But global warming could bring the immigration issue to a boiling point in the coming decades, if a new study holds true.
According a new computer model, a total of nearly seven million additional Mexicans could emigrate to the U.S. by 2080 as a result of reduced crop yields brought about by a hotter, drier climate—assuming other factors influencing immigration remain unchanged.
"The model shows that climate-driven refugees could be a big deal in the future," said study co-author Michael Oppenheimer, an atmospheric scientist at Princeton University in New Jersey.
(Related: "Climate Change Creating Millions of 'Eco Refugees,' UN Warns.")
Using data on Mexican emigration as well as climate and crop yields in 30 Mexican states between 1995 and 2005, Oppenheimer and colleagues created the computer model to predict the effect of climate change on the rate of people crossing the border.
In that ten-year period, 2 percent of the Mexican population emigrated to the U.S. for every 10 percent reduction in crop yield.
Using the model to extrapolate this real-world figure over the next 70 years, the researchers calculated that 1.4 to 6.7 million adult Mexicans—a number roughly equal to 10 percent of Mexico's current adult population—could migrate to the U.S. by 2080.
The research is one of the first attempts by scientists to put hard numbers on how climate change can affect human migration patterns.
"Our study is the first to build a model that can be used for projecting the effects on migration of future climate change," Oppenheimer said.
(Related: "Global Warming Threatens Coffee Collapse in Uganda.")
Global Warming Study "a Simplification"
Though the new global warming study is "original and very interesting," it shouldn't be interpreted as a forecast of what will happen, economist Ian Goldin, who wasn't involved in the project, said via email.
"The [end of the] time range—2080—is a very long time off, and there are many other factors [besides climate change] which may lead to a very different outcome," said Goldin, director of the University of Oxford's James Martin 21st Century School.
Barry Smit, a climate-impact scientist at the University of Guelph in Canada, agreed.
"I wouldn't take these numbers to the bank," said Smit, who also wasn't involved in the research, which is published in this week's issue of the journal Proceedings of the National Academy of Sciences.
To reach their conclusions, the authors had to make some "heroic assumptions," Smit said, such as that the current economic and political situations of the U.S. and Mexico won't change for decades.
Study co-author Oppenheimer acknowledged there are many uncertainties in his team's model. But it's important for scientists to investigate climate change-induced migration quantitatively, he said.
"This is the first time anybody's built a model to do this," Oppenheimer said. "It's a simplification, and there are a lot of assumptions, but it's the start of a learning process. As we learn more, the model will improve, and the numbers will get more reliable."
(Also see "Global Warming Making People More Aggressive?")
U.S. Should Help Mexicans Adapt to Warming?
Despite its limitations, the Mexican-immigration model could help spur governments to start thinking about how they'll deal with so-called eco-migrants created by global warming, the University of Guelph's Smit said.
"The takeaway message for me of this study is that there is indeed a relationship between changes in crop yield and the movement of people," Smit said. "And to the extent that future climate change will introduce more of those stresses on yields, we can expect more pressures on the movement of people."
If the U.S. and other developed nations start thinking about climate change-related immigration now, before it becomes a major problem, they could take steps that would help reduce the amount of immigration in the first place, said Robert McLeman, a geographer who studies climate migration at Canada's University of Ottawa.
Toward this end, developed nations can do a lot to help their poorer neighbors, said McLeman, who wasn't involved in the modeling.
For example, the U.S. could make it easier for Mexican crops to reach U.S. markets, McLeman said. Or the U.S. could help Mexico create new, non-agricultural employment opportunities by encouraging other industries in rural areas.
"One of the things I encourage policymakers to think about is that people don't have to migrate if they have other means of adapting to climate change where they already live," he added.
Emigration is "often a last resort," McLeman said. "The more options you give people, the less 'distress migration' you're likely to encounter."
A real-world example of this is Bangladesh, a country whose coastal inhabitants are currently struggling with climate change-related sea level rises, the University of Guelph's Smit said.
"You might think that this would cause millions and millions of people to move somewhere else, but many of them are adapting," Smit said.
"They're doing things to change their basis of living, such as building floating gardens and harvesting crabs."
No Downside to Preempting Global Warming Immigration?
In some sense, it may not matter whether the study is right or wrong.
The University of Ottawa's McLeman, for example, argues that many of the things the U.S. could do to help Mexico adapt to global warming will also help improve the quality of life for many of Mexico's poor.
"A lot of the things that we could be doing are things that we should be doing anyway," he said.
"Even if it turns out that our future projections about climate change impacts aren't right, it's still a good investment. I don't see any downside to it."