Cleaner skies equals rise in temperatures.
Since 1980, average air temperatures in Europe have risen 1 °C: much more than expected from greenhouse-gas warming alone.
Why?
"The decrease in aerosols probably accounts for at least half of the warming over Europe in the last 30 years," says Rolf Philipona, a co-author of the study at MeteoSwiss, Switzerland's national weather service.
The latest climate models are built on the assumption that aerosols have their biggest influence by seeding natural clouds, which reflect sunlight. However, the team found that radiation dropped only slightly on cloudy days, suggesting that the main impact of aerosols is to block sunlight directly.
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