Will a hotter climate mean more immigration? In some places, yes, that's quite possible. Earlier this week, a team of researchers led by Princeton's Michael Oppenheimer published a study suggesting that as global warming causes agricultural yields in Mexico to decline, an additional 1.4 million to 6.7 million Mexicans could migrate to the United States by 2080. (The team analyzed data on emigration, crop yields, and climate from 1995 to 2005 in order to make their forecasts.)

As always, caveats abound. The social consequences of global warming are always the hardest things to predict. Immigration rates are never driven by physics alone, but depend on plenty of other factors, such as U.S. border policies or the changing structure of Mexico's economy. And it's always difficult to tie specific social trends to climate change. People in rural areas have been migrating for a long time, whether to seek out work or because the rainfall's dried up or the soil's eroded. Global warming will exacerbate these pressures, yes, but it's hard to attribute any single event—or single migrant—to man-made climate change. That's one reason why forecasts of "climate refugees" vary so wildly.

Still, climate-driven migration is a concept that's received a lot of attention in recent years. As the planet heats up, droughts spread, and sea levels rise, millions of people are going to be uprooted from their homes and farms and move elsewhere. According to a 2007 World Bank report, the vast bulk of this migration is expected to take place within developing countries, with people moving from rural villages to urban centers. One big concern here is that places like Lagos or Dhaka are already swelling exponentially, and their infrastructure can barely keep up, which is why so many "megacities" now sport massive slums.

But there's also likely to be a fair amount of migration between countries—and the consequences there are much harder to predict. As the rising oceans chomp away at Bangladesh, for instance, as many as 15 million people may have to abandon their towns and villages by mid-century. Partly in response, India has been constructing a 2,100-mile long fence to barricade itself against the predicted influx of climate refugees. This old Greenwire piece by Lisa Friedman features a number of national security experts in India openly fretting about how rising seas will destabilize the borders between the two countries.

There are even consequences for Western politics. Over in Europe, a variety of ultra-right-wing nativist groups take these climate-migration forecasts very seriously. In his excellent book Forecast, Stephan Faris talked to members of Britain's BNP, which is trying (unsuccessfully) to forge an alliance with greens. A lot of them rant on about how immigration is terrible for the environment, since a person's carbon footprint swells when he or she moves from a poor country to a rich country. Similarly, in France, Jean-Marie Le Pen's National Front has started hitting on environmental themes of late. Few actual environmentalists want anything to do with these parties, and there doesn't seem to be anything comparable in the United States, though if global warming does put pressure on immigration, it's certainly possible that green nativists could find a toehold here.

(Flickr photo credit: ZenZenOK)