Text 12439, 86 rader
Skriven 2005-05-06 05:31:00 av Jeff Binkley (1:226/600)
Ärende: Global Warming
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http://www.nature.com/news/2005/050502/full/050502-8.html


News 
 
 
Published online: 5 May 2005; | doi:10.1038/news050502-8 
Clear skies end global dimming
Quirin Schiermeier 
Earth's air is cleaner, but this may worsen the greenhouse effect. 
 

Our planet's air has cleared up in the past decade or two, allowing more 
sunshine to reach the ground, say two studies in Science this week.

Reductions in industrial emissions in many countries, along with the use 
of particulate filters for car exhausts and smoke stacks, seem to have 
reduced the amount of dirt in the atmosphere and made the sky more 
transparent. 

That sounds like very good news. But the researchers say that more solar 
energy arriving on the ground will also make the surface warmer, and 
this may add to the problems of global warming. More sunlight will also 
have knock-on effects on cloud cover, winds, rainfall and air 
temperature that are difficult to predict. 

The results suggest that a downward trend in the amount of sunlight 
reaching the surface, which has been observed since measurements began 
in the late 1950s, is now over.

The researchers argue that this trend, commonly called 'global dimming', 
reversed more than a decade ago, probably following the collapse of 
communist economies and the consequent decrease in industrial 
pollutants. 

The widespread brightening has remained unnoticed until now simply 
because there wasn't enough data for a statistically significant 
analysis, says Martin Wild, an atmospheric scientist at the Swiss 
Federal Institute of Technology in Zurich and an author on one of the 
reports.

Sunny days

Wild and his team looked at data on surface sunshine levels from 
hundreds of devices around the planet. They found that since the 1980s 
there has been a transition from decreasing to increasing solar 
radiation nearly everywhere, except in heavily polluted areas such as 
India and at scattered sites in Australia, Africa, and South America1. 

A second study, led by Rachel Pinker from the University of Maryland, 
College Park, found a similar trend by looking at satellite data, 
although their research suggests the extent of the brightening is 
smaller2. Unlike ground stations, satellites can sample the whole 
planet, including the oceans. However, satellite data are difficult to 
calibrate, and so are considered less accurate than measurements from 
the ground.

Surprisingly, Wild's study shows a brightening trend in China, despite 
the fact that there is a booming, fossil-fuel-intensive industry in that 
country. Wild says he can only speculate that the use of clean-air 
technologies in China might be more widespread and efficient than has 
been thought.

In contrast, India's vast brown clouds of smog, which result from 
wildfires and the use of fossil fuels, have reduced the sunlight 
reaching the ground.

Just warming up

  
 
Researchers will now focus on working out the long-term effects of 
clearer air. One thing they do know is that black particulate matter in 
the air has been contributing a cooling effect to the ground. "It is 
clear that the greenhouse effect has been partly masked in the past by 
air pollution," says Andreas Macke, a meteorologist at the Leibniz 
Institute of Marine Sciences in Kiel, Germany. 

Uncertainties remain part of the game because scientists have only a 
limited ability to track cloud cover and particulates, says Macke. 
Increased cooperation in programmes such as the NASA-led International 
Satellite Cloud Climatology Project should help to close the gaps in our 
knowledge of how dirty air affects climate, he says. 

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