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Global 4 to 7 Degree Temperature Rise Likely by 2100Overwhelming odds point to global average temperatures that will rise 4 to 7 degrees over the coming century, according to a new probability analysis by scientists in the United States and Europe. As early as 2030 the planet is likely to heat up 1 to 2 degrees, say the scientists. A one-degree temperature rise was observed over the past century. The study is funded in part by the National Science Foundation (NSF) and appears in the July 20 issue of the journal Science. "We are assigning probabilities to long-term projections to aid policy makers in assessing the risks that might accompany various courses of action or non-action," says Tom Wigley of the U.S. National Center for Atmospheric Research (NCAR) in Boulder, Colorado. "If all scenarios are believed to be equally likely, it's difficult to plan." NCAR's primary sponsor is the National Science Foundation. "NSF has supported climate research for well over a decade," adds Cliff Jacobs, program director in NSF's division of atmospheric sciences, which funds NCAR. "Our goal from the beginning has been to improve our understanding of the Earth's systems and ultimately provide useable science to society. This study is another step in that direction." An estimated global warming range of 2.5 to 10.4 degrees F was announced earlier this year by the Intergovernmental Panel on Climate Change (IPCC), composed of hundreds of scientists around the world. But the likelihood that the earth's temperature would warm only 2.5 degrees or as much as 10.4 degrees is very low, say NCAR's Wigley and co-author Sarah Raper of the University of East Anglia in England and the Alfred Wegener Institut for Polar and Marine Research in Germany. Even warming of 4 to 7 degrees F, however, is very large compared with the observed warming over the past century, they write. "Whether or not such rapid warming will occur . . . depends on actions taken to control climate change," they continue. In arriving at their estimates, the scientists assumed that no policies would be implemented to curb climate change before 2100. If a rapid warming and its expected impacts occur in the near future, even swift societal attempts at control would yield little immediate success, say the authors. "The climate's inertia would lead to only a slow response to such efforts and guarantee that future warming would still be large," they write. New estimates of sulfur dioxide and other emissions, along with updated information on carbon storage, ocean circulation, radiation, and other components of the earth system have improved computer models of the earth's climate and led the IPCC to both raise and widen its estimated range of global temperature increase. The latest range of 2.5 to 10.4 degrees F is up significantly from the panel's 1995 estimates of 1.4 to 6.3 degrees. In their analysis, Wigley and Raper attempted to interpret the likelihood of the new estimates, taking into account wide uncertainty about future human activities and the climate's response to them. They identified the main sources of uncertainty and estimated the probability of their values falling within defined ranges. They then used these results to "drive" a simplified climate model and combined the various model results into probability ranges for temperature increases.
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