Is 'global dimming' under way?

By Robert S. Boyd
Knight Ridder Newspapers

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Thanks to thicker clouds and growing air pollution, much of Earth's surface is receiving 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 global dimming, also known as solar dimming, partly 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 NASA'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 Earth's temperature. Clouds and air pollution, on the other hand, block a portion of the heat energy coming from the sun, just as it's cooler sitting 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 few researchers who measure the decline of solar radiation at hundreds of sites globally.

Support for the theory comes from two types of data:

• Radiation meters — black metal plates that absorb the sun's rays — aren't heating up as rapidly as they previously did.

• The rate at which water evaporates from special measuring pans placed in the sunlight has slowed over the years.
 
Roderick, for example, measures the height of the water in his pans at 9 a.m. each day, subtracts rain that may have fallen and calculates how much has evaporated from the previous day.

"There's less evaporation out of pans of water all around the world, and that's consistent with global dimming," he said

The measurements indicate that the amount of energy from the sun — solar radiation — is shrinking by about 3 percent per decade, according to Gerald Stanhill, a biologist at Israel's Agricultural Research Organization.

Liepert said she expects to see the dimming trend continue in places such as China and the western United States, where population and industry are increasing.