Global Warming's Tipping Point

By McKibben, Bill | Tribune-Review/Pittsburgh Tribune-Review, January 6, 2008 | Go to article overview

Global Warming's Tipping Point


McKibben, Bill, Tribune-Review/Pittsburgh Tribune-Review


December may have been the most important month yet in the two- decade history of the fight against global warming. Al Gore got his Nobel in Oslo, Norway; 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.

NASA scientist 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, 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. The number 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.

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