The Intergovernmental Panel on Climate Change predicts that if nothing is done to curb human-emitted carbon dioxide, average temperatures on Earth could increase by up to 6.4 degrees C. (11.5 F.) by century's end.
Until relatively recently, the goal was to stabilize atmospheric co2 concentrations at 450 parts per million. (We're currently at 385 ppm.)
But then NASA's Jim Hansen, who, as The New Yorker's Elizabeth Kolbert pointed out in her recent profile of him, has rather unnervingly made many predictions on climate that came true, began arguing that we had to backtrack to 350 ppm.
You might categorize his reasoning as the precautionary principle writ large: There's a lot we don't know about how Earth's climate responds to higher co2 levels. But what we do know strongly suggests that we shouldn't stray too far into the unknown. The stakes are simply too high.
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