China's skies have darkened over the past 50 years, possibly due to haze resulting from a nine-fold increase in fossil fuel emissions, according to researchers from the U.S. Department of Energy.

The researchers, writing in this month's edition of Geophysical Research Letters, found that the amount of solar radiation measured at more than 500 stations in China fell from 1954 to 2001 despite a decrease in cloud cover.

"Normally, more frequent cloud-free days should be sunnier and brighter but this doesn't happen in our study," said Yun Qian of the energy department's Pacific Northwest National Laboratory in Washington state.

"The pollution (that) resulted from human activity may have created a haze which absorbs and deflects the sun's rays," Qian, the study's lead author, said in an e-mail interview Friday.

Air pollution is widespread in China. Antiquated factories billow smoke, many residents still use coal to heat their centuries-old houses, and a sharp increase in car ownership has bathed the motorways in exhaust fumes.

Using data from more than 500 weather stations in China, researchers found the amount of sunlight hitting the ground has fallen by 3.7 watts per square yard in each of the last five decades amid a nine-fold increase in fossil fuel emissions, the study said.

The cloud cover data used in this study was obtained from the China Meteorological Administration through a bilateral agreement with the U.S. Department of Energy on global and regional climate change, the researchers said.

Herbert G. Fabian, who studies urban pollution and transportation issues for the Asian Development Bank, said the study's conclusion "makes sense" but that more information is needed.

"There really is (an) air pollution problem and a haze problem in China because (of) dust storms and pollution," said Fabian, who was not connected to the study. "But we can't say conclusively that the reduction in sunlight is due to haze."

The study also said haze appears to have masked the impact of global warming by reflecting sunlight back into space and cooling the Earth's surface.

"The haze may have masked the effects of global warming across large parts of China, particularly in the central and eastern regions, where daily high temperatures have actually been decreasing," Qian said. "This may seem like good news, but any success China has in curbing emissions will accelerate the effects of global warming in those areas when the cooling mask is lifted."