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 — like its arid neighbors in the Horn of Africa, where Secretary of State Rex W. Tillerson paid a visit last week, including a stop in Nairobi — has become measurably drier and hotter, and scientists are finding the fingerprints of global warming.

According to recent research, the region has dried faster in the 20th century than at any time during the last 2,000 years. Four severe droughts have walloped the area in the last two decades, a rapid succession that has pushed millions of the world’s poorest to the edge of survival.

Amid this new normal, people long hounded by poverty and strife have found themselves on the front line of a new crisis: climate change. More than 650,000 children under age 5 across vast stretches of Kenya, Somalia and Ethiopia are severely malnourished. The risk of famine stalks people in all three countries; at least 12 million people rely on food aid, according to the United Nations.