemical cycles of atmospheric carbon dioxide and oxygen through the past 600 million years of the history of the earth. The major processes controlling these cycles and their tie to climate are discussed. We will see that for much of this time, the planet has had a more equable climate than at present.

Finally, the text deals with parts of the modern biogeochemical cycles of five of the most important elements essential for life: carbon, nitrogen, phosphorus, sulfur, and oxygen. These elements, along with hydrogen and a suite of nutrient trace elements, interact through the processes of photosynthesis and respiration and/ or decay. Processes and feedbacks within the cycles are described in the context of the potential for a global warming brought about by human activities that have changed the composition of the atmosphere. Keep in mind that the approach can be used to interpret the interaction between biogeochemical cycles and climatic change of any nature - warming or cooling - and at various space and time scales.

An extensive glossary, study questions and answers, and a supplementary reading section conclude the module. The glossary is interdisciplinary and should help in understanding the diverse material of the module. The study questions are designed to enable students to review the text, to integrate the material, and to expand their knowledge of the topics covered. Many of the questions require calculations using standard arithmetic. Mathematics is the foundation of science, and it is necessary for students to get their feet wet. The readings are broad in scope and of a general nature.

I would like to thank John Firor, Dave Schimel, and especially Tom Wigley for their comments on the initial draft of this module. Some of the material in this module comes from research supported by the National Science Foundation and the National Oceanographic and Atmospheric Administration. The final version of this module was written while I was a Fellow at the Wissenschaftskolleg zu Berlin. I thank Prof. Dr. Wolf Lepenies, rector of the institute, for providing space, facilities, and peace of mind to accomplish the task. Many thanks to Michael Shibao for drafting and in so doing substantially improving the original illustrations for this module. Finally, I am extremely indebted to Carol Rasmussen of the University Corporation for Atmospheric Research for her critical and laborious editing. Without her, this module would not have been completed.

Fred T. Mackenzie
School of Ocean and Earth Science and Technology
University of Hawaii
June 1996


Introduction

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The global ecosphere is the thin film around the earth where living things (the biosphere) interact with the atmosphere (air), hydrosphere (water), cryosphere (ice), and lithosphere (soils and shallowly buried rocks) in a complex system involving biological, geological, and chemical processes and cycles (Figure 1). This biogeochemical system of spheres and processes is powered mainly by energy from the sun.

The ecosphere is made up of individual ecosystems, such as tropical forests, grasslands, tundra, coral reefs, and estuaries. Matter and energy flow between and within these ecosystems in interconnected biogeochemical cycles. Gaseous chemical compounds are produced and consumed in the ecosystems and exchanged between them and the air. In the atmosphere, they may react to form other compounds before returning to the earth's surface. Some of these chemical species are greenhouse gases, like carbon dioxide (CO2) and methane, which act in the atmosphere to warm the planet. Others, like dimethylsulfide gas, react with other atmospheric chemicals to form minute airborne particles (aerosols) that directly or indirectly help to cool the climate.

Figure 1. The Ecosphere
The ecosphere, our life support system, showing its relationship to the other important spheres of the surface system of the earth (after Christensen, 1991).

The most common way of studying the global movements of these chemicals is by mathematical modeling of biogeochemical cycles at the earth's surface. Modeling also allows scientists to estimate the effects of human activities on natural biogeochemical cycles. A model is simply a set of equations that describe some of the processes found in the real world. Biogeochemical cycling models generally include processes that move materials and their rates of transfer among a limited number of well-studied spheres of the earth.

Biogeochemical cycles, however, have certain properties that are inherently difficult to describe and model. These include:

  1. irreversibility, that is, the system does not return to its exact previous state if it goes through a disturbance

  2. transitional phenomena, that is, the system tends to switch from one state to another and another and yet others, and perhaps back again, rather than simply moving from "before" to "after"

  3. evolution, in which the system progressively changes in a particular direction

  4. processes that either enhance the original perturbation to the system (positive feedback) or relieve the perturbation (negative feedback).

In Chapter 1 of this module, we shall first consider some examples of biogeochemical processes. In Chapters 2 and 3, we shall discuss how the biogeochemical cycles interact with climate, both in previous eras and at present. In Chapters 4, 5, 6, we shall discuss the present-day global biogeochemical cycles of several elements that are important biologically and that interact with the climate system. The cycles are looked at in the context of global warming from an enhanced greenhouse effect.



Chapter 1: Biogeochemical Processes

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Innumerable biological, geological, and chemical processes cycle elements throughout the ecosphere. The few discussed in this section should give the reader an idea of their variety and complexity. As an example, consider a group of organisms called the prokaryotes: the bacteria and blue-green algae. The processes that these organisms are involved with (summarized in Table 1) include:

This list is given only as an example; some of these processes will not be discussed in the text.

These prokaryotic processes may take place in a variety of ways, such as

  1. autotrophy, in which the organisms convert inorganic carbon in the environment to organic matter

  2. heterotrophy, in which the products from the breakdown of organic compounds are used to make new organic materials

  3. mixotrophy, in which both inorganic and organic compounds are used to make organic matter.



Table 1. Biogeochemical reactions involving prokaryotes
Element Process Summary of partial chemical reactions Examples of organisms involved in process
Carbon CO2 fixation CO2 + H2 => (CH2O)n + A2 (A=O,S) photoautotrophs: cyanobacteria, purple and green sulfur bacter
chemoautotrophs: sulfur and iron oxidizing bacteria
  Methanogenesis COO- + H2 => CH4 methanogenic bacteria
  Methanotrophy CH4 + O2 => CO2 methanotrophic bacteria
  Fermentation (CH2O)n + O2 => CO2 anaerobic heterotrophic bacteria
  Respiration (CH2O)n + O2 => CO2 Aerobic heterotrophic bacteria
Sulfur Sulfur reduction SO4 + H2 =>H2S Sulfur-reducing bacteria
  Sulfur oxidation H2S =>S0 purple and green sulfur phototrophs
    S0 + O2 =>SO4 Sulfur oxidizing bacteria
Nitrogen N2 fixation N2 + H2 =>NH4 phototrophic bacteria, nitrogen-fixing heterotrophic bacteria
  Nitrification NH4 + O2=>NO2, NO3 Nitrifying bacteria
  Denitrification NO2, NO3=>N2O, N2 Denitrifying bacteria
After Stolz et al., 1989

 
Principles chemical reactions
Atoms and Elements

Every object in the universe is composed of matter. Because matter can be converted to energy, it is essentially a form of energy. Matter is composed of atoms, which are the smallest particles of an element that can exist either alone or in combination. An atom is also the smallest particle that can enter into a chemical reaction. Most atoms never change; they only combine with other atoms to make different substances. Radioactive atoms, however, do change and eventually decay into stable, nonradioactive atoms.

Elements consist of atoms of the same kind and, when pure, cannot be decomposed by a chemical change. There are 106 known elements; 103 are listed in the periodic table (Figure 2). The elements most used commercially by people, in order of use, are carbon (C), in the form of coal, oil, and gas; sodium (Na), in table salt and other products; iron (Fe), used in the steel industry; and nitrogen (N), sulfur (S), potassium (P), and calcium (Ca), all used in fertilizers or as soil conditioners for our food supply.

Compounds

When two or more atoms are bonded together in a definite proportion, a compound is formed. Examples of compounds discussed in this text are water (H2O), carbon dioxide (CO2), salt (NaCl), and sugar (e.g., glucose, C6H12O6). (All of the compounds named in the text are listed in Table 2.) The numbers in these chemical formulas are the number of atoms of each substance in the compound. If only one atom of a substance is in the compound, no number is given. The universe is composed of millions of these compounds, all created from the elements given in the periodic table. The smallest particle of a compound that can exist and exhibit the properties of that compound is called a molecule.

A compound is a pure substance that can be decomposed by a chemical change. The atoms in the chemical compound may rearrange themselves, or they may separate from the compound to form different compounds. These changes and interactions among compounds are called chemical reactions.

Chemical Equations

A chemical equation expresses a chemical reaction involving compounds or elements. The chemicals that react together, called reactants, generally are shown on the left-hand side of the equation and the products on the right-hand side. Consider the decay of plant material (represented by the chemical compound CH2O, a carbohydrate), which requires the oxygen gas (the chemical compound O2) in the earth's atmosphere. The simplest chemical equation representing this process is

  CH2O + O2 => CO2 + H2O  (1)

The arrow pointing right indicates that this process is irreversible; the plant material will be completely oxidized to CO2 and H2O in the presence of atmospheric oxygen. Other processes are highly reversible, and these are usually represented by a double arrow. For example, the equilibrium between calcium carbonate and its dissolved calcium and carbonate ions (atoms or molecules that have lost or gained electrons, with the number lost or gained shown as a positive or negative superscript) is represented as

  CaCO3 <=> Ca2+ + CO32-  (2)

In chemical processes, matter cannot be created or destroyed. Thus, when a chemical equation is written, the total number of atoms of any particular element on the left-hand side of a chemical equation must be made to equal the total number of atoms of that element on the right-hand side of the equation. This is the process of balancing a chemical equation. Balancing the equation expresses the fact that molecules usually react in such a way as to bear simple, integral, numerical relationships to one another.

If these relationships are known, it is possible to calculate the masses of reactants and products by using known atomic and molecular weights. In chemical terms, the amount of a substance is expressed in moles. One mole of a substance is the amount that contains as many elementary entities as there are atoms in 12 grams of carbon. This number is termed Avogadro's constant, and its value is equal to 6.022 x 1023. In the chemical equation given above for the equilibrium of CaCO3 and its dissolved chemical species, one mole of CaCO3 will dissolve in water to make one mole of Ca2+ and one mole of CO32-. In terms of mass, 100 grams of CaCO3 will react to give 40 grams of Ca2+ and 60 grams of CO32-. If only 10 grams of CaCO3 were to dissolve, then the same proportions of Ca2+ and CO32- would be present at the equilibrium: 4 and 6 grams, respectively.


 
Figure 2. Periodic Table of the Elements
Periodic table of the elements. Each box includes an element's atomic number, chemical symbol, and atomic weight.

 
Table 2. Chemical formulas and names used in this module
Al2Si2O5(OH)4 kaolinite
Ca2+ calcium ion
CaCO3 calcium carbonate
CaSiO3 calcium silicate
Ca5(PO4)3(OH,F) carbonate fluoroapatite
CH2O carbohydrate
(CH2O)106(NH3)16H3PO4 organic matter in marine phytoplankton
CH4 methane
CO2 carbon dioxide
CO32- carbonate ion
CS2 carbon disulfide
C6H12O6 sugar (glucose)
DIC dissolved inorganic carbon
DMS dimethyl sulfide, (CH3)2S
HCO3- bicarbonate ion
HNO3 nitric acid
H2 molecular hydrogen
H2O water
H2S hydrogen sulfide
H2SO4 sulfuric acid
H3PO4 phosphoric acid
H4SiO40 monomeric silicic acid
KAlSi3O8 orthoclase feldspar
MSA methane-sulfonic acid
NaAlSi3O8 albite
NaCl sodium chloride, common table salt
NH3 ammonia
NH4+ ammonium ion
NH4NO3 ammonium nitrate
(NH4)2SO4 ammonium sulfate
NMHC nonmethane hydrocarbon
NO nitric oxide
NO3- nitrate ion
NOx oxides of nitrogen
N2 diatomic nitrogen
N2O nitrous oxide
OCS carbonyl sulfide
OH* hydroxyl radical
OH- hydroxyl ion
O2 diatomic oxygen (pure oxygen molecules)
O3 ozone
PAN peroxylacetyl nitrate
PH3 phosphine or swamp gas
PO43+ phosphate ion
SO2 sulfur dioxide
SO42- sulfate ion
SOx oxides of sulfur
SiO2 silica

Photosynthesis

We begin with perhaps the most important biogeochemical process of all, photosynthesis. It is a photoautotrophic process, that is, an autotrophic reaction in the presence of light. Nutrients such as phosphate (PO43-) and nitrate ( NO3-) are also necessary for this reaction to occur.

In the early stages of our planet's formation, the atmosphere was very different from that of today. There was no free molecular oxygen (O2), which most of today's life forms require. In fact, oxygen was a very powerful poison for the simple organisms that lived in this early, oxygen-deficient (anaerobic) world. Both the organisms and the earth had to evolve to a stage where the organisms produced oxygen and emitted it to their environment before more advanced life forms could evolve. Photosynthesis, the process of constructing complex organic molecules from simple inorganic ones in the presence of light, was a critical step in the evolution of life and allowed the mass of living organisms to grow to the level of today. In our world, the mass of living organisms on earth is equivalent to about 600 billion tons of carbon. More than 99% of this carbon is in land plants; the remainder is stored in marine plants and in animals.

Photosynthesis is basically a chemical reaction or process in which carbon-, hydrogen-, and oxygen- bearing chemical compounds (carbohydrates) are synthesized from atmospheric CO2 and H2O or another chemical compound that can act as a hydrogen donor. The generalized reaction is

energy + nCO2 + 2nH2A => (CH2O)n + nH2O + 2nA  (3)

where H2A is a hydrogen donor molecule, (CH2O) is a carbohydrate, and n stands for any number. In higher plants, the donor molecule is water, and n = 6. Thus for these plants the specific reaction is

energy + 6CO2 + 12H2O => C6H12O6 + 6H2O +6O2  (4)

For photosynthetic sulfur bacteria the donor molecule is hydrogen sulfide (H2S), and for nonsulfur purple bacteria it is organic compounds.

Incidentally, the carbon dioxide and donor molecule used for photosynthesis are not the only requirements for plant growth. Plants also need nitrogen, phosphorus, sulfur, potassium, and a dozen or so trace elements, like zinc and iron. As we shall see below, human activities are changing the atmospheric concentrations of these nutrients as well as that of carbon, with various possible effects on plants.

The photosynthetic reactions that produce organic matter on land differ from those in the ocean because the proportions of carbon, nitrogen, sulfur, and phosphorus in land vegetation differ from those in marine plankton. The ratio of C:N:S:P in marine plankton is 106:16:1.7:1. Known as the Redfield ratio , this proportion is fairly constant for the surface-dwelling, microscopic plants (phytoplankton) of the world's oceans. The C:N:S:P ratio for land plants is more variable but averages 882:9:0.6:1. The amount of carbon is so much greater in land vegetation because it is stored as cellulose in the structural tissues of trees and grasses.

From this summary, it can be seen that photosynthesis (among other things) links the biogeochemical processes and cycles of the individual organic elements of carbon, nitrogen, phosphorus, and sulfur. These elements, plus hydrogen and oxygen, are the major constituents of organic matter. Those six elements and about a dozen or so minor elements are necessary for the maintenance of organic structures and the physiological functions of living organisms.

Respiration and Decay of Organic Matter

In the life cycle, photosynthesis in plants is balanced by the complementary processes of respiration and decay in plants and animals. In plants, respiration is the breakdown of the complex organic molecules that were formed during photosynthesis. The chemical reactions for respiration and decay are the reverse of those shown above for the production of organic material. The generalized reaction is

C6H12O6 + 6H2O + 6O2 => 6CO2 + 12H2O + energy  (5)

Compare this with reaction 4. The amount of energy released is about 686 kilocalories (kcal) for each mole of C6H12O6 (glucose, the most common form of sugar in living things) that is broken down.

In animals, the respiratory oxidation of foods - that is, the loss of electrons from the carbon in carbohydrates, occurring during digestion - provides energy for a variety of uses, including maintenance of body temperature, muscular movement, and synthesis of complex organic compounds.

During the oxidation of organic matter, CO2, nitrogen-and phosphorus-bearing nutrients, and bioessential trace elements (e. g., iron) are returned to the environment to be used again in the production of more organic matter. When O2 is available, it is the oxidizing agent (oxidant); however, in oxygen-depleted (anoxic) waters, sediments, and soils, other oxidants are used. These include nitrate, sulfate, and iron and manganese oxides.

The chemical equations for respiration and decay, either in an oxygenated or in an anoxic environment, are more complex than the generalized reaction for photosynthesis given above. For example, the chemical composition of average marine phytoplankton - a relatively simple form of life - is (CH2O)106(NH3) 16H3PO4: 106 molecules of carbohydrate, 16 of ammonia, and 1 of phosphoric acid. When dead phytoplankton react with O2 in an oxygenated environment, the products are carbon dioxide, nitric acid, phosphoric acid, and water:

(CH2O)106(NH3) 16H3PO4 + 138O2 => 106CO2 + 16HNO3 + H3PO4 + 122H2O  (6)

For an example of respiration and decay in an anoxic environment, let us consider the reduction of sulfur in sulfate (SO42-) in the pore waters of anoxic sediments. Bacteria use the oxygen originally bound in the sulfate to oxidize organic matter. Again using phytoplankton as the organic matter, the equation for this chemical reaction is

(CH2O)106(NH3) 16H3PO4 + 53SO42- => 106CO2 + 16NH3 + H3PO4 + 53S2- + 106H2O  (7)

This time, in addition to carbon dioxide, phosphoric acid, and water as in reaction 6, the products include ammonia (NH3) and sulfide (S2-).

Weathering of Rocks

Another very important set of biogeochemical processes is that involved with the breakdown of rocks exposed to rain, wind, and ice. Weathering prepares rock for erosion and transportation. Its products are dissolved chemical species and solids derived from changes in the primary minerals of the rock being weathered. The solid products are predominantly clay minerals; there are also dissolved products, predominantly calcium, carbon, and silicon. Ultimately, the products of weathering are either carried by water, blown as dust, or carried by glaciers to the ocean. Of the approximately 20 billion tons of solids and dissolved materials reaching the ocean annually from the land, more than 80% is delivered by rivers. However, high-temperature chemical reactions in the presence of seawater along the great submarine midocean ridges are significant sources of dissolved calcium, silica, and iron for the oceans.

An example of a chemical weathering reaction is the weathering of the mineral albite (the inorganic chemical compound NaAlSi3O8), found in igneous rocks like basalt, to the clay mineral kaolinite [Al2Si2O5(OH)4]. The reaction takes place principally in the presence of soil water and groundwater that contain significant amounts of dissolved CO2. Although the ultimate source of the CO2 is the atmosphere, much of it does not come directly from the air but is produced in soils by the respiration of plants and the decay of dead plants and animals. Because of these processes, the concentration of CO2 in soils may be one or more orders of magnitude greater than that of the atmosphere. The elevated CO2 levels give rise to acidic soil solutions, and these corrosive, low-pH soil solutions are responsible for the weathering of rock minerals like albite:

2NaAlSi3O8 + 2CO2 + 11H2O => Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4H4SiO40  (8)

The products of this reaction, besides the kaolinite, are sodium ion, bicarbonate ion, and monomeric silicic acid.

In regions where human activities such as coal burning release considerable amounts of sulfur and nitrogen oxide gases to the atmosphere, such as the midwestern and eastern United States and southern China, the pH of rainwater and consequently soil water may be lower (more acid) than natural values. This happens because the gases oxidize and react with water in the atmosphere and then rain out as sulfuric and nitric acids, respectively. This phenomenon is the environmental problem of acid deposition (often called acid rain), which in extreme forms can be responsible for increased fish mortalities in lakes and decreased agricultural production.

Deposition in the Oceans

When the solid and dissolved products of weathering reach the ocean, the solids settle out because of their weight and are deposited on the seafloor as gravel, sand, silt, and mud. How long the dissolved products remain in the ocean depends on how long it takes them to enter into a chemical or biochemical reaction. As an example of the periods involved, dissolved sodium in the ocean has a long residence time, about 55 million years. At the other end of the time scale, the residence time of dissolved silica is only 20,000 years.

Calcium and silica

Many of the processes by which dissolved constituents are removed from the ocean involve marine organisms. In today's oceans, dissolved calcium and bicarbonate are precipitated as carbonate minerals in the skeletons of several kinds of marine organisms: planktonic foraminifera (protozoans), pteropods (mollusks), and Coccolithophoridae (algae), and bottom-dwelling (benthic) corals, echinoids, mollusks, and coralline algae. Of the total production of skeletal carbonate in the oceans, equivalent to about 1 billion tons of carbon per year, 80% is redissolved in the ocean as skeletal debris sinks to the seafloor. This efficient recycling is due to the fact that although the surface ocean is oversaturated with respect to calcium carbonate, the deeper sea is undersaturated with respect to this mineral. The remaining 20% of the ocean's carbonate production accumulates in shallow-water and deep-sea sediments.

The amount of carbon in these sediments only accounts for about one-half of the dissolved inorganic carbon brought to the oceans annually by rivers. The other half of the riverborne carbon is released to the ocean and atmosphere when skeletal carbonate minerals are formed.

