Climateprediction.net launched in September 2003. Today there are more than 26,000 personal computers currently running unique versions of the project's GCM. The model runs in the background on PCs and does not affect ordinary computing tasks, such as surfing the Internet, writing a letter, or running a spreadsheet, Stainforth noted.
Ultimately the project aims to produce the world's most comprehensive probability-based forecast for Earth's 21st-century climate. In tomorrow's issue of the science journal Nature, Stainforth and his colleagues will report the results from a test drive of the models.
The first step in the climateprediction.net experiment is for researchers to get a feel for how the differing versions of their climate model work. Stainforth likens it to taking a new car out for a test drive. "When you push down the accelerator, sometimes it makes little difference, and sometimes [the car] shoots away very quickly."
To find out how their climate models "drive," Stainforth and his colleagues sent to project participants more than 2,000 GCM versions. The models realistically represented various past and present climates on Earth but were tweaked by a doubling of carbon dioxide, an atmospheric greenhouse gas.
A model's response to the doubling of carbon dioxide is a scientifically useful measure of the model's sensitivity.
The climateprediction.net models showed a sensitivity range between 2 and 11 degrees Celsius (3.6 and 19.8 degrees Fahrenheit). This was more than twice the sensitivity of the most sensitive GCMs currently used to predict future climate, Stainforth said. Those have a range of 2 to 5 degrees Celsius (3.6 to 9 degrees Fahrenheit).
Gavin Schmidt, a climate modeler with the NASA Goddard Institute for Space Studies in New York, said he is surprised by the sensitivity of the climateprediction.net GCM runs. However, he said the results need to be examined more closely before any implications from the study are drawn.
"The team involved here needs to be congratulated on getting this far," he said.
Stainforth cautioned that the fact that the climateprediction.net GCMs have a sensitivity range of 2 to 11 degrees Celsius does not necessarily mean that global temperatures will rise within that range.
For example, there's no guarantee that carbon dioxide levels will only double. Also, more model runs with more climate variablessuch as an ocean that changes temperature throughout the seasonsare needed to accurately assess the range of future climate scenarios. These will be done in the next stages of the project.
Schmidt, the NASA climate modeler, said that during the last glacial period, about 20,000 years ago, changes in ice sheet cover, vegetation, greenhouse gas concentrations, and atmospheric dust most likely forced a 5-degree-Celsius (9-degree-Fahrenheit) cooling, suggesting a sensitivity of around 3 degrees Celsius (5.5 degrees Fahrenheit).
"If the sensitivity was really 11, the [forces] required to produce 5-degree cooling must have been proportionately less. This appears to be unsupportable, given what we know about glacial climate," he said.
Stainforth and his colleagues have only completed the test-drive phase of their project. They hope future runs with refined and more complex models will lead to an accurate, validated forecast for the 21st-century climate. To participate, log on to www.climateprediction.net.
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