Break out the Ray-Bans. The amount of sunlight reaching Earth's surface appears to be growing.
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.
• 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.
For the most part, "they're placed where people feel like taking measurements - near population centers - so they can be operated and serviced," says Ellsworth Dutton, a scientist with the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo. He took part in two of the three "brightening" studies.
Still, the fact that three groups spot the same trend, however tenuous, gives them confidence that they are seeing something real. "I was quite skeptical of the dimming results when they first came out," Dr. Dutton says, based on the instruments and statistical techniques used. "But there's no instrument deficiency I can think of that would cause data to show an increasing trend" in solar radiation at the surface. "It's hard to get that unless it's real."
He adds that in his mind, the newest results bolster the case for the 30-year decline. "You can't keep going up without starting somewhere and you can't keep going down without turning back up."
What's causing the changes? Scientists are convinced the culprit lies within the atmosphere, rather than with the sun itself. No changes in the sun's output have yet been measured that would account for the measured changes in solar radiation striking the surface, they say.
Dutton, for one, says he suspects that scientists may be seeing an oscillation of some sort that could occur over several decades. Others, such as Charles Long of the Department of Energy's Pacific Northwest National Laboratory in Richland, Wash., suspect aerosols. "They'd be my first guess," he says.
Fossil fuels give off carbon dioxide, a greenhouse gas, when burned. But they also yield sulfur dioxide and oxides of nitrogen, which form tiny particles in the atmosphere. These particles, or aerosols, can reflect incoming radiation back to space and act as seeds around which water vapor can condense to form cloud droplets.
As if to underscore the link, scientists in China published a solar-radiation study in March covering 40 years of measurements there. During the second half of the 20th century, the country was burning enormous amounts of fossil fuels. The team noted "significant decreases" in solar radiation from 1965 into the late 1980s. Solar radiation at the Earth's surface fell by some 5.4 percent per decade. Then it began to pick up again, at least through 1994. The aerosol particles from burning fossil fuels are "likely to have contributed to the decrease in clear days in China," the team reports in the March 17 issue of Geophysical Research Letters.
"That's a nice addition to the growing body of evidence" for the ties between so-called "global dimming" and man-made aerosols, says Lamont-Doherty's Dr. Liepert. Her studies suggest some potentially odd effects from a combination of global warming and aerosols. The result could be less sunshine and less rainfall, despite a warmer, more moist world. In modeling work published last May, she and colleagues at the Max Planck Institute for Meteorology in Hamburg and at Dalhousie University in Halifax, Nova Scotia, found hints that heat can get trapped between clouds and layers of aerosols, reducing the amount of radiation reaching the surface. Less heat at the surface translates into less evaporation to form clouds. Any clouds that do form have droplets too small to rain out, leaving the land below high and dry.
• More than 1 million Earths could fit inside the sun, which measures nearly 1 million miles across.
• With a surface temperature of about 10,000 degrees F., the sun pumps out a lot of energy. Every square meter at the top of Earth's atmosphere receives about 1,400 watts of energy. Some of that gets reflected back to space or absorbed in the atmosphere. The amount reaching Earth depends on latitude, weather, the season, and time of day.
• Solar panels convert that energy into electricity. But actually, most of our conventional energy also originated from solar power. Oil, natural gas, and coal came from decayed plants and animals that got their energy from photosynthesis.
Sources: The Complete Idiot's Guide to The Sun; Solarserver; Arizona State University