Visibility on clear days has declined in much of the world since the 1970s thanks to a rise in airborne pollutants, say scientists.

In a study appearing in the today's issue of Science, the researchers describe a 'global dimming' in particular over south and east Asia, South America, Australia and Africa.

They add visibility remained relatively stable over North America and had improved over Europe.

Aerosols, tiny particles or liquid droplets belched into the air by the burning of fossil fuels and other sources, are responsible for the dimming, the researchers write.

"Aerosols are going up over a lot of the world, especially Asia," says study co-author Professor Robert Dickinson of the University of Texas.

Dickinson and colleagues from the University of Maryland tracked measurements of visibility - the distance someone can see on clear days - taken from 1973 to 2007, at 3250 meteorological stations worldwide.

They say aerosols like soot, dust and sulfur dioxide particles all harmed visibility.

The researchers used recent satellite data to confirm that the visibility measurements from the meteorological stations were a good indicator of aerosol concentrations in the air.

The aerosols from burning coal, industrial processes and the burning of tropical forests can influence the climate and be a detriment to health, say the researchers.

Other pollutants such as carbon dioxide and other greenhouse gases are transparent and do not affect visibility.
Tracking changes

The data will help researchers understand long-term changes in air pollution and how these are associated with climate change, says Dr Kaicun Wang of the University of Maryland.

"This study provides basic information for future climate studies," says Wang.

The scientists highlight increased industrial activity in places like China and India for some of the decreased visibility.

They add air quality regulations in Europe helped improve visibility there since the mid-1980s.

Aerosols can have variable cooling and heating effects on surface temperatures, reflecting light back into space and reducing solar radiation at the earth's surface or absorbing solar radiation and heating the atmosphere, the researchers say.

by:ABC Science