Dissolved silica is also removed from the oceans in the skeletons of marine organisms. Certain of these organisms - planktonic diatoms (algae), radiolarians (protozoans), dinoflaggelates (protozoans), and benthic sponges - use dissolved silica to form their shells of opaline silica. After these organisms die, most of the opal dissolves, because the oceans throughout their extent are undersaturated with respect to this chemical compound. Only about 40% of the total annual production of skeletal silica sinks below the parts of the ocean that daylight reaches (the euphotic zone). Most of this siliceous material dissolves en route to the seafloor; only 5% of that produced in the euphotic zone accumulates in marine sediments. This amount is about equivalent to the annual input of dissolved silica to the oceans by rivers.

Sodium and magnesium

In contrast to carbon and silica, which are removed from the ocean primarily by biological processes, riverborne dissolved sodium and magnesium are removed to a significant extent by inorganic chemical reactions. Both of these elements are involved in hydrothermal reactions between seawater circulating through midocean ridges and the basalt rock making up the ridges. In the hydrothermal reaction process, sodium and magnesium are removed from the seawater. Sodium is also removed from the ocean by the precipitation of halite (common table salt, sodium chloride) from seawater. This process is very important as a removal mechanism for sodium and chlorine, but only occurs when the right set of climatic and tectonic conditions are achieved. Only seawater in relatively isolated arms of the sea can be sufficiently evaporated to reach halite saturation. Thus, because such environments are scarce today, it is likely that sodium and chlorine brought to the oceans by rivers are currently accumulating in seawater.

Some magnesium is also removed from seawater by chemical processes in the pore waters of sediments. These processes taking place during the burial of sediments are collectively referred to as diagenesis. The relative importance of diagenetic and hydrothermal reactions for the removal of magnesium from seawater is a topic of current scientific research and debate.

We can conclude from the above discussion that the circulation of material through the ecosphere is complex and involves myriad chemical, biological, and geological processes. The system is truly biogeochemical in nature. On all time and space scales, if the composition of the ecosphere is regulated, the regulation is controlled by a complex of interwoven inorganic and organic processes. The maintenance of the equable environment, including climate, that is required for life to exist on earth is a product of this interacting and interwoven web of biogeochemical processes and cycles.



Chapter 2: Biogeochemical Cycles and Climate

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In this chapter, we will look at some representative global biogeochemical cycles and their role in climate. The elements whose cycles are discussed are intimately connected through the organic processes of photosynthesis and respiration and/ or decay. These elements are carbon (C), nitrogen (N), sulfur (S), oxygen (O), and - for completeness, because it is an important biological nutrient - phosphorus (P). We will begin with a discussion of gases whose production or consumption on the earth's surface is accomplished by biological reactions (biogenic gases) and the possible effects of these gases on the earth's climate. To set the stage, the greenhouse effect is discussed briefly here.

Greenhouse gases, which are all naturally biogenic in origin, allow incoming shortwave solar radiation to pass through the atmosphere to the earth's surface, but when part of that radiation (about 45%) is reradiated back toward space as heat (infrared radiation), the gases absorb it and thus retain it in the atmosphere. This is the greenhouse effect. We can thank the natural greenhouse effect for the earth's equable climate. Without it, the planet would be about 33C cooler than its mean global temperature of 15C, that is, -18C. (See the Global Change Instruction Module The Sun-Earth System, by John Streete.)

There can be, however, too much of a good thing. In recent years, the concentrations of these gases in the atmosphere have been rising because of fossil fuel combustion, biomass burning, rice paddy cultivation, and other human activities. This buildup may absorb increased amounts of outgoing infrared radiation, leading to an enhanced greenhouse effect and global warming.

It is interesting and informative to put this present-day worry in the context of public and scientific concern about climate during the 1950s and 1960s. Between about 1940 and 1970, global mean temperatures remained nearly constant, or even declined slightly. There was considerable discussion and concern in the scientific literature and in public forums about global cooling and perhaps another ice age. Much of the discussion below in the context of global warming is applicable to a scenario of global cooling as well. The difference is that in a global cooling, many of the feedbacks mentioned would act in the opposite direction and would probably have different magnitudes of change.

The Biogenic Gases and Climate

It is very likely that during the next century the earth's climate will change due to natural causes. Changes in the amount of solar radiation received by the planet, in the circulation of the atmosphere and the oceans, and in volcanism can affect climate on this time scale. On the longer time scale, if left to its own recourse, the planet will most likely enter another ice age about 10,000-30,000 years from now.

On the other hand, human-induced climate change during the next century is also very likely. The flywheel of population growth and fossil fuel burning is turning rapidly and will be difficult to slow in this time. The global population is growing at a rate of 1.5% per year, a doubling time of 45 years. This rate of growth implies a population of about 10 billion by 2050. All these people will require energy to sustain themselves and to develop their industries, farms, and cities. Most scenarios of future global energy use project a continuous heavy reliance on fossil fuel into the 21st century. Continued fossil fuel burning will result in continued emissions of CO2, methane (CH4), and nitrous oxide (N2O) to the atmosphere. These greenhouse gases will be accompanied by emissions of trace metals, non-methane hydrocarbons (NMHCs), oxides of sulfur (SOx), and the most reactive oxides of nitrogen (NO and NO2, collectively known as NOx). The latter three groups of chemical compounds react with other chemical components of the climate system, particularly the hydroxyl radical (OH*). Also, SOx and NOx are the principal constituents in acid deposition, and NOx and NMHCs are involved in the formation of ozone (O3), another greenhouse gas, in the troposphere. The unchecked accumulation of these gases in the atmosphere could lead to an uncomfortably warm planet.

The burning of fossil fuels and the burning of forests and other biomass are the principal human-induced, or anthropogenic, emissions of most biogenic gases to the earth's atmosphere. Also, fossil fuel burning and changes in land use (such as deforestation) are responsible for many of the global environmental problems the people of the world face today. Fossil fuel burning alone accounts for perhaps 80% of sulfur dioxide (SO2) emissions from the land surface to the atmosphere, 50% of carbon monoxide, 50% of NOx, 20% of methane, 20% of NMHCs, 5% of ammonia, and 4% of nitrous oxide. It is also responsible for 70-90% of anthropogenic CO2 emissions to the atmosphere. This amount is equivalent to about 10% of the natural CO2 emissions from respiration and decay.

As mentioned previously, C, N, P, and S, besides O and hydrogen (H), are the principal elements that make up living matter. The biogeochemical cycles of these elements are intimately coupled through biological productivity and respiration and/ or decay. Ecosystems take energy from their surrounding environment. The net result is production of organic matter, more disorder on the planet (increased entropy), and waste. The waste may act as a pollutant. The biogenic gases of carbon, nitrogen, and sulfur are a consequence of this entropy production. Their fluxes maintain the earth's atmosphere in a state of disequilibrium.

The natural sources of these biogenic gases are processes at the earth's surface or chemical reactions in the atmosphere. The processes by which biogenic gases and other components cycle through the coupled C-N-P-S-O system, although in some environments operating close to equilibrium, are principally controlled by the rates at which the processes operate.

During the last half century, scientists have tended to specialize. Consequently, most global environmental systems have been little studied or studied only in a piecemeal fashion. Only recently has attention been paid to the coupled earth-surface system of atmosphere, hydrosphere, biosphere, cryosphere, and shallow lithosphere. Basic information on global reservoir sizes and fluxes (e.g., biological productivity) is lacking or is only partly known. An example of this lack of data is the estimates of tropical forest biomass, which vary by a factor of 2 or 3 for Amazonia alone.

It is very unlikely that the anthropogenic fluxes of gases to the atmosphere will substantially decline as we enter the 21st century. Population growth and our global reliance on fossil fuels as an energy source make such a scenario highly improbable. Thus, continued global environmental change is a virtual inevitability. It is likely that, by the middle of the next century, the atmospheric concentration of CO2 will be double what it was before the Industrial Revolution (to date, it has increased about 30%), and concentrations of other greenhouse gases will also increase. Such a change in the composition of the atmosphere portends a strong probability of climate change.

Historical Framework

It is worthwhile considering at this stage how the earth's biogeochemical cycles and climate system functioned prior to human interference. It is impossible to consider the functioning of all the biogeochemical cycles of concern because of space limitations. Only the global biogeochemical cycles of carbon and oxygen are used as examples in this section. We will end with a brief summary of environmental conditions just prior to major human interference in the biogeochemical cycles and climate system.

Carbon

Carbon composes approximately 50% of all living tissues. In the form of carbon dioxide, it is necessary for plants to grow. Carbon dioxide also helps to sustain an equable climate on earth. The concentration of carbon dioxide in the atmosphere has varied during the geologic past, but has remained within limits that permit life to exist on earth. Carbon dioxide is cycled throughout the spheres of earth on different time scales. We can refer to these scales as short-, medium-, and long-term. Figures 3 and 4 illustrate the processes involved in these time scales.

Figure 3. Biogeochemical Cycle of Carbon Prior to Human Interference
The biogeochemical cycle of carbon prior to human interference, showing (a) the short-term cycle, i.e., photosynthesis and respiration; (b) the long-term cycle, involving accumulation of organic C and CaCO3 in marine sediments, their subduction, their alteration, and the return of CO2 to the atmosphere via volcanism; and (c) the medium-term cycle, involving storage of C in organic materials in sedimentary rocks. Ultimately this carbon is returned to the earth's surface and undergoes weathering; in the process, O2 is taken out of the atmosphere and CO2 is returned.

The short-term carbon cycle

Photosynthesis is part of the short-term carbon cycle (on the order of years). We can look at the short-term cycling of carbon as carbon dioxide by beginning with the producers of organic carbon, the plants. Carbon in the form of atmospheric carbon dioxide is removed from the air by plants. This removal occurs both on land - for example, in forests and grasslands - and in water - for example, in lakes, rivers, and the surface waters of the oceans. The primary producers, the photosynthetic phytoplankton and benthic plants in the oceans and plants on the terrestrial surface, transform inorganic carbon as carbon dioxide into organic carbon within their tissues. Light and nutrients, like phosphate and nitrate, are necessary for this reaction to occur. Some of the energy from the light is used in the growth of plants, and some remains stored in the tissues of plants as carbohydrates.

Plants remove about 100 billion tons of carbon as carbon dioxide from the global atmosphere each year, which is about 14% of the atmosphere's total carbon. Most, but not all, of the carbon dioxide taken from the atmosphere during photosynthesis is returned to the atmosphere during respiration and decay. The annual removal rate of atmospheric carbon dioxide in photosynthesis is slightly larger on land than in the ocean.

Figure 4. Major Reservoirs and Fluxes in the Biogeochemical Cycle of Carbon
The major reservoirs and fluxes in the biogeochemical cycle of carbon. The shapes surrounding the spheres are called boxes. Arrows represent the processes and their directions that transfer carbon from one box to another. The carbon cycle can be conceived of as a series of interlocking circuits in the reservoirs of atmosphere, biosphere, hydrosphere, and shallow lithosphere (crust). In our time, the cycle would be in balance if it were not for human interference by burning of fossil fuels, cement manufacturing, and land-use activities (e.g., deforestation) (after Skinner and Porter, 1987).

After photosynthesis, carbon may next be transferred to a consumer organism if the plant is eaten for food. The carbon stored in the tissue of the plant enters an animal's body and is used as energy or stored for growth. Land animals, such as cows and deer, are the primary consumer organisms. Aquatic plants are eaten by zooplankton (small sea animals) and larger animals. When an animal breathes, some of this carbon that was taken up from plants is released from the animal's body as carbon dioxide gas.

Besides the carbon stored above ground in living and dead vegetation, there is carbon below ground in the root systems of terrestrial plants. When the plants die, some of this carbon may be released as carbon dioxide or methane gas to the air trapped in the soil, or it may accumulate in the soil itself as dead organic material. This dead organic matter may be ingested by consumer organisms, such as insects and worms living in the soil.

Some of the organic carbon generated in land environments is weathered and eroded, and the organic debris is transported by streams to the ocean. In the ocean, some of this debris, along with the organic detritus of dead marine plants and animals, settles to the ocean floor and accumulates in the sediments. However, some of the debris is respired in the ocean to carbon dioxide. This carbon dioxide may leave the ocean and be transported over the continents, where it is used again in the production of land plants.

The long-term carbon cycle

The long-term carbon cycle (on the order of tens to a hundred million years) requires that we consider the earth's history over the last 600 million years or so - the period covered by the fossil record. Figure 5 defines the terms and intervals of geologic time.

The long-term cycling of carbon (Figures 3 and 4) involves interconnections between the cycling of the minerals calcium carbonate (CaCO3) and calcium silicate (CaSiO3). This series of processes dates back to the beginning of plate tectonics. This cycling includes not only the land and ocean reservoirs but also that of limestone rocks. Limestone rocks are mainly composed of calcium carbonate and are the fossilized skeletal remains of marine organisms or, less commonly, inorganic chemical precipitates of calcium carbonate. Limestones are great storage containers for carbon. Most of the carbon near the earth's surface is found in these rocks or in fossil organic matter in sedimentary rocks. Weathering and erosion of the earth's surface result in the leaching of dissolved calcium, carbon, and silica (SiO3) from limestones and rocks containing calcium silicate.

The dissolved substances produced by weathering are transported to the ocean by rivers. As discussed in on p.9, they are then used to form the inorganic skeletons of benthic organisms and plankton, which are composed of calcium carbonate and silica. During formation of the calcium carbonate skeletons, the carbon dioxide derived from the weathering of limestone is returned to the atmosphere.

Figure 5. Geologic Time Scale
The geologic time scale - the calendar of the earth. Geologic time is divided into the intervals of eon, era, period, and epoch. The boundaries of these intervals are based on absolute age dating using the radioactive decay of certain elements (e.g., uranium, potassium, rubidium, and carbon) in rocks; the distribution of fossilized plants and animals found in the rocks; and certain worldwide geologic events recorded in the rocks (after Skinner and Porter, 1987).

When marine animals and plants die, their remains settle toward the seafloor, taking the carbon stored in their bodies with them. En route, their organic matter is decomposed by bacteria, just as on land. Some shells may dissolve. Thus, animal and plant organic and skeletal matter is turned back into dissolved carbon dioxide, nutrients, calcium, and silica in the ocean. This carbon dioxide is stored in the deeper waters of the oceans for hundreds to a thousand or so years before being returned to the atmosphere when the deep water moves upward (upwelling), usually because of divergent movements of surface water.

Some of the animal and plant plankton sinks to the bottom, where the carbon in the organic matter and shells escapes degradation and becomes part of the sediment. As the seafloor spreads through plate tectonics, the sediments containing the remains of marine plants and animals are carried along to subduction zones, where they are transported down into the earth's mantle. At the severe pressures and high temperatures in the subduction zones, organic matter is decomposed and calcium carbonate reacts with the silica found in the subducted rocks to form rocks containing calcium silicate.

During this metamorphism, carbon dioxide is released and makes its way into the atmosphere in volcanic eruptions and via hot-spring discharges. Once in the atmosphere, it can then combine with rainwater. The rainwater falls on the land surface and seeps down into the soils, where it picks up more carbon dioxide from decaying vegetation. This water, enriched in carbon dioxide, weathers and dissolves the compounds of calcium and silica found in rocks of the continents. The cycle begins again.

This series of processes has been active for at least 600 million years, since the advent of the first organisms that made shells (and were therefore the first to leave fossils). The processes were important even earlier in earth's history, when calcium carbonate was deposited in the ocean by inorganic processes.

Figure 6. Model Calculation of Atmospheric Carbon Dioxide During the Last 600 Million Years
Model calculation of atmospheric carbon dioxide during the last 600 million years. The horizontal axis shows time in millions of years before the present (top) and geological time period (bottom). The left vertical axis shows the number of times today's C level that existed in the atmosphere of the time; the right vertical axis shows the amount of CO2 in the atmosphere. For example, 500 million years ago there was about 14 times as much CO2 in the atmosphere as there is today, with a total amount of about 37 x 1018 grams (after Berner, 1991).

Atmospheric CO2 levels of the past

One outcome of changes in the rates of processes in the long-term biogeochemical cycling of carbon is that atmospheric carbon dioxide has varied in a quasicyclic fashion during the last 600 million years of earth's history. Robert A. Berner of Yale University and colleagues have developed models of the carbon cycle to calculate these variations. Figure 6 shows the results of one such calculation.

Periods of high atmospheric carbon dioxide levels are the result mainly of intense plate tectonic activity, with increased metamorphism of limestone and release of carbon dioxide to the atmosphere from volcanoes. These high carbon dioxide periods are often referred to as hothouses or greenhouses. They are also periods of relatively high sea level; for example, in the Cretaceous and Cambrian periods, much of what would become the present continent of North America was covered by water. This flooding of the continental landscapes was due principally to the large size and volume of the midocean ridges, caused by intense plate tectonic activity. The increase in ridge volume led to the displacement of ocean water onto the continents of the time.

Periods with lower atmospheric carbon dioxide levels, such as the Carboniferous through Triassic and much of the Cenozoic, are the outcome of less intense plate tectonic activity and increased removal of carbon dioxide from the atmosphere by weathering. These intervals, which include the present era, are extended cool periods (ice houses) in the climatic history of the earth. They are also times of relatively low sea level due to a decrease in the volume of the midocean ridges.

Biological and other factors also play a role in regulating atmospheric carbon dioxide levels over the long term. For example, the lowering of atmospheric CO2 from the high levels of the mid-Paleozoic era, about 400 million years ago, was not simply the result of the waning intensity of plate tectonic processes. It followed the evolution of land plants, which withdrew CO2 from the atmosphere by photosynthesis. Similarly, the lowering after the Cretaceous period, about 100 million years ago, followed the appearance of flowering plants, which resulted in an increase in weathering rates and withdrawal of CO2 from the atmosphere.

The important point is that atmospheric CO2 has varied by a factor of perhaps more than ten during the last 600 million years of the earth's history. This variation certainly has had climatic implications, because CO2 is an important greenhouse gas. In fact, for much of the last 600 million years, the planet had a different atmospheric composition and a more equable climate than that of today.

The medium-term carbon cycle

Medium-term cycling of carbon dioxide (millions to tens of millions of years) involves organic matter in sediments; coal, oil, and gas; and atmospheric oxygen (Figures 3 and 4). It commences, as does the short-term cycling, with the removal of carbon dioxide from the atmosphere by its incorporation into plants and the accumulation of the dead plant and animal carbon in sedimentary organic matter (the dead and fossilized remains of plants and animals). When dispersed throughout a sedimentary rock, this organic matter is termed kerogen. Shales are very fine-grained sedimentary rocks that are often rich in kerogen. Coal, oil, and gas deposits are also the altered remains of the soft tissues of plants and animals which have accumulated in a geographically restricted area.

Coal is derived mainly from terrestrial plant material, which is often deposited in swampy environments. The plant material is altered when the swamp sediments are buried. If buried deep enough, the dead plant material may be substantially changed because of the increased temperatures and pressures at depth. Different types of coal are formed by varying conditions of temperature and pressure. Anthracite - a hard, dense, black coal - is formed by alteration of plant material at a relatively high temperature and pressure. Bituminous, brown coal is formed under less intense conditions. Peat, used as a fuel in some parts of the world, is little-altered plant material that has not been buried deeply. It is high in carbon. Peat is an important component of tundra areas in the Northern Hemisphere.

Oil and gas represent highly altered organic matter, principally the altered remains of marine phytoplankton that were deposited on the seafloor. During burial, these organic materials are broken down at elevated temperature and pressure, forming oil and gas. The oil and gas may migrate hundreds of miles in the subsurface before coming to rest in large accumulations in the voids of rocks. Often, oil and gas are formed in shales; but as temperature and pressure increase with depth, they move to more coarse-grained rocks like sandstones and limestones. It is in these latter rocks, dating from the Cretaceous and Cenozoic periods, that the great oil and gas reserves of the world are found, like those of the Persian Gulf.

These deposits of coal, oil, and gas come from organic carbon that has escaped respiration and decay. Thus, they represent carbon dioxide that has been removed from the atmosphere. The same is true of the kerogen dispersed as fine-grained materials in the sedimentary rocks. Because these materials were buried, the oxygen that would have been used in their decay has remained in the atmosphere. Eventually, however, the carbon in these deposits and in kerogen is recycled into the atmosphere, returning carbon dioxide to that reservoir and removing oxygen. This happens when these fossil fuel deposits and kerogen are uplifted by plate tectonic forces after millions of years of burial and exposed to the atmosphere. When this occurs, the oxygen that previously accumulated in the atmosphere reacts with the coal, oil, gas, and kerogen. The reaction involves the decay of these organic materials in the presence of oxygen (equation 5). This results in the removal of oxygen from the atmosphere and the return of carbon dioxide to the atmosphere. The ongoing dynamic cycle is complete.

