Researchers have long thought that pollution high in the atmosphere may be putting the brakes on global warming by reflecting sunlight back into space. But new measurements presented this week at the American Geophysical Union meeting in Baltimore are casting doubt on that assumption.
Most climate models predict that increases in sunlight-trapping greenhouse gases should have caused temperatures on Earth to have risen more sharply than they have so far. Scientists have speculated that one reason the predicted warming hasn't occurred is that a haze of sulfates--a major byproduct of burning fossil fuels--in the upper atmosphere is providing a partial sunscreen, reflecting more light than it absorbs. But there have been few data on the actual content of these so-called aerosols to back up that speculation.
To help improve the models, atmospheric scientist Peter Hobbs of the University of Washington, Seattle, and his colleagues studied the aerosol off the Atlantic Coast created by the Washington, D.C. metropolitan area. They used specially equipped airplanes to measure the chemical content and the reflective properties of the particles in the aerosol. To their surprise, they found more carbon particles than sulfate particles. Carbon absorbs more light than it reflects, and therefore would be expected to warm the atmosphere rather than cool it. The carbon seems to come from both humanmade and natural sources, Hobbs says, including burning forests and fossil fuels, industrial emissions, ocean gasses, and forest-produced chemicals called turpines.
The result "clouds the picture" for climate scientists, says atmospheric physicist Andrew Lacis of NASA's Goddard Institute for Space Studies in New York City. "Maybe aerosols can't account for the difference" between expected and observed temperature changes, he says, "which leads to the question of what else is going on."
What may be going on, says Hobbs, is a buildup of clouds. While carbon particles don't reflect as much sunlight as sulfate particles, they can serve as seeds for water droplets and cause an increase in cloud formation, which would--once again--reflect sunlight and cool the atmosphere. But climate modelers know "absolutely nothing" about how particles affect cloud formation or how those clouds affect global warming, says Hobbs. "That's what we're going to be concentrating on for the next few years."