U.S. Water News Online
WASHINGTON -- Scientists call it "global dimming," a
little-known trend that may be making the world darker than it used
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
"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% to 20% 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
"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
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% per decade, according
to Gerald Stanhill, a biologist at Israel's Agricultural Research
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
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.
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