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Powerful new version of CCSM aids in climate analysisScientists studying global climate patterns have a new and important tool at their disposal: NCAR’s Community Climate System Model, version 2 (CCSM-2). The latest version of the powerful model, unveiled in the spring, has passed a major test with flying colors with a hypothetical 1,000 year simulation of Earth’s climate. Researchers are now preparing to use the model to simulate 20th-century climate and to explore “what if” scenarios, such as the impact of continuing carbon dioxide emissions on future climate.
“CCSM-2 represents a significant improvement in simulating phenomena with worldwide climate implications, such as El Niño,” says CGD’s Jeff Kiehl, who played a leading role in the model’s development. “Researchers can use it to study past climate shifts such as the last glacial maximum, and they can examine the changing atmosphere of the future.” CCSM-2 belongs to the elite group of climate tools known as general circulation models, which run on some of the world’s most powerful computers. These models use mathematical formulas to analyze the climatic impact of the atmosphere, oceans, and continents. Increasingly, they are depicting more subtle natural features such as the formation of sea ice and the volume of fresh water deposited into the oceans by major rivers. These models provide a sort of numerical laboratory for conducting experiments on Earth’s climate, since scientists are unable to conduct actual experiments with the climate system. None of these models is perfect, both because of gaps in our understanding of the small-scale processes in the climate system and because of the limited power of computers. But CCSM stands apart from the other models in its ability to simulate 1,000-year of climate without “drifting” from real-world conditions and requiring scientists to correctp its results. CCSM also is unique because it has been developed by an unusually broad collaboration of scientists at NCAR, other national laboratories, andp universities. The underlying computer code of CCSM is freely available on the Web to researchers worldwide. Data from the 1,000-year control simulation are also openly available to the research community. Indeed, NCAR scientists have been refining global climate models for researchers worldwide since 1983. The first version of CCSM was released in 1998. “The CCSM effort is a great example of the trend toward increasing collaboration among research institutions on complex and important scientific problems.” says UCAR president Rick Anthes. New and improved CCSM-2 offers higher resolution than the original version, and is far more successful in reproducing such phenomena as the El Niño–Southern Oscillation and the annual expansion and contraction of sea ice in polar regions. “This 1,000-year run was a real achievement,” says CGD’s Peter Gent. He notes that scientists used the original version of CCSM to model the climate for no more than a 350-year period per run.
Many scientists in CGD have contributed to CCSM-2. At the 2002 CCSM workshop held in June in Breckenridge, discussion leaders included Peter (ocean), Gordon Bonan (land), Bill Collins (atmosphere), Jim Hurrell (climate variability), Bette Otto-Bliesner (paleoclimatology), and Jerry Meehl and Warren Washington (climate change and assessment). SCD’s Cecelia DeLuca co-leads a working group on software engineering. The next step Now that CCSM-2 has proved its worth by simulating many natural modes of climate variability, researchers will use it to probe past and future climate shifts. For example, the model will peer back in time to replicate conditions during the last glacial maximum, a time about 18,000 years ago when much of North America and Europe lay under vast sheets of ice and the atmosphere contained far less carbon dioxide than it does today. CCSM-2 also can help researchers understand other climate shifts, such as the correlation between the North Atlantic Oscillation (a see-saw in atmospheric pressure) and the Little Ice Age that cooled temperatures during the 17th, 18th, and early 19th centuries.
Looking to the future, CCSM-2 likely play an integral role in the fourth of the major climate assessments issued every few years by the Intergovernmental Panel on Climate Change. And the model’s increased capabilities will permit new types of studies, such as the “Flying Leap” experiment. This multi-institutional, multiyear effort will track fossil fuel carbon emissions as they are dissolved in the oceans, processed by the terrestrial ecosystem, and subsequently released back into the atmosphere. But scientists note that, even with the latest version, CCSM lacks the power to capture all the subtle forces that affect world climate. For example, the model does not yet include active chemistry or capture fine-scale ocean processes. Far from stopping at version 2, scientists are working toward future versions that will offer much more detail, possibly even enabling researchers to make climate predictions for specific regions. “Nature is inherently complex, and our models will inevitably evolve in their complexity to more realistically represent Earth’s climate system,” Jeff explains. •David Hosansky
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