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and the University Corporation for Atmospheric Research

Fall 1999— VOLUME 6, NO. 3

Copyright | Note to Teachers




If carbon dioxide continues to accumulate in the atmosphere at its current rate for another hundred years, the sunny Southwest may become soggy and the U.S. Great Plains may be a lot lusher. A new computer projection of the climate of the next hundred years shows an increase in wintertime precipitation in these areas of as much as 40% and a global temperature increase of about 2C (3F) by 2100. If humans manage to cut the buildup of carbon dioxide in half over the next century, rain and snowfall in those areas will remain about the same as now, but the global temperature will still rise some 1.5C. In either case, the sea level will rise 46 to 58 centimeters (17 to 22 inches).

These predictions come from the climate system model (CSM) at the National Center for Atmospheric Research. Scientists at NCAR produced a series of computer runs with the CSM, a global model that includes interacting components for the earth's atmosphere, oceans, land, and sea ice. The first part of the project was an attempt to reconstruct the world's climate from 1870 through 1998, to see how well the CSM could reproduce recorded climate change. The second part went on to 2100, using two scenarios to get insight into how climate is likely to continue changing in response to human behavior. How accurate are these forecasts? The answer to that question, for any climate or weather-forecasting model, depends on two things: the model used and the data that are fed into it.

Rainfall map

The amount of winter rain and snow that falls in the next century will depend on how fast the amount of carbon dioxide in the atmosphere continues to increase, according to the results of two scenarios for 21st-century climate. The colors on this map show the percentage of difference between the scenarios. For example, in the U.S. Southwest, the business-as-usual (BAU) scenario produced in 50% more winter precipitation than the policy-limited scenario, whereas in Maine the BAU scenario resulted in 30% less precipitation than the policy-limited. The differences between the two scenarios indicate how our choices may affect our grandchildren's climate.


How Climate Models Work

Global climate models represent the earth's atmosphere as a grid that covers the surface of the globe and extends upward in layers throughout the entire atmosphere. When the model is running, each corner of every square in this grid is like a tiny weather station where the model calculates atmospheric processes. Models come in coarse and fine resolutions, like coarse and fine screens, depending on how much detail is needed. The finer the grid, the more detailed the resulting simulation, but also the more computing time the model requires. A typical global atmospheric model might have ten vertical layers and 65,000 grid points, making a total of more than half a million points.

To describe the processes that take place in the atmosphere, the model uses mathematical equations. It also includes equations to mimic the input of solar energy and the effects of the earth's rotation. The scientist tells the model how often to perform calculations--for example, to adjust temperature, humidity, barometric pressure, and wind velocity every half-hour of "earth time." In that case, the computer running the model would have to perform at least 40 million calculations for every hour in the model. For a hundred-year-long model run, such as the CSM simulation of the 21st century, a computer must perform some 100 trillion calculations. And remember, these are just the calculations in the atmospheric component; the model also includes relevant processes for land, oceans, and sea ice.

To do all this crunching, a model needs numbers--lots of them. Modern data sources include the highly accurate radar and satellite observations of the last several decades, intensive data sets from field experiments, and records from weather stations throughout the world. Historical documents range from national meteorological records to old ship logs.

A Great Model and Great Data

Scientists who study climate agree that, worldwide, a handful of models stand head and shoulders above the rest. The CSM is one of these elite few. Every model has its own strengths and weaknesses, but the CSM has a special advantage: it does not require flux corrections, the in-flight adjustments that many other models need to keep their simulated climates within the range of possibility. This means that the equations the CSM uses to reproduce climate processes are very close to reality.

In preparing for these simulations, the CSM Chemistry and Climate Change Working Group spent a couple of years putting together the best possible data sets for four important greenhouse gases (carbon dioxide, ozone, methane, and nitrous oxide), a set of halocarbons, and sulfate aerosols. Coleaders Jeffrey Kiehl of NCAR and Susan Solomon of the National Oceanic and Atmospheric Administration pinpointed experts in each of these fields and invited them to participate in the project. Many models use only carbon dioxide to stand in for all greenhouse gases to save computer time, but in reality each of the gases modeled in the CSM absorbs and emits radiation at a different rate and has different chemical reactions.

Besides a realistic model and excellent data set, predicting the future requires a reasonable approximation of human actions that affect climate. For that part of the puzzle, Tom Wigley and Steve Smith of ACACIA (A Consortium for the Application of Climate Impact Assessments), an NCAR-industry program, and Hugh Pitcher of Battelle Pacific Northwest Laboratories took a new approach to the problem of sulfur dioxide emissions. These emissions, which are the byproducts of burning coal, form sulfate aerosols--tiny particles that react in the atmosphere to make acid rain. "Sulfur emissions are very costly to a nation's economy," says Wigley. "In North America, we've introduced legislation to drastically reduce sulfur dioxide emissions because we know how expensive they can be." To prepare the new scenario, the ACACIA team considered the history of environmental regulation in developed countries, the consequences of unchecked pollution, and the likely growth in developing nations' gross domestic products (which will allow industries to invest in the modifications needed, for example, to clean up dirty factories). These factors led them to believe that the Asian countries will begin introducing controls over sulfur emissions in the next few decades.

