Deserts Expand Faster as Earth Brightens

Aug. 19, 2009 -- As the world warms over the next century, global deserts could expand by as much as 34 percent, according to a new study, swallowing an area roughly the size of the United States.

Predicting what the future holds for climate is a tricky business; nothing is ever certain.

But our best guess is that human-induced global warming is going to dry out much of the arid lands in Africa's Sahel region, areas around the Mediterranean Sea, the fringes of China's Gobi desert, parts of South America and southwestern North America.

Moderate estimates by the Intergovernmental Panel on Climate Change (IPCC) predict that deserts will expand 10 percent worldwide by the year 2100. But it could be much worse.

The computer models the IPCC used for their latest estimate neglected a key aspect of climate: When trees dry out and die en masse, they leave behind a brighter landscape that reflects more sunlight back out into space.

This causes land to stay cooler, a death sentence for arid regions that rely on heat to draw in nourishing monsoonal winds and rain from the ocean.

According to Ning Zeng and Jinho Yoon of the University of Maryland, College Park, the increasing brightness could triple the amount of desert expansion expected by the end of the 21st century. Such a change would imperil millions of people who depend on already scarce water resources for drinking, growing crops and raising livestock.

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"The implications are tremendous," Zeng said. "Over the next few decades, we might see very rapid degradation of marginal zones near deserts."

Zeng and Yoon's study is due to be published in the journal Geophysical Research Letters.

Despite the dire prediction, the fate of these lands isn't set in stone. Carbon dioxide (CO2) is a key nutrient for plants, and it's possible that as humans pump ever more of it into the atmosphere, the shrubbery will thrive. Trees and grasses may even become more resistant to drought as CO2 concentrations increase.

It's a major, unsolved question whether this so-called "CO2 fertilization effect" is strong enough to rescue threatened regions from plunging into desert, or whether the predicted drying will be too severe for plants to survive.

Either way, there is another layer that Zeng and Yoon couldn't add to their models: the direct impact of human presence in these regions. Intensive farming, livestock overgrazing and water consumption could combine with natural changes to touch off ecological crises similar to the infamous Dust Bowl that struck the central United States in the 1930's.

"The same areas that will be drying because of global warming also happen to have lots of land use change," Benjamin Cook of Columbia University said. "Potentially those sorts of Dust Bowl conditions could happen again."

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