for National Geographic News
Even as the ice caps melt, global warming threatens to leave a billion people high and dry, says a team of U.S. climate scientists.
If the Earth warms just a degree or two Celsius in coming decades, regions that depend on runoff from mountain snows for drinking water and farming will face shortages, according to a study published in the November 17 issue of the journal Nature.
A companion article supports the claim, showing that mountain runoff has already decreased in some regions of the world.
"We found that, no surprise, less snow falls in a warmer world," said Timothy Barnett of the Scripps Institute of Oceanography in La Jolla, California. "And what snow there is melts earlier."
The two factors combine to push peak runoff from summer into spring, when reservoirs are already at capacity.
"The dams get filled earlier in the year, and they can only be filled to a certain level, so there's still some flood control," Barnett said.
The result: Much of the early runoff goes to waste, prompting shortages in late summer and autumn.
"It's like squeezing six months of snowmelt into four," Barnett said.
A Billion Dry
To develop their climate model, which looks ahead 30 to 40 years, Barnett and Jenny Adam of the University of Washington in Seattle looked at 30 years of global precipitation records.
They mapped the globe into squares and calculated how much precipitation fell in each square as rain and how much as snow. They overlaid that data onto a map of regions that depend on snow for at least 50 percent of their water supply, including the western United States.
"In California, Mother Nature holds the snow for us up in the Sierras," Barnett said. "It's basically a massive reservoir."
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