Is a Two-Degree Limit on Global Warming Off Target?

At Cancun in 2010, climate delegates made 2C one of the organizing principles of their 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 a substantial number.  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 provoking enormous damage all over the planet, from dying forests to collapsing sea ice to savage heat waves to torrential rains.  

And scientists are realizing they may have underestimated the vulnerability of the great ice sheets in Greenland and Antarctica.
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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 as rising temperatures play out around the world, 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.

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