"We are assigning probabilities to long-term projections to aid policy makers in assessing the risks that might accompany various courses of action or nonaction," says first author Tom Wigley of the U.S. National Center for Atmospheric Research (NCAR). "If all scenarios are believed to be equally likely, it's difficult to plan." NCAR's primary sponsor is the National Science Foundation.
An estimated global warming range of 2.5-10.4 degree 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 degree or as much as 10.4 degree is very low, say NCAR's Wigley and coauthor 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 4-7 degree 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 2.5-10.4 degree F is up significantly from the panel's 1995 estimates of 1.4-6.3 degree.
In their analysis Wigley and Raper attempted to interpret the likelihood of the new estimates, taking into account the wide uncertainties 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.
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of 66 universities offering Ph.D.s in atmospheric and related sciences.
Visuals: Illustration available at ftp://ftp.ucar.edu/communications. Filename: temps.xls
Caption: Wigley and Raper used several mathematical procedures to arrive at a 90% likelihood that, over the period 1990-2100, the earth's temperature will rise between 1.7 and 4.9 degrees Celsius, or 3 and 9 degrees Fahrenheit (right hand curve). The corresponding 1990-2030 range (left hand curve) is 0.5 to 1.2 degree Celsius, or 1 to 2 degrees Fahrenheit.