Fossil fuel is a nonrenewable energy source, because coal, oil, and gas deposits take millions of years and specific environmental conditions to form. The mining of these deposits brings these materials back to the surface much more rapidly than natural processes do. The stored energy from the long-dead organisms is released in the form of heat when the coal, oil, and gas are burned. This fossil fuel energy keeps us warm, powers our cars, and moves the machinery of industry. It is also a main cause of environmental pollution, because a byproduct of fossil fuel burning is the release of gases and particulate materials into the environment. Climatic change is an important potential global environmental effect of the release of carbon dioxide and other gases to the atmosphere by combustion.

In summary, carbon is found in all four major surface spheres of the earth. In the ecosphere, it is essential to every life form, occurring in all organic matter. In the atmosphere, it is found as the gases carbon dioxide, methane, and several other compounds. It occurs as carbon dioxide dissolved in lakes, rivers, and oceans in the hydrosphere. In the earth's crust - part of the lithosphere - it is found as calcium carbonate originally deposited on the seafloor, as kerogen dispersed in rocks, and as deposits of coal, oil, and gas. It is because carbon is stored in the large sedimentary reservoirs of limestone and fossil organic carbon, and not in the atmosphere, that life on earth is possible. If all this carbon were stored in the atmosphere, there would be about 30 times the existing amount of CO2. The result would be a substantial heating of the atmosphere because of the increased absorption of the long-wave infrared radiation emitted from the surface of the earth. The greenhouse effect would be very strong, and our planet would probably have a temperature like that of Venus: 460C!

Oxygen

Oxygen composes 20.9% of the gases of the atmosphere. The cycling of oxygen (Figure 7) is strongly coupled to that of carbon (Figure 4). Oxygen is produced by plants during photosynthesis, when carbon dioxide is consumed. It is removed by respiration and decay, when carbon dioxide is produced. This is a short-term, nearly balanced cycle on land, because the amount of oxygen produced yearly by land plants is about equivalent to the amount they use in the processes of respiration and decay. On average, it takes about a decade for oxygen and carbon dioxide to cycle through living plants. However, a little terrestrial organic matter that has not undergone respiration or decay, such as leaves and trunks of trees and smaller-sized organic debris, leaks from the land to the ocean via rivers. In the ocean, some of this organic detritus escapes destruction and is deposited in the sediments.

In the oceans, phytoplankton produce slightly more oxygen than is consumed during the respiration and decay of marine life. As a result, oxygen is released to the atmosphere. The organic carbon not decayed by this oxygen, along with some of the terrestrial organic detritus mentioned in the preceding paragraph, is deposited on the seafloor and accumulates in the sediments of the ocean. If this accumulation were not counteracted by other processes, and no other factors were to intervene, all the carbon dioxide in the atmosphere would disappear in less than 10,000 years. The oxygen content of the atmosphere would double in less than several million years. Fortunately, the overproduction of oxygen in the oceans is balanced by the weathering of fossil organic carbon and other materials in rocks on land. During this process, carbon dioxide is returned to the atmosphere.

If the burial of organic carbon in sediments were enhanced, oxygen might accumulate in the atmosphere. In fact, it seems to be the case that times of high organic carbon burial in the past gave rise to high atmospheric oxygen levels. Figure 8 is a model calculation of atmospheric oxygen concentration variations during the last 600 million years. Just before the Carboniferous, vascular plants evolved and spread over the continents. Their remains were a new source of organic matter resistant to degradation by atmospheric oxygen. During the Carboniferous and Permian, large quantities of vascular plant organic matter were buried in the vast coastal lowlands and swamps of the time. This material became the coal deposits mined from rocks of Carboniferous and Permian age today. This large accumulation of organic matter gave rise to the high atmospheric oxygen levels of the late Paleozoic. Coal deposits are also important in Cretaceous- and early Cenozoic-age rocks, another time of high atmospheric oxygen concentrations.

Figure 7. Biogeochemical Cycle of Oxygen
The biogeochemical cycle of oxygen. This cycle is strongly coupled to that of carbon (Figure 4). The boxes represent the major reservoirs of oxygen, and the arrows the fluxes of oxygen from one box to another. The heavier the arrow, the larger the flux. The broken lines show the flow of carbon in sedimentation on the ocean floor, burial in sediments, and uplift by plate tectonic processes. When uplifted, organic C is oxidized by oxygen in the atmosphere, and CO2 is released (after Andreae, 1987).

Environmental Conditions Before Human Interference

From the above discussion, we can conclude that the carbon and oxygen biogeochemical cycles have changed throughout geologic time. These changes have led to changes in atmospheric composition and climate. Just prior to significant human interference in the ecosphere, environmental conditions were also changing naturally because of a variety of factors. These factors include external forcings, such as changes in the orbit of the planet earth and in the flux of solar radiation to the planetary surface, and internal forcings, involving the behavior of the earth's ocean-atmosphere-land-biota-cryosphere system.

For about 1.6 million years, the earth has experienced a series of cold and icy times known as ice ages or glacial stages. These periods of extensive glaciation of the continents alternated with shorter and warmer periods, interglacial stages like today. During the interglaciations, the great glaciers that covered the continents of Europe and North America to depths of two kilometers or more melted. The glaciers retreated to geographical positions similar to those of the present continental ice sheets of Greenland and Antarctica. As the glaciers waxed and waned through the last 2 million years, global temperatures went up and down, as did sea level. The composition of the atmosphere and other environmental conditions changed.

The earth, before extensive human interference in its biogeochemical cycles, was recovering from the climax of the last great glaciation 18,000 years ago. The recovery has not been smooth. There have been times in the past 18,000 years when the planet cooled quickly. Also, there have been periods when the earth was warmer than today. However, during the past several centuries, the global environment has changed substantially and rapidly. Atmospheric trace gas concentrations, matter in runoff, temperature, and other indicators have increased in magnitude. A major reason for all the changes is the impact of human activities on the environment.

Figure 8. Model Calculation of Atmospheric Oxygen During the Past 600 Million Years
Model calculation of atmospheric oxygen during the past 600 million years. The dashed line across the figure shows today's atmospheric concentration of O2. The left vertical axis shows the atmospheric O2 level; the right vertical axis shows O2 as a percentage of the total atmospheric gases. The horizontal axis shows time in millions of years before the present and geological time period (after Berner and Canfield, 1989).



Chapter 3: The Modern Coupled C-N-P-S-O System

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The modern coupled C-N-P-S-O system is multidimensional and complex, with numerous processes, reservoirs, and fluxes. All these attributes are difficult to portray in detail. To see the relationships between biogeochemical cycles and climate, we will examine those biogeochemical compounds that play a role in climatic change. In particular, we will focus on the problem of an enhanced greenhouse effect and global warming brought about by human activities. Keep in mind that the cycles also respond to a global cooling and can feed back into any such cooling. The processes discussed below would respond to cooling in an opposite sense, to some extent, and probably with different magnitudes of change.

Figures 9 - 20 show the biogeochemical cycles of interest. The figures show that the burning of forests and other biomass and fossil fuel combustion are the major human sources of most biogenic gases in the earth's atmosphere. Many of the natural exchanges of gases between the earth's surface and the atmosphere in these cycles are driven by biological processes, which involve bacteria. In the atmosphere, many of the gases are oxidized, usually by hydroxyl radical (OH*), a trace gas that is the atmosphere's main cleansing mechanism. The oxidized materials then return to the earth's surface, through biological production of plants on land and in the ocean and by wet and dry deposition.

Figure 9. Modern Global Biogeochemical Cycle of Methane
Part of the modern global biogeochemical cycle of methane, emphasizing exchanges of methane between the earth's surface and atmosphere and the fate of the gas in the atmosphere. The atmospheric reservoir is shown as a circle, including the amount of carbon in teragrams (1012grams), equivalent to million metric tons (Mt). Flux ranges are also given in Mt C/yr; arrows indicate the directions of the fluxes. The proportion of C in the atmosphere is given in parts per million by volume (ppmv), the residence time in years, and the accumulation rate per year in both Mt and parts per billion by volume (ppbv). The chemical reaction at the top of the figure shows the fate of methane that escapes to the stratosphere (after Mackenzie, 1995; Houghton et al., 1996).

Many of the natural processes shown in these figures have feedbacks that affect the accumulation of trace gases in the atmosphere and hence affect the global climate. Because these feedbacks are linked to biological processes, they are termed biotic feedbacks. We shall explore the nature of the feedbacks in the C-N-P-S-O system below.

Figure 10. Modern Global Biogeochemical Cycle of Carbon Monoxide
Part of the modern global biogeochemical cycle of carbon monoxide, emphasizing exchange of the gas between the earth's surface and its atmosphere and the fate of the gas in the atmosphere. See Figure 9 for an explanation of the units and abbreviations used. Notice, as with methane, the role of OH* as an agent of oxidation of the reduced carbon gases (after Mackenzie, 1995; Houghton et al., 1996).



Chapter 4: Carbon Cycles

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The CH4-CO-CO2 Connection

Figures 9, 10, 11 illustrate the biogeochemical cycles of methane (CH4), carbon monoxide (CO), and carbon dioxide between the earth's surface and atmosphere. We have a reasonably good understanding of the major processes associated with the exchange of these gases between the surface and the atmosphere and of the gases' atmospheric reservoir sizes. However, the fluxes associated with the various processes are less well quantified.

All three of these gases have been accumulating in the atmosphere because of human activities. Since the late 18th century, global atmospheric CO2 concentrations have risen from about 280 to about 360 parts per million by volume (ppmv), a change of approximately 30%. This increase is principally the result of fossil fuel burning and land-use activities such as deforestation, which add CO2 to the atmosphere when the burning carbon combines with atmospheric molecular oxygen. Because of this increase in CO2 from human activities, the atmospheric reservoir of oxygen gas has been reduced by less than 1%. Thus, in the burning of fossils fuels, there is no concern about depletion of atmospheric CO2.

The global concentration of CH4 during this period has more than doubled from about 800 to 1,720 parts per billion by volume (ppbv). As with CO2, this increase is due to human activities, and again as with CO2 those activities include burning of biomass and fossil fuels. For methane, though, rice paddies, farm and ranch animals (which produce methane as they chew their cud), and rotting landfills are also important. Landfills emit CH4 as methanogenic bacteria decompose the wastes in an anaerobic environment.

Carbon monoxide has also been increasing in concentration in the atmosphere, particularly in the Northern Hemisphere where important anthropogenic emission sources are located. The rate of increase in the 1980s was approximately 1 ppbv per year, principally because of the same culprits, fossil fuel and biomass burning (Figure 10).

The rates of increase of all three atmospheric carbon gases slowed during the late 1980s and early 1990s because of a variety of natural and human-induced causes.

Major processes in the CH4 and CO cycles

CH4 and CO are reduced gases, that is, they may react with other chemical compounds by the loss of electrons from the carbon in the compounds. The carbon in CH4 has a valence of -4; the carbon in CO has a valence of +2. The biogeochemical cycles of these two gases and CO2 are connected because OH* oxidizes CH4 and CO to CO2, which has a valence of +4. This CO2 can then be used by plants and other organic matter. The decay of this organic matter, in turn, leads to the release of CH4 and CO from the earth and oceans.

The major natural processes involving exchange of CH4 between the earth's surface and the atmosphere (shown on the right-hand side of Figure 9) are

The major sink of CH4 is the atmosphere, where it is oxidized principally by reaction with OH* (top right-hand side of Figure 9). Approximately 300-450 million tons of carbon annually are oxidized this way. During the past hundred years, the change in the concentration of methane in the atmosphere could be responsible for about 20% of the warming due to an enhanced greenhouse effect. If CH4 concentration were to double in the atmosphere, one would expect, based on the ability of the gas to warm the atmosphere, a rise of approximately 0.3C in global mean temperature.

Soils are both a natural source of CO to the atmosphere and a natural sink of the gas from the atmosphere (Figure 10). The source is the bacterial decomposition of organic matter in soils, and bacteria and algae in the ocean. The sink is also the result of bacterial processes.

The atmosphere is also both a source and sink of CO. It is the most important sink of CO, as with CH4, through the oxidation of CO to CO2 via OH*. This process transfers approximately 1,200 million tons of carbon to the atmospheric CO2 reservoir annually (Figure 10). The atmosphere is a source of CO because nonmethane hydrocarbons (NMHCs) are oxidized to that compound by OH*.

CH4 and CO feedbacks to global climate change

Respiration and the bacterial decomposition of organic matter emit CH4 and CO from soils to the atmosphere. Because the rates of these processes increase with increasing temperature, the emissions might increase in a world that was warming. This would be a positive feedback on any initial warming of the earth. However, because soils are also sinks for atmospheric CH4 and CO, and emissions of these gases are sensitive to the amount of moisture in the soil, the situation is more complex.

In general, it is likely that warmer temperatures in the high northern latitudes will lead to an increase in CH4 fluxes from CH4 trapped in permafrost, decomposable organic matter frozen in permafrost, and decomposing CH4 gas hydrates - substances in which CH4 is locked in a structure of water ice. These hydrates are stable only at low temperatures or under pressures exceeding ten atmospheres. They are stored in sediments under shallow seas, particularly in the Arctic region, and also in permafrost. The flux of methane escaping to the atmosphere from this source is poorly known today (Figure 9).

Because CH4 and CO react easily with OH* in the atmosphere, increases in their atmospheric concentrations (and in those of the reduced N and S gases) would affect the concentration of OH*, thus having an impact on the atmosphere's ability to cleanse itself. It is possible that an increase in the flux of CO to the atmosphere, because of the gas' short residence time there, could deplete OH* and consequently cause less CH4 to be removed from the atmosphere. If so, CH4 would accumulate faster in the atmosphere, leading to an increased rate of global warming. However, the overall effect is likely to be small. Furthermore, increased atmospheric CH4 would probably lead to increased production of water vapor in the stratosphere (Figure 9), where methane and OH* react to form water vapor and CH3. Water vapor is a greenhouse gas, and its increased production in the stratosphere is a positive, but minor, feedback in a scenario of a warming earth.

In summary, although the various effects are difficult to quantify, it is likely that an initial warming of the climate would lead to a net increase in fluxes of CH4 and CO to the atmosphere and accumulation there. This situation constitutes a positive feedback on the concentration of these gases in the atmosphere and hence on global warming. Any global cooling would probably result in an opposite effect.

The CO2 Cycle

The most important trace gas contributing to the potential of an enhanced greenhouse effect is CO2. From 1765 to 1995, CO2 accounted for about 60% of the human-induced warming due to the accumulation of greenhouse gases in the atmosphere. A doubling of the concentration of CO2 in the atmosphere could eventually lead to an increase in global mean temperature of about 1.5-4.5 C.

Figure 11 shows the major processes, both natural and human, affecting the exchange of CO2 between the atmosphere and earth's surface. Estimates of the fluxes associated with the processes are also shown. Let us begin our exploration of Figure 11 by considering the ocean fluxes. The total exchange of CO2 between the ocean and atmosphere amounts to about 90 billion tons of carbon per year. Much of this carbon exchange involves

Almost half of this exchange - close to 45 billion tons of carbon annually - is involved in net primary production due to CO2 fixation by photoautotrophic marine plankton. A slightly larger amount of CO2 is returned to the atmosphere by respiration and decay in the ocean. The difference between the two amounts implies that the ocean, prior to human interference in the global biogeochemical cycle of carbon, was a net heterotrophic system and supplied CO2 to the atmosphere. The CO2 released from the ocean was subsequently used in organic production on land.

Figure 11. Modern Global Biogeochemical Cycle of Carbon Dioxide
Part of the modern global biogeochemical cycle of carbon dioxide, emphasizing its exchanges between the earth's surface and its atmosphere. See Figure 9 for an explanation of the units and abbreviations used. If there were a doubling of the CO2 concentration in the atmosphere, one would expect a rise in temperature of 2-3C at the earth's surface and in the lower atmosphere. CO2 accounts for about 60% of the enhanced greenhouse effect. Note that fossil fuel burning and land-use practices released to the atmosphere about 7,700 Mt of carbon in 1995. The atmosphere was a major sink for this anthropogenic carbon, but the ocean and the terrestrial biosphere took up about 50% of it. Compare this figure with Figure 4 (after Mackenzie, 1995; Houghton et al., 1996).

In contrast, the terrestrial biota were a net autotrophic system before humans interfered with the carbon cycle. In other words, the land biota produced more organic carbon than was consumed. (Compare the rates for net primary production and respiration-decay in Figure 11.) This excess organic carbon, approximately 400 million tons annually, eventually made its way into river waters in particulate and dissolved forms. It was then transported to the oceans, where some portion of it decayed. The CO2 generated by this decay led to the pristine heterotrophic state of the ocean. In addition, the precipitation of calcium carbonate in the ocean also led to the evasion of CO2 to the atmosphere (Figure 11).

During the past several centuries, as fossil fuel burning and land-use changes added CO2 to the atmosphere, the ocean went from a net source to a net sink of CO2 from the atmosphere. The concentration of CO2 in the atmosphere rose, and the gradient of CO2 concentration between the atmosphere and ocean changed. This change favored the net uptake of CO2 in the ocean by solution of the gas in surface seawater (see Equation 9, below). This oceanic sink of anthropogenic CO2 was on the order of 2,000 million tons of carbon per year during the 1980s, although it varies annually.

Continuing with the exploration of Figure 11, we see that volcanic emissions of CO2 from land and under the sea amount to about 60 million tons of carbon annually. This flux varies each year with the intensity of volcanic activity. The processes of diagenesis and metamorphism create a flux of about the same size as that of volcanism. These processes occur as buried sedimentary materials change; calcium carbonate and silica are converted to calcium silicate and CO2. The CO2 is consequently released to the atmosphere via volcanism, hot springs, and seepage from deep within sedimentary basins.

Weathering of minerals on land removes CO2 from the atmosphere, and weathering of sedimentary organic matter (kerogen and fossil fuel) at the land surface by oxygen adds CO2 to the atmosphere (Figure 11). As mentioned in Chapter 2, the balance between the weathering fluxes and those of volcanism, diagenesis, and metamorphism has changed throughout geologic time. These changes are in part responsible for the long-term variation in atmospheric CO2 during geologic time (Figure 6).

Keep in mind, as mentioned in the previous section, that CO2 is coupled to the reduced carbon gases through the oxidation of CH4 and CO by OH* in the atmosphere. Therefore, any initial warming of the planet and consequent enhanced emissions of CH4 and CO from the earth's surface because of human activities could potentially lead to an increase in the accumulation of CO2 in the atmosphere, a positive feedback.

The imbalance in the cycle and feedbacks

The notorious problem with atmospheric CO2 today is the difficulty in balancing the known sinks with the source from fossil fuel combustion and land-use activities such as biomass burning. The problem is illustrated in Figure 11.

We know that carbon is accumulating in the atmosphere at the rate of 3,300 million tons annually. We also know that the amount of carbon put into the atmosphere from land-use activities is 1,600 million tons of carbon per year (with an uncertainty of 1,000 million tons, plus or minus), and the flux from fossil fuel burning plus cement manufacturing is 6,100 million tons per year - a total of around 7,700 million tons. Subtracting the known atmospheric accumulation of carbon from the known source, we see that the fate of roughly 3,000 to 4,000 million tons of human-produced carbon per year is not known. The lower number is within the rather large error margins of recent estimates of how much anthropogenic CO2 the ocean takes up annually, but the upper amount is out of the range of these estimates.

These numbers suggest that there may be another important sink (sometimes called the "missing sink") of anthropogenic CO2 besides the atmosphere and ocean. This sink has been hypothesized to be the Northern Hemisphere's midlatitude forests and soils. If that is the case, a rather strange situation has developed with respect to the modern carbon cycle. While tropical rain forest ecosystems are sources of carbon to the atmosphere because of deforestation at rates of about 1% of the world's forested area per year, higher-latitude terrestrial ecosystems may be sinks of carbon.

Such a net sink implies that some processes of carbon storage on land have changed. Some or all of the following changes may have occurred.

The first process on this list, carbon dioxide fertilization, is a potentially strong negative feedback on the accumulation of anthropogenic CO2 in the atmosphere and hence on global warming. We know from experiments on plants and small ecosystems that almost all agricultural crops and a few perennial plants, when subjected to increased CO2 levels, will increase their rates of photosynthesis and growth. If this enhancement should also occur in large ecosystems, like forests, CO2 would be withdrawn from the atmosphere and stored in plant organic matter.

Humans have put excess phosphorus, nitrate, and ammonia nutrients into the environment by applying fertilizers to the land surface, burning fossil fuels and biomass, and discharging sewage containing nitrogen and phosphorus. These nutrients can stimulate increased plant growth in the soil and aquatic environments. This eutrophication of both land and marine environments is a negative feedback on accumulation of carbon in the atmosphere from anthropogenic sources and hence is a negative feedback on warming of the earth. Total land and ocean eutrophication may amount to a billion tons of carbon per year.

