Princeton study links climate change, crop yields and cross-border migration

July 26, 2010, 6:33 p.m.

Climate change is expected to cause mass human migration, including immigration across international borders, according to a new study by three Princeton University professors and researchers. The researchers -- all from the University's Woodrow Wilson School of Public and International Affairs -- examined the linkages between variations in climate, agricultural yields and people's migration responses.

"Changes in crop yields that result from climate change occur over broad geographical areas and are likely to lead to long-term population shifts," said Shuaizhang Feng, a research associate at the Woodrow Wilson School's Program in Science, Technology and Environmental Policy, and one of the three researchers. "Such a phenomenon is especially relevant to developing countries, which typically have large rural populations that derive a living directly from agriculture."

Along with Feng, the research was conducted by Alan B. Krueger, the Bendheim Professor of Economics and Public Affairs -- who is currently on leave as assistant secretary for economic policy at the U.S. Treasury Department -- and Michael Oppenheimer, the Albert G. Milbank Professor of Geosciences and International Affairs. The study focused on Mexico from 1995 to 2005, a time when a substantial increase in emigration to the United States occurred.

A statistical estimator, a tool that uses only the portion of variations in crop yields across states that is predicted by changes in climate (e.g., temperature and rainfall), was used to estimate the sensitivity of emigration to crop yields. Projections of the effect of climate change on crop yield in the future then were used to estimate future migration flows, assuming all other factors except climate would be unchanged. The researchers estimated that, depending on whether agriculture improvements help farmers adapt to a hotter, drier climate, an additional 1 million to 7 million Mexicans (up to 10 percent of the adult Mexican population) could migrate to the United States in the next 70 years due to climate-induced declining agricultural productivity, if other conditions affecting immigration were otherwise similar to today's.

According to the researchers, no study has directly associated a component of the increase in emigration with changes in climate, despite numerous reports and anecdotes of Mexican farmers fleeing to the United States because they no longer could maintain their previous way of life because of climate-driven crop failure. Their work was published online Monday, July 26, in the Proceedings of the National Academy of Sciences.

"The purpose of the study is to determine if the magnitude of immigration flows due to climate change could be significant, to underscore areas where further research is needed, and to provide insights for policymakers who may need to plan to manage the multitude of problematic outcomes of climate change, such as this one," said Oppenheimer.

Mexico was chosen as the focus of the study because it is one of the biggest migrant-source countries, it has undergone diverse degrees of climate variability across regions, and it has relatively accurate state-level data on emigration. Furthermore, the researchers noted in the published study, "Mexico is unique in that the relative ease of migration to the United States allows us to better capture the full potential emigration response to changes in crop yields than in many other cases."

Krueger said, "The methods that we developed in the study could be applied more generally to understand and project worldwide migration trends over the next 70 years, and to help governments and nongovernment organizations prepare for population inflows and outflows."
The researchers concluded in the published study, that their projections indicate that "certain migrant-receiving countries, including the United States, are expected to face increased migration streams as a result of existing transnational networks with migrant-sending countries that are particularly vulnerable to climate change."