Model Results

The two scenarios for the future both use the reduced sulfur emissions. The difference comes in the treatment of greenhouse gases. In the "business-as-usual" scenario, emissions of these gases will continue to increase at about their current rate throughout the model run. The "policy-limited" scenario assumes that, starting in 2010, new climate policies will gradually slow greenhouse-gas increases until carbon dioxide finally stabilizes at 50% above today's atmospheric concentrations by 2150.

The researchers found that, despite this difference, global temperature increased and precipitation patterns changed at about the same rate in the policy-limited case as in the business-as-usual scenario until around 2060. The half-century gap before the policies' effects appear is the result of "thermal inertia." In other words, it takes that long for the earth--especially the oceans--to heat up or cool down noticeably.

After 2060, the effects of climate change on precipitation in the two scenarios differed considerably (see figure). The difference in global temperature increase between the two scenarios by the end of the century was only half a degree Celsius, but in some regions the effects were more marked. For example, the model showed Europe's temperature as increasing 50% more under business as usual than under the policy-limited case.

global temperature map

Global temperature differences (in Kelvins, or degrees C) between the two 21st-century scenarios. For the next hundred years, the effect on temperature of limiting carbon dioxide emissions is considerably less marked than the effect on precipitation. The differences between the two scenarios are most marked in western Europe, North Africa, and Australia.


The modeled decrease in sulfur emissions allowed the warming effect of the greenhouse gases to emerge more clearly. Sulfate aerosols are known to cool the atmosphere; they themselves reflect solar radiation back into space before it can warm the earth, and they lead to denser clouds, which also reflect more solar radiation back into space.

And now, a few words about sulfates. . .

Sulfate aerosols are a hot topic these days in global climate modeling. Climate models haven't included their effects until recently, and many still do so only partially and crudely. Many scientists believe that this lack explains why earlier models predicted that, given our greenhouse gas emissions, the earth should have warmed considerably more than it has in reality. Modeling the aerosols' effects was one of the bigger challenges in preparing the CSM to simulate the climate of the 20th and 21st centuries.

If sulfate aerosols are so important in the climate system, why haven't modelers been accounting for them all along? For one thing, a real-world sulfate particle isn't like a molecule from a chemistry lab. Perhaps it's more like some of the students--dirty, mixed-up, and hard to understand. Kiehl says, "It has some water attached to it, and what we call organics, which come from trees or pollution. In polluted places like the U.S. East Coast, 50% may be sulfate and another 50% may be organics." The organic molecules themselves are extremely complex, and scientists are only beginning to attempt to model them. Also, compared to the greenhouse gases, sulfur has a short lifetime in the atmosphere. Therefore, it doesn't get distributed evenly over the whole globe. Kiehl says, "That's made quantifying the problem more difficult than [for] greenhouse gases; you have to know how much of the aerosol is located in a certain region." And there's vertical movement as well; the species that react to produce sulfate may be carried upward in clouds before they convert into aerosol.

Perhaps the bottom line is that, until recently, few field projects collected data about sulfate aerosols, so scientists have been uncertain how to model even such basic facts as the number and composition of the particles in various parts of the world. Last spring, however, the multinational Indian Ocean Experiment made a wealth of observations on aerosol effects that, once analyzed, should help scientists nail down many of the remaining uncertainties.

What the future holds . . .

Even with the years of effort that Kiehl, Wigley, and their colleagues have spent on creating a realistic model, collecting the best available data, and developing plausible future scenarios, they are well aware that we cannot know with certainty what 2099's climate will be. But we do know that climate change is already under way, and future changes are likely to be much greater than any other climate change since the end of the last Ice Age, in both their magnitude and the speed at which they take place. Model results like these help us realize that our own behavior may make the difference between a sunny day and a torrential downpour--if not for our children, then certainly for our great-grandchildren.



has written a report called "The Science of Climate Change" for the Pew Center on Global Climate Change, describing how global climate has already changed and discussing likely changes in U.S. climate, extreme weather, etc. The report is available free from the:

Pew Center on Global Climate Change
2111 Wilson Boulevard, Suite 350
Arlington, VA 22201
or on the World Wide Web at:

The CSM results reported here will be used as part of the U.S. National Assessment of global climate change, sponsored by the U.S. Global Change Research Program. The Web address is http://www.nacc.usgcrp.gov.

The Public Utilities Commission of Ohio offers links to a wide range of global-change Web sites, including educational resources: http://www.puc.state.oh.us/consumer/gcc/index.html

The National Oceanic and Atmospheric Administration's report, Our Changing Climate, is written at a high school level: http://www.ogp.noaa.gov/OGPFront/Mono4/RTN4.html

The National Climate Data Center, http://www.ncdc. noaa.gov, is the world's largest archive of weather data, offering lists of extreme weather events such as tornadoes and hurricanes and many other resources.



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