Other processes and feedbacks involving CO2

What about other processes and feedbacks in the biogeochemical cycle of CO2 that might change with a global warming? Perhaps one of the strongest potential positive biotic feedbacks in the terrestrial environment is the effect of increasing temperature on photosynthesis and respiration. The rates of both processes in vegetation and microbial life increase with increasing temperature. However, respiration rates are more sensitive to temperature change. In a warming world, increased respiration could temporarily increase the flux of CO2 to the atmosphere by as much as 1-3 billion tons of carbon per year. This increased flux is a potentially strong positive feedback on CO2 accumulation in the atmosphere and hence on global warming.

A change in the ocean temperature would affect the amount of dissolved inorganic carbon in seawater. The equation relevant to this effect is

CO2(g) + H2O + CO32- = 2HCO3-  (9)

The (g) is gas, and the equal sign implies the reaction is an equilibrium representation of the process.

For a warmer ocean surface, this reaction moves from right to left - the bicarbonate ions break down into carbon dioxide, water, and carbonate. Thus the concentration of CO2 in the water will increase, and CO2 will evade from the seawater to the atmosphere. For a temperature increase of 1C, the change in CO2 concentration is on the order of 10 x 10-6 atmospheres (10 atm). This is a positive feedback that could amplify a future atmospheric CO2 increase by about 5%.

At a constant temperature, as you add CO2 to the ocean, equation 9 goes from left to right, consuming carbonate ion and producing bicarbonate ion. This process too has the effect of reducing the ocean's ability to take up CO2.

Feedbacks involving ocean circulation are strongly linked to the biogeochemical cycling of carbon and nutrients in the ocean. If the ocean began to warm, the waters at the surface would warm more than those at depth. This change would decrease the amount of nutrients rising from the deep ocean into the euphotic zone. With fewer nutrients, there could be a consequent decrease in biological productivity, followed by less organic carbon escaping the euphotic zone and settling into the deep sea in a set of processes known as the biological pump. It is also conceivable that changes in wind patterns and wind intensities along the western coastal margins of continents could lead to changes in the upwelling of CO2 and nutrients to the surface ocean.

There are several other potential feedbacks involving CO2, the ocean, and climatic change, but it is difficult to determine their importance:




Chapter 5: The Important Nutrient Nitrogen

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Nitrogen forms part of the molecules that make up living things, such as amino acids (the building blocks of proteins) and DNA. The nitrogen in proteins bonds together various amino acids to form the protein structure. The amount of nitrogen in the atmosphere is very large compared to that in the oceans or rocks. Of the elements C, N, P, S, and O, only nitrogen is found in more abundance in the atmosphere than in rocks.

The complete biogeochemical cycle of nitrogen is very complex. Figures 12-17 show only portions of it. There are six major forms of atmospheric nitrogen: the gaseous forms of diatomic nitrogen (N2), ammonia (NH3), nitrous oxide (N2O), and NOx (NO and NO2), and the aerosols of ammonium (NH4+) and nitrate (NO3-). In this chapter, we will focus on the cycles of the first four of these forms, and also discuss nonmethane hydrocarbons, the cycles of which are closely related to those of NOx.

Figure 12. Modern Global Biogeochemical Cycle of Nitrogen
Part of the modern global biogeochemical cycle of nitrogen, emphasizing interactions among the land, atmosphere, and ocean. Fluxes between the ocean, land, and groundwater are shown as arrows, with quantities given in Mt N/yr. Fluxes within reservoirs are shown as circling arrows. "Ind.fix" is industrially fixed N (for the manufacture of fertilizers), "Bio.fix" is biologically fixed N, DN is dissolved N, PON is particulate organic nitrogen, and "pollutant" is the excess nitrogen that has resulted from human activities (modified from Mackenzie, 1995).

N2

The overwhelming majority of nitrogen in the atmosphere is in the form of N2. The other forms exist only in small quantities. Biological fixation and denitrification are the major processes leading to exchange of nitrogen between the earth's surface and the atmosphere (Figure 12). Biological fixation is the process whereby N2 is withdrawn from the atmosphere and converted to N compounds that plants can use (e.g., NH3 and subsequently NO3-). Denitrification is the process by which nitrogen as N2 or as N2O is returned to the atmosphere. Both processes are mediated by a variety of bacteria living in soils and water.

The exchange fluxes between the earth's surface and the atmosphere are small compared to the internal recycling of nitrogen within the land and ocean realms (Figures 12 and 13). Combustion practices, the production of commercial fertilizers, and rice paddy cultivation add fixed nitrogen to the earth's surface. Because of these human activities, the amount of nitrogen on land is increasing (Figure 12). About 30 million tons of nitrogen are leached from agricutural fertilizers and human waste each year and added to groundwater systems and runoff. Some of this nitrogen makes its way to rivers and then to lakes and the coastal oceans. On a global scale, rivers may already carry more nitrogen from human activities than was transported in the natural state (Figure 12). This increased nitrogen flux to lakes, rivers, and coastal marine environments is one cause of increased regional and global eutrophication of these systems. Note, however, that rivers supply only a small percentage of nitrogen to the coastal zone (Figure 13). Most of the nitrogen there, other than that recycled in the zone, upwelled from the deep ocean to the surface.

Figure 13. River Input of Nitrogen to the Ocean
River input of N to the ocean compared to the fluxes involved with the internal recycling of N in the ocean due to biological productivity and decay. Besides the ocean fluxes shown, about 90% of the nitrogen involved in biological production is simply recycled in the shallow surface waters of the coastal and open oceans. Some nitrogen escapes from the surface ocean in organic matter that settles to the deep ocean, where the organic matter is decayed and the nitrogen released. It then returns to the surface via upwelling in the coastal zone and vertical mixing in the open ocean. Some nitrogen, about 30 Mt per year, is buried in marine sediments in organic matter (see Figure 12). Some N is transported to the open ocean from the coastal environment (after Mackenzie, 1995; Houghton et al., 1996).

Scientists have calculated how much this human-caused increase in nitrogen is likely to be changing ocean productivity and the flux of organic carbon from the ocean's euphotic zone. The calculations show that in the 1980s there may have been an increased organic carbon flux from the atmosphere to the oceanic environment of about 200 million tons of carbon per year (Figure 12), which is buried in marine sediments. This flux takes from the atmosphere about 3% of the increase occurring today as a result of fossil fuel burning. While relatively small, this is a possible negative biotic feedback on atmospheric CO2 and hence global climate change.

N2O

N2O is a natural product of biological denitrification in soils and in the ocean (Figure 14). The N2O produced by denitrification is only about 15% of all N returned to the atmosphere; the rest is in the form of N2.

Figure 14. Modern Global Biogeochemical Cycle of Nitrous Oxide
Part of the modern global biogeochemical cycle of nitrous oxide. Symbols and units are as in Figure 9. This gas is responsible for 5% of the enhanced greenhouse effect. A doubling of its atmospheric concentration could lead to about a 0.4C increase in global temperature. Notice the reaction of this long-lifetime gas in the stratosphere, leading to the destruction of stratospheric ozone. The fluxes in this cycle are not well known (modified from Mackenzie, 1995).

N2O is an important greenhouse gas, accounting for about 9% of the enhanced greenhouse effect since the 18th century. It has a present atmospheric concentration of 312 ppmv and a residence time of about 130 years. This concentration is about 8% greater than in preindustrial time and is increasing at a rate of 0.2-0.3% per year because of human activities, including the combustion of fossil fuels, burning of biomass, and emissions from urea and ammonium nitrate applied to croplands. These emissions amount to 0.13 to 2.8 million tons of nitrogen annually (Figure 14).

N2O is chemically inert in the troposphere. In the stratosphere, it can be converted photochemically to nitric oxide (NO), which acts as a catalyst in the destruction of stratospheric ozone (see sidebar). The series of reactions by which this is accomplished has been one of the regulators of stratospheric ozone concentration through geologic time.

N2O reactions leading to the destruction of stratospheric ozone
NO formation in the middle stratosphere (20-30 km):

   N2O + ultraviolet light => NO + N   (10)
 
   N2O + O => 2NO   (11)

Ozone destruction:

  NO + O3 => NO2 + O2
 
  O3 + ultraviolet light => O2 + O
 
  NO2 + O = > NO + O2

  --------------------------------------

  2O3 + ultraviolet light => 3O2 (net reaction)  (12)

The flux of N2O from the earth to the atmosphere has been increasing because of the rapidly increasing use of industrially fixed nitrogen (up to the late 1980s), increases in fossil fuel burning and biomass burning, and increases in organic carbon in coastal waters. This last process is an important link between the carbon and nitrogen cycles. The rate of denitrification and consequently of N2O emissions from coastal waters may have increased because rivers are bringing more organic carbon to these systems or because these systems are undergoing eutrophication as they receive increased inputs of nutrients from fertilizer, sewage, and the atmosphere.

With warming, the most important biotic feedbacks involving N2O are changes in the denitrification (and nitrification) rates in soils, sediments, and ocean water. Also, N2O fluxes from nitrogen-bearing fertilizers applied to the land surface and sewage discharges into aquatic systems will be affected by warming. Because the reactions involving N2O are bacterially mediated, it is likely that an increase in temperature will lead to enhanced evasion rates of N2O from the earth's surface. This is a positive biotic feedback on accumulation of N2O in the atmosphere and, hence, on global warming. It could also lead to a small enhanced destruction of stratospheric ozone (Figure 14).

NH3

The biogeochemical cycle of ammonia (NH3) is shown in Figure 15. Ammonia is released to the atmosphere by organic decomposition and volatilization. There, it reacts with water droplets to form ammonium ion (NH4+) and hydroxyl ion (OH-). NH4+ appears to be removed from the atmosphere mainly by being deposited back on the earth in the aerosols of ammonium sulfate [(NH4)2SO4] and ammonium nitrate (NH4NO3). Incidentally, (NH4)2SO4 links the nitrogen and sulfur biogeochemical cycles, since its deposition on the earth is also one of the ways oxidized sulfur is removed from the atmosphere; the other is by deposition of sulfuric acid (H2SO4).

Two interactions in the NH3 cycle are important in considerations of global warming. The first is its interaction with OH* to produce NOx. In a warmer world, the decomposition that releases NH3 would probably be enhanced, which would slightly increase the stress on the OH* concentration of the atmosphere and enhance production of NOx (Figure 16). The effects of increased NOx concentrations are discussed in the following section. The second important interaction is NH3's reaction with NO3 and SO4 to produce aerosols containing ammonium (Figure 15). Aerosols are known to cool the planet, although the amount of the effect is unclear. An increase in atmospheric NH3 could lead to a small negative feedback on potential warming.

The ammonia cycle also gives us information on nitrogen fertilization of the terrestrial biosphere. About four-fifths of the N released to the atmosphere each year in NH3 comes from human activities - 50 out of 62 million tons. Only about 12 million tons of ammonia nitrogen per year comes from natural bacterial decomposition in soils. About 25% of the human-produced flux is transported away from the continents to the oceanic atmosphere. The rest, about 37 million tons of nitrogen per year, falls back on the land surface and may be available for terrestrial organic productivity.

Now it's time for a back-of-the-envelope calculation. If this 37 million tons of nitrogen were to fertilize land plant production with a ratio of C to N of 100 to 1, the plants would require more than 3 billion tons of carbon per year. The phosphorus accumulating on land each year from agricultural fertilizers and sewage amounts to about 8.5 million tons (see Figure 18, below) - just about the amount of phosphorus needed to sustain this magnitude of land plant production. This calculation gives some idea of the potential of fertilization of the land as a sink for the excess CO2 that we are emitting to the atmosphere by fossil fuel burning and land-use practices.

Figure 15. Modern Global Biogeochemical Cycle of Ammonia
Part of the modern global biogeochemical cycle of ammonia, including that of the ammonium ion (NH4+). See Figure 9 for an explanation of the units and symbols used. Most of the ammonia emissions from the land surface are due to human activities. Ammonia is removed from the atmosphere mainly in rain and as small, solid aerosol particles after reaction with water and with nitrate and sulfate. Through reaction with OH*, a small amount of NH3 is converted to nitrogen oxides, e.g., NO and NO2 (modified from Mackenzie, 1995).

NOx and NMHCs

This brings us to the cycles of NOx and the NMHCs (Figures 16 and 17). We will begin with NOx. It has several natural sources: on the earth, bacterial decomposition of organic matter in soils; in the atmosphere, lightning, mixing from the stratosphere, and oxidation of ammonia. NOx also has anthropogenic sources: fossil fuel and biomass burning. The main sink of NOx is deposition on earth of chemical products that were produced in the atmosphere by photochemical reactions with NOx, such as HNO3 and organic nitrates.

NMHCs (also called volatile organic compounds, or VOCs) are natural byproducts of plant productivity in terrestrial and marine environments. Thus, their fluxes to the atmosphere change greatly with the seasons. They also have anthropogenic sources - once again, fossil fuel and biomass burning. Their main sink is in the atmosphere, through oxidation with OH*.

Effects of NOx on ozone

Increasing temperature alone would probably increase the flux of NOx from soils to the atmosphere, potentially depleting OH* and forming more methane and ozone in the troposphere (Figure 16). For tropospheric ozone, however, the effect of increasing temperature is not at all straightforward. The concentration of ozone in the troposphere depends in a complex way on the atmospheric concentrations of several other biogenic trace gases, including CH4, CO, and the NMHCs.

In general, when there is little NOx in the troposphere (5-30 pptv), increases in the concentrations of CH4, CO, and NMHCs lead to a decrease in the concentration of O3. At high NOx concentrations (generally greater than about 90 pptv), increases in these three gases lead to an increase in ozone. The combination of high NOx and NMHCs in the troposphere disrupts the natural cycle of production and destruction of ozone, and ozone accumulates. In urban areas, this contributes to air pollution.

Figure 16. Modern Global Biogeochemical Cycle of the Nitrogen Oxides
Part of the modern biogeochemical cycle of the nitrogen oxides. About one-half of the emissions of these gases to the atmosphere comes from the combustion of fossil fuels. In the atmosphere, the gases are converted to several other chemical species, mainly HNO3, and removed from the atmosphere both in rain and in dust. Notice the tie to tropospheric O3 (see text), a greenhouse gas and a component of smog (modified from Mackenzie, 1995).

Effects of NOx on OH*

The concentration of OH*, which is mainly responsible for cleansing the atmosphere, depends on the concentrations of trace gases, tropospheric ozone, and water vapor. Elevated concentrations of O3, NOx, and H2O will increase OH* levels. (Generally, changes in NOx concentrations affect OH* in the same way they affect ozone, described above, except to a lesser degree.) On the other hand, increases in CH4, CO, and NMHCs will lead to lower levels of OH*.

One critical positive feedback is that increases in CO concentrations in the atmosphere could lead to a reduction in OH*, because NOx has too short a lifetime to counteract that effect on a global scale. Decreased concentrations of OH* could lead to an increase in the lifetime of CH4, a positive, but small, feedback on the accumulation of CH4 in the atmosphere and hence global warming.

Figure 17. Modern Global Biogeochemical Cycle of the Nonmethane Hydrocarbons
Part of the modern global biogeochemical cycle of the nonmethane hydrocarbons. Land vegetation and phytoplankton naturally produce these compounds. Their human sources include industrial practices, transportation, and fossil fuel combustion. These compounds react in the atmosphere with OH* and are important in controlling that compound's concentration in the troposphere. They are also responsible for disrupting the natural production and destruction of the ozone cycle in the troposphere. In conjunction with NOx, they can lead to increased concentrations of O3 in the troposphere (modified from Mackenzie, 1995; Guenther et al., 1995).



Chapter 6: Phosphorus and Sulfur

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The Important Nutrient Phosphorus

Phosphorus is a key nutrient, fueling organic productivity on land and in water. A portion of its cycle is shown in Figure 18. The P cycle is considered here both because it is closely coupled with those of carbon and nitrogen and for completeness.

Controversy still exists, particularly between marine biologists and geochemists, as to whether phosphorus or nitrogen is the limiting nutrient in the marine environment. Globally and on a long time scale, phosphorus is probably the limiting nutrient, if for no other reason than that the atmosphere contains essentially an unlimited supply of nitrogen for fixation in aquatic systems.

Figure 18. Modern Global Biogeochemical Cycle of Phosphorus
Part of the modern global biogeochemical cycle of phosphorus, emphasizing the exchange of P among the land, atmosphere, and ocean. Units and symbols are as in Figure 9. Notice, as with nitrogen (Figure 12), that the internal recycling fluxes in the ocean and land reservoirs by organic production and decay are much larger than the exchanges. Also as with nitrogen, the land is gaining P because of its mining, its use in the manufacture of fertilizers and detergents, and sewage inputs. "Wet-dry fallout" is the precipitation of phosphorus on the ocean as particles and in rain. DP is dissolved phosphorus, and "pollutant" is excess phosphorus from human activities (modified from Mackenzie, 1995).

The major difference between the phosphorus cycle and the carbon, nitrogen, and sulfur cycles is that no biological process generates an important gas flux of phosphorus from the earth's surface to the atmosphere (Figure 18). A phosphorus gas known as phosphine or swamp gas (PH3gas) causes a small flux, but the amount is insignificant compared to other fluxes of phosphorus. As with nitrogen, fluxes of phosphorus between the land, ocean, and atmosphere are small compared to the amounts that cycle within the land and ocean systems.

Sinks and sources

Unlike nitrogen, phosphorus accumulates in both organic and inorganic sediments. Because of the direct tie between N and P in organic matter, the only organic sink shown in Figure 16 is the important phosphorus flux to sediments as P incorporated in dead organic matter (bottom right-hand corner of the figure). The inorganic sinks, which are not shown in the figure, involve the precipitation of the mineral carbonate fluoro-apatite [Ca5(PO4)3(OH,F)], scavenging by iron oxy-hydroxides, and incorporation in oxidized iron coatings on the surfaces of calcium carbonate.

Phosphorus mining is the largest source of P to the land surface. Much of the mined phosphorite rock is used to make commercial fertilizers. The major impact of humankind on the phosphorus cycle results from the application of both commercial and organic fertilizers to croplands and the disposal of sewage. Phosphorus from these sources is introduced to streams via direct sewage discharge or by leaching. The increased amount of phosphorus in rivers, lakes, and coastal marine waters results in increased rates of eutrophication of these systems in cases where phosphorus is the limiting nutrient.

Feedbacks

Two major biological feedbacks in the phosphorus cycle may be of importance in global climate change. One involves the enhanced eutrophication of aquatic systems as a result of human activities, as just mentioned. The global dissolved phosphorus flux, carried by rivers to the coasts, has doubled since preindustrial times because of these activities (Figure 18). The resulting eutrophication of the coastal regions has led to a potential accumulation of organic carbon in these systems of about 140 million tons of carbon per year (Figure 18, top right). This enhanced flux is a negative feedback on the accumulation of anthropogenic CO2 in the atmosphere today and, as with nitrogen, could become more important in the future.

The second biological feedback has to do with the land reservoir of phosphorus. Notice in Figure 18 that phosphorus is accumulating on land because of mining, fertilizer use, and sewage discharges. Because most chemical weathering and biological decomposition rates increase with increasing temperature, a climate warming would mean that phosphorus in this reservoir may be more easily leached into aquatic systems. This additional phosphorus could increase eutrophication in these systems and lead to increased accumulation of organic matter. This flux is a negative feedback on CO2 accumulation in the atmosphere and hence on global warming. However, the effect is quite small. If all the phosphorus now stored on land were leached and transported by rivers to coastal systems, and all the other nutrients in coastal marine systems could be used for plant growth (which may not be the case), the increase in organic carbon storage would be 800 million tons. This is equivalent to only 13% of one year's flux of carbon to the atmosphere because of the burning of fossil fuels.

Reduced and Oxidized Sulfur Gases

The reduced and oxidized sulfur cycles (Figures 19 and 20) are closely tied, because the reduced sulfur gases that dominate the earth's biological sulfur emissions are oxidized in the atmosphere. These reduced gases are dimethyl-sulfide or DMS [(CH3)2S] and carbonyl sulfide (OCS), which are emitted by the ocean surface, and hydrogen sulfide (H2S), emitted by decaying terrestrial vegetation. Oxidation converts these gases to sulfur dioxide (SO2) gas and sulfate aerosol, that is, microscopic sulfur-containing particles. Sulfur enters the atmosphere as the earth emits the reduced sulfur gases and leaves the atmosphere as sulfate aerosol floats or is washed to the earth.

The chemistry of DMS and OCS has two major connections with climate, by way of cloud condensation nuclei and stratospheric sulfate.

Cloud condensation nuclei

DMS is produced by bacteria in phytoplankton. Its concentration in the oceans is very low, but it is found nearly everywhere at the sea surface, where it may escape into the troposphere. Once airborne, it is oxidized by OH* to sulfate within a few days. Along with other chemical species in the atmosphere, it condenses into small (micron-sized) aerosol particles. These atmospheric sulfate aerosols act as cloud condensation nuclei (CCN) - the centers on which water droplets may form, facilitating cloud formation. In the remote marine atmosphere, DMS emissions are likely the main source of the aerosols that act as CCN.

