Visibility on clear days has declined in much of the world since the 1970s thanks to a rise in airborne pollutants, scientists said.
They described a "global dimming" in particular over south and east Asia, South America, Australia and Africa, while visibility remained relatively stable over North America and improved over Europe, the researchers said.
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 said.
"Aerosols are going up over a lot of the world, especially Asia," Robert Dickinson of the University of Texas, one of the researchers, said.
Aerosols all harm visibility
Dickinson and two University of Maryland researchers tracked measurements of visibility -- the distance someone can see on clear days - taken from 1973 to 2007 at 3,250 meteorological stations worldwide.
Aerosols like soot, dust and sulphur dioxide particles all harmed visibility, they said in the journal Science. 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, the researchers said.
Other pollutants such as carbon dioxide and other so-called greenhouse gases are transparent and do not affect visibility. The data will help researchers understand long-term changes in air pollution and how these are associated with climate change, said Kaicun Wang of the University of Maryland.
More air quality regulations needed
"This study provides basic information for future climate studies," Wang said.
The scientists blamed increased industrial activity in places like China and India for some of the decreased visibility, while they said air quality regulations in Europe helped improve visibility there since the mid-1980s.
The 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, they added. – (Reuters Health, March 2009)
Pollution affects life expectancy
Traffic jams harm the heart