Watch out for 'global dimming' - World may be darkening as clouds, air pollution dim the sun's rays, says climate researcher


Knight Ridder Newspapers
Monterey Herald


WASHINGTON - Scientists call it "global dimming," a little-known trend that may be making the world darker than it used to be.

Thanks to thicker clouds and growing air pollution, much of the 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, 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, the 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 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 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 handful of researchers who measure the decline of solar radiation a hundreds of sites around the globe.

Liepert, Roderick and several other scientists will discuss their findings at an international geophysical conference in Montreal later this month.

"Initially, people were very skeptical, but now there's other pieces of evidence that all fit together," Roderick told a radio interviewer last December. Reductions in sunlight of 10 percent to 20 percent have been observed in many places over the past 50 years, he said.

"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."

NASA, the Scripps Research Institute in La Jolla, Calif., and the National Center for Atmospheric Research, a university-sponsored organization in Boulder, Colo., among others, are showing interest in global dimming.

"The conclusion that, on average, there has been a reduction in surface solar irradiance over the past half-century is pretty clear," NASA's Hansen said in an e-mail.

Support for the theory comes from two types of data collected in recent decades:

_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 any rain that may have fallen and calculates how much has evaporated from the day before.

"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.

According to Liepert, about two-thirds of the dimming is caused by more water vapor in the clouds, a byproduct of global warming.

Less sunlight reaches the ground, she said, because "the clouds are optically thicker. As global warming increases, clouds can hold more water. There's not more rain; it just stays up there."

The rest of the dimming is due to increasing air pollution - minute particles in the atmosphere known as aerosols. This problem affects the world, not just smoggy cities such as Houston and Los Angeles.

For example, NASA scientists reported in early May that air pollution can travel on high-speed winds from the Indian Ocean clear across the Pacific and into the southern Atlantic.

"When I fly from New York to California, I see very high brownish layers. That's old aerosol layers hanging on," Liepert said. "As we get more aerosols and more warming, we get more dimming."

She 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. In contrast, economic decline in the former Soviet Union has begun to clear the air somewhat in Eastern Europe.


For more information, see the National Oceanic and Atmospheric Administration's surface radiation Web site at



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