Even for Americans -- who are constitutionally convinced that there will always be a second act, and a third, and a do-over after that, and, if necessary, a little public repentance and forgiveness and a Brand New Start -- even for us, the world looks a little terminal right now.

It’s not just the economy: We’ve gone through swoons before. It’s that gas at $4 a gallon means we’re running out, at least of the cheap stuff that built our sprawling society. It’s that when we try to turn corn into gas, it helps send the price of a loaf of bread shooting upward and helps ignite food riots on three continents. It’s that everything is so tied together. It’s that, all of a sudden, those grim Club of Rome types who, way back in the 1970s, went on and on about the “limits to growth” suddenly seem ... how best to put it, right.

All of a sudden it isn’t morning in America, it’s dusk on planet Earth.

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 weeks ago, NASA’s chief climatologist, James Hansen, submitted a paper to Science magazine with several coauthors. The abstract attached to it argued -- and I have never read stronger language in a scientific paper -- 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. It’s like watching the tachometer edge into the red zone and knowing that you need to take your foot off the gas before you hear that clunk up front.

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. Two weeks ago came the news that atmospheric carbon dioxide had jumped 2.4 parts per million last year -- two decades ago, it was going up barely half that fast.

And suddenly the news arrives that the amount of methane, another potent greenhouse gas accumulating in the atmosphere, has unexpectedly begun to soar as well. It appears that we’ve managed to warm the far north enough to start melting huge patches of permafrost, and massive quantities of methane trapped beneath it have begun to bubble forth.

And don’t forget: China is building more power plants; India is pioneering the $2,500 car; and Americans are buying TVs the size of windshields, which suck juice ever faster.

Here’s the thing. Hansen didn’t just say that if we didn’t act, there was trouble coming. He didn’t just say that if we didn’t yet know what was best for us, we’d certainly be better off below 350 ppm of carbon dioxide in the atmosphere.

His phrase was: “if we wish to preserve a planet similar to that on which civilization developed.” A planet with billions of people living near those oh-so-floodable coastlines. A planet with ever-more vulnerable forests. (A beetle, encouraged by warmer temperatures, has already managed to kill 10 times more trees than in any previous infestation across the northern reaches of Canada this year. This means far more carbon heading for the atmosphere and apparently dooms Canada’s efforts to comply with the Kyoto protocol, which was already in doubt because of its decision to start producing oil for the U.S. from Alberta’s tar sands.)

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.”

In the next two or three years, the nations of the world are supposed to be negotiating a successor treaty to the Kyoto accord (which, for the record, has never been approved by the United States -- the only industrial nation that has failed to do so). When December 2009 rolls around, heads of state are supposed to converge on Copenhagen to sign a treaty -- a treaty that would go into effect at the last plausible moment to heed the most basic and crucial of limits on atmospheric CO2.

If we did everything right, Hansen says, we could see carbon emissions start to fall fairly rapidly and the oceans begin to pull some of that CO2 out of the atmosphere. Before the century was out, we might even be on track back to 350. We might stop just short of some of those tipping points, like the Road Runner screeching to a halt at the very edge of the cliff.

More likely, though, we’re the coyote -- because “doing everything right” means that political systems around the world would have to take enormous and painful steps right away. It means no more new coal-fired power plants anywhere, and plans to quickly close the ones already in operation. (Coal-fired power plants operating the way they’re supposed to are, in global warming terms, as dangerous as nuclear plants melting down.) It means making car factories turn out efficient hybrids next year, just the way U.S. automakers made them turn out tanks in six months at the start of World War II. It means making trains an absolute priority and planes a taboo.

It means making every decision wisely because we have so little time and so little money, at least relative to the task at hand. And hardest of all, it means the rich countries of the world sharing resources and technology freely with the poorest ones so that they can develop dignified lives without burning their cheap coal.

It’s possible. The United States launched a Marshall Plan once, and could do it again, this time in relation to carbon. But at a time when the president has, once more, urged drilling in the Arctic National Wildlife Refuge, it seems unlikely. At a time when the alluring phrase “gas tax holiday” -- which would actually encourage more driving and more energy consumption -- has danced into our vocabulary, it’s hard to see. And if it’s hard to imagine sacrifice here, imagine China, where people produce a quarter as much carbon apiece as Americans do.

Still, as long as it’s not impossible, we’ve got a duty to try to push those post-Kyoto negotiations in the direction of reality. In fact, it’s about the most obvious duty humans have ever faced.

After all, those talks are our last chance; you just can’t do this one lightbulb at a time.

We do have one thing going for us -- the Web -- which at least allows you to imagine something like a grass-roots global effort. If the Internet was built for anything, it was built for sharing this number, for making people understand that “350" stands for a kind of safety, a kind of possibility, a kind of future.

Hansen’s words were well-chosen: “a planet similar to that on which civilization developed.” People will doubtless survive on a non-350 planet, but those who do will be so preoccupied, coping with the endless unintended consequences of an overheated planet, that civilization may not.

Civilization is what grows up in the margins of leisure and security provided by a workable relationship with the natural world. That margin won’t exist, at least not for long, as long as we remain on the wrong side of 350. That’s the limit we face.

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 ( www.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.