What is the Carbon Cycle?
Modified with permission from Global Climates - Past, Present, and Future,
S. Henderson, S. Holman, and L. Mortensen (Eds.). EPA Report No. EPA/600/R-93/126,
U.S. Environmental Protection Agency, Office of Research and Development, Washington,
DC. pp. 59 - 64.
In this exercise, students will explore the carbon cycle and be able to identify
carbon sources, sinks, and release agents.
All living organisms are based on the carbon atom. Unique among the common
elements of the earth's surface, the carbon atom has the ability to form bonds
with as many as four other atoms (including other carbon atoms) and to form
double bonds to itself. Carbon compounds can be solid, liquid, or gas under
conditions commonly found on the earth's surface. Because of this, carbon can
help form solid minerals (such as limestone), 'squishy' organisms (such as plants
and animals), and can be dissolved in water or carried around the world through
the atmosphere as carbon dioxide gas. The attributes of the remarkable carbon
atom make possible the existence of all organic compounds essential to life
Carbon atoms continually move through living organisms, the oceans, the atmosphere,
and the crust of the planet. This movement is known as the carbon cycle. The
paths taken by carbon atoms through this cycle are extremely complex, and may
take millions of years to come full circle.
Consider, for example, the journey of a "typical" carbon atom that existed
in the atmosphere as part of a carbon dioxide molecule some 360 million years
ago, during the Carboniferous Period. That
molecule drifted into the leaf of a large fern growing in the extensive tropical
swamp forests of that time.
Through photosynthesis, the oxygen from the
molecule was released back into the air and the carbon atom was removed from
the molecule and used to build
a molecule of sugar.
The sugar could have been broken down by the plant at a later time to release
the energy stored inside, but this particular sugar molecule was transformed
instead into a long-lived structural part of one of the plant cells. Soon after,
the fern died and the remains sank into the muck at the bottom of the swamp.
Over thousands of years, more plants grew in the swamp and their remains also
sank into the swamp, forming a layer of dead plant material many meters thick.
Gradually, the climate changed, becoming drier and less tropical. Sand, dust,
and other materials slowly covered the ancient swamp and sealed the decaying
vegetation under an ever-thickening layer of sediment. The sediment hardened,
turning to sedimentary rock. The carbon atom stayed trapped in the remains of
the long-vanished swamp while the pressure of the layers above slowly turned
the material into coals.
Some 360 million years later, in the 1900s, the coal bed was mined by humans
and burned to fuel industrial civilization.
The process of burning released the energy stored in the carbon compounds in
the coal and reunited the carbon atom with oxygen to form
The was released to the atmosphere through the smokestack and the journey
continues. Many other paths are possible, some taking only hours or days to
trace, others, like the one above, many millions of years.
The aggregation of the possible paths of carbon, where it may be stored for
extended periods (the "sinks"), where it is likely to be released to the atmosphere
(the "source"), and what triggers those sources (the "release agents"), together
defines the carbon cycle.
Carbon sinks include long-lived trees, limestone (formed from the carbon-containing
shells of small sea creatures that settle to the ocean bottoms and build up
into thick deposits), plastic (a modern invention, but very long-lived), and
the burial of organic matter (such as those that formed the fossil fuels we
use today). Carbon sources include the burning of fossil fuels and other
organic matter, the weathering of limestone rocks (which releases ),
and the respiration of living organisms. Release agents include volcanic
activity, forest fires, and many human activities.
In this activity, students will use resources provided by the teacher to construct
the essential elements of the global carbon cycle. Students may work alone or
in small groups, but should discuss their findings with the whole class. The
goal is to develop a class consensus on the important elements of the cycle.
Your role is to encourage and question, but not to give away answers or suggest
that there is only one right way of looking at the issue.
- Students will understand that carbon is critical to the biosphere and must
continue cycling to support life on earth.
- Students will understand and be able to identify carbon sources, sinks,
and release agents in the carbon cycle.
Alignment to National Standards
National Science Education Standards
- Life Science, The Interdependence of Organisms, Grades 9 to 12, pg. 186,
Item #1: "The atoms and molecules on the earth cycle among the living and
nonliving components of the biosphere."
- Earth and Space Science, Geochemical Cycles, Grades 9 to 12, pg. 189, Item
#1: "The earth is a system containing essentially a fixed amount of each stable
chemical atom or element. Each element can exist in several different chemical
reservoirs. Each element on earth moves among reservoirs in the solid earth,
oceans, atmosphere, and organisms as part of geochemical cycles."
