Will Climate Go Over The Edge?(International Edition; WORLD AFFAIRS)(climate change)

Byline: Fred Guterl

Even a miracle of diplomacy wouldn't put global warming back in its box.

There is something compelling, in a ghoulish sort of way, about the notion that earth's climate may be headed toward a tipping point. The idea gained broad currency in 2007, when a panel of scientists, including Harvard environmental expert John Holdren--now the White House science adviser--warned that the planet is approaching a threshold beyond which damage to the environment would be irreversible. As policymakers work toward a climate treaty in Copenhagen in December that will include new limits on emissions, the question in the back of everyone's mind is whether an agreement can halt the warming trend, or at least stave off the worst consequences. Or is it already too late? A definitive answer isn't forthcoming, but the signs in recent months have been gloomy.

The truth is shrouded by a big scientific unknown: how quickly does climate respond to changes in carbon levels? After 30 years of research, the link between the two is still imprecise. That's why temperature trends are expressed within wide confidence intervals. The Intergovernmental Panel on Climate Change, the U.N. group, puts the odds at two in three that a doubling of carbon levels in the atmosphere from pre-industrial levels would raise average temperatures anywhere from 2 degrees C to 4.5 degrees C. The difference between the top and bottom of this range, according to the 2007 report, spells the difference between bad and catastrophic. (Some scientists believe, for instance, that crop yields decline 10 percent for each degree rise in temperature.) Where future generations wind up on the scale--or even if they fall on the scale at all--is still a roll of the dice.

Empirical evidence is worrying, but not particularly enlightening. Melting glaciers, changing bird-migration patterns and other observations suggest that warming is proceeding at a pace that may exceed past estimates--in other words, we may be heading toward the top of the IPCC forecast. But they don't tell us much about natural variability. Arctic sea ice, for instance, is clearly shrinking faster than the climate-change computers predicted. How much is due to carbon and how much would the ice have retreated anyway? There's too little data to know--comprehensive records on Arctic sea ice go back only to 1979. "The most likely bet is that the acceleration is due to greenhouse warming," says David Battisti, an atmospheric scientist at the University of Washington in Seattle. "But I'd be nervous about making that bet. To know for certain we'd want a couple hundred years of data. We have 30 years of really good data."

Since the real world is so messy, climate scientists Gerard Roe and Marcia Baker turned for insight to the distinctly neater world of mathematics. Last year, they ...