Two new models that account for uncertainties surrounding global climate change forecasts both conclude that global temperatures are likely to rise strongly by 2020–30 [Nature, 416, 719 and 723 (2002)].

One study by Peter A. Stott predicts average global temperatures will rise 0.3–1.3 ºC (0.5–2.3 ºF) and the other study by Thomas F. Stocker forecasts a rise of 0.5–1.1 ºC above the average temperature for the 1990–2000 decade. In contrast, the average global temperature increased by only 0.6 +– 0.2 ºC over the entire 20th century.

The two studies take quite different approaches. Stott, of the Hadley Centre for Climate Prediction & Research, Bracknell, U.K., used a complex state-of-the-art model that simulates in detail the circulation of the atmosphere and the oceans. It incorporates variations in solar output, volcanic activity, greenhouse gas emissions, and aerosols. "It runs on a Cray supercomputer and is able to represent in some detail the atmospheric and oceanic processes," Stott says.

In contrast, Stocker, professor of climate and environmental physics at the University of Bern, in Switzerland, and his coworkers employed a much simpler model that contains only a limited number of processes but "is capable of simulating the observed large-scale changes over the past 150 years," Stocker says. By running many simulations, his team was "able to consistently assess uncertainties."

Stott's results show that changing emission scenarios makes a great deal of difference in the projected temperature rise. "For the fossil-fuel-intensive scenario, we estimate warming between 3 and 7 ºC by 2100 compared with 1 to 3 ºC for the scenario in which efforts are made to reduce fossil-fuel use," he says.

In his work, Stocker attempted to quantify uncertainties in the United Nations Intergovernmental Panel on Climate Change (IPCC) projection of a temperature rise of 1.4–5.8 ºC by 2100. His results show there is a 40% probability that the global mean temperature will exceed the range projected by IPCC, but only a 5% chance it will fall under that range.

Drew Shindell, research physicist at the National Aeronautics & Space Administration's Goddard Institute for Space Studies, says that Stott and Stocker made reasonable predictions with the information they had available. What could invalidate those predictions are the "things that most assuredly we don't understand," he says. One unknown is the sun's output, which has been measured by satellite for only 20 years. "We do not know for sure whether the sun will grow brighter or cooler over the next few decades," Shindell says. "However, the odds are high that we'll see a warming of 0.4–0.5 ºC by 2020–30."

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