In Search of the Climate's Tipping Point

When asked to quantify the impact of climate change, scientists come up with a lot of interesting answers, no two of them quite the same. For the lay person, then, perhaps the simplest way to understand it is to imagine a distant asteroid, somewhere out in space, on a collision course with Earth. It's not clear when or where the asteroid will hit, or exactly how severe the consequences will be. But it is clear that when it happens, the consequences will be far worse — and last far longer — than any natural disaster humanity has ever known.

That is the threat to the planet that many scientists can agree is posed by climate change. Yet the global response to global warming — one of fits and starts, with more hot air than real focus — doesn't exactly resemble the mobilizing opening scenes of disaster flicks like Armageddon or Deep Impact. Quite the opposite, as fears over the recession grow, climate change may be receding from the public consciousness. A Gallup poll released last week found that a record-high 41% of Americans believe that the threat of global warming is exaggerated in the news media, up from 30% in 2006. Though a majority of Americans are still a "fair amount" or a "great deal" concerned about climate change, that proportion has hardly changed in recent years, even as the preponderance of scientific evidence has increasingly supported the danger of global warming and the speed with which it is occurring. The asteroid is out there, and yet we remain reluctant to heed the warnings. Why?

For one thing, most people imagine global warming to be a gradual process, like water slowly coming to boil — so, it follows that averting the worst effects of climate change should be as straightforward as turning down the heat. But the climate, immensely complex as it is, doesn't work that way. The real danger could come from "tipping points" — sharp, sudden changes that could result in the complete collapse of the ice sheets on Greenland or West Antarctica, or the desiccation of the Amazon rainforest. If we cross those thresholds, the effects could be too swift and terrible for us to cope — the ice on Greenland alone contains enough water to raise global sea levels by more than 20 feet, which would swallow the coasts. Passing a tipping point would be irreversible and that is why the possibility keeps climatologists up at night.

The key is to figure out where that tipping point lies: how much carbon do we have to pump into the atmosphere and how much warmer does the climate need to be before get there? A team of researchers led by Elmar Kriegler of the Potsdam Institute for Climate Impact Research in Germany posed those questions to 43 of world's top climatologists. Their answers, reported in an intriguing study published in the Mar. 16 issue of the Proceedings of the National Academy of Sciences (PNAS), basically amounted to, We don't really know. The scientists agreed that there was a significant chance that tipping points would be crossed this century — or the next — especially if we fail to reduce carbon emissions and if global temperatures rise considerably over the coming decades. (Which, as we know, is likely to happen without a concerted global effort to cut CO2.) But the reality is that the climate system remains too complex, the future to difficult to predict, to peg exactly when we've passed the point of no return. We may not know the asteroid is about to hit us until it's over our heads.

That uncertainty has an obvious impact on the world's response to global warming. Scientists can warn us repeatedly that adding billions of tons of CO2 to the atmosphere will eventually cause severe, even irreparable damage to the planet and to us — but they can't tell us exactly when the catastrophe will occur. (Small pieces of it already have — the Arctic ice cap has already shrunk considerably, and dozens of frog species alone have gone extinct thanks to warming.) So climate change inevitably fades from the news and the public agenda, replaced by more urgent concerns: the recession, wars, terrorism. It's perfectly understandable that many of us would take refuge in the uncertainty, and hope that we'll somehow have the climate problem solved before we cross the tipping point. That would be a mistake. It may come later — or it may come sooner. We'll never get an exact date. But the asteroid is on its way.

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