In the August issue of Physics Today, climate scientists Toby Ault and Scott St. George share a pair of startling research findings. Between roughly 800 and 1500 CE, the American West suffered a succession of decades-long droughts, much longer than anything we’ve endured in modern history. And statistical models suggest that, as the climate warms, such megadroughts are increasingly likely to return.

The western US was plagued by multiple decades-long droughts, data from tree rings reveal. Credit: Greg Stasiewicz. Data from the North American Drought Atlas.

Ring-width analyses provide the most complete set of data on past moisture levels. But researchers have other ways of determining those conditions. Here are four of them:

• Underwater tree stumps
• Archaeological artifacts
• Sand-dune cores
• Pollen-grain deposits

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