That earth will get warmer this century is pretty much a given, but how much warmer is tough to pin down. The Intergovernmental Panel on Climate Change (IPCC) announced earlier this year that an increase of anywhere from 2.5 to 10.4 degrees Fahrenheit was likely by 2100. Now two scientists are trying to help policy makers place calculated bets by giving odds to the wide range of predictions. "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," Tom Wigley of the U.S. National Center for Atmospheric Research says. "If all the scenarios are believed to be equally likely, it's difficult to plan."

Wigley and co-author Sarah Raper of the University of East Anglia report today in Science that there is a nine-out-of-ten chance that global temperatures will rise between four and seven degrees this centuryquite a jump over the one-degree increase seen between 1900 and 2000. The scientists' probability analysis further indicates that our planet will heat up a degree or two as soon as 2030. "Whether or not such rapid warming will occur...depends on actions taken to control climate change," the authors write. But if it does, it will be hard to correct with quick interventions. "The climate's inertia would lead to only a slow response to such efforts," they add, "and guarantee that future warming would still be large."