BY many measurements, this summer’s drought is one for the record books. But so was last year’s drought in the South Central states. And it has been only a decade since an extreme five-year drought hit the American West. Widespread annual droughts, once a rare calamity, have become more frequent and are set to become the “new normal.”

Until recently, many scientists spoke of climate change mainly as a “threat,” sometime in the future. But it is increasingly clear that we already live in the era of human-induced climate change, with a growing frequency of weather and climate extremes like heat waves, droughts, floods and fires.

Future precipitation trends, based on climate model projections for the coming fifth assessment from the Intergovernmental Panel on Climate Change, indicate that droughts of this length and severity will be commonplace through the end of the century unless human-induced carbon emissions are significantly reduced. Indeed, assuming business as usual, each of the next 80 years in the American West is expected to see less rainfall than the average of the five years of the drought that hit the region from 2000 to 2004.

That extreme drought (which we have analyzed in a new study in the journal Nature-Geoscience) had profound consequences for carbon sequestration, agricultural productivity and water resources: plants, for example, took in only half the carbon dioxide they do normally, thanks to a drought-induced drop in photosynthesis.