Places like California are already facing very dry conditions, but these are quite gentle compared with some periods in the 12th and 13th Centuries.
Scientists have now compared these earlier droughts with climate simulations for the coming decades.
The study suggests events unprecedented in the last millennium may lie ahead.
"These mega-droughts during the 1100s and 1200s persisted for 20, 30, 40, 50 years at a time, and they were droughts that no-one in the history of the United States has ever experienced," said Ben Cook from Nasa's Goddard Institute for Space Studies.
"The droughts that people do know about like the 1930s 'dustbowl' or the 1950s drought or even the ongoing drought in California and the Southwest today - these are all naturally occurring droughts that are expected to last only a few years or perhaps a decade. Imagine instead the current California drought going on for another 20 years."