Air pollution has caused skies above most of the world's land areas to dim slightly over the past 30 years, says a study out today in the journal Science.
Scientists found that most of the blame can be traced to aerosols — suspended airborne pollution — that are released from the burning of fossil fuels. Aerosols in the atmosphere block sunlight from reaching the Earth's surface.
The dimming has been nearly worldwide. The report says that while visibility worsened only slightly in North America, it "decreased substantially over south and east Asia, South America, Australia and Africa, resulting in net global dimming over land."
Europe, however, has actually experienced brightening skies, the authors say, where visibility has increased since the mid-1980s. This is because of declines in pollution over that continent, likely because of controls on using sulfur in coal, says study co-author Robert Dickinson of the University of Texas.
When averaged globally, Dickinson says, the dimming has been rather subtle, akin to a 100-watt bulb dimming to a 99-watt bulb. But it has been more noticeable in the most polluted locations.
The research was conducted by Dickinson along with Kaicun Wang and Shunlin Liang of the University of Maryland. Wang used a database from the National Climatic Data Center in Asheville, N.C., to collect visibility measurements from 3,250 meteorological stations worldwide from 1973 to 2007.
While decreases in atmospheric visibility have been reported in the past, the new study compiles satellite and land-based data for a longer period than had been available.
Ellsworth Dutton, a scientist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo., countered in an e-mail that the locations of the weather stations, in many cases near population centers, do not represent the entire land-area distribution of the planet.
"Also, their methodology does not account for any changes in aerosols above the lowest 10 or so meters of the atmosphere, which can be strongly affected by rain and wind," Ellsworth writes.
While the effects of increased greenhouse gases (such as carbon dioxide) on global warming are clear, the effects of increased aerosols are not, the scientists report. Studies of the long-term effects of aerosols on climate change have been largely uncertain up to now, due to limited aerosol measurements over land, Wang and his team say.
However, with this study, Wang says, researchers now can compare temperature, rainfall and cloud-cover data from the past 35 years with the aerosol measurements in the new database. Wang says, "This is the first time we have gotten global long-term aerosol information over land to go with information already available on aerosol measurements over the world's oceans."