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Earth's brightness is dimming
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Kenneth Chang NYT
Friday, May 28, 2004
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Tracking the brightness of the Earth by looking at its reflection on the Moon, scientists have concluded that sunshine on Earth brightened in the 1990s, then dimmed after 2000.
The findings, reported on Friday in the journal Science, add a new level of mystery to the recent debate about "global dimming" and its causes. Measurements by ground-based instruments have shown a decrease of up to 10 percent in sunlight between the late 1950s and early 1990s.
"This would say that it reversed through the '80s and '90s to a global brightening and now it's flattening," said Philip Goode, a professor of physics at the New Jersey Institute of Technology and one of the authors of the Science article. "And the suggestion is," Goode said, "that the trend is turning back to the other direction."
The output of the Sun varies only slightly, so scientists believe global dimming has probably resulted from air pollution. Some light bounces off soot particles in the air. The pollution also causes more water droplets to condense out of air, leading to thicker, darker clouds, which block light. For that reason, the dimming appears to be more pronounced on cloudy days than sunny ones. Some less polluted regions have experienced little or no dimming.
The notion remains disputed because it runs counter to expectations of global warming (less sunlight should mean lower temperatures), and some scientists wonder how widespread the dimming effects are.
Unlike the earlier data, the research did not look at sunshine on Earth. Instead, Goode and colleagues at Big Bear Solar Observatory in California and the California Institute of Technology used a principle described by Leonardo da Vinci. The bright side of the moon is lighted by sunshine. The dark side is not completely dark. Rather, as da Vinci deduced, it is dimly illuminated by light reflected off the Earth.
Using a small telescope at Big Bear, the astronomers have for the past five years measured the relative brightness of the two sides of the moon, which tells how much light is bouncing off Earth back into space, what the scientists call "earthshine." The reflectivity is largely a measure of clouds, which are much shinier than the ocean or ground. Thus, a brightening of earthshine means a dimming on the Earth's surface, because less light is reaching the ground. On average, Earth reflects about 30 percent of the incoming sunlight, but that varies day to day and hour to hour. When the Sun rises over a cloudy Asia, earthshine might brighten by 10 percent.
Over all, reflectivity increased - and sunshine dimmed at the surface - from 1999 to 2003, with an especially sharp upward jump in 2003. However, the reflectivity was lower than what the scientists measured during an earlier round of observations in 1994 and 1995. They then took data on cloud cover from 1985 to 2000 to calculate the reflectivity. The calculations indicate that the Earth reflected 32 percent of sunlight in 1985 and that the reflectivity declined 7 percent over the next 15 years, which would correspond to a brightening of sunshine on Earth.
"It gives at least a good argument that the clouds are not getting thicker globally," said Beate Liepert, a research scientist at the Lamont-Doherty Earth Observatory of Columbia University.
The researchers said they could not say what was driving the changes in reflectivity. "What we say is somehow the cloud properties have changed," Goode said. The brightening coincided with accelerated warming of global temperatures in the late 1990s.
Other scientists have also reported signs that global dimming halted in the 1990s. At a joint meeting of American and Canadian geological societies in Montreal this month, Martin Wild, a climatologist at the Swiss Federal Institute of Technology in Zurich, reported that sunshine measurements from 10 stations worldwide showed no signs of dimming during the '90s.
Since 2000, reflectivity has since risen to close to the 1985 level.
The New York Times
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