Science: Airborne Particles Have Reduced Clear Sky Visibility Over Most of the World

Average Atmospheric Optical Depth

Global aerosol optical depth (0.55 µm) from visibility data collected at 3250 meteorological stations from 2000 to 2007.

[Image copyright Science/AAAS]

Visibility in most of the world’s skies has decreased in recent decades, and higher levels of soot and other fine particles called aerosols are probably to blame, researchers report in the 13 March issue of Science.

Atmospheric aerosols have a large effect on how the atmosphere is warmed by solar radiation, but uncertainty about their abundance has been a major obstacle for scientists trying to develop a more nuanced understanding of global warming.

Kaicun Wang of the University of Maryland in College Park and colleagues analyzed records of aerosol concentrations and “optical depth,” a term that describes visibility in the clear sky, gathered worldwide since 1973.

They found that skies have become less transparent over most, but not all, of the world in that time. The exception to the general trend is Europe, where visibility has increased since the mid-1980s.

The results correspond well to previous reports showing that the amount of solar radiation reaching the Earth’s surface decreased in the years up to the mid-1980s but subsequently increased in Europe.

Although increases in the concentrations of many types of aerosols may have contributed to this effect, by far the largest documented changes in aerosols are those from the increased use of fossil fuels, in particular the burning of coal and other fuels that release sulfur dioxide.

Kathleen Wren

12 March 2009