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Clear skies end global dimming

May 5, 2005 By Quirin Schiermeier This article courtesy of Nature News.

Earth's air is cleaner, but this may worsen the greenhouse effect.

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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.

References

  1. Wild M., et al. Science, 308. 847 - 850 (2005).
  2. Pinker R. T., et al. Science, 308. 850 - 854 (2005).

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