The scientists write in a study in the journal Science Advances that global warming will lead to "unprecedented drought conditions" -- the worst since the 1100s. The study's author spoke with the BBC:
"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."
The study is based on current increasing rates of rising emissions of carbon dioxide and complex simulations run by 17 different computer models, which generally agreed on the outcome.
The Southwest will see less rain. But the biggest problem in both regions will be the heat, which will increase evaporation and dry out the soil.
"We're going to have to think about a much drier future in western North America," Cook said. He spoke more about the study in this NASA-produced video:
