Understanding Climate Change

Topics

• Global warming or climate change?
  
  
• What is the average global temperature now?
  
  
• How much has the global temperature risen in the last 100 years?
  
  
• How much carbon dioxide (and other kinds of greenhouse gas) is already in the atmosphere?
  
  
• What does the ozone hole have to do with climate change?
  
  
• Aren’t the computer models used to study climate really simplistic?
  
  
• Weren't scientists warning us about global cooling a few years ago?
  
  
• It's been freezing cold where I live. Doesn't that prove global warming is a hoax?
  
  
• Don't all the government reviewers and negotiations over wording make the IPCC report a political, not a scientific, document?
  
  
• What can we do about global warming?
  
  
• What's in the Working Group I Summary for Policymakers on the Physical Science of Climate Change? (opens new window)
  
  
• What's in the Working Group II Summary for Policymakers on Climate Change Impacts? (opens new window)
  
  
• What does the American Geophysical Union have to say about human impacts on climate? (opens new window)
  
  
• What does the American Meteorological Society have to say about climate change? (opens new window)

Global warming or climate change: Which is it?

Two 19th-century scientists are associated with the discovery that increasing carbon dioxide in the atmosphere warms the entire planet: French researcher Jean Baptiste Fourier and Swedish scientist Svante Arrhenius. Their identification of what came to be called the greenhouse effect (see box at right) applies to both natural and human-produced additions of CO₂.

As measurements of atmospheric CO₂ levels showed steady increases after World War II (see below), Earth system scientists looked for a corresponding rise in global average temperatures, basing their studies on the physical laws governing the greenhouse effect. By the early 1980s, climate scientists were calling this atmospheric response global warming. Not every place on Earth was expected to warm at the same rate, and rising temperatures were not the only impacts anticipated. So some researchers began talking about global climate change to convey that the situation was more complex.

To some ears, "climate change" sounds less ominous than "global warming," so the question of how best to convey the seriousness of the problem continues. Suggestions include "global heating" and "global water crisis."

 back to top

What is the average global temperature now?

The average air temperature near Earth’s surface in 2006 was estimated by the World Meteorological Organization to be about 57.9°F (14.4°C) as of December 14, 2006.

There are several different techniques for coming up with a global average, depending on how one accounts for temperatures above the data-sparse oceans and other poorly sampled regions.

Since there is no universally accepted definition for Earth’s average temperature, several different groups around the world use slightly different methods for tracking the global average over time. Yet the trends that emerge are remarkably similar—more so than the averages themselves. This is why global warming is usually described in terms of anomalies (variations above and below the average for a baseline set of years) rather than in absolute temperature. A Web site from NASA's Goddard Institute for Space Studies goes into more detail on the topic of The Elusive Absolute Surface Air Temperature.

 back to top

How much has the global temperature risen in the last 100 years?

Averaged over all land and ocean surfaces, temperatures have warmed about 1.35°F (0.75ºC) over the last century. Much of this warming—about 0.72°F (0.4°C)—has occurred since 1979. Because oceans tend to warm and cool more slowly than land areas, continents have warmed the most (about 0.7ºC or 1.26°F since 1979), especially over the Northern Hemisphere.

The year 1998 was the warmest year in U.S. records, followed closely by 2006 and 1934, according to the National Climatic Data Center. Looking at the average temperature during floating five-year periods (for example, 2003–2007, 2002–2006, and so on), the last nine (since 1995) were the warmest in 113 years of U.S. record keeping, also according to the NCDC.

There are slight differences when it comes to the global record, but the data from around the world point to the decade between 1998 and 2008 as the hottest since 1850. Globally, 2007 was the fifth warmest year in records extending back to the late 19th century, according to NCDC. At NASA, scientists place 2007 in a tie with 1998 for second warmest (and 2005 at the top for warmest year on record). Groups at NASA and the University of East Anglia also calculate global temperature year by year, using slightly different techniques. The East Anglia group places 2007 as eighth warmest since 1850, with 1998 as the hottest, 2005 at second, and the other five hottest years all within this past decade.

This graph from NOAA shows the annual trend in average global air temperature in degrees Celsius, through 2007. For each year, the range of uncertainty is indicated by the vertical bars. The blue line tracks the changes in the trend over time. Click here or on the image to enlarge. (Image courtesy NOAA's National Climatic Data Center.)

 back to top

 How much carbon dioxide (and other kinds of greenhouse gas) is already in the atmosphere?

