A group led by top atmospheric geochemist Susan Solomon has found that higher aerosol levels during the first decade of this century may account at least in part for why global warming has been slower than climate models predicted on the basis of greenhouse gas buildup. 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 with 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, indicated 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."
To put the findings in a very long perspective, during the Cretaceous period—120 to 90 million years ago, when Earth turned into a "hothouse," as the current issue of Scientific American puts it—warming took place at an average rate of 0.00025 degrees C per 100 years. During the Paleocene-Eocene Thermal Maximum, about 56 million years ago, the warming rate was 1,000 times as abrupt, at 0.025 per 100 years. The current rate is 1–4 degrees per 100 years, at least 400 times as high as in PETM.