Wolf and his colleagues combed through thousands of newspaper clippings and historical accounts of anything having to do with water at international borders. He ranked every event on a scale of -7 to +7, with numbers below zero indicating worsening degrees of conflict, and numbers above zero pointing to increasing degrees of collaboration. “It turns out two-thirds of the time we do anything over water, we cooperate,” Wolf says. “There was a whole rich history that isn’t covered anywhere.”

In a warming world, the fight for water can push nations apart—or bring them together