In December 1997, Nadia Flores-Yeffal awoke early in a small town in the state of Guanajuato, in the heart of Mexico. She pulled on her shoes and followed a local guide down a cactus-fringed dirt path, past old adobe houses intermixed with newer construction. They walked for more than an hour, out of the small town where Flores-Yeffal was spending a month to research her senior thesis, until they came to a rocky, snake-infested hill. At the top, she found what she was looking for: the 100-square-foot garden plots where local families farmed their staple crops. The rows of corn and beans were sparse and dry; many of the plots were empty.

“It just looked really bad,” Flores-Yeffal remembers.

The town, which sits on the river Lerma, was in the grip of a drought that would extend through the early 2000s, drying up the river and the soil along with it. There was no irrigation, few crops, nothing to eat. “In one day, as many as 30 people left,” says Flores-Yeffal, who is now a population scientist at Texas Tech University and an expert on the sociology of migration. In the words of local farmers, Flores-Yeffal says, “It used to rain all of the time, and then all of a sudden it didn’t.”