Permanent drought in the US southwest

\David Funkhouser in Columbia University News: The American Southwest has seen naturally induced dry spells throughout the past, but now human-induced global warming could push the region into a permanent drought in the coming decades, according to Lamont-Doherty scientist Richard Seager and others who have been studying the area’s climate.

Seager, who focuses on climate variability and climate change, began his work studying droughts by looking into the past using sea surface temperature records gathered by ships plying the oceans in the 19th century. He and colleagues used computer models to recreate a climate history that showed periodic droughts. Focusing on North America, they also used tree rings to look back as far as the Middle Ages, when the Southwest experienced a drought lasting hundreds of years.

“You begin to see that there’s a natural cycle of droughts, large and small,” says Seager, the Palisades Geophysical Institute/Lamont research professor at Lamont-Doherty Earth Observatory. “But when you add in the human effects from rising greenhouse gases, we could be pushing subtropical regions like the American Southwest into a permanent state of aridity. There are signs it’s already underway.”

In a 2007 paper, Seager and colleagues used computer models to show the Southwest is on the verge of a transition to a more arid climate. And in the December 2010 issue of Proceedings of the National Academy of Sciences, Seager and Gabriel Vecchi of NOAA pinned the drying to a drop in winter precipitation and showed how this is caused by changes in atmospheric circulation and water vapor transports induced by warming temperatures.

The warming also shortens the snow season, reduces the snow mass that serves as natural storage for water, and forces an earlier spring melt, disrupting the supply system that waters much of the Southwest—the region from the western Great Plains to the Pacific, and the Oregon border to southern Mexico.

That is ominous news for a region that has seen explosive growth in population, land use and water demands in recent decades. A reduction in the flow of important water resources such as the Colorado River will have serious consequences….

Lake Powell with the bathtub ring caused by low water, shot by PRA, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported license