KAKUMA, Kenya - These barren plains of sand and stone have always known lean times: times when the rivers run dry and the cows wither day by day, until their bones are scattered under the acacia trees. But the lean times have always been followed by normal times, when it rains enough to rebuild herds, repay debts, give milk to the children and eat meat a few times each week. Times are changing, though. Northern Kenya has become measurably drier and hotter, and scientists are finding the fingerprints of global warming. According to recent research, the region dried faster in...