The United Nations forecasts that by 2050 the world’s population will grow by more than two billion people. Half will be born in sub-Saharan Africa, and another 30 percent in South and Southeast Asia. Those regions are also where the effects of climate change—drought, heat waves, extreme weather generally—are expected to hit hardest. Last March the Intergovernmental Panel on Climate Change warned that the world’s food supply is already jeopardized. “In the last 20 years, particularly for rice, wheat, and corn, there has been a slowdown in the growth rate of crop yields,” says Michael Oppenheimer, a climate scientist at Princeton and one of the authors of the IPCC report. “In some areas yields have stopped growing entirely. My personal view is that the breakdown of food systems is the biggest threat of climate change.”