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.”

Across Mexico, farmers still wait for rain that doesn’t come. Severe droughts, punctuated by intense storms and flooding, are huge environmental challenges for Mexico in the coming century. By 2080, agricultural declines are expected to drive 1.4 million to 6.7 million adult Mexicans out of the country. Most of those people will come to the United States.

Hardly the drug lords and criminals of Trumpian myth, most Mexican climate migrants are struggling rural people (not unlike Trump’s own supporters). They depend on predictable weather to grow the crops they eat. When they know the rain isn’t coming, or their home has been destroyed in a flood, or some other climate-fueled event has upended their lives, families face limited options: starve or move.

Right now, the primary factors driving Mexicans to migrate to the United States are better economic prospects or family connections — both of which can be twisted up with climate change. But in Mexico and around the world, climate changes tend to reshuffle populations within borders, too. Droughts and floods, in particular, often trigger shorter-distance moves, largely from rural areas into cities.