Droughts are weather extremes that are hostile enough that plenty of sci-fi and post-apocalyptic stories use near-permanent droughts for apocalyptic backdrops (Waterworld notwithstanding). And for good reason—drought is part of the reality-based picture of modern climate change, as combined trends in rainfall and evaporation are bringing drier conditions to some regions. But understanding trends is a challenge: more rain is being delivered to other regions, drought conditions are naturally variable, and historical rainfall data is limited.

Researchers have typically turned to tree rings for archives of past droughts. By compiling records from many trees, historical maps called “drought atlases” have been built for a number of regions and can cover nearly a millennium. These can provide incredible historical information, including events like the megadroughts of the Western United States between 800 and 1300 CE. But each drought atlas is only one piece of the global picture.