Clear Skies Have Become Less So Over Time, Data Show

U.S. researchers find aerosol levels have diminished visibility

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THURSDAY, March 12 (HealthDay News) -- More than three decades of data showing how clear, or unclear, the sky over land has been should reveal how changes in air pollution have affected climate change, according to a University of Maryland-led team of researchers.

The data show that what the researchers call clear sky visibility over land has decreased worldwide since the early 1970s because of an increase in aerosols, which are solid particles or liquid droplets suspended in air. Aerosols, which include soot, dust and sulfur dioxide particles, are created by the burning of fossil fuels, industrial processes and burning of tropical rain forests. Aerosols pose a threat to human health and the environment, the researchers said.

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"Creation of this database is a big step forward for researching long-term changes in air pollution and correlating these with climate change," Kaicun Wang, an assistant research scientist in the geography department at Maryland and the research team leader, said in a university news release. "And it 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."

Carbon dioxide and other greenhouse gases are transparent and have no effect on visibility. Though the impact of greenhouse gases on climate change is well-established, scientists don't fully understand the effects of aerosols. The database is expected to provide answers.

The data include sky visibility measurements taken between 1973 and 2007 at 3,250 meteorological sites worldwide. Visibility refers to the distance an observer can clearly see from a measurement source. The more aerosols in the air, the shorter the visibility distance.

A preliminary analysis of the database revealed a steady increase in aerosols between 1973 and 2007. Increased aerosols in the atmosphere block solar radiation from the Earth's surface, causing an overall "global dimming," the researchers said.

The findings were published in the March 13 issue of Science.

More information

The World Health Organization has more on climate change and human health.

-- Robert Preidt