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. . . A decade of subsequent research added scientific support to the notion that 2C was a dangerous threshold. Experts realized, for example, that at some increase in global temperature, the immense Greenland ice sheet would begin an unstoppable melt, raising the sea by as much as 23 feet over an unknown period. Their early calculations suggested that calamity would be unlikely as long as global warming did not exceed about 1.9 degrees Celsius.
“Risking a loss of the whole Greenland ice sheet was considered a no-go area,” said Stefan Rahmstorf, head of earth system analysis at the Potsdam Institute for Climate Impact Research in Germany. “We are talking about really sinking a lot of coastal cities.”
As the economic and scientific arguments accumulated, the Germans managed to persuade other countries to adopt the 2C target, turning it into official European policy. The proposal was always controversial, with African countries and island states, in particular, arguing that it was too much warming and would condemn them to ruin. The island states cited the potential for a large rise of the sea, and African countries feared severe effects on food production, among other problems.
But as a practical matter, the 2C target seemed the most ambitious possible, since it would require virtually ending fossil fuel emissions within 30 to 40 years. At Cancun in 2010, climate delegates made 2C one of the organizing principles of negotiations.
The talks culminating in Paris next year are seen as perhaps the best chance ever to turn that pledge into meaningful emissions limits, in part because President Obama has gone far beyond his predecessors in committing the United States, the largest historical producer of greenhouse gases, to action. That, in turn, has lured China, the largest current producer, into making its first serious commitments.
Yet even as the 2C target has become a touchstone for the climate talks, scientific theory and real-world observations have begun to raise serious questions about whether the target is stringent enough.
For starters, the world has already warmed by almost one degree Celsius since the Industrial Revolution. That may sound modest, but as a global average, it is actually substantial. For any amount of global warming, the ocean, which covers 70 percent of the earth’s surface and absorbs considerable heat, will pull down the average. But the warming over land tends to be much greater, and the warming in some polar regions greater still.
The warming that has already occurred is causing enormous damage all over the planet, from dying forests to collapsing sea ice to savage heat waves to torrential rains. And scientists realize they may have underestimated the vulnerability of the ice sheets in Greenland and Antarctica.
Those ice sheets now appear to be in the early stages of breaking up. For instance, Greenland’s glaciers have lately been spitting icebergs into the sea at an accelerated pace, and scientific papers published this year warned that the melting in parts of Antarctica may already be unstoppable.
“The climate is now out of equilibrium with the ice sheets,” said Andrea Dutton, a geochemist at the University of Florida who studies global sea levels. “They are going to melt.”
That could ultimately mean 30 feet, or even more, of sea level rise, though scientists have no clear idea of how fast that could happen. They hope it would take thousands of years, but cannot rule out a faster rise that might overwhelm the ability of human society to adapt.
Given the consequences already evident, can the 2C target really be viewed as safe? Frightened by what they are seeing, some countries, especially the low-lying island states, have been pressing that question with fresh urgency lately.
So, even as the world’s climate policy diplomats work on a plan that incorporates the 2C goal, they have enlisted scientists in a major review of whether it is strict enough. Results are due this summer, and if the reviewers recommend a lower target, that could add a contentious dimension to the climate negotiations in Paris next year.
Barring a technological miracle, or a mobilization of society on a scale unprecedented in peacetime, it is not at all clear how a lower target could be met.
Some experts think a stricter target could even backfire. If 2C already seems hard to achieve, with the world on track for levels of warming far beyond that, setting a tighter limit might prompt political leaders to throw up their hands in frustration.
In practice, moreover, a tighter temperature limit would not alter the advice that scientists have been giving to politicians for decades about cutting emissions. Their recommendation is simple and blunt: Get going now.
“Dealing with this is a little bit like saving for retirement,” said Richard B. Alley, a climate scientist at Pennsylvania State University. “All delay is costly, but it helps whenever you start.”
* * *Justin Gillis, “3.6 Degrees of Uncertainty,” The New York Times, December 15, 2014