"Major 20th-century droughts pale in comparison to droughts documented in paleoclimatic records over the past two millennia," the researchers wrote, noting that high temperatures coincided with lengthy dry spells in medieval times.
"We're not saying future droughts will be worse than what we see in the paleo record, but we are saying they could be as bad," said lead author Connie Woodhouse, an associate professor of geography and regional development. "However, the effects of such a worst-case drought, were it to recur in the future, would be greatly intensified by even warmer temperatures."
In the mid-12th century, a six-decade-long heat and dry spell covered most of the western United States and northern Mexico, the researchers found.
During 25 years of that period, the Colorado River -- an important tributary that today feeds seven states including the big cities of Los Angeles, Las Vegas, Denver, Phoenix, Tucson and Albuquerque -- flowed at a rate of 15 percent below normal.
Over the past decade, sampling of the Colorado shows the river is at its lowest point since measurement records were first kept in 1906.