One hypothesis argues that a warming of earth's climate could lead to enhanced phytoplankton growth and thus greater emission of DMS from the sea surface. The increased DMS flux could result in increased production of sulfate aerosols and CCN in the remote marine atmosphere, creating more and denser clouds. Besides leading to more rain, clouds also reflect incoming solar radiation and have a cooling effect on the lower atmosphere and surface of the earth. Thus an increase in cloud cover would give rise to a cooling of the troposphere, a negative biotic feedback on global warming. To counteract a warming owing to a doubling of atmospheric CO2 would require a 25% increase in the number of cloud condensation nuclei.

Figure 19. Global Biogeochemical Cycle of Reduced Sulfur
Part of the global biogeochemical cycle of reduced sulfur. See Figure 9 for an explanation of the units and symbols used. Reduced sulfur gases are naturally produced both on land and in the ocean by biological processes. They are eventually removed from the atmosphere by reaction with OH* and conversion to SO2 and thence to sulfate aerosol (see Figure 20). Sulfate aerosol is produced by the oxidation of OCS in the stratosphere to form the sulfate veil during times of low volcanic activity. Sulfate aerosol is responsible for the scattering and reflection of solar energy and thus cools the planet (modified from Mackenzie, 1995).

The validity of this hypothesis is not yet confirmed by real-world evidence. For example, in ice cores collected by drilling at Vostok Station in Antarctica, a record of methane-sulfonic acid (MSA) has been obtained from the ice. MSA is a product of DMS oxidation. If this theory were correct, one would expect to find more MSA in ice that dates from warmer periods, since increased levels of the compound would wash out of the atmosphere. However, the record shows just the opposite: higher levels of MSA in the ice from the last ice age, which culminated 18,000 years ago, than in this and past interglacial stages.

Stratospheric sulfate

OCS is produced mainly by the photolysis of organic sulfur in the surface waters of the ocean and by photochemical oxidation of the biogenic gas carbon disulfide (CS2) in the atmosphere. It is the most abundant reduced sulfur species in the remote marine atmosphere. Because it is chemically inert, it has a long residence time in the troposphere and can enter the stratosphere. Once in the stratosphere, OCS is destroyed by reacting with ultraviolet light and atomic oxygen. It is converted to SO2 and then on to sulfate aerosol. This reduced sulfur gas supplies about half of the mass of sulfate aerosol found in the lower stratosphere (the so-called Junge layer).

If global warming causes any change in the flux of OCS to the stratosphere, it will have an effect on climate through change in the Junge layer. An increased stratospheric sulfate burden would give rise to cooling of the atmosphere; a decreased burden would lead to warming. The likely feedback effect on an initial warming is difficult to predict.

Figure 20. Global Biogeochemical Cycle of Oxidized Sulfur
Part of the global biogeochemical cycle of oxidized sulfur. See Figure 9 for an explanation of the units and symbols used. Notice that the flux of oxidized sulfur from land as SO2 is dominated by the combustion of fossil fuels and biomass burning. This sulfur reacts with OH* to produce sulfate aerosols of ammonium and hydrogen. About 40% of the sulfur falling back on the earth's surface is derived from these sources. Volcanism can add sulfate aerosols to the stratosphere and produce a temporary cooling of the planet, as after the eruption of Mt. Pinatubo in 1991. Sulfate aerosols derived from combustion of fossil fuels and biomass burning exert a strong cooling effect, but only in the regions where they are produced (modified from Mackenzie, 1995).

Oxidized sulfur

To complete the picture of the global cycle of sulfur gas species, the earth surface-atmosphere oxidized sulfur cycle is shown in Figure 20. Natural sources of oxidized sulfur in the atmosphere include oxidation of reduced sulfur gases as mentioned above, volcanism, and aerosols from the sea. The oxidized sulfur is removed from the atmosphere by deposition of aerosols.

One of the most dramatic demonstrations of the connection between volcanism, sulfate aerosols, and climate was the eruption of Mt. Pinatubo in the Philippines in 1991. The eruption plume rose high into the stratosphere, where sulfate aerosol was generated and distributed over much of the globe. The aerosol scattered and reflected solar energy back to space. This event led to a cooling of the planet of about 0.5C during 1991-93. At its maximum in 1993, the cooling reached -3-4 watts per square meter. This is considerably more than the enhanced greenhouse forcing of +2.5 watts per square meter.

Human activities have strongly interfered with the global biogeochemical cycle of oxidized sulfur. This disturbance in the cycle has led to the acidification of land and freshwater systems - the problem of acid precipitation. Of the total global deposition of oxidized sulfur today, approximately 40% is derived from the human activities of combustion of fossil fuels, biomass burning, and smelting of sulfide ores. The deposits from these activities vary enormously spatially.

Recently a new hypothesis has emerged linking our sulfur emissions with climate. In the Northern Hemisphere, emissions of SO2 have led to an increased mass of sulfate aerosol. Because of the aerosol's short lifetime in the atmosphere, it remains concentrated over the eastern half of the continent and the western Atlantic Ocean. This regional enhancement could be cooling the atmosphere enough to explain the discrepancy between the observed temperature record of the past 100 years and the higher temperatures predicted by climate models that include only the effects of increased greenhouse gases and not increased aerosols. On the other hand, when climate modelers attempt to introduce the cooling effect of sulfate aerosols into their models, the modeled temperatures are generally lower than those observed in the recent past. This difficulty implies that sulfate aerosols are an important component of the temperature record, but that the amount by which they cool the planet is not yet known.



Chapter 7: The Water Cycle

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The water cycle (Figure 21) is important in the context of biogeochemical cycling in the C, N, P, S, and O system, as well as being important in its own right. Water circulating through the ecosphere is part of a continuous hydrologic cycle that makes life on earth possible. The water cycle is the driver cycle for transport of many elements at the earth's surface. In the atmosphere, water vapor is the most important greenhouse gas, and its behavior during a global warming is of concern.

Figure 21. The Hydrologic Cycle
The global biogeochemical cycle of water (the hydrologic cycle), showing the major processes of water movement. Numbers in parentheses show the water budget in thousands of cubic kilometers per year (after Maurits la Rivire, 1990.)

The water cycle is a balanced system, with water stored in many places at any one time (Table 3). The cycle involves the transfer of water in its various forms of liquid, vapor, ice, and snow through the land, air, and water environments. Both matter and energy are involved in the transfer. The transfer begins when heat from the sun warms the ocean and land surfaces and causes water to evaporate. The water vapor enters the atmosphere and generally moves with the circulation of the air. On a global scale, warm air rises in the atmosphere and cooler air descends. The water vapor rises with the warm air. The farther from the warm planetary surface the air travels, the cooler it becomes. Cooling causes water vapor to condense on small particles (cloud condensation nuclei) in the atmosphere and to precipitate as rain, snow, or ice and fall back to the earth's surface. When the precipitation reaches the land surface, it is evaporated directly back into the atmosphere, runs off or is absorbed into the ground, or is frozen in snow or ice. Also, plants require water and absorb it, retaining some of the water in their tissue. The rest is returned to the atmosphere through transpiration. Precipitation on land is equal to evapotranspiration plus runoff to the ocean. That is, over the land, there is more precipitation than evapotranspiration.

Table 3. Distribution of water in the ecosphere
Reservoir Volume (106km3) Percent of total
Ocean 1370 97.25
Cryosphere (ice caps and glaciers) 29 2.05
Groundwater 9.5 0.68
Lakes 0.125 0.01
Soils 0.065 0.005
Atmosphere 0.013 0.001
Rivers 0.0017 0.0001
Biosphere 0.0006 0.00004
Total 1408.7 100
After Berner and Berner, 1996.

For the ocean, the situation is reversed. Much of the water evaporated from the ocean returns there directly; however, a small amount (about 8% of that evaporated) is carried by atmospheric winds over the continents, where it precipitates. Once on the ground, the water finds its way to streams, lakes, or rivers in runoff or by percolation into and through groundwater. In due time, the water will return to the ocean, mainly in stream and river flows and less importantly in groundwater. This return flow balances the net loss from the ocean surface by evaporation.

Snow and ice may remain on the land for a long time before the water in these forms of precipitation evaporates to the atmosphere or returns via rivers or as direct glacial meltwater to the oceans. The snow and ice that feed glaciers may remain locked up in the cryosphere for thousands of years, but finally the ice will melt, and the water will travel to another part of the hydrologic cycle.

Water vapor in the atmosphere is the principal greenhouse gas. Because the amount of water vapor in the atmosphere is dependent on the temperature of the planet, any initial warming of the planet will probably lead to more water vapor in the atmosphere. The increased water vapor has the potential to absorb more infrared radiation reradiated from the planetary surface and thus lead to further warming. This is another example of a positive feedback.

Water droplets form in the troposphere by condensation of water on cloud condensation nuclei. These particles may absorb or reflect energy. The amount of water vapor in part determines the types and distribution of clouds that form in the atmosphere. In terms of predicting the effects of increasing concentrations of greenhouse gases in the atmosphere on temperature and other climatic variables, general circulation models (GCMs) and other types of models have been used. The GCMs are very complex computer representations of the atmosphere or atmosphere-ocean system that are used in the modeling of global climate change. In these models, the effects of clouds on the radiant energy budget of the planet are a major source of uncertainty in attempting to predict future climate change.

Clouds regulate the radiative heating of the planet. They reflect a significant part of the incoming solar radiation. Clouds also absorb longwave, infrared radiation emitted by the earth. At the cold tops of clouds, energy is emitted to space. In 1984 the Earth Radiation Budget Experiment (ERBE) was launched. This experiment involves a system of three satellites that provide data on incoming and outgoing radiation. One result of this experiment so far is the demonstration that clouds have a net cooling effect on the earth. On a global scale, clouds reduce the amount of radiative heating of the planet by -13 watts per square meter. This is a large number when compared to the +2.5 watts per square meter attributed to the increase in atmospheric greenhouse gases during the last century. It is also large compared to the radiative heating that could arise from a doubling of atmospheric carbon dioxide concentrations in the next century - about +4 watts per square meter. Thus, a small change in the types and distribution of clouds may have a large effect on the radiation budget compared to the effect of changing greenhouse gas composition owing to human activities.

In a world already warmed by greenhouse gases released from human activities, it is difficult to predict what will happen to the types and distribution of clouds. Too little is known about the complex processes of cloud formation and, in particular, the response of clouds to a warming earth. Also, these complex processes are difficult to simulate in the general circulation models. One difficulty lies with the problem of reproducing cloud physics in the models. A second difficulty is that GCMs only perform calculations at widely separated points over the globe and relatively infrequently. Cloud formation involves very dynamic processes at short time and spatial scales.

Clouds may act as a positive or negative feedback in a future earth warmed by the enhanced greenhouse effect. This ambiguity accounts in part for the range of estimates of the average temperature increase predicted by the GCMs for a doubling of the atmospheric carbon dioxide concentration.

A final comment on the water cycle is that it is being significantly affected by water usage and contamination of water stocks. Currently, humans use an amount of water equivalent to about 25% of total terrestrial evapotranspiration and 55%, or 6,800 cubic kilometers per year, of the runoff of water from the continents that is accessible. Only about 20% of the world's drainage basins have pristine water quality. There is little doubt that the world's water resources will be significantly strained in the 21st century.



Conclusion

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An analogy is commonly made between the earth's surface system and a giant chemical engineering factory. In the natural system, material circulation is driven by energy from the sun and, to a much lesser extent, from radioactive decay of elements in the earth's interior and motions of its tides. This is a mechanical and inorganic view of the earth. In another and more realistic sense, the earth has a natural metabolism; materials have circulated about its surface for millions of years in a complex, interconnected web of biogeochemical cycles. An array of physical, chemical, and biological processes weather and erode rocks and transfer materials in and out of the atmosphere, from the atmosphere to the biota and back again, to the oceans via rivers, and to the continents by uplift. Each element has a natural biogeochemical cycle. It is these cycles and their relationship to the physical climate system that have led to the development of a relatively stable and resilient surface system during geologic time. Life has evolved in this system and plays a strong role in the development and maintenance of the system through processes, fluxes, and feedbacks.

Human activities have contributed materials to the biogeochemical cycles. Some of these materials enter element cycles already naturally in operation; they are the same chemical species that have circulated for millions of years. Other materials are synthetic compounds and are foreign to the natural environment. These anthropogenic fluxes are leading to a number and variety of environmental issues, including the possibility of global climate change. There is no doubt that human activities have interfered in biogeochemical cycles and have modified the composition of the atmosphere. Understanding the consequences of this interference requires better quantitative descriptions of these cycles, their interconnections, and, in particular, their coupled response to perturbations, such as a change in climate.


Study Questions and Answers

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Chemistry Questions Answers
The Oceans, Atmosphere, Sediments, and Rocks Questions Answers
Ecology Questions Answers
Biogeochemical Cycles Questions Answers

 

Questions

Chemistry (To answers)

  1. The most simple chemical expression for the production of organic matter in plants is
    CO2 + H2O = CH2O + O2

    The chemical compound CH2O is organic matter; CO2 is carbon dioxide gas; O2 is diatomic oxygen; and H2O is water. The atomic weights of the elements C, H, and O are, respectively, 12, 1, and 16.

    A. What are the gram molecular weights of the compounds of CH2O, CO2, O2, and H2O?

    B. What is the weight of one mole of each of these compounds?

    C. If 10 moles of plant matter have been produced, how many moles of CO2 did it take to produce the plant matter? How many grams of CO2?

    D. If the C:N ratio of the plant material in moles is 106:16 (that of marine phytoplankton), how many moles of nitrogen were needed to produce the plant matter?

    E. If the source of the nitrogen were nitrate (NO3-) in the euphotic zone of the ocean, how many grams of nitrate were consumed in the process?

  2. Write a balanced chemical reaction for the weathering of the mineral orthoclase feldspar (KAlSi3O8) by water containing dissolved CO2. The products that are formed in this weathering reaction are: kaolinite [(Al2Si2O10(OH)4], bicarbonate (HCO3-), monomeric silicic acid (H4SiO40), and dissolved potassium ion (K+). (Hint: to begin, balance the reaction on aluminum.)

  3. The concentration of dissolved potassium (K+) in the ocean is 390 mg/kg. The atomic weight of potassium is 39. The average density of seawater is 1.027 g/cm3.

    A. What is the concentration of K+ in seawater in moles/kg?

    B. What is its concentration in parts per million by volume of seawater?

  4. The average molecular weight of the gases in the atmosphere is 29 and the mass of the atmosphere is 52 x 1020 grams.

    A. What is the total number of moles of gases in the atmosphere?

    B. Carbon dioxide gas makes up 0.036% of the atmosphere in moles. How many moles of CO2 are there in the atmosphere? How many grams?

  5. Water vapor in the atmosphere averages about 0.2% of the atmosphere in weight.

    A. What is the total mass of water vapor in the atmosphere?

    B. How many moles of H2O vapor are there in the atmosphere?

Questions

The Oceans, Atmosphere, Sediments, and Rocks (To answers)

  1. The average depth of the ocean is 3.8 kilometers and the average upwelling rate of deep water into the surface open ocean and into coastal environments is 4 m/yr. The area of the ocean is 3.6 x 1018 cm2.

    A. About how long would it take to upwell the entire volume of the ocean?

    B. The average nitrogen content of deep water is about 40 x 10-6 moles per liter (L). What is the annual rate of addition of nitrogen to the surface water due to upwelling?

    C. At a molar ratio of C:N of 106:16, what is the productivity in the global surface ocean in grams (g) C/m2/yr sustained by the upwelling of nitrogen?

  2. One source of the deep water of the world's oceans is in the Norwegian Sea. Here water is sufficiently dense to sink to the bottom. This water mainly forms from the cooling by evaporation of water carried northward by the Gulf Stream. The Gulf Stream carries heat to the high latitudes of the North Atlantic Ocean which helps to moderate the climate of Europe. There is considerable interest in the rate of formation of North Atlantic deep water (NADW) because of the role it plays in climate. Possible changes in the rate of its formation have been cited as the cause of rapid climate change in the past. Any global warming could modify the rate of deep water formation.

    A. How would you expect the residence time of the deep water to change from the Atlantic Ocean to the Pacific Ocean?

    B. How does the deep water return to the surface?

    C. If the continental glaciers were to begin melting because of a global warming, what would you expect might happen to the rate of deep water formation and the climate of Europe?

  3. The winds of the earth tend to blow along latitudinal lines around the planet. The major wind belts are the Polar Easterlies, Westerlies, Trade Winds, and Equatorial Easterlies in both the Northern and Southern Hemispheres. The winds drive the surface currents of the ocean. Cool air that has descended at midlatitudes moves toward the equator as the Northeast and Southeast Trade Winds in the Northern and Southern Hemispheres, respectively. These winds exert a force on the sea surface, and currents are generated that flow toward the equator. Both the winds of the atmosphere and the surface currents of the ocean converge near the equator. The warm equatorial air rises and moves north and south toward the poles. The converging ocean currents generate westward-flowing equatorial currents.

    A. Based on the above, would you expect a pollutant gas like carbon monoxide with major anthropogenic sources in the Northern Hemisphere and a short atmospheric residence time to be evenly distributed in the troposphere?

    B. Is there an equatorial barrier to the dispersal of floating tar balls produced when petroleum tankers spill oil in the shipping lanes of the North Atlantic?

    C. Does such a barrier exist for the deep waters of the ocean?

  4. The very finest particles of airborne dust carried by winds off the Sahara Desert travel in the troposphere for long distances westward across the Atlantic Ocean. These particles are deposited on the ocean surface and settle out at a rate of 500 cm/yr. How long would it take such particles to reach the bottom of the Atlantic Ocean at 4 km?

  5. The global atmosphere in 1996 contained about 360 ppmv of CO2. This concentration is equivalent to a partial pressure of CO2(PCO2) of 10-3.45 atmosphere (atm).

    A. What is the concentration of dissolved CO2 in the surface ocean in equilibrium with atmospheric CO2 at a temperature of 25C?

    B. In the year 1700, when atmospheric CO2 was at a concentration level of 280 ppmv (PCO2 = 10-3.55 atm), what was the concentration of CO2 in the surface ocean?

    C. What was the percentage change in atmospheric CO2 concentration between 1700 and 1996?

  6. The primary energy sources for the earth are:

    About 49% of the solar radiation is absorbed by earth's surface and reradiated to space as longwave, infrared radiation.

    A. What percentage of the total comes from the combination of heat from the interior of the earth and tidal energy?

    B. In units of W/m2, how much solar radiation reaches the earth's surface and is absorbed there? What does this energy do?

    C. Of the 388 W/m2 of longwave radiation emitted to space by the earth's surface, 326 W/m2 are absorbed by water vapor, CO2, and other greenhouse gases in the atmosphere and reradiated back to the earth. What happens to this energy, and what effect does it have on the earth?

  7. The area of land today is 150 x 1012 m2, and the mean elevation of the continents is 0.84 km. The continents are being eroded at a rate of 200 x 1014 g/yr. The average density of rock is 2.7 g/cm3.

    A. At the current rate of erosion, how long would it take to wear the continents down to sea level?

    B. Your answer to question 7A was a period of time much less than the age of the earth of 4.6 billion years. In that case, why are there any continents?

  8. The total mass of sedimentary rock younger than 600 x 106 years is 1.8 x 1018 metric tons. Its mean residence time is 400 x 106 years.

    A. In an unchanging system, what is the flux of sediment in and out of the sedimentary rock reservoir?

    B. How does sediment get into the sedimentary rock reservoir and how does it get out?

  9. What chemical species of nitrogen, sulfur, and carbon might you expect to find in a water-logged (anoxic) soil? In a well-aerated soil?

  10. What does the weathering of silicate minerals subtract from or add to the atmosphere?

Questions

Ecology (To answers)

  1. Tropical rain forests have a total area of 17 x 1012 m2, and estuaries have an area of 1.4 x 1012 m2. Their mean net primary production is 2,000 g dry plant matter/m2/yr and 1,800 g dry plant matter/m2/yr, respectively. Their mean plant biomasses in kg C/m2 are 20 and 0.45, respectively. Forty-five percent of dry plant matter is carbon.

    A. What is the total net primary production of tropical rain forests and estuaries in metric tons of dry plant matter and carbon per year?

    B. What is the total plant mass of these ecosystems in metric tons of dry plant matter and carbon?

    C. The tropical rain forests of the world lost 9% of their area due to cutting in the 1980s. How much dry plant matter does this cutting represent? How much carbon?

    D. If all of the carbon in the cut trees of question 1C were emitted to the atmosphere by burning and slow oxidation, how many grams of CO2 would this represent? What fraction of the atmospheric CO2 reservoir is this (see Figure 11)?

