"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."