One of the strongest pieces of evidence for human-induced climate change is the consistent rise in carbon dioxide (CO₂) in modern times, as measured at the Mauna Loa Observatory in Hawaii, where CO₂ has been observed since 1958. As of 2005, the concentration of CO₂ in Earth’s atmosphere was about 378 parts per million (ppm).

This graph, known as the Keeling curve in honor of its originator, Charles David Keeling of Scripps Institution of Oceanography, shows an annual seasonal cycle and a steady upward trend since measurements began at Mauna Loa in 1958. The seasonal cycle is due to the vast land mass of the Northern Hemisphere, which contains the majority of land-based vegetation. The result is a decrease in atmospheric carbon dioxide during northern spring and summer, when plants are absorbing CO2 as part of photosynthesis. The pattern reverses, with an increase in atmospheric carbon dioxide during northern fall and winter. The yearly spikes during the cold months occur as annual vegetation dies and leaves fall and decompose, which releases their carbon back into the air. Click here or on the image to enlarge. (Image courtesy Scripps CO2 Program.)

Current atmospheric concentrations of CO₂ are about 30% higher than they were about 150 years ago at the dawn of the industrial revolution. According to the Scripps Institution of Oceanography, ice core reconstructions going back over 400,000 years show concentrations of around 200 ppm during the ice ages and about 280 ppm during the warm interglacial periods. In other words, our current CO₂ levels are higher than they've been in at least the last 400 millenia. See the Scripps Web site for a graphic illustrating this trend.

Almost a quarter of the carbon dioxide emitted by human activities is absorbed by land areas; another quarter is absorbed by the ocean. The remainder stays in the atmosphere for a century or longer. Because CO₂ stays in the air so long, it becomes very well mixed throughout the global atmosphere. This makes the Mauna Loa record an excellent indication of long-term trends.

Carbon dioxide accounts for more than half of the human-produced enhancement to Earth’s greenhouse effect.  Among the other gases involved is methane, which has increased dramatically over the last century. Methane concentrations rose about 1% a year in the 1980s, but since about 2000 the concentration has leveled off. The reasons for this are not yet clear, although one possibility is a drop in the amount of methane leaked from natural gas pipelines and plants. Methane stays in the atmosphere for much less time than carbon dioxide (around a decade) and there is much less of it, but molecule for molecule, it is a far more powerful greenhouse gas. As of 2005 the concentration of methane in Earth’s atmosphere was about 1772 parts per billion.

Other important greenhouse gases include nitrous oxide and near-surface ozone. Water vapor is actually the most prevalent greenhouse gas, but human activity has not directly increased its concentration in the atmosphere, unlike the other chemicals above. However, as global temperatures increase, more water vapor is released by oceans and lakes, and this in turn helps to increase temperatures further. This is one of many feedback loops that help to reinforce and intensify climate change.

 back to top

What does the ozone hole have to do with climate change?

There are a few connections between the two, but they are largely separate issues.

First, it's important to know that ozone plays two different roles in the atmosphere. At ground level, "bad ozone" is a pollutant caused by human activities; it's a major component of health-damaging smog. The same chemical occurs naturally in the stratosphere, and this "good ozone" acts as a shield, filtering out most of the ultraviolet light from the Sun that could otherwise prove deadly to people, animals, and plants.

The ozone hole refers to the seasonal depletion of the ozone shield in the lower stratosphere above Antarctica. It occurs as sunlight returns each spring, triggering reactions that involve chlorofluorocarbons (CFCs) and related molecules produced by industrial processes. These reactions consume huge amounts of ozone over a few weeks' time. Later in the season, the ozone-depleted air mixes with surrounding air and the ozone layer over Antarctica recovers until the next spring. Other parts of the globe have experienced much smaller losses in stratospheric ozone.

Because of international agreements to limit CFCs and related emissions instituted with the Montreal Protocol, it's expected that the ozone hole will be slowly healing over the next few decades.

The ozone hole does not directly affect air temperatures in the troposphere, the layer of the atmosphere closest to the surface, although changes in circulation over Antarctica related to the ozone hole appear to be changing surface temperature patterns over that continent. Ozone is actually a greenhouse gas, and so are CFCs, meaning that their presence in the troposphere contributes slightly to the heightened greenhouse effect. The main greenhouse gas responsible for present-day and anticipated global warming, however, is carbon dioxide produced by burning of fossil fuels for electricity, heating, and transportation.

 back to top

Aren’t the computer models used to study climate really simplistic?

