Aerosols, which can be projected into the atmosphere from both natural sources like volcanoes and human sources like coal-fired power plants, shield the Earth from incoming solar radiation. They wash out of the atmosphere quickly, however, compared to most greenhouse gases and so their effect is relatively short-lived.
An earlier paper by another Solomon group found that lower concentrations of stratospheric water vapor have been another factor in slowing the rate of global warming. That paper, which appeared early last year, found that concentrations probably increased between 1980 and 2000, on the other hand, adding to the warming trend.
The Solomon paper, published in this week's Science magazine online, estimates the radiative impact of higher aerosol levels in the last decade at minus 0.1 Watt per square meter. That is enough to reduce the amount of expected warming by 25 percent. The findings are based on direct atmospheric measurements, and the paper makes no attempt to determine the natural versus human sources of the recent aerosol buildup. Its main message indeed is that aerosol levels are "persistently variable." Read the full report here.
Source: by Bill Sweet, Spectrum.Ieee.com.
