Cleaner air makes brighter skies
May 9, 2005
The amount of sunlight reaching the Earth's surface is increasing, two new studies in Science magazine suggest.
Using different methods, they find that solar radiation at the surface has risen for at least the last decade.
Previous work had found the opposite trend, leading to a popular theory known as "global dimming".
But the latest Swiss and US research indicates the dimming in the past has now been reversed, possibly because of reduced atmospheric pollution.
Hard-won data
The idea of global dimming holds that tiny particles - aerosols - in the atmosphere are reflecting sunlight back into space, and the effect is to cool the Earth's surface.
The aerosols - a large proportion of which come from human activities - are therefore acting against any human-induced greenhouse effect. And only when societies clean up the production of aerosols will the true extent of global warming become apparent.
That is the theory, but global dimming has been hard to study definitively.
As with many other issues relevant to climate science, the answers researchers seek are not easily obtained, because previous generations did not build the instruments and set up the experiments that present-day investigators now suddenly need.
With the growing realisation that climate change may be a major hazard for the planet and for human society, the gaps which are left in this area are quickly being plugged.
Martin Wild's team, from the Swiss Federal Institute of Technology in Zurich - one of the groups reporting in this week's edition of the journal Science - has taken advantage of recent developments.
"We needed a system which allowed high-quality measurements of solar radiation," he told BBC News.
"Since 1990, many stations giving high-quality data have been built - and overall, they show an increase in radiation reaching the Earth's surface."
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