Global climate models—the software packages that simulate the past, present, and future of our atmosphere—have grown in complexity and quality over the last 10 to 20 years. Yet even the earliest models of the 1960s, which were quite crude by today’s standards, showed that a doubling of carbon dioxide in the atmosphere could increase global temperature by around 5°F (3°C). That projection remains close to the modern consensus, and temperatures over the last 30 years have risen at a rate consistent with this early estimate.

This illustration shows how the amount of detail in climate models has increased in recent years, largely because of the calculation power provided by newer supercomputers. In the 1990s, high-resolution global climate models operated on the T42 resolution scheme (upper left). At this resolution, temperature, moisture, and other features were tracked in grid boxes that each spanned about 200 by 300 kilometers at midlatitudes (120 x 180 miles), an area roughly as large as West Virginia. In more recent modeling that led up to the 2007 IPCC Working Group I report, the NCAR-based Community Climate System Model (CCSM) routinely operated at T85 resolution (upper right), with midlatitude grid points of about 100 by 150 km (60 x 90 miles)—the size of Connecticut. Better resolution not only provides a more true-to-life depiction of atmospheric processes, but also allows for more realistic topography, which makes regional climate projections more accurate. For example, the highest Rocky Mountains appear as two coarse grid points at T42 but as a more diverse assortment of high peaks at T170 (lower left). Enhancements in computing power will help scientists explore the use of higher resolutions, such as T170 and T340 (lower right). Click here or on the image to enlarge. (Illustration courtesy Warren Washington, NCAR. ©UCAR. News media terms of use*)

Far more information is available from today’s models, such as the NCAR-based Community Climate System Model, because they now include many more aspects of the Earth system, including ice sheets, vegetation, cloud areas, and soil moisture.

Research conducted for the 2007 IPCC Working Group 1 assessment compared the output from major models at research centers around the world. While these models are far from perfect, scientists are confident that they capture the key processes that drive climate. For example, models now replicate the ups and downs of 20th-century global temperature quite accurately.

As in other areas of science, rigorous testing and continual improvement are part and parcel of climate modeling. Researchers can test models against reality, identify and correct flaws, and compare their models with others.

The complexity of global climate models has increased enormously over the last 20 years, as shown in the flow chart at left. Beneath each time period is a list of the components included in state-of-the-art models such as the NCAR-based Community Climate System Model. Click here or on the image to enlarge. (Illustration courtesy Warren Washington, NCAR. ©UCAR. News media terms of use*)

Now in its third generation, the NCAR-based Community Climate System Model (CCSM) is one of the world’s most sophisticated and widely used models of global climate. The graphic at left illustrates the many components included in the CCSM, ranging from cirrus and stratus clouds to ocean currents and soil moisture. These components are also typical of many of the dozen or so climate models at other major research centers around the globe. Click here or on the image to view a larger image with more details on each component. (Illustration by Paul Grabhorn, ©UCAR. News media terms of use*)

 back to top

Weren't scientists warning us about global cooling a few years ago?

After rising in the early 20th century, global surface temperatures cooled slightly from just after World War II (the mid-1940s) into the 1970s.

Scientists already knew that carbon dioxide was accumulating in the atmosphere and that it could lead to eventual global warming. In 1975, Wallace Broecker (Lamont-Doherty Earth Observatory) published the first major study with "global warming" in the title.

But a few researchers believed that pollution from burgeoning postwar industry was shielding sunlight and shading the planet, causing the observed cooldown. Some even theorized that a "snow blitz" could accelerate the cooling and bring on the next ice age. Their statements got major play in the media.

Starting in the 1970s, new clean-air laws began to reduce sulfates and other sunlight-blocking pollutants from U.S. and European sources, while greenhouse gases continued to accumulate unchecked. Global temperatures began to warm sharply in the 1980s and have continued rising since then.

Increasingly detailed models suggest that the more recent warmup can be attributed to greenhouse gases overpowering the effect of sunlight-shielding pollution.  Computer simulations also suggest that today's atmosphere would be even warmer still, were it not for that air pollution.

 back to top

It's been freezing cold with lots of snow where I live. Doesn't that prove global warming is a hoax?

