Global warming's sooty smokescreen revealed
Fred Pearce, New Scientist Print Edition, 04 June 03

BERLIN - Smoke is clouding our view of global warming, protecting the planet from perhaps three-quarters of the greenhouse effect. That might sound like good news, but experts say that as the cover diminishes in coming decades, we are in for a dramatic escalation of warming that could be two or even three times as great as official best guesses.

This was the dramatic conclusion reached last week at a workshop in Dahlem, Berlin, where top atmospheric scientists got together, including Nobel laureate Paul Crutzen and Swedish meteorologist Bert Bolin, former chairman of the UN's Intergovernmental Panel on Climate Change (IPCC).

IPCC scientists have suspected for a decade that aerosols of smoke and other particles from burning rainforest, crop waste and fossil fuels are blocking sunlight and counteracting the warming effect of carbon dioxide emissions. Until now, they reckoned that aerosols reduced greenhouse warming by perhaps a quarter, cutting increases by 0.2 °C. So the 0.6 °C of warming over the past century would have been 0.8 °C without aerosols.\

Two views of future warming
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But the Berlin workshop concluded that the real figure is even higher - aerosols may have reduced global warming by as much as three-quarters, cutting increases by 1.8 °C. If so, the good news is that aerosols have prevented the world getting almost two degrees warmer than it is now. But the bad news is that the climate system is much more sensitive to greenhouse gases than previously guessed.

As those gases are expected to continue accumulating in the atmosphere while aerosols stabilise or fall, that means "dramatic consequences for estimates of future climate change", the scientists agreed in a draft report from the workshop.

Parasol effect
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Past calculations of the cooling effect of aerosols have been inferred from "missing" global warming predicted by climate models. But direct measurements reported in Science (vol 300, p 1103) in May by Theodore Anderson of the University of Washington in Seattle show a much greater parasol effect. Anderson says climate sensitivity could be larger than climate models suggest.

The Berlin meeting also heard evidence that past warm eras had higher temperatures than they ought to, if estimates of the atmospheric composition at the time and greenhouse models are correct. Again this suggests greater sensitivity.

"It looks like the warming today may be only about a quarter of what we would have got without aerosols," Crutzen told New Scientist. "You could say the cooling has done us a big favour. But the health effects of many aerosols in smog are so great that even in the poor world, they are already cutting emissions." For good reasons, aerosol levels look set to fall.

Moreover, most aerosol emissions only stay in the atmosphere for a few days. Most greenhouses gases remain for a century or longer. So as time goes on, aerosols will protect us less and less from global warming. "They are giving us a false sense of security right now," said Crutzen.

'Sooner, not later'
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One tentative estimate put warming two or even three times higher than current middle-range forecasts of 3 to 4 °C based on a doubling of greenhouse gases in the atmosphere, which is likely by late this century. That suggests global warming well above the IPCC maximum forecast of 5.8 °C. Back-of-the-envelope calculations now suggest a "worst case" warming of 7 to 10 °C.

Will Steffen of the Swedish Academy of Sciences says the message for policy makers is clear: "We need to get on top of the greenhouse gas emissions problem sooner rather than later."