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Can Earth take the heat of 'global brightening'?

By Peter N. SpottsStaff writer of The Christian Science Monitor / May 12, 2005

Break out the Ray-Bans. The amount of sunlight reaching Earth's surface appears to be growing.

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The phenomenon, which some dub "global brightening," presents scientists with a puzzle. If the trend is real and global, how long will it last and what are the consequences for climate change, the planet's water cycle, and other processes that draw energy from sunlight?

At first glance, the answer might seem obvious: More sunlight reaching the ground in a warming world means that temperatures will get warmer still. Not so fast, some researchers say. Additional warming would be certain only if nothing else in the climate system changes. And the climate system is anything but static. Some combinations of changes could reinforce the heating; others could offset it. Unraveling these interactions and forecasting their course require an accurate accounting of the sunlight reaching the surface and the radiation the surface sends skyward. Moreover, researchers say, measurements of the sun's strength at Earth's surface are potentially powerful tools for gauging human influences on the climate.

Earth's radiation "budget" represents an "extremely important parameter that is poorly known," says Robert Charlson, an atmospheric scientist at the University of Washington at Seattle. "It needs to be quantified much better than it is."

Concerns about the amount of sunlight reaching Earth's surface were first raised in 1974. Researchers from the United States and Israel recorded a 12 percent drop in sunlight over 40 years at a monitoring station in the southern Sinai Peninsula. Since then, others have used a variety of techniques to try to track incoming sunlight. Three years ago, for example, a team led by Beate Liepert at Columbia University's Lamont-Doherty Earth Observatory gathered data from ground stations around the world and found that solar radiation reaching the surface fell by 4 percent from 1961 to 1990.

Evidence for a turnaround comes from three groups using ground instruments and satellites.

They found:

• Since 1990, the amount of solar energy striking Earth's surface at selected sites rose by an average of just over half a watt for each square meter of surface area, according to Swiss climatologist Martin Wild and colleagues, using data from a network of groundbased sensors.

• The surface has experienced an average rise of 0.16 watts per square meter for each year from 1983 to 2001, according to inferences by a team led by University of Maryland atmospheric scientist Rachel Pinker, using long-term satellite measurements. A large upswing since 1990 accounts for the bulk of the increase.

• Earth's albedo - the proportion of incoming radiation reflected back into space - has been shrinking over the past few years, calculates a third group led by Bruce Wielicki at NASA's Langley Research Center in Hampton, Va., using satellite measurements. This suggests more radiation is hitting the surface, instead of being reflected back into space by snow, ice, clouds, or tiny particles known as aerosols. Ironically, the results contradict a study published last year that used changing brightness in "earthshine" on the moon as a surrogate for the amount of energy Earth was reflecting into space. That study suggested the planet's albedo was increasing over the same period.

The studies appear in the current edition of the journal Science.

For all the detailed work that went into gathering and analyzing the information, the results represent only a start at tackling the issue, scientists say. Although the teams say they used the best available instruments and techniques for figuring out what the data mean, each approach has its shortcomings. These can yield uncertainties that can be as large as the quantity researchers are trying to measure. To infer surface information from satellites, which can see only the top of the atmosphere, scientists must use models of interactions they think are taking place between there and the surface. And surface instruments measure only what's taking place directly above them - they don't necessarily represent global conditions.