Even for Americans, constitutionally convinced thatthere 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 swoonsbefore. It's that gas at $4 a gallon means we're running out, at least of the cheapstuff that built our sprawling society. It's that when we try toturn corn into gas, it sends the price of a loaf of bread shooting upwards andstarts food riots on three continents. It's that everything is so inextricablytied together. It's that, all of a sudden, those grim Clubof 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 duskon planet Earth.

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

A few weeks ago, our foremost climatologist, NASA'sJim Hansen, submitted a paper to Science magazine with several co-authors. The abstractattached to it argued -- and I have never read stronger language in a scientific paper -- "if humanity wishesto preserve a planet similar to that on which civilization developed and towhich life on earth is adapted, paleoclimate evidence and ongoing climatechange suggest that CO2 will need to be reduced from its current 385 ppm to atmost 350 ppm." Hansen cites six irreversible tipping points -- massive sealevel rise and huge changes in rainfall patterns, among them -- that we'll passif we don't get back down to 350 soon; and the first of them, judging by lastsummer's insane melt of Arctic ice, may already be behind us.

So it's a tough diagnosis. It's like the doctortelling you that your cholesterol is way too high and, if you don't bring itdown right away, you're going to have a stroke. So you take the pill, you swearoff the cheese, and, if you're lucky, you get back into the safety zone beforethe coronary. It's like watching the tachometer edge into the red zone andknowing that you need to take your foot off the gas before you hear that clunkup front.

In this case, though, it's worse than that becausewe're not taking the pill and we are stomping on the gas -- hard. Instead ofslowing down, we're pouring on the coal, quite literally. Two weeks ago came thenews that atmospheric carbon dioxide had jumped 2.4 parts permillion last year -- two decades ago, it was going up barely half that fast.

And suddenly, the news arrives that the amount ofmethane, another potent greenhouse gas accumulating in the atmosphere, hasunexpectedly begun to soar as well. Apparently, we've managed to warm the farnorth enough to start melting huge patches of permafrost, and massivequantities 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 converting to TVsthe size of windshields that suck juice ever faster.

Here's the thing. Hansen didn't just say that, if wedidn't act, there was trouble coming; or, if we didn't yet know what was bestfor 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 onwhich civilization developed." A planet with billions of people livingnear those oh-so-floodable coastlines. A planet with ever more vulnerableforests. (A beetle, encouraged by warmer temperatures, has already managed to kill 10 times more trees than inany previous infestation across the northern reaches of Canada this year. Thismeans far more carbon heading for the atmosphere and apparently dooms Canada'sefforts to comply with the Kyoto Protocol, already in doubt because of itsdecision to start producing oil for the U.S. from Alberta's tar sands.)

We're the ones who kicked the warming off; now, theplanet is starting to take over the job. Melt all that Arctic ice, forinstance, and suddenly the nice white shield that reflected 80 percent ofincoming solar radiation back into space has turned to blue water that absorbs80 percent of the sun's heat. Such feedbacks are beyond history, though not inthe sense that Francis Fukuyama had in mind.

And we have, at best, a few years to short-circuitthem -- to reverse course. Here's the Indian scientist and economistRajendra Pachauri, who accepted the Nobel Prize on behalf of the IntergovernmentalPanel on Climate Change last year (and, by the way, got his job when the Bushadministration, 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 nexttwo to three years will determine our future. This is the definingmoment."

In the next two or three years, the nations of theworld are supposed to be negotiating a successor treaty to the KyotoAccord. When December 2009 rolls around, heads of state are supposedto converge on Copenhagen to sign a treaty -- a treaty that would go intoeffect at the last plausible moment to heed the most basic and crucial oflimits on atmospheric CO2.

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

More likely, though, we're the Coyote -- because"doing everything right" means that political systems around theworld would have to take enormous and painful steps right away. It means nomore new coal-fired power plants anywhere, and plans to quickly close the ones already inoperation. (Coal-fired power plants operating the way they're supposed to are,in global warming terms, as dangerous as nuclear plants melting down.) It meansmaking car factories turn out efficient hybrids next year, just the way we madethem turn out tanks in six months at the start of World War II. It means makingtrains an absolute priority and planes a taboo.

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

That's possible -- we launched a Marshall Plan once, and we could doit again, this time in relation to carbon. But in a month when the Presidenthas, once more, urged us to drill in the Arctic National Wildlife Refuge, thatseems unlikely. In a month when the alluring phrase "gas tax holiday"has danced into our vocabulary, it's hard to see (though it was encouraging tosee that Clinton's gambit didn't sway many voters). And if it's hard to imaginesacrifice here, imagine China, where people produce a quarter as much carbonapiece as we do.

Still, as long as it's not impossible, we've got aduty to try. In fact, it's about the most obvious duty humans have ever faced.

A few of us have just launched a new campaign, 350.org. Itsonly goal is to spread this number around the world in the next 18 months, viaart and music and ruckuses of all kinds, in the hope that it will push thosepost-Kyoto negotiations in the direction of reality.

After all, those talks are our last chance; you justcan't do this one lightbulb at a time. And if this 350.org campaign is a Hail Mary pass,well, sometimes those passes get caught.

We do have one thing going for us: This new tool, theweb, which at least allows you to imagine something like a grassroots globaleffort. If the internet was built for anything, it was built for sharing thisnumber, for making people understand that "350" stands for a kind ofsafety, a kind of possibility, a kind of future.

Hansen's words were well-chosen: "a planetsimilar to that on which civilization developed." People will doubtlesssurvive on a non-350 planet, but those who do will be so preoccupied, copingwith the endless unintended consequences of an overheated planet, thatcivilization may not.

Civilization is what grows up in the margins ofleisure and security provided by a workable relationship with the naturalworld. That margin won't exist, at least not for long, this side of 350. That'sthe limit we face.

Bill McKibben is a scholar-in-residence atMiddlebury College and co-founder of 350.org. His most recent book is The Bill McKibben Reader.

Copyright2008 Bill McKibben

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