Climbing temperatures are expected to raise sea levels and increase droughts, floods, heat waves and wildfires.
Now, scientists are predicting another consequence of climate change: mass migration to the United States.
Between 1.4 million and 6.7 million Mexicans could migrate to the U.S. by 2080 as climate change reduces crop yields and agricultural production in Mexico, according to a study published online this week in the Proceedings of the National Academy of Sciences. The number could amount to 10% of the current population of Mexicans ages 15 to 65.
"Assuming that the climate projections are correct, gradually over the next several decades heading toward the end of the century, it becomes one of the more important factors in driving Mexicans across the border, all other things being equal," said study author Michael Oppenheimer, professor of geosciences and international affairs at Princeton University.
Of course, Oppenheimer acknowledged, all things will not remain equal. Changes could occur in U.S. immigration and border policy or in Mexico's economy and its reliance on agriculture. But he said this was a simplified first step in studying the effect of global warming on migration.
"Our primary objectives were, No. 1, to give policymakers something to think about and, No. 2, to give researchers a spur to start answering some of the more complicated questions," Oppenheimer said.
Oppenheimer teamed up with two economists, Alan B. Krueger and Shuaizhang Feng, to study the connection between agricultural decline and migration. They looked at Mexican emigration, crop yield and climate data from 1995 to 2005 to make estimates about the next 70 years.
In the past, Oppenheimer said, Mexican farmers from rural areas fled to the United States when they could no longer grow their crops. If the rising temperatures dry out the land and reduce yield as expected, many more farmers could do the same.
Philip Martin, an expert in agricultural economics at UC Davis, said that he hadn't read the study but that making estimates based solely on climate change was virtually impossible.
"It is just awfully hard to separate climate change from the many, many other factors that affect people's decisions whether to stay in agriculture or move," he said.
Over the last 20 years, Mexico has seen a decline in the percentage of people who live in rural areas, Martin said. But much of that is because of economic growth in the nation. "As countries get richer, people leave agriculture," he said.
Nevertheless, Martin agreed that global warming could make farming more difficult and lead to more emigration.
Douglas Massey, a sociology professor at Princeton, also agreed that climate change could lead to emigration from Mexico, but much of that will depend on labor demand in the U.S.
"Environmental change is not going to produce migrants from Mexico unless there are jobs to go to," he said in an e-mail.
According to the study, other countries and regions dependent on agriculture could also see a similar exodus. Among the areas mentioned are much of Africa, India, Bangladesh and Latin America.
But Massey expressed skepticism about generalizing to other countries, because Mexico and the U.S. have a 60-year migrant history and share a 2,000-mile border. Mexico is also well connected to the U.S. labor demand through social networks.
"When economic shocks like climate change hit other developing countries … displacements from the countryside may happen, but they are not likely to produce so many international migrants," Massey said.