The images of Pakistan's 2010 devastating flood still haunt. Women up to their necks in the waters of the swollen Indus River, carrying children to safety. Men wading through the brown currents with bags of rice on their heads and young ones on their backs. More fierce floods followed with the next monsoon season and then the next, each year with headline-grabbing figures of millions left homeless. Reports from environmental groups as well as global development agencies cited the floods as early examples of the mass migration that global warming could induce. But now a major new study finds that uncharacteristically high temperatures are actually the most serious driver of migration.