But in the most comprehensive study to date of overcast versus cloud-free days in China, a team led by the Department of Energy's Pacific Northwest National Laboratory, reporting in the current advance online issue of Geophysical Research Letters, has found that cloud cover has been decreasing for the past 50 years.
Eliminating clouds from the dimming equation now leaves little doubt that human activity, in the form of a nine-fold increase in fossil fuel emissions over the same half-century period, has entrenched China in a foggy haze that absorbs and deflects the sun's rays.
For the study, PNNL senior research scientist and lead author Yun Qian and colleagues surveyed records from more than 500 weather stations across China for the years 1954 to 2001. To remove the subjectivity and ambiguity of "partly cloudy" reports, they consulted only records of an either/or nature--cloud-free and overcast days. Their statistical analysis noted a clear trend: overcast days decreased .78 percent each decade while cloud-free days increased .6 percent for the same period.
The results, said co-author Ruby Leung, a PNNL laboratory fellow, "strongly suggest that increasing aerosol concentrations (particles, mainly soot and sulfur, that pollute the air) in the past has produced a fog-like haze that has reduced solar radiation (surface heat from sunshine), despite more frequent clear days that should lead to increased solar radiation."
In fact, a report in Science last year showed that most of the planet's surface is brightening, attributable to air-pollution regulation throughout most of the industrialized world. The report even showed a curious blip in surface brightness in China in the mid-90s, Leung noted. "Our results showed a similar trend in the mid-90s, consistent with the Science paper.
Contact: Bill Cannon
DOE/Pacific Northwest National Laboratory