PEOPLE IN OSUKURU, Uganda, say that climate change arrived in 2007. That spring, it rained without stopping. Floods washed away houses, crops, and livestock. By April, hundreds of the newly homeless had taken shelter in a community hall, which was growing increasingly squalid. Alarmed, government officials sent a group of local volunteers trained in public health to visit the hall and talk about basic sanitation.

Among those who came to help was a tall, soft-spoken woman named Constance Okollet. She was stunned by the devastation that the storms had brought to her neighbors, who had always been poor but self-sufficient. They had relied on the predic