Scientists will contribute results to the next assessment by the Intergovernmental Panel on Climate Change (IPCC), an international research body that advises policymakers on the likely impacts of climate change.

The system, known as the Community Climate System Model, version 3 (CCSM3), indicates in a preliminary finding that global temperatures may rise more than the previous version had projected if societies continue to emit large quantities of carbon dioxide into the atmosphere.

NCAR developed the model in collaboration with researchers at universities and laboratories across the country, with funding from NSF as well as the Department of Energy, the National Oceanic and Atmospheric Administration, and the National Aeronautics and Space Administration. It is releasing the model results and the underlying computer codes to atmospheric researchers and other users worldwide.

"The release of CCSM3 marks a significant milestone in development of climate models," said Jay Fein, director of NSF's climate dynamics program. "The investment by the NSF, the Department of Energy and the scientific community is yielding new insight into the complexities of the Earth system and the likely responses of our planet to natural and anthropogenic influences."

CCSM3 shows global temperatures could rise by 2.6 degrees Celsius (4.7 degrees Fahrenheit) in a hypothetical scenario in which atmospheric levels of carbon dioxide are suddenly doubled. That is significantly more than the 2 degree Celsius (3.6 degree Fahrenheit) increase that had been indicated by the preceding version of the model.

William Collins, an NCAR scientist who oversaw the development of CCSM3, says researchers have yet to pin down exactly what is making the model more sensitive to an increased level of carbon dioxide. But he says the model overall is significantly more accurate than its predecessor.

"This model makes substantial improvements in simulating atmospheric, oceanic and terrestrial processes," Collins says. "It has done remarkably well in reproducing the climate of the last century, and we're now ready to begin using it to study the climate of the next century."

As scientists learn more about the atmosphere, the world's most powerful climate models generally agree about the climatic effects of carbon dioxide, an important greenhouse gas emitted by motor vehicles, power plants, and other sources.

Observations show that atmospheric levels of carbon dioxide have increased from 280 parts per million by volume (ppmv) in preindustrial times to more than 370 ppmv today, and the increase is continuing.

A doubling of carbon dioxide over present-day levels would significantly increase global temperatures, according to all the major models. The models do not always agree, however, on the complex impacts of clouds, sea ice, and other pieces of the climate system.

CCSM3 is one of the world's leading general-circulation climate models, which are extraordinarily sophisticated computer tools that incorporate phenomena ranging from the effect that volcanic eruptions have on temperature patterns to the impact of shifting sea ice on sunlight absorbed by the oceans.

Climate models work by solving mathematical formulas, which represent the chemical and physical processes that drive Earth's climate, for thousands of points in the atmosphere, oceans, sea ice, and land surface. CCSM3 is so complex that it requires about 3 trillion computer calculations to simulate a single day of global climate.

With CCSM3, scientists were able to add four times as many points for the land and atmosphere than had existed in the previous version of CCSM, thereby producing far more information about regional variations in climate and climate change.

The new version also captures such features as continental land temperatures and upper atmospheric temperatures far more accurately than the previous version. In a test, the model closely simulated changes in global temperatures over the last century.

In addition to simulating temperatures over the next century, scientists will use the model to study climate patterns of the past, such as the peak of the last ice age 21,000 years ago. They will also use it to probe chemical processes and the cycling of carbon between the atmosphere, ocean, and land, as well as the localized impacts of sulfates and other pollutants on climate.

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