Community-scale rainfall extremes influence undocumented migration patterns between Mexico and the United States, according to research published in the Proceedings of the National Academy of Sciences of the United States of America. The findings confirm previous studies that showed a link between extreme weather and migration to the United States and show that extreme weather affects migration back to Mexico, too.
“We see a lot of fluctuations in migration from Mexico, and the climate story is a part of that.”
“We see a lot of fluctuations in migration from Mexico, and the climate story is a part of that,” said Filiz Garip, a sociologist at Princeton University and a coauthor on the new study.
Garip and her colleagues used estimates of daily rainfall and precipitation on a 1-square-kilometer scale throughout Mexico to determine which years had normal and anomalous weather. They combined these data with annual surveys of households in agricultural communities in Mexico, which detailed information about when and where individuals migrated each year between 1992 and 2018. Those data came from the Mexican Migration Project (MMP).
The researchers used the data to estimate two things: the link between extreme weather in Mexican communities and migration from Mexico to the United States and the link between extreme weather in a person’s origin community and return migration to Mexico.
The researchers analyzed weather patterns only during the local corn growing season (May to August) because the impacts weather can have on crop growth can cause economic stress that spurs migration. The study also focused on only undocumented migrants, who experience the greatest danger from border crossings, Garip said. “It’s really important to study this group, and our data is unique in capturing this group,” she said.
Seasonal rainfall extremes—defined as seasonal rainfall more or less than twice the average deviation from the 1980–1990 mean—affected movement both out of Mexico and back. Extremely dry conditions in an individual’s home community increased the likelihood they would migrate from Mexico to the United States, whereas extremely wet and extremely dry seasons significantly lowered an individual’s likelihood of returning to Mexico in their second year of residing in the United States. Wet seasons, between 1 and 2 times the average rainfall deviation, also lowered the likelihood of return migration.
Temperature extremes did not play a significant role in migration patterns. That may be because the researcher’s calculations had a very stringent definition of “extreme heat,” Garip said. These events are very rare, so there were not enough instances for the researchers to capture their effects. Another possibility is that extreme heat does not affect crops, and therefore the economic health of a community, as starkly as rainfall extremes do.
Return Migration
Return migration is an “important but under-studied aspect” of climate migration research, Alex de Sherbinin, a geographer at Columbia University who was not involved in the new study, wrote in an email.
One “key question” in migration studies is how changes in migration patterns can become permanent, said David Wrathall, a geographer at Oregon State University who was not involved in the new study. The new study’s results on return migration indicate that weather deviations drive changes to the permanence of migration between Mexico and the United States, he said—extreme drought causes people to stay in the United States rather than returning.
“They’re helping us see this migration permanence question a little bit more clearly,” he said.
The results confirm previous findings that weather stress in agricultural areas can lead to increased migration from those areas, according to Hélène Benveniste, an environmental and social scientist at Stanford University. Benveniste was not involved in the new research but is currently working on another project with Garip using the same dataset.
The weather impacts were concentrated in communities without irrigation systems, which are less resilient when drought hits. For example, in one such community in Mexico, the share of the population who migrated was less than 2% each year until 2002, when extremely dry conditions occurred. In 2003, 14% of those living in the community decided to migrate.
Garip said the findings regarding irrigation show that there are things communities can do to give people multiple options when extreme weather occurs, such as installing irrigation systems. Making home communities more prosperous might help people avoid dangerous border crossings, she explained.
“We’re going to have to help people stabilize their livelihoods in drought-affected areas,” Wrathall said.
Data Decisions
Though previous studies have linked weather extremes to migration patterns between the two countries, the new research uses more spatially granular migration data, which the researchers were able to couple with highly local weather data, Benveniste said. “The MMP data is highly valuable in that regard,” she wrote.
“We came to learn that the scale you use really, really matters.”
The results also show how useful small-scale weather data can be in understanding migration, Garip said. “We came to learn that the scale you use really, really matters.”
Garip added that the some of the weather data are interpolated because weather stations do not evenly cover communities in Mexico. Even more granular data, as well as a better understanding of conditions that affect crops, such as measurements of soil moisture and weather variability within a season, could give an even more accurate picture of how migration decisions are made, she said.
Such collaboration between geoscientists and social scientists may be especially useful as the climate warms because climate-induced weather changes may continue to spur migration through dangerous crossings, the authors wrote in the study. “We should, across the sciences, become more knowledgeable about how to measure the changes that we’re experiencing,” Garip said.
—Grace van Deelen (@GVD__), Staff Writer
Citation: van Deelen, G. (2024), Weather extremes influence human migration between Mexico and the United States, Eos, 105, https://doi.org/10.1029/2024EO240504. Published on 4 November 2024.
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