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Clear skies raise global-warming estimates

Cleaner air could remove a vital brake on climate change.

Munich

For more than a century, dust and aerosols in the atmosphere have been blocking some of the Sun's radiation, shielding us from the worst effects of global warming. The question has always been: how much? Now, as cuts in pollution allow the skies to clear, an attempt to quantify the effect on future temperatures has produced an alarming conclusion.

Even under relatively cautious assumptions about past and present aerosol cooling, the study suggests that global warming could easily exceed the upper extreme predicted by the Intergovernmental Panel on Climate Change (IPCC), as clean-air measures take effect.

How much has pollution shielded us from the Sun?

“Things could get really uncomfortable,” says lead author Meinrat Andreae, an atmospheric researcher at the Max Planck Institute of Chemistry in Mainz, Germany. “The climate system is much more sensitive to human perturbations than has been thought. If our model is right, things could become totally uncontrollable in the second half of the century.”

That is quite a big ‘if’, however. As cars, industries and power plants worldwide become cleaner, atmospheric concentrations of emitted aerosols are expected to drop substantially. But how global temperatures will respond depends on how big the masking effect was in the first place — and that is the wild card in the climate game.

The problem is that different methods of estimating the cooling effect arrive at vastly different values. Trying to work it out from our understanding of how aerosol particles behave in the atmosphere suggests that the amount of solar energy reaching the ground will be reduced by anything from 0 to 4.5 watts per square metre. Working it out from a best guess of how sensitive the atmosphere is to greenhouse gases and how much warming we have seen so far gives 2 watts per square metre.

Such uncertainty has deterred researchers from estimating the effect of losing our aerosol shield. But as the skies are already starting to brighten (see Nature 435, 135; 200510.1038/435135a), the question has become critical.

Andreae, along with German and British colleagues, used a climate model specifically designed to simulate aerosol effects. After calibrating it against a series of more complex global models, they plugged in a range of values for aerosol cooling, and ran the model to simulate future temperatures as the skies clear (see Strong present-day aerosol cooling implies a hot future).

For a present-day cooling of 1.5 watts per square metre, which most climatologists agree is a relatively conservative value, the model implies that temperatures could rise between 6 and 10 °C by 2100. That is well in excess of the current IPCC predictions, which suggest a temperature rise of between 1.4 and 5.8 °C over the same time period.

Andreae acknowledges that there are many uncertainties about his study. But he points out that it is the best estimate we have so far. “This forces us to accept that pessimistic climate scenarios are much more plausible than had been thought,” he says.

Other experts are more cautious. “Climate modellers like playing around with values,” says Theodore Anderson, an atmospheric scientist and aerosol researcher at the University of Washington in Seattle. “It is legitimate to engage in speculative reasoning. But I object to conclusions based on the assumption that our knowledge is better than it actually is.”

Anderson points out that, for high values of aerosol cooling, Andreae's model breaks down, predicting unrealistically high or infinite temperature rises. He says this could mean that our understanding of what is driving the climate system is wrong. Or, he suggests, natural climate variability might be much larger than most scientists assume.

“The predictions may look more dramatic than what we actually expect,” agrees Olivier Boucher, head of climate chemistry and ecosystems at the Hadley Centre for Climate Prediction and Research in Exeter, UK. “But this is still an alarming hint at the upper bound of what can happen.”

All agree that precise observations about the vertical distribution of aerosols are required. Calypso, a satellite funded by the United States and France, will provide such data after its launch in August, although scientists warn that it could take 20 years to get a clear picture.

In the meantime, Andreae says he hopes his results will rouse political debate, especially as the G8 summit looms. “Mankind must fight CO2 emissions more aggressively,” he says.

The uncertainty surrounding the effects of global warming has been widely used to imply that things might not be as bad as projected, says Michael Grubb, an expert on policy responses to climate change at Imperial College, London. “This study is a timely reminder that uncertainty also means things could be a lot worse,” he points out. “Politically, this is a hugely important message.”

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