For the past 21 years, researchers have been studying the migration patterns of people in Pakistan. (Similar studies are done in America—that's how we know that most emigrants from New York are going to Florida.) Migration data in hand, the scientists, led by the International Food Policy Research Institute's Valerie Mueller, measured the relationship between Pakistanis' movements and changes in a handful of environmental variables, from the quantity and timing of rainfall, to temperature, the strength of the annual monsoon and the occurrence of floods.
Instead, they found, high temperatures, particularly during the spring and winter farming season, were the dominant driver of mass migration. It's not that it suddenly became too hot for people to live. But as temperature and weather patterns change, previously productive ground may become uneconomical to work. High heat wipes out the farming economy, the researchers suggest, causing Pakistani men to pack up and leave for greener pastures.
“Thus, we are left with an overall picture that heat stress—not high rainfall, flooding, or moisture—is most strongly associated with migration. The risk of a male, non-migrant moving out of the village is 11 times more likely when exposed to temperature values in the fourth quartile,” they wrote.
The failure of the farm and the exodus that follows, the scientists say, sends a rippling shock through the rest of the economy as people stop buying and start leaving.