Extreme weather contributes to undocumented migration and return between Mexico and the United States, suggesting that more migrants may risk their lives crossing the border as climate change fuels droughts, storms and other hardships, according to a new study.

People from agricultural areas in Mexico were more likely to cross the border illegally after drought and were less likely to return to their original communities as extreme weather continued, according to research this week in the journal Proceedings of the National Academy of Sciences.

Across the world, climate change – caused by the burning of fossil fuels such as coal and natural gas – is exacerbating extreme weather. Droughts are longer and drier, the heat is deadlier, and storms are rapidly intensifying, dumping record-breaking rain.

In Mexico, a country of nearly 130 million people, the drought has drained reservoirs dry, creating severe water shortages and drastically reduced corn productionthreatens livelihoods.

Researchers said Mexico is a remarkable country to study the links between migration, return and weather stressors. Its average annual temperature is expected to rise up to 3 degrees Celsius (5.4 degrees Fahrenheit) by 2060, and extreme weather is likely to economically devastate rural communities that depend on rain-fed agriculture. The United States and Mexico also have the largest international migration flow in the world.

Scientists predict that migration will grow as the planet warms. Over the next 30 years, 143 million people worldwide are likely to be uprooted by rising seas, droughts, scorching temperatures and other climate disasters, according to a report by the UN’s Intergovernmental Panel on Climate Change.

The new migration research comes as Republican Donald Trump was re-elected to the US presidency this week. Trump has called climate change a “hoax” and promised mass deportations of an estimated 11 million people in the US illegally.

Researchers said their findings highlight how extreme weather drives migration.

Filiz Garip, a study researcher and professor of sociology and international affairs at Princeton University, said advanced countries have contributed much more to climate change than developing countries, which bear the burden.

Migration “is not a decision that people take lightly … and yet they are forced to do it more, and they are forced to stay longer in the United States” as a result of extreme weather conditions, Garip said.

The researchers analyzed daily weather data along with survey responses from 48,313 people between 1992 and 2018, focusing on about 3,700 individuals who crossed the border without documents for the first time.

They looked at 84 agricultural communities in Mexico where the cultivation of corn depended on the weather. They correlated a person’s decision to migrate and then return with abnormal changes in temperature and precipitation in their communities of origin during the May-to-August corn growing season.

The study found that communities experiencing drought had higher migration rates compared to communities with normal rainfall. And people were less likely to return to Mexico from the United States when their communities were unusually dry or wet. This was true for recent arrivals to the United States and people who had been there longer.

People who were better off financially were also more likely to migrate. There were also people from communities with an established history of migration where friends, neighbors or family members who had previously migrated could offer information and help.

These social and economic factors influencing migration are well known, but Garip said the study’s findings underscore the inequities in climate adaptation. With extreme weather events, not everyone is affected or reacts in the same way, she said, “and the typical social and economic advantages or disadvantages also shape how people experience these events.”

For Kerilyn Schewel, co-director of Duke University’s Program on Climate, Resilience and Mobility, the economic factors highlight that some of the most vulnerable people are not those displaced by climate extremes, but rather are “trapped in place or lacking the resources to move. “

Schewel, who was not involved in the study, said that analyzing regions with migration histories can help predict where migrants come from and who is more likely to migrate due to climate shocks. In “places where people are already leaving, where there is a high level of migration prevalence, … that’s where we can expect more people to leave in the future,” she said.

The survey data used from the Mexican Migration Project makes this study unique, according to Hélène Benveniste, a professor in Stanford University’s Department of Environmental Sciences. Community-specific migration data of its scale is “rarely available,” she said in an email. So is information about a person’s entire migration journey, including their return.

The finding that decisions about return migration were delayed by weather stress in indigenous communities is “important and novel,” said Benveniste, who studies climate-related human migration and was not involved in the study. “Few data sets allow for an analysis of this question.”

But increased monitoring and enforcement along the U.S.-Mexico border is making it harder to return home—and to move back and forth—says Michael Méndez, assistant professor of environmental policy and planning at the University of California, Irvine. And once undocumented migrants are in the U.S., they often live in dilapidated housing, lack health care or work in industries such as construction or agriculture that leave them vulnerable to other climate impacts, he said. Méndez was not involved in the study.

As climate change threatens social, political and economic stability around the world, experts said the study highlights the need for global cooperation on migration and climate resilience.

“So much of our focus has kind of been on the border and securing the border,” Duke’s Schewel said. “But we need much more attention to not only the reasons why people leave, but also the demand for immigrant workers within the United States.”