Warming temperatures, Hansen says, not only increase the amount of meltwater on the surface of the ice but also increase rainfall. “Ice-sheet growth,” he says, “is a dry process. Disintegration is a wet process, so it goes a lot faster.”
If today’s CO2 levels would lead to several meters of sea-level rise—putting many coastal areas, housing hundreds of millions of people, completely underwater—then letting CO2 rise to 560 ppm could lead to a disaster of unimaginable proportions. Even a rise to 450 ppm could be catastrophic, according to Hansen’s team’s analysis. Before about 35 million years ago, the planet was completely ice free, so warm-water alligators and lush redwood forests thrived above the Arctic Circle. The transition to large-scale glaciation in Antarctica began, the researchers estimate, when CO2 dropped to 425 ppm, plus or minus 75 ppm. Most of the ice should therefore disappear again if we reach that point—and if all of Antarctica’s and Greenland’s glaciers were to melt, sea level would rise many tens of meters. The only way to keep CO2 concentrations as low as that, Schneider says, is to have the entire world adopt California’s strictest-in-the-nation proposals for limiting carbon emissions—something that is hard to imagine even the other U.S. states agreeing to, let alone developing nations such as India and China.
That’s only taking the feedback from melting glaciers into account. “Changes in vegetation, in atmospheric and ocean chemistry, and in aerosols and dust in the atmosphere all appear to be positive feedbacks on temperature changes,” Schmidt says. “If global average temperatures change for any reason, those other elements will amplify the change.” Other positive feedbacks include the release of CO2 dissolved in the oceans, which will happen as they warm up, and the accelerated release of other greenhouse gases—methane, for example, from biomass that will begin rotting as permafrost melts in the Arctic.
Given Hansen’s eminence as a climate scientist, one might expect that his analysis would have triggered a general panic. And it has—but not among scientists. “This month may have been the most important yet in the two-decade history of the fight against global warming,” wrote journalist and author Bill McKibben in the Washington Post this past December, shortly after Hansen spoke about his new calculations at a conference: “[350 ppm is] the number that may define our future.” McKibben has even created an organization he calls 350.org to spread the word. Other activists and bloggers have reacted with similar alarm.
Most climate experts turn out to be much less exercised, even though they take the danger of global warming very seriously. The reason: Hansen and his colleagues base their new estimate of climate sensitivity and of the various tipping points represented by different feedback mechanisms on a record of ancient conditions that are not really well understood. “The problem,” Schmidt says, “is that the further back you go, the less you really know. The error bars get very large.” The planet’s ice-free periods are, he admits, “very interesting—they’re like the periods we think we’re heading for, and in principle they can tell us a lot about the climate sensitivity to CO2 changes.”
And although you can infer atmospheric CO2 levels indirectly, from changes in the acidity of ocean sediments, for instance, Schmidt notes that this involves assumptions that might be wrong. “People generally think CO2 was higher then, but you can’t get a precise number or a precise time series. Jim would say that the true climate sensitivity is twice the Charney sensitivity, but it could be three times, it could be one.” Similar uncertainties surround the ebb and flow of continental ice sheets. “It’s quite possible,” Schmidt says, “that the ice sheets across North America might have been more sensitive than those in Greenland, which would explain why Greenland’s have persisted.”