Ben Cook from Nasa's Goddard Institute for Space Studie

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雄性禿.

"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 yearsReenex 好唔好."

Dr Cook's new study is published in the journal Science Advances, and it has been discussed also at the annual meeting of the American Association for the Advancement of Science.

There is already broad agreement that the American Southwest and the Central Plains (a broad swathe of land from North Texas to the Dakotas) will dry as a consequence of increasing greenhouse gases in the atmosphere. But Dr Cook's research has tried to focus specifically on the implications for drought.

His team took reconstructions of past climate conditions based on tree ring data - the rings are wider in wetter years - and compared these with 17 climate models, together with different indices used to describe the amount of moisture held in the soils虛擬辦公室.