Climate change could cause up to 6.7 million Mexicans to migrate to the United States over the next 70 years beyond those who would come to this country for other reasons, a new study predicted Monday.

The study, by Princeton University researchers, based that on a second forecast: that warmer, drier weather will reduce Mexican crop yields by 10 percent to 48 percent by 2080. That alone would cause between 1.4 million and 6.7 million Mexicans to move to the U.S., the study said.

Depending on how much crop yields are reduced, the resulting migration would reduce Mexico's population between the ages of 15 and 65 by 2 percent to 10 percent, the study said. It was published in the Proceedings of the National Academy of Sciences. The study was based in part on the researchers' conclusion that a drought in the 1990s helped to trigger a flood of Mexican immigration into this country from 1995 to 2005.

Currently, about 12.5 million Mexican immigrants live in the United States, says the Pew Hispanic Center, a nonprofit research center based in Washington, D.C.

The U.S. Census Bureau, the United Nations and the Mexican government have predicted, without discussing climate-change issues, that another 12 million to 15 million Mexican residents will emigrate, mostly to the United States, by 2050. Extend those forecasts another 30 years and the projected total migration by 2080 could rise another 7 million to 9 million, said Jeffrey Passel, a senior demographer at Pew.

"Our intention was to show that this problem is a substantial one," said one of the study's authors, Michael Oppenheimer, a Princeton professor of geosciences and international affairs. "Our goal was not to project specific outcomes 80 years from now but to show the magnitude of problems that policymakers ought to pay more attention to. I don't want to say that this will be the single biggest factor driving immigration, but it could become among the largest factors."

However, the study has drawn criticism in the United States and Europe from researchers who say other factors besides climate could account for huge swings in migration - including declining fertility rates among Mexican women.

Because fertility rates have dropped sharply in Mexico since 1970, forecasters predict future Mexico-U.S. migration will range from the low 200,000s to the low 300,000s annually, compared with 500,000 a year from 2005 to 2008.

If that happens, the number of "climate migrants" would be lower than the new study predicted, acknowledged Shuaizang Feng, a co-author and an economist at Princeton and at Shanghai University. The study noted that fertility declines could reduce Mexican immigration, and it said its specific forecasts were based on the theory that other factors will remain equal from now until 2080.

Some outside researchers said that factors other than climate could have caused the increase in Mexican-U.S. migration during the 1990s. Diana Liverman, a University of Arizona climate researcher, criticized the new study for basing its forecasts in part on research that she worked on in the early 1990s that looked at crop yields in only two central Mexico sites.

In reply, Oppenheimer said the Princeton study found similar results in a second crop-yield study, and the crop reductions predicted for Mexico are typical of what has been predicted for other countries in that latitude.

Liverman said that while she believes climate change could cause widespread migration, she has seen no study documenting it. Having studied the problems of Mexican farmers for two decades, she said she has found that a bad economy, the government's withdrawal of agricultural subsidies and the North American Free Trade Agreement have caused problems far greater than climate change.

While disagreeing with the study's specific forecasts, several outside researchers said climate migration should be considered over the coming years.

"The study should be seen for what it is, which is an original and rigorous modeling of the relationship between climate change, agricultural yields and migration," said Ian Goldin, director of the James Martin 21st Century School at the University of Oxford, in Britain.


Contact reporter Tony Davis at or 806-7746.