Talk with farmers in Mexico and they will tell you they’re already feeling the brunt of climate change.

Persistent heat and meager rainfall are drying out the land, posing a significant challenge to indigenous small farmer in southern states like Oaxaca, Chiapas, and Quintana Roo. Complicating matters, these farmers, called “campesinos,” often lack access to irrigation, either because they don’t live near lakes, rivers, or other sources of fresh water, or because they can’t afford to build irrigation systems.

This isn’t just a problem for Mexico. These growers are the custodians of rare varieties of maize that may hold the secret to more sustainable agriculture. If they lay down their tools, their crops could begin to vanish.

“The last four years, harvests have been very bad,” said Aldo Gonzalez Rojas, a spokesperson for the Union of Organizations of the Sierra Juarez of Oaxaca, a collection of indigenous Zapotec farmers that is helping campesinos cope with drought, building irrigation systems where possible.

“The rainy season begins later and is much shorter than it used to be,” he said. “The rains don’t start until September. A campesino cannot wait till September to start planting, because the cold in December will kill his maize.”

Campesinos typically grow native varieties of maize, beans, squash, and chili peppers, primarily to feed their own extended families. At least 20 million Mexicans rely on rain-fed maize, which is imperiled by worsening drought, a consequence of climate change. By the end of this century, production of rain-fed maize could drop by as much as 30% in parts of Mexico, according to a 2018 study.

Roberto Cortez Viliegas, a 64-year old campesino from the town of Totontepec in the Sierra Mixe region of Oaxaca, said he has struggled with dwindling rainfall.

“A period of drought in August never used to last longer than two days,” he said “This year, in August, we had 10 days in a row without a drop of rain.” He said that farming rain-fed crops is so precarious that many campesinos have sought other sources of income.

“Today, a campesino also has to do other jobs to feed his family,” Cortez Viliegas said, adding that he sometimes works as a butcher. Others may find work in construction or road repair or, occasionally, as musicians. For many, however, the best option is to emigrate.