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The science of climate change

Heating up

Feb 2nd 2007
From Economist.com

A UN-backed report on the science of climate change concludes that global warming is increasing and the sea level is up

THE fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC), published in Paris on Friday February 2nd, is important, unsurprising and will probably be uncontroversial. It is important because the IPCC is the body set up under the auspices of the United Nations so that governments should have an agreed view of the science on which to base policy. It is unsurprising because, while some of the figures differ from those in the third assessment report published in 2001, the changes are minimal and its broad conclusion, that something serious is happening and man is in part responsible, remains the same (though the authors now say that man is “very likely” responsible, rather than just “likely”). It will probably be uncontroversial because the few remaining climate-change sceptics prepared to speak out against the consensus argue not so much about the climate science as about its consequences. Those arguments will take place mostly around the IPCC’s two follow-up reports, to be published later this year, on the impact of climate change and on what to do about it.

Part of the report’s job is to consider studies of the speed of change so far. Warming seems to be accelerating somewhat. Eleven out of the dozen years from 1995-2006 were among the 12 hottest years since 1850, when temperatures were first widely recorded. So the estimate for the average increase in global temperature for the past century, which the third assessment report put at 0.6C, has now risen to 0.74C.

The sea level, which rose on average by 1.8mm a year in 1961-2003, went up by an average of 3.1mm a year between 1993-2003. The numbers are still small, but the shape of the curve is worrying. And because the deadline for scientific papers to be included in the IPCC’s report was some time ago, its deliberations have excluded some alarming recent studies on the acceleration of glacier melt in Greenland.

Some trends now seem clear. North and South America and northern Europe are getting wetter; the Mediterranean and southern Africa drier. Westerly winds have strengthened since the 1960s. Droughts have got more intense and longer since the 1970s. Heavy rainfall, and thus flooding, has increased. Arctic summertime sea ice is decreasing by just over 7% a decade.

In some areas where change might be expected, however, nothing much seems to be happening. Antarctic sea ice, for instance, does not seem to be shrinking, probably because increased melting is balanced by more snow.

The other part of the report’s job is to make predictions about what will happen to the climate. In this, it illustrates a curious aspect of the science of climate change. Studying the climate reveals new, little-understood, mechanisms: as temperatures warm, they set off feedback effects that may increase, or decrease, warming. So predictions may become less, rather than more, certain. Thus the IPCC’s range of predictions of the rise in the temperature by 2100 has increased from 1.4-5.8C in the 2001 report to 1.1-6.4C in this report.

That the IPCC should end up with a range that vast is not surprising given the climate’s complexity. But it leaves plenty of scope for argument about whether it’s worth trying to do anything about climate change.