By Bill McKibben
Friday, December 28, 2007; A21

This month may have been the most important yet in the two-decade history of the fight against global warming. Al Gore got his Nobel in Stockholm; international negotiators made real progress on a treaty in Bali; and in Washington, Congress actually worked up the nerve to raise gas mileage standards for cars.

But what may turn out to be the most crucial development went largely unnoticed. It happened at an academic conclave in San Francisco. A NASA scientist named James Hansen offered a simple, straightforward and mind-blowing bottom line for the planet: 350, as in parts per million carbon dioxide in the atmosphere. It's a number that may make what happened in Washington and Bali seem quaint and nearly irrelevant. It's the number that may define our future.

To understand what it means, you need a little background.

Twenty years ago, Hansen kicked off this issue by testifying before Congress that the planet was warming and that people were the cause. At the time, we could only guess how much warming it would take to put us in real danger. Since the pre-Industrial Revolution concentration of carbon in the atmosphere was roughly 275 parts per million, scientists and policymakers focused on what would happen if that number doubled -- 550 was a crude and mythical red line, but politicians and economists set about trying to see if we could stop short of that point. The answer was: not easily, but it could be done.

In the past five years, though, scientists began to worry that the planet was reacting more quickly than they had expected to the relatively small temperature increases we've already seen. The rapid melt of most glacial systems, for instance, convinced many that 450 parts per million was a more prudent target. That's what the European Union and many of the big environmental groups have been proposing in recent years, and the economic modeling makes clear that achieving it is still possible, though the chances diminish with every new coal-fired power plant.

But the data just keep getting worse. The news this fall that Arctic sea ice was melting at an off-the-charts pace and data from Greenland suggesting that its giant ice sheet was starting to slide into the ocean make even 450 look too high. Consider: We're already at 383 parts per million, and it's knocking the planet off kilter in substantial ways. So, what does that mean?

It means, Hansen says, that we've gone too far. "The evidence indicates we've aimed too high -- that the safe upper limit for atmospheric CO2is no more than 350 ppm," he said after his presentation. Hansen has reams of paleo-climatic data to support his statements (as do other scientists who presented papers at the American Geophysical Union conference in San Francisco this month). The last time the Earth warmed two or three degrees Celsius -- which is what 450 parts per million implies -- sea levels rose by tens of meters, something that would shake the foundations of the human enterprise should it happen again.

And we're already past 350. Does that mean we're doomed? Not quite. Not any more than your doctor telling you that your cholesterol is way too high means the game is over. Much like the way your body will thin its blood if you give up cheese fries, so the Earth naturally gets rid of some of its CO2each year. We just need to stop putting more in and, over time, the number will fall, perhaps fast enough to avert the worst damage.

That "just," of course, hides the biggest political and economic task we've ever faced: weaning ourselves from coal, gas and oil. The difference between 550 and 350 is that the weaning has to happen now, and everywhere. No more passing the buck. The gentle measures bandied about at Bali, themselves way too much for the Bush administration, don't come close. Hansen called for an immediate ban on new coal-fired power plants that don't capture carbon, the phaseout of old coal-fired generators, and a tax on carbon high enough to make sure that we leave tar sands and oil shale in the ground. To use the medical analogy, we're not talking statins to drop your cholesterol; we're talking huge changes in every aspect of your daily life.

Maybe too huge. The problems of global equity alone may be too much -- the Chinese aren't going to stop burning coal unless we give them some other way to pull people out of poverty. And we simply may have waited too long.

But at least we're homing in on the right number. Three hundred and fifty is the number every person needs to know.

Bill McKibben is a scholar in residence in environmental studies at Middlebury College and the author of the forthcoming "Bill McKibben Reader."