The American Southwest is a subtropical area of the world, but one that has seen a cycle of droughts – long and short – over the past thousand years. Columbia University Lamont-Doherty Earth Observatory professor Richard Seager has been studying the climate, and believes that human-induced global warming could push the natural variability to a point where the region enters into a time of permanent drought in the comig decades.
Seager and colleagues who have been studying the area’s climate have looked into the past by using sea surface temperatures collected by ships that plied the oceans in the 19th century to create computer models recreating the climate history. On top of that they have used tree rings to look back as far as the Middle Ages.
“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.”
They found that there were periodic droughts throughout their climate models, as well as a drought in the Southwest that lasted hundreds of years, sometime around the Middle Ages.
This is not the first time that Seager has sounded the alarm. In a 2007 paper Seager and colleagues showed that the Southwest is on the verge of a transition to a more arid climate, and then in December of 2010 Seager along with Gabriel Vecchi of the NOAA narrowed down the causes behind the drop in winter precipitation to a change in atmospheric circulation and water vapour transport induced by warming temperatures.
Warming doesn’t just have impact when the climate switches into arid, but has slowly increasing effects as time goes on.
The warming being seen in the region is shortening the snow season, reducing the amount of snow that falls and forcing an early spring, which minimizes the amount of water which flows into the water system that supplies much of the Southwest, a region from the western Great Plains to the Pacific, and the Oregon border to southern Mexico.
Worse still is the population boom that the region has seen over the past decades. A reduction in available water will stretch resources to the limit.
“I’m curious how the Southwest is going to handle this,” Seager says.
Seager says the natural variations between wet and dry periods are driven mostly by the El Niño/La Niña cycle of sea surface warming and cooling in the Pacific. “The anthropogenic signal is currently small compared to the natural variability,” Seager says. “But you can see it, and it’s consistent with the climate models. It works across the whole subtropics. Right now the human effect is small, but it will become a serious problem in the decades down the road.”