Scientists have long argued that human activity may be warming the Earth through a process known as the greenhouse effect. Now studies show we may also be having a different kind of impact: global dimming.
Researchers with the Pacific Northwest National Laboratory in Washington found the amount of sunlight hitting the ground over China dropped by 3.7 watts per square yard over the last 50 years. This coincided with another drop in cloud cover during the same time. Since increased cloudiness would normally explain a solar dimming, the researchers say another factor is probably to blame: pollution.
"The main suspect is increasing air pollution, which produces haze and cuts down on the solar radiation reaching the surface," said L. Ruby Leung of the U.S. Energy Department. Leung points out there has been a ninefold increase in fossil-fuel emissions from China over the last half-century.
"This study shows that air pollution could also have large impacts on solar radiation, which can affect plant growth [or agriculture]," she added.
Is pollution truly turning down the dimmer switch over the Earth? Other work over the last few years has found a similar decline in the amount of solar radiation hitting the entire planet since the 1950s, while noting a slight increase since the 1990s.
These most recent findings, published in the journal Geophysical Research Letters, support that past work and were based on the readings of nearly 500 ground-based instruments throughout China.
But some scientists argue we can't yet be sure what's going on.
Potential Clues Lost as 'Goresat' Satellite Scrapped
"The observations we have at this point just aren't good enough," said Robert Charlson of the Department of Atmospheric Sciences and Department of Chemistry at the University of Washington in Seattle. "The biggest single problem we have now is a lack of adequate satellite measurements, and the platforms that could be moving us toward answers are either pending or being killed."
In fact, last month NASA scrapped a program that might have offered key evidence one way or the other. Initially dubbed "Goresat" by Republicans in Congress because it was first promoted by Vice President Al Gore in the late 1990s, the Deep Space Climate Observatory was designed to hover and observe an entire sunlit side of the Earth for long periods.
The satellite, Charlson argues, might finally have offered solid data about so-called global dimming -- as well as warming. The device was built and scheduled to launch in 2001. The 2001 terror attacks and then the loss of Columbia in 2003 pushed the launch date farther and farther away. Finally, NASA science chief Mary Cleave wrote scientists early this year saying that "the context of competing priorities and the state of the budget for the foreseeable future precludes continuation of the project."
As a result, researchers looking into the possibility of global dimming are limited to basing their work on scant satellite data and ground-based instrument readings.
Judging What's Real
Rachel Pinker of the University of Maryland in College Park has done studies based on satellite readings and argues that the numbers suggest something is going on but there's too little data to know just what.
"It may or may not be pollution levels," she said. "It might also be aerosol cloud interaction or different instruments that are doing the readings. It's not yet clear why we're seeing this."
Others are more confident that the dimming is happening and that pollution is the main cause. Martin Wild, an atmospheric scientist at the Swiss Federal Institute of Technology in Zurich and lead author of a May 2005 study in the journal Science, showed there was significant dimming over Earth until about 1990 when data suggested there had been a gradual brightening. Wild attributes the recent brightening to successful efforts to stem pollution. He adds it's important to take the phenomenon into account because it can influence the effects of global warming.
"From the 1960s to the 1980s, the dimming may have been large enough to counterbalance the greenhouse-induced increase in downward long-wave radiation," Wild said.
If these preliminary numbers are right and the brightness of the skies is being altered by pollution levels, the effects could be widespread, Leung points out. From slower plant growth to less frequent precipitation, further dimming could eventually lead to food and water shortages in some regions. The fact that the phenomenon might also be skewing the intensity of global warming also holds consequences, argues Charlson, since it's important to carefully measure effects in order to curb them.
"It makes sense -- there are good reasons why we expect man-made particles to affect the Earth globally," Charlson said. "We just need the observations, and we need to do them much better if we want to come up with some solutions."