Droughts lasting tens of years are known to have occurred in the southwestern United States during the past two millennia, but model simulations suggest that much longer ‘megadroughts’ might occur in a future, warmer climate. A new analysis of a lake sediment core from northern New Mexico reveals that such extreme droughts have occurred before: millennial-scale megadroughts were a regular feature of the warmest phases of Pleistocene interglacials. Mean annual temperatures during these droughts were comparable with or higher than today’s. Comparison of the mid-Pleistocene climate record with that of the Holocene shows many similarities and suggests that were it not for anthropogenic warming, the southwestern United States would probably be entering a cool and wet phase by now.
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