IT MAY come as a surprise to a walrus wondering where all the Arctic’s summer sea ice has gone. It could be news to a Staten Islander still coming to terms with what he lost to Hurricane Sandy. But some scientists are arguing that man-made climate change is not quite so bad a threat as it appeared to be a few years ago. They point to various reasons for thinking that the planet’s “climate sensitivity”—the amount of warming that can be expected for a doubling in the carbon-dioxide level—may not be as high as was previously thought. The most obvious reason is that, despite a marked warming over the course of the 20th century, temperatures have not really risen over the past ten years.
It is not clear why climate change has “plateaued” (see article). It could be because of greater natural variability in the climate, because clouds dampen warming or because of some other little-understood mechanism in the almost infinitely complex climate system. But whatever the reason, some of the really ghastly scenarios—where the planet heated up by 4°C or more this century—are coming to look mercifully unlikely. Does that mean the world no longer has to worry?
No, for two reasons. The first is uncertainty. The science that points towards a sensitivity lower than models have previously predicted is still tentative. The error bars are still there. The risk of severe warming—an increase of 3°C, say—though diminished, remains real. There is also uncertainty over what that warming will actually do to the planet. The sharp reduction in Arctic ice is not something scientists expected would happen at today’s temperatures. What other effects of even modest temperature rise remain unknown?
The second reason is more practical. If the world had based its climate policies on previous predictions of a high sensitivity, then there would be a case for relaxing those policies, now that the most hell-on-Earth-ish changes look less likely. But although climate rhetoric has been based on fears of high sensitivity, climate policy has not been. On carbon emissions and on adaptation to protect the vulnerable it has fallen far short of what would be needed even in a low-sensitivity world. Industrial carbon-dioxide emissions have risen by 50% since 1997.No, for two reasons. The first is uncertainty. The science that points towards a sensitivity lower than models have previously predicted is still tentative. The error bars are still there. The risk of severe warming—an increase of 3°C, say—though diminished, remains real. There is also uncertainty over what that warming will actually do to the planet. The sharp reduction in Arctic ice is not something scientists expected would happen at today’s temperatures. What other effects of even modest temperature rise remain unknown?
Any emissions reductions have tended to come from things beyond climate policy—such as the economic downturn following the global financial crisis, or the cheap shale gas which has displaced American coal. If climate policy continues to be this impotent, then carbon-dioxide levels could easily rise so far that even a low-sensitivity planet will risk seeing changes that people would sorely regret. There is no plausible scenario in which carbon emissions continue unchecked and the climate does not warm above today’s temperatures.
Good news we must use
Bad climate policies, such as backing renewable energy with no thought for the cost, or insisting on biofuels despite the damage they do, are bad whatever the climate’s sensitivity to greenhouse gases. Good policies—strategies for adapting to higher sea levels and changing weather patterns, investment in agricultural resilience, research into fossil-fuel-free ways of generating and storing energy—are wise precautions even in a world where sensitivity is low. So is putting a price on carbon and ensuring that, slowly but surely, it gets ratcheted up for decades to come.
If the world has a bit more breathing space to deal with global warming, that will be good. But breathing space helps only if you actually do something with it.