350

There's a number -- a new number -- that makes this point most powerfully. It may now be the most important number on Earth: 350. As in parts per million of carbon dioxide in the atmosphere.

A few month's ago, NASA's chief climatologist, James Hansen, submitted a paper to Science magazine with several coauthors. The abstract attached to it argued that "if humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm."

Hansen cites six irreversible tipping points -- massive sea level rise and huge changes in rainfall patterns, among them -- that we'll pass if we don't get back down to 350 soon; and the first of them, judging by last summer's insane melt of Arctic ice, may already be behind us.

So it's a tough diagnosis. It's like the doctor telling you that your cholesterol is way too high and, if you don't bring it down right away, you're going to have a stroke. So you take the pill, you swear off the cheese, and, if you're lucky, you get back into the safety zone before the coronary.

In this case, though, it's worse than that because we're not taking the pill and we are stomping on the gas -- hard. Instead of slowing down, we're pouring on the coal, quite literally.

We're the ones who kicked the warming off; now the planet is starting to take over the job. Melt all that Arctic ice, for instance, and suddenly the nice white shield that reflected 80% of incoming solar radiation back into space has turned to blue water that absorbs 80% of the sun's heat. Such feedbacks are beyond history, though not in the sense that Francis Fukuyama had in mind.

And we have, at best, a few years to short-circuit them -- to reverse course. Here's the Indian scientist and economist Rajendra Pachauri, who accepted the Nobel Prize on behalf of the Intergovernmental Panel on Climate Change last year (and, by the way, got his job when the Bush administration, at the behest of Exxon Mobil, forced out his predecessor): "If there's no action before 2012, that's too late. What we do in the next two to three years will determine our future. This is the defining moment."

Bill McKibben, a scholar in residence at Middlebury College and the author, most recently, of "The Bill McKibben Reader," is the co-founder of Project 350 ( 350.org), devoted to reducing carbon dioxide in the atmosphere to 350 parts per million. A longer version of this article appears at Tomdispatch.com.