Washington Post (pg. A21)  [Printer-friendly version]
December 28, 2007


By Bill McKibben

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

Copyright 2007 The Washington Post Company