Cleaner skies leave global warming forecasts uncertain

Will reduced air pollution hasten climate change?

Munich

Sun block: ‘global dimming’ caused by air pollution may have been masking the greenhouse effect. Credit: D. MILLER/DMI

Finally, some good news about the state of our planet. Studies of the amount of sunlight making it through the atmosphere suggest that our air is getting cleaner, thanks to reduced industrial emissions and the use of particulate filters.

But there's a nasty sting in the tail. Scientists are concerned that aerosols and dust in the air may have been shielding us from the worst of global warming. They don't know how extra solar radiation will affect future temperatures.

A downward trend in the amount of sunlight reaching the planet's surface, known as ‘global dimming’, has been noticed since measurements began in the late 1950s, but consensus that it was a global phenomenon was reached only last year (see Nature doi:10.1038/news040517-7; 2004 ). Many scientists have been reluctant to discuss the effect, fearing it would be used as an excuse to ignore the consequences of global warming.

They don't need to worry about that any more. Two studies, reported in Science, conclude that since 1990 the dimming has been replaced by brightening (M. Wild et al. Science 308, 847–850; 2005 and R. T. Pinker, B. Zhang and E. G. Dutton Science 308, 850–854; 2005). It has taken years to collect enough data for a statistically significant analysis, says Martin Wild, an atmospheric scientist at the Swiss Federal Institute of Technology in Zurich.

Wild and an international team of scientists analysed data from hundreds of ground stations around the world. They found that the amount of radiation reaching Earth's surface fell by 4–6% between 1960 and 1990, but that the trend has since reversed nearly everywhere — although the total amount of radiation has not yet reached 1960 levels.

The result is backed up by a second study, led by Rachel Pinker from the University of Maryland, College Park, which infers a similar, albeit smaller, trend from satellite data.

“The good news is that the atmosphere has become cleaner and more transparent,” says Andreas Macke, a meteorologist at the Leibniz Institute of Marine Sciences in Kiel, Germany. The collapse of communist economies in the late 1980s and the subsequent decrease in industrial pollutants released in the area was probably a major factor.

Wild and his team did detect continued dimming in some highly polluted areas, such as India, where vast clouds of smog from burning fossil fuels and wildfires darken the sky for long periods each year. But there was a brightening trend in China, despite the country's booming, fossil-fuel-intensive industry. “I am surprised,” says Wild, adding that he can only speculate that the use of clean-air technologies in China may be more widespread and efficient than previously thought.

The question now is how the trend towards cleaner air will affect global temperatures. “It is clear that the greenhouse effect has been partly masked in the past by air pollution,” says Macke.

Wild is investigating just how much was masked. He has yet to publish his results but he estimates that, until 1990, air pollution protected us from at least 50% of the warming that would have otherwise occurred.

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Schiermeier, Q. Cleaner skies leave global warming forecasts uncertain. Nature 435, 135 (2005). https://doi.org/10.1038/435135a

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