There are always cold spells and warm spells going on in one place or another. But even where weather is cold, what's considered "typical" is changing. For example, the heavy snow that struck Colorado and Kansas at the end of 2006 was actually more characteristic of that area's autumn or spring weather than a typical December.

Globally, Earth's natural processes don't follow a linear pattern, so the global average temperature may be slightly cooler or warmer from one year to the next. Different parts of Earth's ecosystem also respond to the greenhouse effect in different ways. The oceans, for example, hold more heat and respond to atmospheric chnages more slowly than land masses do. Average temperatures of the land, oceans, and atmosphere also vary from year to year as well as from each other.

To examine long-term warming, climate scientists look at large areas and longer time periods. The maps below help illustrate the global nature of climate change.

Temperature anomalies (variations above and below average, in degrees Celsius) for Wednesday, January 17, 2007 (top). Despite the cold across Texas and New England, most of the Northern Hemisphere was running well above average that day. Click here or on the image to enlarge. (Image courtesy NOAA/ESRL PSD Map Room.)

The picture becomes even more striking when looking at the period from mid-December to mid-January (center) and December as a whole (bottom). The warming is especially strong in the high-latitude countries of Canada and Russia, which is consistent with the long-term trends predicted by computer models that take increased greenhouse gases into account. Click here or on the image to enlarge. (Image courtesy NOAA/ESRL/PSD Map Room.)

December as a whole. Click here or on the image to enlarge. (Image courtesy NOAA/NESDIS/NCDC.)

 back to top

Don't all the government reviewers and negotiations over wording make the IPCC report a political, not a scientific, document?

The unique structure of the IPCC includes both scientific and governmental review, but the input of diplomats to the final report is designed to be distinct and different from that of scientists.

Scientists who are experts in their subject matter prepare the chapters that go into the full assessment reports. Those chapters are scrutinized by individual scientists and scientist panels, whose questions must be addressed before a chapter can be approved for inclusion.  The chapters making up the 2007 draft report from IPCC Working Group 1 report run to over 1,600 pages.

It is only the Summary for Policymakers, typically around 30 pages, that receives a word-by-word review, during the final plenary session, by diplomats from almost every nation in the world. The lead authors of the report are on hand at the plenary to make sure that any changes are scientifically valid. The diplomats have a say in how the Summary for Policymakers is worded, but the scientists have the last word on what is said.

 back to top

What can we do about global warming?

There are two basic types of response to climate change. Mitigation is reducing the emissions of greenhouse gases responsible for climate change, so that less change occurs. Adaptation is dealing with the consequences of warming and other aspects of climate change, such as changes in extreme weather events.

Because some amount of climate change has already occurred, and more change is inevitable based on the greenhouse gases already emitted, society will need to adapt. Yet in order to prevent even more-extreme climate change from happening, mitigation will be required.

Policymakers are now examining these two types of responses, including how much attention and what resources to devote to each one and how to find a balance between mitigation and adaptation.

"Business as usual" is also a choice. This option saves expenditures for mitigation in the near term, but risks higher adaptation costs to wildlife, human populations, infrastructure, and economies later on. It also increases the odds of unforeseen consequences from unchecked climate change.

The 2007 IPCC report helps policymakers weigh these options. To promote discussion of policy choices in our democracy, NCAR's parent organization, UCAR, has joined with professional societies and other members of the atmospheric sciences community to offer policy-relevant Advice to the Administration and Congress: Making Our Nation Resilient to Severe Weather and Climate Change.

As impacts on natural systems are being felt, human adaptation is already happening on some fronts. Many insurance companies are examining their practices and taking climate change into account in setting their rates and their policies. Air conditioning is becoming more widespread in North America and Europe. Some communities on small islands are already making plans to abandon their homes due to rising sea levels. The fate of plants and animals that cannot readily adapt is being discussed.

The best-known mitigation plan to date is the Kyoto Protocol, ratified by most countries in the world, though not the United States and Australia. Often cited out of context, NCAR scientist Tom Wigley's research has shown that adherence to the Kyoto Protocol alone, without subsequent action, would have a minimal impact on global warming. However, he notes, "This does not mean that the actions implied by the Protocol are unnecessary."

Many U.S. cities and states have committed to reducing their output of greenhouse gases over the coming decades. Mitigation is also happening on the personal level (buying a fuel-thrifty or hybrid vehicle, for instance, or installing energy-saving light bulbs) and in private industry (a growing number of businesses and organizations have pledged to become carbon neutral).