NewScientist, July 9, 2008

GOODBYE air pollution and smoky chimneys, hello brighter days. That's been the trend in Europe for the past three decades -- but unfortunately cleaning up the skies has allowed more of the sun's rays to pierce the atmosphere, contributing to at least half the warming that has occurred.

Since 1980, average air temperatures in Europe have risen 1 °C: much more than expected from greenhouse-gas warming alone. Christian Ruckstuhl of the Institute for Atmospheric and Climate Science in Switzerland and colleagues took aerosol concentrations from six locations in northern Europe, measured between 1986 and 2005, and compared them with solar-radiation measurements over the same period. Aerosol concentrations dropped by up to 60 per cent over the 29-year period, while solar radiation rose by around 1 watt per square metre (Geophysical Research Letters, DOI: 10.1029/2008GL034228). "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.