    E. Estuaries receive 1.5 x 1012 g of pollutant dissolved phosphorus annually (see Figure 18). This amount of P could support how much additional plant productivity as dry matter and as C/m2 per year?

    F. If all the additional plant matter in 1A were buried in the sediments of the estuaries, would this flux qualify as a biological feedback to the accumulation of CO2 in the atmosphere? Explain. What percentage of the atmospheric flux of 6 x 1015 g C from fossil fuel burning in 1995 does the additional total plant production in estuaries represent?

  2. The net primary production of the earth's surface is 0.37 kg dry matter/m2/yr.

    A. With a total area of 510 x 1012 m2, what is the total production of dry matter?

    B. How much carbon does the production in 2A represent?

    C. Total production on land exceeds that in the ocean by perhaps as much as two times. What is the annual total production on land and in the ocean?

  3. What is the difference between autotrophy and heterotrophy?

  4. What are three biogeochemical processes performed by the prokaryotes?

  5. What is cultural eutrophication? Why may this phenomenon qualify as a negative feedback to the accumulation of CO2 in the atmosphere?

Questions

Biogeochemical Cycles (To answers)

  1. Construct a diagram showing the reservoirs and fluxes in the global biogeochemical cycle of water. Use the information in Figure 21 and Table 3, and the reservoirs of atmosphere, land, and ocean.

    A. What is the residence time of water in the ocean with respect to the flux of water via the rivers to the ocean?

    B. If the concentration of dissolved calcium is 400 ppm in the ocean and 15 ppm in average river water, what is the residence time of calcium in the ocean?

  2. A nearly rectangular-shaped lake is 5 km long, 2 km wide, and 100 m deep and contains 0.001 mg/L of dissolved mercury. A river discharges 2 x 1012 L/yr of water with a concentration of mercury of 0.0005 mg/L into the lake.

    A. What is the water volume in the lake (in liters)?

    B. What is the total mass of mercury in the lake?

    C. What is the residence time of mercury in the lake?

    D. An industrial plant near the mouth of the river accidentally discharges mercury into the lake. How long will it take for most (95%) of the mercury contamination to work its way out of the lake?

  3. The terrestrial living biomass contains 600 x 109 tons of carbon. It is argued that during the decade of the 1980s, about 135 x 1012 moles of anthropogenic CO2 were taken up annually by the terrestrial biomass.

    A. What would be the average annual percentage increase in the mass of carbon in the terrestrial biosphere?

    B. Do you believe such an increase would be detectable by doing field studies to determine the increase in biomass?

  4. The mixed layer of the ocean is the vertical layer that is well stirred by winds blowing across the surface of the sea. Chemical and physical characteristics of the water column are rather uniform over the depth of the mixed layer. The average thickness of the mixed layer throughout the world's oceans is about 100 meters but varies from 50 to 300 meters. The average total dissolved inorganic carbon (DIC, HCO3- + CO32- + CO2) content of the mixed layer is 2.2 micromoles (mmol) per kg.

    A. What is the total mass (reservoir) of DIC in the mixed layer of the ocean in moles of carbon? (The area of the ocean is 360 x 1012 m2.)

    B. The ocean takes up about 2 billion tons of carbon annually from the human activities of fossil fuel combustion, cement manufacturing, and deforestation. What is the annual increase in dissolved carbon in the mixed layer of the ocean in mmol/kg due to this absorption of anthropogenic CO2?

    C. Use the annual increase you derived from 4B and assume the DIC content of the mixed layer was 2.2 mmol/kg in 1975. What would be the percentage increase in the DIC content of the mixed layer from 1975 to 1996?

  5. Total evaporation is 496 x 103 km3 H2O/yr over the earth's surface.

    A. What is the residence time of water in the atmosphere?

    B. If a pollutant is susceptible to being rained out (washed out) of the atmosphere, would you expect it to mix evenly throughout the troposphere?

  6. What are the three major processes controlling atmospheric CO2 concentration levels on a long time scale? Write a balanced set of chemical equations demonstrating these processes.

  7. What is the connection between organic matter and atmospheric O2?

  8. What is the major difference between the biogeochemical cycle of phosphorus and those of carbon, nitrogen, and sulfur?

  9. What is the major difference in the reactivity of DMS and OCS in the atmosphere?

  10. The mean atmospheric lifetime (residence time) of NH3 is 14 days; that of CO is 60 days. How do the rates of destruction of these gases in the atmosphere by OH* compare qualitatively?

  11. Referring to Figures 9, 10, 11, what is the major reaction in the atmosphere that couples the biogeochemical cycles of CH4, CO, and CO2? What percentage of the enhanced greenhouse forcing on climate is due to CO2? To CH4? What are the sources of emissions of CH4 to the atmosphere from human activities? Of CO2 and CO? If the concentration of CO2 in the atmosphere were doubled, what would be the potential increase in temperature? How much more effective is CH4 than CO2 as a greenhouse gas?

  12. Referring to Figure 12, what is the ratio of anthropogenic N fluxes on land involving fixation of atmospheric N to natural biological fixation? What is the minimum percentage of the N fixed by human activities that is discharged to the ocean by rivers? What is the important environmental problem related to this additional N flux to the ocean?

  13. Referring to Figure 13, what is the ratio of these two fluxes: the upwelling flux of N to the coastal zone and the dissolved N flux to the ocean via rivers? Which flux would you expect to change during the next century?

  14. Referring to Figure 14, based on the summation of the high and low estimates of fluxes of N2O from earth's surface to the atmosphere, what is the range in residence time estimates for N2O in the atmosphere? What percentage of the enhanced greenhouse forcing of climate is due to N2O?

  15. Referring to Figure 16, of the total flux of NOx to the atmosphere, what percentage is from human activities? What is the principal type of reaction that destroys NOx in the atmosphere? Write the reaction for the destruction of NO2 and its subsequent removal as HNO3.

  16. Referring to Figure 18, the total mass of P in land plants is 1,800 million tons and that in marine plankton is 73 million tons. What is the ratio of the internal recycling flux in the ocean to that on land? What is the residence time of P in the land biota relative to the recycling flux? In the marine plankton?

Answers

Chemistry (To questions)

  1. This question is designed to enable the student to review basic knowledge of the meaning of a chemical equation. Many biogeochemical processes are simply described by chemical reactions. In this reaction, there are four pure substances that can be decomposed by a chemical change, that is, four chemical compounds: CO2(gas), H2O(liquid), CH2O(solid), and O2(gas). The atoms in these compounds may separate from one another and rearrange themselves during a chemical reaction. In this case, in the presence of light and available nutrients, plant material has formed from CO2 and H2O, and the carbon atom in CO2 has been rearranged into the carbon atom of CH2O. This reaction further shows that all chemical equations must balance. The total number of atoms of an element on the left-hand side of an equation must equal the total number of atoms of that element on the right-hand side of the equation.

    A. The atomic weights of the elements in these compounds are C = 12, H = 1, O = 16. Thus, the summation of these weights, or the gram molecular weight, for CH2O is 12 + (2 x 1) + 16 = 30 g; for O2, 2 x 16 = 32 g; for CO2 , 12 + (2 x 16) = 44 g; and for H2O, (2 x 1) + 16 = 18 g.

    B. The weight of one mole of these compounds is CH2O = 30 g; O2 = 32 g; CO2 = 44 g; and H2O = 18 g.

    C. The relationships between the compounds as shown in the equation must always be preserved; thus one unit - that is, one mole of the reactant compound CO2 - must react with an equivalent number of moles of the reactant compound H2O to give you a ratio of product compounds of CH2O:O2 = 1:1. Thus, the formation of 10 moles of plant material requires 10 moles, or 10 moles x 44 g/ mole = 440 g of CO2.

    D. At a molar ratio (that is, a ratio of moles) of C:N = 106:16, the moles of nitrogen are 16/106 x 10 moles = 1.51 moles.

    E. The atomic weight of N is 14. The reaction required 1.51 moles of N or 1.51 moles of NO3-. (There is 1 mole of N in 1 mole of NO3-.) The molecular weight of NO3- is 14 + (3 x 16) = 62 g. Thus, the grams of nitrate consumed were 1.51 moles NO3- x 62 g NO3-/mole = 93.6 g NO3-.

  2. This question again deals with chemical reactions. The equation to be balanced represents the weathering of a common mineral found at the surface of the earth. The student not only learns to balance a reaction on the basis of atoms but also on the basis of charge. The reason for starting the balance on aluminum is that solids containing aluminum are very insoluble at the temperature and pressure of weathering. Thus, it is assumed that all the aluminum from the weathering of orthoclase feldspar is simply transferred to the solid weathering product kaolinite. Of course, this is not true, but it is a reasonable approximation.

    2KAlSi3O8 + 2CO2 + 11H2O = Al2Si2O5(OH)4 + 2K+ + 2HCO3-+ 4H4SiO40

  3. Dissolved potassium is the sixth most important dissolved constituent in seawater. Its concentration is important to biological processes, although it is a minor essential element for phytoplankton productivity.

    A. The concentration of K+ in the ocean in moles/kg = 390 x 10-3 g/kg seawater 39 g/mole = 10 x 10-3 moles/kg seawater or 10 millimoles.

    B. The average density of seawater is 1.027 g/cm3. Thus, 1 kg of seawater is equivalent to 1,000 g of seawater 1.027 g/cm3 = 973.7 cm3 of seawater. The concentration in parts per million by volume of seawater is 390 x 10-3 g/kg seawater x 1.027 x 10-3 kg/cm3 x 1,000 cm3/L seawater = 400.5 x 10-3 g/L = 400.5 mg/L = 400.5 ppmv of seawater.

  4. The atmosphere is an important reservoir that exchanges materials with the earth's surface. Its composition has changed on a geological time scale and on the human scale of generations because of the activities of human society. This problem illustrates the fact that although atmospheric CO2 accounts for 60% of the enhanced greenhouse effect, it represents a relatively small part of the total atmospheric mass.

    A. The mass of the atmosphere can be obtained from the weight of air above 1 cm2 of the earth's surface and the total area of the earth: 1,031 g/cm2 x 5.1 x 1018 cm2 = 52 x 1020 g. The total number of moles of gases in the atmosphere is 52 x 1020 g 29 g/mole = 1.8 x 1020 moles.

    B. The number of moles of CO2 is 0.00036 x 1.8 x 1020 moles = 64.8 x 1015 moles. The mass of CO2 in grams is 64.8 x 1015 moles x 44 g/mole = 2,850 x 1015 g = 2,850 x 109 metric tons = 2,850 gigatons x (12 44) = 777 gigatons of C 12 g/mole = 64.8 x 1015 moles C.

  5. Water vapor is the most important greenhouse gas, yet as the following calculation shows, it is a small portion of the mass of the atmosphere.

    A. The total mass of water vapor in the atmosphere is 0.002 x 52 x 1020 g = 10.4 x 1018 g.

    B. here are 10.4 x 1018 g 18 g/mole = 57.8 x 1016 moles of water vapor.

Answers

The Oceans, Atmosphere, Sediments, and Rocks (To questions)

  1. Upwelling is an important process in the ocean. It involves the upward movement of water and dissolved constituents from depth in the ocean to the surface. Upwelling occurs in the coastal regions of offshore Peru, California, Namibia, Mauritania, and Somalia, and in open ocean equatorial regions and the high latitudes of the Southern Hemisphere.

    A. To upwell the volume of the ocean with an average depth of 3,800 m at the mean upwelling rate of 4 m/yr would take 3,800 m 4 m/yr = 950 years.

    B. 400 cm of water rises 1 cm2/yr. With an ocean area of 3.6 x 1018 cm2, this is equivalent to a volume of water of 1,440 x 1018 cm3/yr or 1,440 x 1015 L/yr x 40 x 10-6 moles N/L = 57.6 x 1012 moles N/yr x 14 g/mole = 806 x 1012 g N/yr.

    C. 806 x 1012 g N/yr 14 g/mole = 57.6 x 1012 moles of N/yr. At a C:N ratio of 106:16 = 382 x 1012 moles C/yr x 12 g/mole = 4.6 x 1015 g C/yr 360 x 1012 m2 = 13 g C/m2/yr. This productivity is only about 10% of global marine net primary productivity (Figure 11). This calculation illustrates the fact that much of the nitrogen used in biological productivity in the euphotic zone of the ocean comes from recycling of the N within this zone.

  2. The formation of the deep water of the ocean is part of the conveyor belt circulation pattern of the world oceans. This pattern is not well known from observations; our understanding of it is based mainly on theoretical models. The North Atlantic deep water (NADW) flows southward at depth from its source in the high latitudes of the North Atlantic Ocean and meets a northward-flowing current (the Antarctic Bottom Water, ABW) whose water originated by sinking in the Weddell Sea near Antarctica. The currents merge and part of the water flows into the deep Indian Ocean and Pacific Ocean. Water upwells to the surface in all basins of the ocean. In addition, warm water returns from the Pacific and Indian Oceans into the Atlantic Ocean at about the depth of the thermocline. We believe these return flows to the Atlantic Ocean occur through the Drake Passage between Antarctica and South America and through the Bering Sea into the Arctic Ocean and thence to the Atlantic Ocean. The return flows in the high latitudes of the North Atlantic sink as the NADW.

    A. Because of this pattern of circulation, the deep water of the world's oceans generally gets older from the Atlantic Ocean to the Indian Ocean to the Pacific Ocean. The time the water stays out of contact with the atmosphere, that is, its residence time, increases toward the Pacific Ocean. The residence time of the bottom waters of the Atlantic Ocean is 200-500 years; that of the Pacific Ocean is 1,000- 2,000 years.

    B. By upwelling as described above.

    C. This is a difficult question and one that is asked by a number of scientists today. The answer is of concern to the world's people because of the link to climate. It is likely that any substantial melting of sea ice and the continental glacier of Greenland would add fresh water to the surface of the ocean in the high latitudes of the North Atlantic for some period of time. This would affect the rate of deep water formation because of the change in the salt content of surface ocean water. In turn, the pattern of the conveyor belt circulation of the ocean could be altered. One suggestion is that there would be a less intense flow of warm water northward by the Gulf Stream, and the climate of Europe would cool.

  3. A. Because of the short residence time of CO in the atmosphere (about 70 days) and the fact that the upwelling of air near the equator effectively separates air exchange in the troposphere between the two hemispheres, the gas would not be evenly distributed. Higher concentrations would be found in the Northern Hemisphere troposphere than in the Southern Hemisphere. In fact, observations show a strong gradient in the concentration of CO between the two hemispheres, with higher concentrations in the North.

    B. The barrier is the convergence of the North Equatorial Current and the South Equatorial Current near the equator and their westward flows. This converging pattern inhibits exchange of surface waters between the Northern Hemisphere and the Southern Hemisphere. Floating tar balls in the North Atlantic would be caught in the North Equatorial Current and transported westward to the northward-moving Gulf Stream.

    C. No, the NADW moves southward at depth in the North Atlantic Ocean into the South Atlantic Ocean. The ABW moves north at depth. Neither current is involved with surface ocean currents.

  4. The fine dust particles would take 4 km x 105 cm/km = 4 x 105 cm 500 cm/yr = 800 years. In actual fact, they settle much faster because they are encapsulated in the fecal pellets of animal plankton (zooplankton) in the ocean. During feeding the zooplankton inadvertently pass them through their guts and excrete them contained in bigger mucilaginous fecal particles that sink at rates of 350 m/day.

  5. To answer this question requires an understanding of some basic chemistry. The equilibrium between a gas and a solution is normally given by Henry's Law, which states that the concentration of the gas in the solution equals a constant (known as the Henry's Law constant) times the partial pressure of the gas. For CO2, the expression is
        [CO2] = KHPCO2

    The brackets around CO2 denote its concentration in moles/L. The partial pressure of CO2(PCO2) is in atmospheres. The value of KH varies with the composition of the solution and the temperature. For seawater at 25C, KH equals 10-1.54.

    A. Carbon dioxide dissolves in seawater to an extent determined by the CO2 concentration in the atmosphere and the reactions that occur in the seawater. At 25C and with an atmospheric CO2 concentration of PCO2 = 10-3.45 atm, the relationship is
        [CO2] = 10-1.54 x 10-3.45 = 10-4.99 mole/L

    B. In 1700 for a PCO2 = 10-3.55 atm, we have
        [CO2] = 10-1.54 x 10-3.55 = 10-5.09 mole/L

    C. The percentage change is [( 10 -4.99 mole/L - 10-5.09 mole/L) 10-5.09 mole/L] x 100 = 26%. Thus, the dissolved CO2 concentration in the surface ocean has changed by this percentage over the past 300 years because of fossil fuel combustion and biomass burning.

  6. A. The percentage is [(0.9 x 10-4 cal/cm2/min + 0.9 x 10-5 cal/cm2/min) 0.5 cal/cm2/min + 0.9 x 10-4 cal/cm2/min + 0.9 x 10-5 cal/cm2/min] x 100 = 0.02%.

    B. The amount of energy reaching the earth's surface and absorbed is 343 W/m2 x 0.49 = 168 W/m2. This energy is used to heat the atmosphere and surface of the earth; to evaporate water; to generate rising air masses (thermals); to drive wind, waves, and currents; and for photosynthesis.

    C. The energy is reabsorbed, keeping the earth warm.

  7. A. Provided there were no processes restoring the continents, they would be reduced to sea level by erosion in 150 x 1012 m2 x 840 m = 126 x 1015 m3 x 106 cm3/m3 = 126 x 1021 cm3 x 2.7 g/cm3 = 340 x 1021 g 200 x 1014 g/yr = 17 x 106 years.

    B. This is a question that has plagued geologists for two centuries. There must be processes that add mass to the continents to keep them above sea level. Certainly lavas originating in the interior of the earth add material to the continents. In subduction zones, not all the rock of the oceanic crust is transported down toward the interior of the earth. Some of it is added to the continents and increases their area and thickness. Finally, continents often collide during their movement about the earth's surface. In the collisions, the continents act as a great vise, squeezing sediments originally derived from their erosion and other sources into high mountain ranges. This action adds volume back to the continents. The collision of India with Asia followed by the formation of the Himalayan Mountains is an example of such an event. Thus, there is a great rock cycle at work in which the continents are eroded and their materials deposited in the ocean. The sediments of the oceans are buried to great depths or transported to subduction zones. In either case, the sedimentary material is eventually returned to the continents to be uplifted to their surfaces and eroded, completing the cycle.

  8. A. This question simply uses the concept of residence time, where λ = mass flux; therefore, flux = mass λ = 1.8 x 1018 metric tons 600 x 106 yr = 3 x 109 metric tons/yr = 30 x 1014 g/yr. Interestingly, this flux is much less than that of erosion and thus deposition in the oceans today. This implies that today is somewhat unusual in terms of geologic history. We know that to be the case from other geological information.

    B. Sediment (clay, mud, silt, sand, gravel, and skeletal and organic materials) enters the sedimentary rock reservoir by erosion of rocks, transportation of the eroded debris, and subsequent deposition on the seafloor, followed by burial in some cases to depths of 12 km. The sediment "gets out" by uplift due to plate tectonic movements and re-erosion of the sedimentary mass.

  9. The chemical species in anoxic soils are chiefly reduced forms of the compounds CH4, CO, NH3, and H2S (see Figures 9, 10, 15, and 19). CO2 occurs as well, although it is a relatively oxidized form of carbon. In a well-aerated soil, we might expect O2, CO2, N2, and SO2.

  10. Your answer to Question 2 in the Chemistry section shows that the weathering of silicate minerals subtracts CO2 from the atmosphere. This subtraction must be balanced by an addition of CO2 from other processes or the atmosphere would run out of CO2 in about 6,000 years. Those processes involve hydrothermal reactions at midocean ridges and the subduction and/ or burial of carbonate minerals to realms of higher pressure and temperature, where carbonates are converted to silicates, and CO2 is released back to the atmosphere by volcanism and other processes.

Answers

Ecology (To questions)

  1. This question is designed to give the student a feeling for some of the characteristics of two important ecosystems that are being severely impacted by human activities. These activities are the deforestation of tropical rain forests and their conversion to pasture and urban areas and the eutrophication of coastal marine environments.

    A. The total net primary production of tropical rain forests and estuaries is:
    Rain forests: 17 x 1012 m2 x 2,000 g dry matter/m2/yr = 34 x 1015 g dry matter x 0.45 = 15.3 x 1015 g C. Dividing by 106 g/metric ton, we get 34 x 109 metric tons dry matter and 15.3 x 109 metric tons C.
    Estuaries: 1.4 x 1012 m2 x 1,800 g dry matter/m2/yr = 2.52 x 1015 g dry matter x 0.45 = 1.13 x 1015 g C. Dividing by 106 g/metric ton, we get 2.52 x 109 metric tons dry matter and 1.13 x 109 metric tons C. Notice that although the NPP of tropical rain forests and estuaries is similar, the smaller area of estuaries leads to more than an order of magnitude difference between the total net primary production of the two ecosystems.

