Ancient Drought Holds Clues to Warming Future

The distant past holds some bad news for western water managers in a warming world. Researchers have found evidence of a drought that lasted 60 years between the years 1118 and 1179 in the Colorado River basin.

The length of this ancient drought “dwarfs any drought previously documented for a region that includes areas of Arizona, Colorado, New Mexico, Utah and Wyoming,” according to researchers from the University of Arizona.Even though the drought was nearly a thousand years ago, it indicates that natural variability of drought periods is larger than thought earlier. In the ancient drought, the Colorado was below normal for 13 consecutive years at one interval, and there were no “very wet” years. In contrast over the lasts 100 years, there have been only five consecutive years of below-normal flows in the river.

Water managers rely on very wet years to refill reservoirs after dry years. The Colorado is a major water supplier for seven states in the U.S. and two in Mexico. Several cities , including Denver, Las Vegas, Los Angeles, and Phoenix rely on the river for at least a portion of their water supplies.

U of A geography professor Connie A. Woodhouse, who co-wrote the paper, said, “We have natural variability that includes this time in the 1100s. If we have warming it will exacerbate these kinds of droughts.” The newly documented droughts “could be an analogue for what we could expect in a warmer world.”

The paper will be published in the American Geophysical Union’s journal Geophysical Research Letters on May 24.