'Global dimming' may stop the Earth overheating.
19 May 2004

MARK PEPLOW


Scattered light helps plants suck up carbon dioxide.
© GettyImages
It's official: the world is getting darker. Scientists now agree that as cloud cover and particles in the atmosphere increase, the amount of radiation reaching us from the Sun is falling. And although they are nervous about raising the idea, they think the effect may help protect us from global warming.

The phenomenon, called global dimming, has been quietly discussed in scientific circles for the past decade or so. Since the late 1950s, scientists have observed a 2-4% reduction each decade in the amount of solar radiation reaching the Earth's surface, which is thought to be caused by particles and clouds in the atmosphere scattering the light.

But previous studies have been confined to the Northern Hemisphere, so many scientists have questioned whether this is a localized effect, or if it even exists at all.

Advocates of the idea now have the evidence they need to convince the sceptics. A presentation by Australian scientists to the Joint Assembly of the American and Canadian Geophysical Unions in Montreal on Monday has shown that the effect is also found south of the equator.

The researchers found that evaporation rates in Australia have fallen significantly over the last 30 years, a sure sign that less thermal radiation is reaching the surface. "This proves that it is a global phenomenon," says Michael Roderick, an environmental scientist at the Australian National University, Canberra, who led the research.

But Roderick is not dismayed by the result. He has recently advised the Australian government that global dimming may be a good thing. "The standard dogma is that Australia will dry out [with global warming], but that's just not right. The world is actually getting less arid," he insists.

In fact, Roderick sees global dimming as part of a negative feedback loop that allows the atmosphere to regulate itself. Burning fossil fuels not only increases carbon dioxide levels in the atmosphere; it also pumps tiny particles into the air. At the same time, higher temperatures increase the amount of cloud cover. The clouds and particles help to block the Sun's rays, and the scattered light they allow through actually boosts plants' absorption of carbon dioxide, the principle greenhouse gas. This helps to keep carbon dioxide levels stable, argues Roderick, protecting the planet from runaway global warming

Grey expectations

"Diffuse light is like putting plants on steroids," Roderick explains. Scattered light takes a zigzag path, bathing every part of a plant's leaf in light instead of just one surface. Even if the overall amount of light is lower, this increases the plant's rate of photosynthesis and more carbon dioxide is removed from the atmosphere.

The only plants that might suffer are those in northern European greenhouses, because they are so desperate for light that any reduction could affect their growth.

This week's conference is the first time that all the leading scientists in the field have met, and it puts global dimming firmly on the research map. But Shep Cohen from the Institute of Soil, Water and Environmental Sciences in Bet Dagan, Israel, says that some of his colleagues are still anxious about discussing the effect in public, in case it is seen as an excuse not to worry about global warming.

However, Cohen points out that understanding global dimming is essential for working out the best strategies for tackling pollution and climate change. For example, the half measure of filtering out particles while continuing to pump carbon dioxide into the air could destabilize the relationship between the two and actually increase global warming, he warns.

© Nature News Service / Macmillan Magazines Ltd 2004