WASHINGTON - Scientists call it "global dimming," a little-known trend that may be making the world darker than it used to be.
Because of thicker clouds and growing air pollution, much of the Earth's surface gets about 15 percent less sunlight than it did 50 years ago, according to Michael Roderick, a climate researcher at Australian National University in Canberra.
"Global dimming means that the transmission of sunlight through the atmosphere is decreasing," Roderick said.
"Just look out the window when you fly into New York or to California—it's dimmer," said Beate Liepert, a climatologist at the Lamont-Doherty Earth Observatory of Columbia University in New York.
Researchers say that global dimming, also known as solar dimming, partially offsets the global warming that most scientists agree is produced by greenhouse gases such as auto exhaust and emissions from coal-burning power plants.
The solar dimming effect is "about half as large as the greenhouse gas warming," said James Hansen, director of the National Aeronautics and Space Administration's Goddard Institute for Space Studies in New York.
In global warming, gases in the atmosphere, such as carbon dioxide, trap some of the sun's heat and keep it from radiating back out to space, thereby raising the Earth's temperature. Clouds and air pollution, on the other hand, block a portion of the heat energy that's coming from the sun, just as it's cooler to sit under a beach umbrella than under a bright sky.
Although global warming has been widely accepted, global dimming remains controversial. The theory has been advanced in recent years by a handful of researchers who measure the decline of solar radiation at hundreds of sites around the globe.
Liepert, Roderick and several other scientists will discuss their findings at an international geophysical conference in Montreal this month.
"We still face a lot of controversy, but it's [solar dimming] getting accepted," Liepert said in a telephone interview. "We've found it in the United States, Europe, Israel and Asia. Already, major research institutions are changing their point of view."
Support for the theory comes from two types of data collected in recent decades:
source: http://www.dfw.com/mld/dfw/news/nation/8620703.htm 8may04
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