    B. The total plant mass (biomass) of these ecosystems is:
    Rain forests: 17 x 1012 m2 x 20 kg C/m2 = 340 x 1012 kg C 103 kg/metric ton = 340 x 109 metric tons C 0.45 = 755 x 109 metric tons dry matter.
    Estuaries: 1.4 x 1012 m2 x 0.45 kg C/m2 = 630 x 109 kg C 103 kg/metric ton = 6.3 x 108 metric tons C 0.45 = 1.4 x 109 metric tons dry matter. Notice the orders-of-magnitude difference between the biomass of tropical rain forests and estuaries.

    C. In ten years, 9% of the area of rain forests was lost; this represents 755 x 109 metric tons dry matter x 0.09 = 68 x 109 metric tons dry matter x 0.45 = 30.6 x 109 metric tons C.

    D. The grams of CO2 emitted to the atmosphere in this ten-year period would be 30.6 x 109 metric tons C x 106 g/metric ton = 30.6 x 1015 g C x (44 12) = 112 x 1015 g CO2. This flux represents (Figure 11) the size of the atmospheric CO2 reservoir of 744 x 1015 g C x (44 12) = 2,730 x 1015 g CO2. 112 x 1015 g CO2 2,730 x 1015 g CO2 = 0.041, or 4% of the atmospheric reservoir.

    The flux would represent about 3 x 1015 g C/yr for the decade of the 1980s. The magnitude of the flux is too high, meaning that all the woody plant material was not burned to CO2. Some remains on the ground as waste from the cutting, and some has gone into lumber. The flux of CO2 to the atmosphere from all land-use activities in tropical rain forests for the 1980s was on the order of 1 - 1.6 x 1015 g C/yr.

    E. At a C:P ratio of 106:1, the additional plant productivity supported by the pollutant P would be 1.5 x 1012 g P/yr 31 g/mole = 48.4 x 109 moles P/yr x (106 1) = 5.13 x 1012 moles C/yr. Divided by the estuary area of 1.4 x 1012 m2 = 3.7 moles C/m2/yr = 44 g C/m2/yr 0.45 = 98 g dry matter/m2/yr, or about a 5% increase in the productivity of the world's estuaries.

    F. In fact, because the pollutant P will be used more than once in plant productivity in estuaries, the total organic carbon that could be buried in the sediments of estuaries amounts to 140 x 1012 g C/yr (Figure 18). This flux does qualify as a negative biological feedback because the additional P has been added to the earth's surface by the human activities of fertilizer application to croplands and sewage discharge. Some of this P makes its way to lakes and coastal marine environments and increases the productivity of such aquatic systems. The flux represents (140 x 1012 g C/yr 6 x 1015 g C/yr) x 100 = 2% of the fossil fuel flux in 1995. Not much!

  2. A. The total production of dry matter is 510 x 1012 m2 x 0.37 kg dry matter/m2/yr = 188.7 x 1012 kg dry matter/yr.

    B. 188.7 x 1012 kg dry matter/yr x 0.45 = 84.9 x 1012 kg C/yr.

    C. Let total production in the ocean = X; then production on land = 2X. Thus X + 2X = 188.7 x 1012 kg dry matter/yr, i. e., 3X = 188.7 x 1012 kg dry matter/yr; X = ocean production = 62.9 x 1012 kg dry matter/yr and 2X = land production = 125.8 x 1012 kg dry matter/yr.

  3. Autotrophy is the biochemical pathway by which an organism uses CO2 as a source of carbon and simple inorganic nutrient compounds of N and P for synthesis of organic matter. In heterotrophy, more complex organic materials are used as the source of carbon for metabolic processes.

  4. Prokaryotes - the Kingdom Monera, including the bacteria and cyanobacteria - take part in a variety of biogeochemical processes (see Table 1). We often forget the fact that the bacteria are responsible for the decay of organic matter both on land and in the ocean. In other words, it is their metabolic activity that returns CO2 and other gases and nutrients back to the environment. The cyanobacteria are photoautotrophic and produce oxygen as a metabolic byproduct. These organisms were responsible for the initial growth of oxygen in the earth's atmosphere. The processes include CO2 fixation, nitrogen fixation, and oxidation of sulfur as examples.

  5. Eutrophication is the set of processes leading to the overnourishment of a lake, river, or marine environment; consequent rapid plant growth and death; and oxygen deficiency of the system. This is a natural set of processes in certain environments. When it occurs because the excess nutrients come from fertilizers, sewage, detergents, etc., it is called cultural eutrophication. This term distinguishes the natural situation from that produced by the activities of people.

Answers

Biogeochemical Cycles (To questions)

  1. See Figure 22. Reservoirs and Fluxes in the Global Biogeochemical Cycle of Water

    A. The residence time = λ = mass of water in the ocean divided by the flux of water to the ocean by rivers: 1,370 x 106 km3 40 x 103 km3/yr = 34,250 years.

    B. λ of Ca in the ocean is (1,370 x 106 km3 x 1012 L/km3 x 400 mg/ L) (40 x 103 km3/yr x 1012 L/km3 x 15 mg/L) = 913,000 years.

  2. Here we apply the concept of residence time in the context of how long it takes for a lake to recover from a single input of a chemical into it. This is an environmental problem often encountered in developing, as well as developed, countries.

    A. The volume of the lake is 5 km x 2 km x 0.1 km = 1 km3 x 1012 L/km3 = 1 x 1012 L.

    B. The mass of mercury (Hg) is 1 x 1012 L x 1 x 10-6 g Hg/L = 1 x 106 g Hg.

    C. The flux of mercury by the river to the lake is 2 x 1012 L/yr x 5 x 10-7 g Hg/L = 1 x 106 g Hg/yr; thus the residence time of Hg (λHg) in the lake is 1 x 106 g Hg 1 x 106 g Hg/yr = 1 year.

    D. The time to reach a new equilibrium - that is, for the Hg input to work its way through the lake - is 3 x (λHg) = 3 x 1 yr = 3 yr. In this period of time, 95% of the Hg input would be gone.

  3. This question provides the student with a feeling for the problem of "looking for a needle in a haystack" that scientists have when observing the change in the size of the terrestrial living biomass due to uptake and storage of anthropogenic CO2.

    A. 600 x 109 tons C x 106 g/ton = 600 x 1015 g C. 135 x 1012 moles C/yr x 12 g/mole = 16.2 x 1014 g C/yr. The annual % change would be (16.2 x 1014 g C/yr 600 x 1015 g C) x 100 = 0.27% per year.

    B. Most unlikely. For the decade of the 1980s, this would be only a 2.7% change in the mass of living biomass globally. This amount of change would be difficult to measure by field studies, even if they were aimed at seeing a change in the amount of organic carbon stored in terrestrial vegetation. However, if the storage continues, such measurements could provide information in a few years.

  4. A. The total DIC in the mixed layer is 360 x 1012 m2 x 102 m = 360 x 1014 m3 x 106 cm3/m3 = 360 x 1020 cm3 x 1.027 g/cm3 = 37 x 1021 g 103 g/kg = 37 x 1018 kg seawater x 2.2 x 10-3 moles C/kg seawater = 81.4 x 1015 moles C.

    B. 2 x 109 tons C/yr = 2 x 1015 g/yr 12 g/mole = 166.7 x 1012 moles C/yr 37 x 1018 kg seawater = 4.5 x 10-6 moles C/kg seawater/yr = 4.5 micromoles C/kg seawater/yr. Actual measurements of the change in the DIC content of seawater over time show an increase of about 1 micromole per year. The difference is due to the fact that the anthropogenic carbon taken up by the ocean mixes deeper in the ocean than the average depth of the mixed layer used in the problem, on average about 300-400 meters.

    C. Using 1 micromole per year as the average carbon uptake since 1975, we get 1 x 10-6 mole C/kg/yr x 21 yr = 21 x 10-6 moles C/kg 2.2 x 10-3 moles C/kg x 100 = 0.95%. It was not until the late 1980s that scientists were able to measure these small changes in seawater DIC accurately and precisely.

  5. A. Referring to Table 3, the total amount of water in the atmosphere as water vapor is 0.013 x 106 km3; thus the residence time of water vapor in the atmosphere calculated with respect to total evaporation (must equal precipitation) is 0.013 x 106 km3 H2O 496 x 103 km3 H2O/yr = 0.026 yr x 365 days/yr = 9.6 days.

    B. No, the pollutant would not mix evenly throughout the troposphere because of water's very short residence time in the atmosphere. In general, the dust and aerosol content of the troposphere exhibits a regional pattern and is most concentrated near sources, downwind from sources, and in regions of relatively dry climate. However, the dust plume in the troposphere derived from the Sahara and Sahel areas of Africa can be seen in satellite images extending all the way across the Atlantic Ocean.

  6. The three major processes are (1) inputs of volcanic CO2 derived from the metamorphism of CaCO3 to CaSiO3, (2) weathering of silicate minerals like CaSiO3 and then deposition of calcium carbonate and silica in the ocean, and (3) the evolution of plants and their effect on weathering. The reactions are

    1. CaCO3 + SiO2 => CaSiO3 + CO2
    2. CaSiO3 + 2CO2 + 3H2O => Ca2+ + 2HCO3- + H4SiO40, and then
      Ca2+ + 2HCO3- + H4SiO40 =>CaCO3 + SiO2 + 3H2O + CO2
    3. CO2 + H2O <=> CH2O + O2
  7. The connection is simply the fact that when organic matter is buried in sediments of the ocean, oxygen not used to oxidize the organic matter is left in the atmosphere. With the continuous burial of organic matter on the seafloor, the oxygen content would increase in a few millions of years to levels that would lead to burning of forests and grasslands. This does not happen because the buried organic matter is returned to the earth's surface through uplift by plate tectonic processes. On exposure to the atmosphere, the organic matter is oxidized, and the oxygen is removed from the atmosphere.

  8. The major difference is that there is not a major gas of phosphorus that resides in the atmosphere or is transported through it (see Figure 18). This statement is not true of carbon, nitrogen, and sulfur. All of these elements have important gaseous compounds in the atmosphere that exchange with the earth's surface. (See the text sections on the cycles of C, N, and S.)

  9. DMS in the troposphere reacts fairly rapidly with hydroxyl radical in the presence of light, water, and oxygen to make sulfate aerosol. On the contrary, OCS is inert in the troposphere but is converted in the stratosphere to sulfate aerosol (see Figure 19).

  10. The difference in residence time implies that ammonia reacts more rapidly than carbon monoxide in the atmosphere (see Figures 10 and 16). This is simply a consequence of the fact that the shorter the residence time (lifetime) of a chemical compound in a reservoir, the more reactive the substance.

  11. This question and the following five questions relate to relationships depicted in the figures of the biogenic trace gases. They are designed to help the student to study the figures and interpret them.

    The major reaction in the atmosphere coupling the biogeochemical cycles of CH4, CO, and CO2 is the oxidation of the reduced carbon gas CH4 to CO and then on to CO2 by OH*. About 60% of the enhanced greenhouse forcing is due to CO2, and about 20% is due to CH4. The major anthropogenic sources of emissions of CH4 to the atmosphere are fossil fuel burning and leakage from gas transmission pipelines, biomass burning, landfills, rice paddies, and enteric fermentation in domesticated animals. CO emissions come from fossil fuel and biomass burning. Land-use activities and fossil fuel burning are the major anthropogenic sources of CO2 to the atmosphere. A doubling of atmospheric CO2 concentration could lead to a 2.5C increase in mean global temperature. Molecule for molecule, CH4 is about 20 times more effective as a greenhouse gas than CO2. (Compare the amount of temperature change per ppbv for the two gases.)

  12. The ratio is (42 + 20 + 78) x 106 metric tons N/yr 126 x 106 metric tons N/yr = 1.1:1. The anthropogenic nitrogen fluxes on land slightly exceed the natural biological fixation flux! The minimum percentage is (21 x 106 metric tons N/yr 140 x 106 metric tons N/yr) x 100 = 15%. The additional nitrogen flux to the ocean is a cause of eutrophication of coastal marine environments.

  13. The ratio is 206 x 106 metric tons N/yr 62 x 106 metric tons N/ yr = 3.3:1. This is a difficult question to answer. On the time scale of a century, it is very likely that the flux of N to the ocean from rivers and groundwaters will increase because of continuous use of industrial fertilizers, atmospheric deposition of anthropogenic nitrogen, and disposal of sewage. If there is climatic change on this time scale, it is uncertain whether the upwelling rate of the world's oceans will change. However, a warming of the global surface layer of the ocean would most likely lead to a slowing of upwelling and thus delivery of nutrients to the surface ocean.

  14. The range is 143 to 339 years. 73 x 106 tons N (2.9 + 0.1 + 0.02 + 0.01 + 1.4) x 106 tons N/yr = 339 years. 1,500 x 106 tons N (5.2 + 0.3 + 0.2 + 2.2 + 2.6) = 143 years. N2O accounts for about 9% of the enhanced greenhouse effect.

  15. The percentage from human activities is [( 21 + 3) x 106 tons N/yr (21 + 3 + 20) x 106 tons N/yr] x 100 = 54.5%. Human activities substantially interfere with the fluxes of NOx. Environmental problems associated with the anthropogenic fluxes are acid deposition, photochemical smog, and increased tropospheric ozone, a greenhouse gas. The principal reaction leading to destruction of NOx in the atmosphere is photochemical. The products of the reaction are nitric acid, peroxylacetyl nitrate, and organic nitrates. The reaction is NO2 + OH*+ light => HNO3.

  16. The ratio is 1,085 x 106 tons P/yr 186 x 106 tons P/ yr = 5.8:1. The residence time of P in the marine biota relative to the recycling flux is 73 x 106 tons P 1,085 x 106 tons P/yr = 0.07 years. That for P in the land biota is 1,800 x 106 tons P 186 x 106 tons P/yr = 9.6 years. The phytoplanktyon of the ocean "turn over" much more rapidly than do terrestrial plants. There are more generations of death and birth for marine plankton than for most plants growing on land.



Glossary

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Acid deposition - the fallout of acidic substances, primarily of nitrogen and sulfur, from the atmosphere on to the earth's surface in rain, snow, or other forms. Acid rain has a pH generally less than 5 (averaged over a year).

Aerosol - the suspension of very fine, generally micrometer-sized, solid and liquid particles in the atmosphere.

Albedo - the amount of incident radiation that is reflected by a surface and thus does not contribute to the heating of the surface. The albedo of the whole earth is approximately 30%. The albedo of clean snow is about 90% and that of water is about 10%.

Anaerobic - an organism that does not need oxygen to carry on its metabolism or an environment without oxygen.

Anoxic - without oxygen.

Anthropogenic - of, relating to, or influenced by the impact of humans on nature.

Atom< - the smallest component of an element having the chemical properties of the element. An atom consists of a nucleus of neutrons and protons and one or more electrons bound to the nucleus by electrical attraction.

Autotrophy - the biochemical pathway by which an organism uses carbon dioxide as a source of carbon and simple nutrient compounds for synthesis of organic matter.

Autotrophic system - an environment in which the difference between gross photosynthesis and gross respiration is positive. In such a terrestrial or aquatic environment, the net transfer of carbon dioxide is into the system.

Bacteria - one-celled organisms having a spherical, spiral, or rod shape belonging to the Kingdom Monera.

Benthic - of, relating to, or occurring at the bottom of a body of water.

Bioessential - required by virtually all living organisms. The major bioessential elements are oxygen, carbon, nitrogen, phosphorus, sulfur, potassium, magnesium, and calcium. Minor or trace quantities of iron, manganese, copper, zinc, boron, silicon, molybdenum, chlorine, vanadium, cobalt, and sodium are also required by organisms.

Biogenic gas - a gas whose production or consumption on earth is accomplished by biological reactions.

Biogeochemical cycle - representation of biological, geological, and chemical processes that involve the movement of an element or compound about the surface of the earth.

Biogeochemical system - the interactive system of biogeochemical processes and cycles of elements and compounds.

Biogeochemistry - the discipline that links various aspects of biology, geology, and chemistry to investigate the surface environment of the earth.

Biological productivity - the rate of production per unit area of organic matter by producer organisms. For example, the rate may be given as grams of carbon per square meter per year for a marine grass community. There are several kinds of productivity. Gross primary production (GPP) refers to the total amount of plant material produced by photosynthesis in a defined area in an interval of time. Net primary production (NPP) is the net amount of plant material produced per unit area per unit time and is the difference between GPP and cell respiration. Net ecosystem production (NEP) is the difference between GPP and cell respiration plus heterotrophic processes of decay.

Biological pump - the set of processes by which organic carbon is exported from the surface ocean to the deep sea.

Biomass - the amount of living matter in a unit area or volume of habitat. For example, the total biomass of the world's tropical rain forests is 42 kilograms of dry matter per square meter of forest, or a total of 420 billion tons of dry matter (equivalent to approximately 170 billion tons of carbon).

Biosphere - the living and dead organic components of the earth. Sometimes this term is used in the same way as the term ecosphere in this module, and sometimes for only the living animals and plants.

Climate - the characteristic long-term environmental conditions of temperature, precipitation, winds, etc., in a region or for the globe at present or in the past (paleoclimate).

Cloud condensation nuclei - airborne particles of very small size, generally less than one micrometer in diameter, that serve as sites on which liquid cloud droplets condense when an air mass is supersaturated with water vapor. The particles are commonly composed of water-soluble material.

Coccolithophoridae - a family of planktonic algae that build skeletons of micrometer-sized disc-shaped plates of calcite, called coccoliths.

Concentration - the fraction of the total of a substance made up of one component. For example, seawater contains 400 parts per million by weight of calcium. Concentration is also expressed in moles per liter or kilogram or in percent (that is, parts per hundred), parts per thousand (/), per million (ppm), per billion (ppb), and so forth, either by weight or by volume.

Coupled - the condition in which information from one part of the system is provided to, and influences the behavior of, other parts. The biogeochemical cycles of the elements necessary for life are coupled through processes that are essential for life, e.g., photosynthesis and respiration.

Crust - the outer layer of the earth, enriched in silicon, sodium, and potassium and having a thickness of 35 kilometers beneath the continents and 10 kilometers beneath the oceans.

Cryosphere - the icy part of the earth; its continental and mountain glaciers, ice sheets, and ice shelves; a reservoir in the earth's surface system.

Decay - the oxidative process of conversion of organic tissue to simpler organic and inorganic compounds. The oxidizing agent may be diatomic oxygen (O2), nitrate (NO3-), or other chemical compounds.

Denitrification - the conversion, principally by bacteria, of compounds of nitrogen in soils and aquatic systems to nitrogen gas (N2) and nitrous oxide gas (N2O) and the eventual release of these gases to the atmosphere.

Diagenesis - the collection of physical, chemical, and biological processes that operate on a sediment after deposition.

Diatom - planktonic and benthic freshwater and marine algae that commonly use silicon to build a skeleton of opal.

Dry deposition - the deposition of materials from the atmosphere on to the earth's surface in the form of solid particles. Such particles also may be "washed out" of the atmosphere by rain.

Ecosphere - the system that includes the biosphere and its interactions with the air, water, and soils and sediments of the earth.

Ecosystem - complex of a community or group of communities and the environment that functions as an ecological unit in nature.

Electromagnetic spectrum - the entire range of radiation. The wavelengths (distances between adjacent peaks) of the electromagnetic waves within the spectrum range from kilometers for radio waves to billionths of a meter (nanometers) for X rays.

Entropy - a scientific measure of the degree of disorder in a system. The greater the disorder the greater is the entropy of the system. The Second Law of Thermodynamics states that entropy is always increasing.

Equilibrium - stable, balanced state in which all influences on a system are countered by others.

Erosion - the set of processes by which the surface of the earth is worn away by the action of water, wind, glacial ice, etc.

Euphotic zone - the upper, lighted zone of the ocean or a lake in which most of the productivity of plants occurs. In the ocean, the euphotic zone extends from the surface to a depth where the light intensity is reduced to about 0.1- 1.0% of that available at the surface. The depth of the euphotic zone depends on season and latitude.

Eutrophication - the set of processes leading to overnourishment of an aquatic system in nutrients, rapid plant growth and death, and oxygen consumption and deficiency in the system. These processes occur naturally in some aquatic systems but may be speeded up by additions of nutrients from human activities (e.g., fertilizer application) to the systems. Human-induced eutrophication is often called cultural eutrophication.

Evaporation - the physical process by which water is converted from liquid to vapor and is transported into the atmosphere.

Evapotranspiration - the combined processes of evaporation and transpiration.

Evasion - the escape or release of a gas from the surface of the ocean or land to the atmosphere.