- Earth and Space Science, Geochemical Cycles, Grades 9 to 12, pg. 189, Item
#2: "Movement of matter between reservoirs is driven by the earth's internal
and external sources of energy. These movements are often accompanied by a
change in the physical and chemical properties of the matter. Carbon, for
example, occurs in carbonate rocks such as limestone, in the atmosphere as
carbon dioxide gas, in water as dissolved carbon dioxide, and in all organisms
as complex molecules that control the chemistry of life."
Benchmarks for Science Literacy, Project 2061, AAAS
- The Physical Setting, Processes That Shape the Earth, Grades 9 to 12, pg.
74, Item #1: "Plants alter the earth's atmosphere by removing carbon dioxide
from it, using the carbon to make sugars and releasing oxygen. This process
is responsible for the oxygen content of the air."
- The Living Environment, Flow of Matter and Energy, Grades 6 to 8, pg. 120,
Item #2: "Over a long time, matter is transferred from one organism to another
repeatedly and between organisms and their physical environment. As in all
material systems, the total amount of matter remains constant, even though
its form and location change."
- The Living Environment, Flow of Matter and Energy, Grades 9 to 12, pg. 121,
Item #1: "At times, environmental conditions are such that plants and marine
organisms grow faster than decomposers can recycle them back to the environment.
Layers of energy-rich organic material have been gradually turned into great
coal beds and oil pools by the pressure of the overlying earth. By burning
these fossil fuels, people are passing most of the stored energy back into
the environment as heat and releasing large amounts of carbon dioxide."
- Grade level: 6 to 9
- Teacher introduction: 20 minutes
- Student brainstorming: 20 minutes
- Student preliminary carbon cycle: 15 minutes
- Student research time: 45 minutes to 60 minutes
- Discussion/assessment: 30 minutes
Materials (for extension activity)
- Magazines and newspapers
- Tagboard or poster board for collages
- Blank paper
- Distribute blank paper to students.
- Using class discussion, brainstorming, or question and answer methods, have
students try to trace the movement of a carbon atom. You may wish to record
while students come up with ideas. The result should be a version of a simplified
carbon cycle, including at least eating and respiration in animals and photosynthesis
in plants. It is common for the students to get stuck on the animal-to-plant-to-animal
cycle because most have been exposed to the concept several times. You may
need to move the cycle development along by encouraging them to think about
oceans and whether plays
a role in them. Have them consider rocks and minerals - do they contain carbon?
When you feel that they have at least several important elements of the cycle,
have them work with a partner to illustrate their version of the carbon cycle.
- Next, have students compare their cycle with more elaborate ones illustrated
in texts, encyclopedias, or on-line. Have them list the differences and modify
their own cycle if they think it necessary. Through class discussion, students
can share what they have discovered.
- Introduce the concepts of 'sink,' 'source,' and 'release agents.' Challenge
students to label these factors on their modified cycles. Why might these
be important factors to identify?
- Have students respond to the discussion questions below.
Questions and Observations
- What gas do humans and animals exhale? Write the formula for this exhaled
- Can humans be considered carbon sinks? If so, for how long? What living
organisms are better long-term sinks than humans?
- List two important 'sinks' (things that store carbon), two important 'sources'
(things that release carbon), and one important 'release agent' (things that
trigger sources) for carbon.
- We are currently worried that
sources are out of balance with
sinks. If sources produce more
than sinks can remove, in
the atmosphere increases, possibly leading to global warming. What might happen
if the reverse were true and sinks took up more
than sources? (Answers will vary but students may note that less
would be available for plant growth or that less atmospheric
might result in a slightly cooler atmosphere.)
- Explain how understanding the carbon cycle helps atmospheric scientists
understand and prepare for global climate changes.
EXTENSION (Highly recommended): Divide the class into small groups. Ask each
to develop a collage using magazines and newspapers that illustrates the carbon
cycle. They should label the sinks, sources, and release agents. Each group
should present its collage to the class.
- Consider using the last two discussion questions as a short quiz. Thoughtful
answers to these should indicate that the students met the learning goals.
Modifications for Alternative Learners
- English Language Limited (ELL) students should be able to grasp the concepts
in this activity as long as they can express them in graphic form and are
not forced to rely entirely on text resources for their information. They
may be paired together and given selected ELL-appropriate reading material,
if available, or paired with students who will help explain the readings.
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