The longevity of CO₂ in the atmosphere is probably the least well understood part of the global warming issue," says paleoclimatologist Peter Fawcett of the University of New Mexico. "And it's not because it isn't well documented in the IPCC report. It is, but it is buried under a lot of other material."

It doesn't help, though, that past reports from the UN panel of climate experts have made misleading statements about the lifetime of CO₂, argue Archer, Caldeira and colleagues. The first assessment report, in 1990, said that CO₂'s lifetime is 50 to 200 years. The reports in 1995 and 2001 revised this down to 5 to 200 years. Because the oceans suck up huge amounts of the gas each year, the average CO₂ molecule does spend about 5 years in the atmosphere. But the oceans also release much of that CO₂ back to the air, such that man-made emissions keep the atmosphere's CO₂ levels elevated for millennia. Even as CO₂ levels drop, temperatures take longer to fall, according to recent studies.