In Mexico, the conditions that have contributed to the largest sustained movement of humans across any border in the world will get only more common. This spring, at the start of the corn-growing season, 76 percent of Mexico was in drought, and the country was sweltering under a deadly heat dome. Finally, after too many months, summer rains started to refill reservoirs. But years and droughts like this promise to become more intense: Mexico is slated to warm 1 to 3 degrees Celsius by 2060.

When drought strikes rural corn farmers in Mexico during the growing season, they are more likely to attempt to immigrate to the United States the following year out of economic desperation, according to a study released this month in the journal PNAS. This is just the latest example of a signal in migration data that keeps getting clearer: Climate change is pushing people to cross borders, and especially the southern border of the United States. Many live on the edge of financial stability; if one of their few options to support themselves is jeopardized, they might not recover. “And climate extremes are taking away whatever option there is there,” one of the study’s co-authors, Filiz Garip, a professor of sociology and public affairs at Princeton University, told me.

Donald Trump and his incoming administration have said that limiting immigration into the United States is a priority; the president-elect intends to both close the southern border and deploy the military in order to carry out mass deportations. He is also poised to ignore the climate altogether, and likely hasten the pace of change with policies that increase oil and gas drilling. That combination is “sort of like turning the heat up on a boiling pot and then forcing the lid shut,” Ama Francis, a lawyer and the climate director of the International Refugee Assistance Project (IRAP), told me. Drought and other climate disasters will help propel more people north; U.S. immigration policies will attempt to block them, but migrants won’t stop coming. Part of the argument for dealing with climate change, and doing so in partnership with the rest of the world, is that it will mitigate these sorts of pressures before they become even more dramatic conflicts. The next administration could be setting the country up for the opposite.

Climate isn’t usually the only factor that drives people to move, but it can be a tipping point that clinches their decision. Like many places in the world, Mexico is becoming a harder place to live because of both drought and extreme rainfall, which leads to flooding. These are particular challenges for rural farmers whose crop depends on the seasons progressing as they have for hundreds of years. More may make the desperate choice to leave. And more who have left may stay for longer in the United States. Garip’s study found that climate extremes will delay migrants from returning to their communities. “I was really taken aback by how strong the return results were,” she said. “These weather extremes continue to shape, it seems, how people think about whether to remain a migrant or whether to go back to their communities.”

Climate factors are not what many immigrants first cite as a reason for leaving their home. Violence and racial or political persecution will often come up before drought, for example. But start talking through the deeper roots, and in many cases, “climate-related factors do come up,” Alexander de Sherbinin, an expert on climate and human migration at Columbia University, told me. Francis’s organization, IRAP, which gives migrants legal support, recently co-published a report based on interviews with more than 3,000 clients, nearly half of whom had experienced a climate disaster in their home country before leaving. The most common of these was extreme rainfall, followed closely by extreme heat.

Even when demographers control for other characteristics in a person’s life, climate change still emerges as a statistically significant factor of migration, says Lori Hunter, the director of the Institute of Behavioral Science at the University of Colorado at Boulder, who has studied migration data for decades. The pattern is clear, Hunter told me: “If we disinvest from the climate, the pressure to migrate will intensify.”

Conversely, a certain subset of the potential immigrant population, if their climate desperation could be alleviated, may not choose to come to the United States. In the long term, dramatically lowering the U.S.’s emissions would help limit climate stresses, but the warming the world has already experienced is driving weather extremes right now. Adapting to new climatic normals is now necessary. Migration is one way of adapting. But people could, with assistance, adapt in place. Among the corn farmers Garip and her colleagues studied, those who had access to some form of irrigation infrastructure, such as a reservoir, were less likely to leave, even when faced with drought conditions. It was mostly rural, smallholder farmers entirely dependent on rainfall who decided to make the perilous trek north. With investment for projects to install irrigation in those communities, “these decisions could really be different,” Garip said. “Unless we do something, then we’re just pushing more people into this dangerous journey.”

Indeed, the biggest topic at the global COP29 climate negotiations, under way in Baku, Azerbaijan, is the dollar amount that developed countries, responsible for the majority of historical emissions, will transfer to developing countries, which are bearing the brunt of the climate crisis and require at least $1 trillion of outside funding per year to build more renewable energy and respond to climate-driven disasters. Many at COP assume that the U.S. won’t contribute to those funds at all, and the meeting, now at its halfway point, is by all accounts at a deadlock, with little leadership from wealthy countries materializing. The Biden administration had plans to fund $3 billion worth of climate adaptation internationally each year, with a special focus on water security—and explicitly framed that as a tool to “address key drivers of migration.” Those plans are unlikely to continue into the next Trump presidency.

Climate finance is a nebulous category, and a lack of transparency about how the funds get spent can undermine the process. But other research has found that remittances—money that migrants send home—tend to be spent on things that improve climate resilience, such as air-conditioning. To Hunter, that remittance data suggest that international climate finance could be spent in ways that would help people adapt to climate change where they live, and remove one of the factors that force them to leave. If a motivated government made a real effort to supply that funding in the first place, perhaps those communities would not feel that they had to send a family member north. It wouldn’t stop migration altogether, but it could help reduce the pressures the incoming Trump administration is so eager to address.