Evolution - the pattern of development and change in a variable from one state to another. Biological evolution describes the pattern of emergence, development, and extinction of organic species through geologic time.

Feedback - a process or mechanism in which some fraction of the output is returned or "fed back" to the input. Feedback loops may either stabilize (negative feedback) or destabilize (positive feedback) a system undergoing a perturbation. These feedback loops exist in both the biogeochemical cycles and the climate system.

Fermentation - the bacterial process of conversion of sugars to carbon dioxide.

Fixation - see Nitrogen fixation.

Flux - the movement of a variable or a substance into or out of a reservoir.

Foraminifera - animal plankton (zooplankton) in the ocean belonging to the Phylum Protozoa that commonly have a shell of calcium carbonate.

Forcing - the ability of a variable, like the concentration of a greenhouse gas in the atmosphere, to induce a change in a system. A forcing function controls the behavior of a system and often makes it regular and predictable.

General circulation model (GCM) - a simulation, usually performed on a large computer, of the large-scale, or general, wind and ocean systems on earth to calculate climate and its changes.

Geologic time scale - a calendar of earth history. The time scale is divided into variable time units of eon, era, period, and epoch (see Figure 5).

Glacial stage - an extended cold interval of time within the Pleistocene Epoch in which continental glaciers covered much of the Northern Hemisphere continents, atmospheric CO2 concentrations were low, and sea level was low.

Greenhouse effect - the warming of the earth's atmosphere and surface by the atmospheric greenhouse gases. These gases absorb and reradiate longwave radiation from the earth, keeping it in the atmosphere and thus warming the global temperature. Without the natural greenhouse effect, the planet would be about 33C cooler than its global mean annual temperature of 15C, that is, -18C. Because of inputs from human activities, these gases are increasing in concentration in the atmosphere. This may lead to an enhanced greenhouse effect and warming of the planet.

Greenhouse gas - an atmospheric gas that absorbs and radiates energy in the infrared part of the electromagnetic spectrum. Such gases include water vapor, carbon dioxide, methane, nitrous oxide, tropospheric ozone, and the synthetic chlorofluorocarbon gases. These gases warm the atmosphere and the earth's surface below.

Groundwater - the water beneath the ground, largely formed by the seepage of surface water downward.

Heterotrophic system - an environment in which the difference between gross photosynthesis and gross respiration is negative. In such a terrestrial or aquatic environment, the net transfer of carbon dioxide is out of the system.

Heterotrophy - a biochemical pathway in which organic substrates are used by organisms to make organic matter.

Hothouse - an extended period of geologic time during which the earth was warm.

Hydrosphere - the watery envelope surrounding the earth; a reservoir in the earth's surface system.

Hydrothermal reaction - a chemical reaction involving hot water and minerals in a rock.

Hydroxyl radical (OH*) - the excited chemical compound of hydrogen and oxygen in the atmosphere with an imbalance of electric charge. The hydroxyl radical is responsible for the oxidation of many chemically reduced gases emitted from the surface of the earth.

Ice age - a glacial stage, especially within the Pleistocene Epoch, beginning about 1.8 million years ago.

Ice house - an extended period of geologic time in which the earth was cool.

Infrared radiation - the region of the electromagnetic spectrum with wavelengths longer than visible light (about 1 micrometer) but shorter than microwaves (about 1 millimeter). Commonly known as heat. Radiation emitted from the earth back to space is predominantly infrared radiation.

Interglacial stage - an extended warm interval of time within the Pleistocene Epoch in which the continental glaciers retreated and atmospheric CO2 concentrations and sea level were low.

Ion - an electrically charged atom or group of atoms formed by the loss or gain of one or more electrons. A positive ion, the cation, is created by an electron loss, and a negative ion, the anion, is created by an electron gain.

Irreversible process - a process in which the entropy change is greater than zero. After the process is complete, the system is more disordered than and different from its initial state.

Kerogen - fossil organic matter dispersed throughout a rock.

Leaching - the selective removal of substances from a substrate, usually with water. For example, rainwater percolating through a soil can dissolve nitrogen and transport it to the groundwater. This is leaching.

Lifetime - a measure of the reactivity of an atmospheric chemical compound. The more reactive the compound, the shorter its atmospheric lifetime. Analogous to residence time.

Limestone - a sedimentary rock consisting predominantly of calcium carbonate minerals.

Limiting nutrient - the chemical compound, generally inorganic, that limits productivity in a terrestrial or aquatic environment. Examples are nitrate, phosphate, and iron.

Lithosphere - the dynamic subdivision of earth on the order of 100 kilometers in thickness forming the outer, rigid part of the planet. Also, the solid portion of the earth, composed of minerals, rocks, and soils; a reservoir in the earth's surface system.

Mantle - the portion of earth between its crust and its innermost zone (the core). The mantle is enriched in magnesium and iron and has a thickness of about 2,900 kilometers.

Metamorphism - the set of processes that lead to a change in the structure or composition of a rock due to pressure and temperature. A metamorphic rock is formed from a preexisting rock by an increase in pressure and temperature.

Methanogenesis - the conversion of organic material to methane, principally by bacteria.

Methanotrophy - the conversion of methane to carbon dioxide, principally by bacteria.

Midocean ridge - any of several seismically active, submarine mountain ranges that are found in the Atlantic, Indian, and Pacific Oceans. These ridges are regions where the seafloor originates and are the source of the lithospheric plates.

Mixotrophy - the use of both organic and inorganic materials to make organic matter.

Mole - one gram atomic weight of an element or one gram molecular weight of a compound. One gram atomic weight of an element is its atomic weight expressed in grams (i. e., the atomic weight of oxygen is 16; its gram atomic weight is 16 grams). One gram molecular weight of a compound is its molecular weight expressed in grams (i.e., the molecular weight of carbon dioxide is 44; its gram molecular weight is 44 grams).

Molecule - the smallest physical unit of an atom or compound, consisting of one or more similar atoms in an element and two or more different atoms in a compound.

Negative feedback - a process or mechanism that relieves or subtracts from an initial perturbation to a system.

Net primary production - see Biological productivity.

Nitrification - the conversion of ammonium to nitrite and nitrate by nitrifying bacteria.

Nitrogen fixation - the conversion of diatomic nitrogen gas (N2) to ammonium by bacteria. Also, the industrial conversion of free nitrogen into combined forms used as starting materials for fertilizers and explosives.

Nutrient - a substance that supplies nutrition to a living organism, like phosphorus and nitrogen.

Organic - pertaining to a class of chemical compounds that include carbon as a component; characteristic of or derived from living organisms.

Oxidation - the removal of electrons from an atom or molecule. Oxidizing capacity is the intrinsic ability of a system to oxidize reduced substances.

pH - the negative logarithm of the effective hydrogen ion concentration, used in expressing both acidity and alkalinity on a scale whose values run from 0 to 14, with 7 representing neutrality. Numbers less than 7 denote increasing acidity, and numbers greater than 7 increasing alkaline (basic) conditions.

Photoautotrophy - the conversion of inorganic carbon into organic matter in the presence of light.

Photochemical - pertaining to chemical reactions involving chemical compounds in the presence of light. Urban smog is the result of a complex series of photochemical reactions involving ozone, nitrogen and sulfur oxides, and hydrocarbons.

Photolysis (photodissociation) - pertaining to chemical reactions triggered by light that convert a complex compound to more simple products. The photolysis of ammonia is an example: 2NH3 => N2 + 3H2.

Photosynthesis - the synthesis of complex organic materials (e. g., carbohydrates) from carbon dioxide, water, and nutrients, using sunlight as a source of energy with the aid of chlorophyll and associated pigments.

Phytoplankton - minute plant life that passively floats in a body of water. The phytoplankton are at the base of the food chain in the ocean.

Plankton - minute plant and animal life of the ocean ranging in size from 5 micrometers to 3 centimeters. The plant plankton are the phytoplankton; the animal plankton are the zooplankton.

Plate tectonics - the theory of global tectonics in which the lithosphere is divided into a number of crustal plates that move on the underlying plastic asthenosphere. These plates may collide with adjacent plates, slide under or over them, or move past them in a nearly horizontal direction. The sources of the plates are the great midocean ridges of the world's oceans, where hot molten material upwells from within the earth. The plates are destroyed at subduction zones, like that along the western margin of the Pacific Ocean, where they sink down into the underlying asthenosphere.

Positive feedback - a process or mechanism that reinforces or adds to an initial perturbation of a system.

Precipitation - the removal of water from the atmosphere and its deposition on the earth's surface in the form of rain, ice, or snow.

Prokaryote - any cellular organism that has no membrane about its nucleus and no organelles in the cytoplasm except ribosomes. Prokaryotic genetic material is in the form of single, continuous strands forming coils or loops, characteristic of all organisms of the Kingdom Monera, such as bacteria or cyanobacteria.

Protozoan - eukaryotic organism of the Kingdom Protoctista, Phylum Protozoa, with a membrane-bound nucleus and organelles within a mass of protoplasm. Planktonic foraminifera and radiolarians which secrete shells of calcium carbonate and opal, respectively, are members of the group.

Radical - an electronically excited compound with an imbalance of electric charge, which enables it to react rapidly with another molecule.

Redfield ratio - the relatively constant ratio of 106:16:1 of the bioessential elements carbon, nitrogen, and phosphorus in marine plankton. The concept of the Redfield ratio has been applied to the terrestrial realm as well as to organic matter in soils and sediments.

Reduction - the chemical process by which an atom or a molecule gains electrons.

Reservoir or stock - a part of a system that can store or accumulate and be a source of one of the substances that compose the system. For example, the atmosphere is a reservoir of the surface system of the earth (the ecosphere). It can store water vapor released to it from the land by evapotranspiration and from the ocean by evaporation, and return the water to the earth's surface as precipitation.

Residence time - the total mass of a substance in a reservoir divided by its rate of inflow or outflow. The residence time is a measure of the reactivity of the substance in the reservoir. For example, the residence time of sodium in the ocean is very long (55 million years). Therefore, sodium does not enter into chemical or biochemical reactions that remove it very rapidly from the ocean. In contrast, the residence time of dissolved silica in the ocean is about 20,000 years. This compound is readily taken up by certain types of plankton to build their skeletons.

Respiration - the physical and chemical processes by which an organism supplies its cells and tissues with the oxygen needed for metabolism and releases carbon dioxide formed in the energy-producing reactions.

Reversible process - a process in which the change in entropy is zero. In general, after the process is complete, the state of the system is as it was initially.

Saturation - the degree to which a solution or a gas is at equilibrium with one of its components. It is measured in several different ways. For example, a humidity of 125% would be a supersaturation of 25% with respect to water vapor in the air. Saturation of seawater with respect to the mineral calcite (CaCO3) of 50% would mean that the seawater was 50% undersaturated with respect to calcite. If a lake water contained exactly enough dissolved CO2 to be in equilibrium with the atmosphere, it would have a saturation of 100% with respect to CO2.

Sedimentary rock - a rock formed from the erosion of preexisting rocks and the deposition of the eroded materials as sediment. Sedimentary rocks are also formed by inorganic or biological precipitation of minerals from natural waters.

Shortwave radiation - generally, the region of the electromagnetic spectrum with wavelengths shorter than 0.5 micrometers. Solar radiation has an important component of shortwave radiation of varying intensity.

Solar radiation - the electromagnetic radiation emitted by the sun. It includes energy wavelengths from the very short ultraviolet (<0.2 micrometers) to about 3 micrometers.

Stratosphere - the region of the upper atmosphere extending upward from the troposphere to about 30 kilometers above the earth's surface. This region is characterized by an increase in temperature as altitude increases.

Subduction zone - the juncture of two lithospheric plates where the collision of the plates results in one plate's being drawn down or overridden by another plate. This region is the sink of the crustal plates of the earth.

System - a selected set of interactive components. An example of a simple system is an air conditioning unit. A biogeochemical system consists of reservoirs, processes and mechanisms, and associated fluxes involving material transport. The global climate system is very complex and involves all the physical, chemical, and biological interactions that control the long-term environmental conditions of the world.

Thermocline - the depth range in the ocean where the temperature decreases rapidly with increasing depth. The thermocline is about one kilometer thick and extends from the base of the surface layer of the ocean at a depth of 50- 300 meters to a depth of about 800- 1,000 meters.

Trace gas - a gas present in the atmosphere in a very low concentration (less than 1% of the composition of the atmosphere). For example, methane, nitrous oxide, and carbon monoxide are considered trace gases.

Transitional phenomenon - a feature of a system that changes from one state to another. Such a change may be relatively slow or more generally abrupt. In the boiling of water, the change from the state of little water motion to that of turbulence is a transitional phenomenon.

Transpiration - the process by which water in plants is excreted through a plant membrane as water vapor.

Troposphere - the lowest level of the atmosphere, up to 8- 13 kilometers high, within which there is a steady drop in temperature with increasing altitude. It is the region where most cloud formations occur and weather conditions manifest themselves.

Ultraviolet radiation - the region of the electromagnetic spectrum with wavelengths longer than 0.5 nanometers but shorter than 0.5 micrometers. Solar radiation has an important component of ultraviolet radiation of varying intensity.

Uptake - generally, the incorporation of a substance into a solid or liquid. For example, the invasion of CO2 into the ocean represents the uptake of CO2 from the atmosphere.

Upwelling - the upward movement of water from depths of typically 50 - 150 meters at speeds of approximately 1 - 3 meters per day. The upwelling of water generally results from the lateral movement of surface water. Upwelling zones in the ocean are found along the western margins of the continents, in equatorial regions, and at high latitudes of the Southern Hemisphere.

Vascular plant - either a plant with seeds that are not enclosed in a fruit or seed case, such as pine, fir, spruce, and other cone-bearing trees or shrubs (gymnosperm), or a flowering plant that produces encased seeds, such as oak, maple, and eucalyptus trees (angiosperm).

Volatilization - the conversion of a substance into the gas or vapor state and its emission into the environment.

Washout - the scavenging of particles from the atmosphere by rainfall and their subsequent deposition on the surface of the earth.

Weathering - the set of chemical, physical, and biological processes that lead to the disintegration of minerals, kerogen, and rocks.

Wet deposition - the deposition on the earth's surface of solid particles and dissolved chemical compounds in rain.

Zooplankton - minute animal life in a body of water that generally drift passively or swim very weakly.



Supplementary Reading

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Berner, E. K., and R. A. Berner, 1996, Global Environment: Water, Air, and Geochemical Cycles. Prentice Hall, Upper Saddle River, New Jersey.
 
Broecker, W. S., 1983, The ocean. Scientific American, vol. 1249, no. 3, 100-112.
 
Chiras, D. D., 1988, Environmental Science. Benjamin Cumming Publishing Co., Redwood City, California.
 
Ehrlich, P. R., A. H. Ehrlich, and J. P. Holden, 1977, Ecoscience: Population, Resources, Environment. W. H. Freeman and Co., San Francisco, California.
 
Garrels, R. M., F. T. Mackenzie, and C. H. Hunt, 1975, Chemical Cycles and the Global Environment: Assessing Human Influences. William Kaufmann, Inc., Los Altos, California.
 
Graedel, T. F., and P. J. Crutzen, 1993, Atmospheric Change: An Earth System Perspective. W. H. Freeman and Co., San Francisco, California.
 
Gross, M. G., 1987, Oceanography: A View of the Earth, 4th ed. Prentice Hall, Englewood Cliffs, New Jersey.
 
Holland, H. D., and U. Patterson, 1995, Living Dangerously: The Earth, Its Resources, and the Environment. Princeton University Press, Princeton, New Jersey.
 
Mackenzie, F. T., Biogeochemistry. In Encyclopedia of Environmental Biology, vol. 1, Academic Press, Inc., New York, 249-276.
 
Mackenzie, F. T., and J. A. Mackenzie, 1998 (2nd ed.), Our Changing Planet: An Introduction to Earth System Science and Global Change. Prentice Hall, Englewood Cliffs, New Jersey.
 
Schlesinger, W. H., 1991 (2nd ed.), Biogeochemistry: An Analysis of Global Change. Academic Press, San Diego, California.
 
Skinner, B. J., and S. C. Porter, 1987, Physical Geology. John Wiley and Sons, New York.
 
Smil, V., 1997, Cycles of Life: Civilization and the Biosphere. Scientific American Library, New York.
 
Turekian, K. K., 1996, Global Environmental Change: Past, Present and Future. Prentice Hall, Upper Saddle River, New Jersey.
 
Wayne, R. P., 1991, Chemistry of Atmospheres. Oxford University Press, New York.
 
 
 


References

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Andreae, M. O., 1987, The oceans as a source of biogenic gases. Oceanus, vol. 29, 27-35.
 
Berner, R. A., 1991, A model for atmospheric CO 2 over geologic time. American Journal of Science, vol. 291, 339-376.
 
Berner, R. A., and D. E. Canfield, 1989, A new model for atmospheric oxygen over Phanerozoic time. American Journal of Science, vol. 289, 333-361.
 
Christensen, J. W., 1991, Global Science. Kendall/ Hunt Publishing Co., Dubuque, Iowa.
 
Guenther, A., C. N. Hewitt, D. Erickson, R. Fall, C. Geron, T. Graedel, P. Harley, L. Klinger, M. Lerdau, W. A. McKay, T. Pierce, B. Scholes, R. Steinbrecher, R. Tallamraju, J. Taylor, and P. Zimmerman, 1995, A global model of natural volatile organic compound emissions. Journal of Geophysical Research, vol. 100, 8873-8892.
 
Houghton, J. T., G. J. Jenkins, and J. J. Ephraums (eds.), 1990, Climate Change: The IPCC Scientific Assessment. Cambridge University Press, Cambridge, U. K.
 
Houghton, J. T., L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg, and K. Maskell (eds.), 1996, Climate Change: The Science of Climate Change. Cambridge University Press, Cambridge, U. K.
 
Mantoura, R. F. C., J.-M. Martin, and R. Wollast (eds.), 1991, Ocean Margin Processes in Global Change. Wiley-Interscience, New York.
 
National Research Council, 1986, Global Change in the Geosphere-Biosphere. National Academy Press, Washington, D. C.
 
Skinner, B. J., and S. C. Porter, 1987, Physical Geology. John Wiley and Sons, New York.
 
Socolow, R., C. Andrews, F. Berkhout, and V. Thomas, 1994, Industrial Ecology and Global Change. Cambridge University Press, New York.
 
Stolz, J. F., D. B. Botkin, and M. N. Dastoor, 1989, The integral biosphere. In: Global Ecology, M. B. Rambler, L. Margulis, and R. Fester, eds. Academic Press, San Diego, California, 31-50.
 
Wollast, R., F. T. Mackenzie, and L. Chou, 1993, Interactions of C, N, P and S Biogeochemical Cycles and Global Change. Springer-Verlag, New York.
 
Woodwell, G. M., and F. T. Mackenzie, 1995, Biotic Feedbacks in the Global Climatic System. Oxford University Press, New York.

 
 



Bibliographic Information

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University Corporation for Atmospheric Research
National Center for Atmospheric Research / UCAR Office of Programs

Understanding Global Change: Earth Science and Human Impacts

GLOBAL CHANGE INSTRUCTION PROGRAM

Global Biogeochemical Cycles and the Physical Climate System
by Fred T. Mackenzie

An instructional module produced by the Global Change Instruction Program of the University Corporation for Atmospheric Research with support from the National Science Foundation.

GCIP Staff Advisory Committee
Tom M. L. Wigley, Scientific Director
National Center for Atmospheric Research
Arthur Few
Rice University
Lucy Warner, Program Manager
University Corporation for Atmospheric Research
John Firor
National Center for Atmospheric Research
Carol Rasmussen, Editor
University Corporation for Atmospheric Research
William Moomaw
Tufts University
Linda Carbone, Secretary
University Corporation for Atmospheric Research
Ellen Mosley-Thompson
The Ohio State University
  Jack Rhoton
East Tennessee State University
  John Snow
University of Oklahoma

1999 by the University Corporation for Atmospheric Research. All rights reserved.

Any opinions, findings, conclusions, or recommentations expressed in this publication are those of the authors and donot necessarily reflect the views of the National Science Foundation.

For more information on the Global Change Instruction Program, contact the
UCAR Communications Office, P. O. Box 3000, Boulder, CO 80307-3000
Phone: 303-497-8600; Fax: 303-497-8610;
http://www.ucar.edu/communications/gcip/


A note on this series

This series has been designed by college professors to fill an urgent need for interdisciplinary materials on global change. These materials are aimed at undergraduate students not majoring in science. The modular materials can be integrated into a number of existing courses - in earth science, biology, physics, astronomy, chemistry, meteorology, and the social sciences. They are written to capture the interest of the student who has little grounding in math and technical aspects of science but whose intel-lectual curiosity is piqued by concern for the environment. For a complete list of materials contact UCAR Communications (see previous page).

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