EARTHSHINE, the reflection of the earth on the unlit part of the moon, is one way to determine how much sunlight makes it to the surface of the earth. Brighter earthshine would suggest increasing cloudiness, which reflects sunlight away. Image: FRED ESPENAK Science Photo Library
Much to their surprise, scientists have found that less sunlight has been reaching the earth's surface in recent decades. The sun isn't going dark; rather clouds, air pollution and aerosols are getting in the way. Researchers are learning that the phenomenon can interact with global warming in ways that had not been appreciated.
"This is something that people haven't been aware of," says Shabtai Cohen of the Institute of Soil, Water and Environmental Sciences in Bet Dagan, Israel. "And it's taken a long time to gain supporters in the scientific world." Cohen's colleague Gerald Stanhill first published his solar dimming results 15 years ago.
Estimates of the effect vary, but overall ¿the magnitude has surprised all of us," comments climatologist Veerabhadran Ramanathan of the University of California at San Diego. Stanhill and Cohen have pegged the solar reduction at 2.7 percent per decade over the period from 1958 to 1992. Put another way, the radiation reduction amounts to 0.5 watt per square meter per year, or about one third (in magnitude) of the warming that takes place because of carbon dioxide buildup in the atmosphere.
A separate analysis by climatologist Beate Liepert of Columbia University and her colleagues has found a 1.3 percent per decade decrease in solar radiation over the period from 1961 to 1990, with especially strong declines in North America. That's a total decline of up to 18 watts per square meter, out of the 200 watts per square meter or so that reaches the earth's surface.
Sometimes called global dimming, the reduction in solar radiation varies from region to region, and no measurements have yet been made over the world's oceans. It has also been deduced from evaporation rates around the world-the amount of water that evaporates from specially calibrated pans has been dropping for at least five decades in the Northern Hemisphere. At the May American Geophysical Union meeting in Montreal, Michael Roderick and Graham Farquhar of the Australian National University presented results that extend the finding across the Southern Hemisphere as well.
A key culprit appears to be aerosols-micron-size particles (or smaller) consisting of sulfates, black and organic carbon, dust, and even sea salt. Aerosols have already been implicated in cooling tendencies, such as the slight decrease in global temperatures seen from about 1945 to 1975. Besides keeping temperatures from rising even higher than they already have, the aerosols complicate the modeling of global warming. The particulates act as the nuclei points for cloud condensation. They can lead to more cloudiness--a phenomenon called the indirect aerosol effect--which reflects sunlight away.
Solar dimming has consequences for the hydrological cycle as well. By the conventional wisdom, higher global temperatures mean that more water evaporates from the seas and falls as rain on land. But on a planet dimmed by aerosols and clouds, water vapor and rain stay in the atmosphere about half a day longer than they would in a nonaerosol world, according to Liepert's simulations. ¿All this debate on global warming is always discussed in terms of temperature," Liepert remarks. ¿I think we really have to discuss it more in terms of energy balance and water balance."
Cohen notes that the dimming effect could have consequences on farming-as a rule of thumb, agricultural productivity of light-loving plants such as peppers and tomatoes declines by 1 percent for each 1 percent decline in sunlight. Some plants, though, do better in more limited, diffuse light.
For now, scientists continue to gather data on solar dimming and puzzle through the climatological consequences. ¿It's going to be extremely difficult," says Ramanathan, noting the vagaries of readings. ¿We don't know the quality of the measurements."
David Appell is based in Newmarket, N.H.