At least 9,000 years ago, humans began domesticating corn for the first time near Tehuacan, in the central Mexican state of Puebla, laying the foundation for permanent settlements in the Americas.

But in the past few years, more frequent and more prolonged droughts have forced many farmers in the area to give up corn and other cereals in favor of alternatives requiring less water such as pistachio nuts or cactus.

Agricultural experts predict parts of Mexico will feel the effects of climate change more than many countries, not least because its location between two oceans and straddling the Tropic of Cancer expose it to weather volatility.

Sol Ortiz, director of the agriculture ministry’s climate change group, noted that 75 percent of Mexico’s soil is already considered too dry to cultivate crops. In regions such as Tehuacan, temperatures may rise more than the global average.

“We know there are areas where the increase is going to be greater. That will obviously affect rain patterns, and in turn, agriculture and food security,” Ortiz said.

The area under corn cultivation in Tehuacan decreased 18 percent to about 40,000 hectares between 2015 and last year, a Reuters calculation using statistics by the agriculture ministry shows, outstripping a nationwide decline.

In the five years before that, the area planted with corn had been slowly increasing in Tehuacan.

Nationally, the area under corn cultivation declined 4 percent from 2015 to 7.4 million hectares last year. While factors leading farmers to switch crops are complex, in Tehuacan, farmers and local officials describe a fast-changing climate as a leading cause.

Mexico’s rainy season last year was the driest since 2011, which in turn was one of the driest on record, numbers from the country’s national water agency showed.

Climate change is expected to cause substantial declines in yields of corn globally, especially in the tropics, a 2018 study published in the U.S. Proceedings of the National Academy of Science concluded.

In Tehuacan, conditions are fast changing for the worse. In a field where dried out plants have been lingering in the dust since the last drought, farmer Porfirio Garcia, holding a stunted cob, was struggling to make sense of it all.

Garcia, who has 12 children, half of them working with him on the farm, recalls how one hectare in some years yielded as many as four tonnes (8,800 lbs) of corn In the past five years, he said, with luck it produced 700 kg (1,543 lbs).

“The corn harvest has shrunk because in the months of June, July, August and September there was no rain,” said Garcia, 59, who uses ancestral farming techniques to grow corn, beans and pumpkin, an ancient system called a “milpa.”

“Our lives center on corn, so what do we do without it?”

Eusebio Olmedo, director of rural development, agriculture and livestock in Tehuacan, recalls that it began to get hotter at the turn of the millennium.

Having worked in the department for five years, Olmedo said the area used to be characterized by a “very pleasant, very benevolent” climate.

Last year was the warmest on record in the state of Puebla - where Tehuacan is located - with thermometers reaching an average maximum temperature of 26.8 Celsius (80 F). In 1985, the first year the available state records show, Puebla, registered an average maximum temperature of 24.7 Celsius (76 F).

A 2016 study commissioned by the environment ministry and backed by the U.N. Development Program concluded climate change in Mexico would mean less rain, lower yields for basic grains such as corn, beans and wheat, as well as “unexpected effects on food security.”

“When rain patterns change, agriculture becomes risky,” Olmedo said.

Mexican corn farmers have suffered significant shocks in the past - most notably the arrival of cheap imports from the United States under the NAFTA free trade agreement in the 1990s.

In the north of Mexico, where vast cornfields are irrigated, climate change may initially have little impact, studies show.

But in the south, where the oldest corn strains on earth are grown using traditional methods without irrigation, the changing rain patterns and temperatures are already being felt.

Agricultural consulting group GCMA estimates Mexican corn production will continue to decline in 2020, and that corn imports mainly from the United States will reach a record 18 million tons.

Mexico is now the world’s second-largest corn importer thanks to a reliance on U.S. grain for animal feed. President Andres Manuel Lopez Obrador calls that “a contradiction” and has implemented programs to boost national production.

Garcia, however, chose to diversify into other crops, planting 300 trees of pistachio, a desert plant that can withstand temperatures between minus 10 and 40 degrees Celsius.

Nearby farmer Natalio De Santiago also abandoned the corn that he, his father and his grandfather used to plant for other crops that require less water. Those include maguey, a raw ingredient for mezcal, a Mexican liquor.

“I stopped sowing (corn) because the weather is changing,” said De Santiago, 56. “Now, I plant maguey because it needs less water.”

Wearing a cowboy hat to shield his face from the sun, he said he irrigates 400 maguey plants every month with a liter of water each. When he planted corn, he said, his crops needed four months of rain.

Others in the area gave up agriculture altogether and sold the land to real estate developers.

In an attempt to stop this trend, local authorities developed a bank of native corn seeds more resistant to pests and that need less water.

“We have to adapt to climate change, and these are the best varieties to recover food self-sufficiency,” Olmedo said of the seeds.

Other government measures meant to help farmers adapt to and mitigate the effects of climate change include agricultural insurance, alternative crops and campaigns to reduce agricultural burning.

“It’s very difficult to reverse the tendency to increase CO2 (carbon dioxide) in the atmosphere,” said the agricultural ministry’s Ortiz. “That’s why we’re prioritizing adaptation.”

“Climate change is here to stay,” he added.