Until the rains failed in Darfur, the region's pastoralists lived amicably with the settled farmers. The nomadic herders grazed their camels on the rocky hillsides between the fertile plots and fed their animals on the leavings from the harvest. But with the land crippled by a decades-long drought, the region was no longer able to support both. Farmers began to fence off their fields, and clashes broke out between sedentary and nomadic tribes. When a rebellion broke out in 2003, it began as a reaction to the Sudanese government's neglect. But although the rebels initially sought a pan-ethnic front against a distant, uncaring regime, the schism between those who opposed the government and those who supported it soon broke along ethnic lines. The camel-herding Arabs--those most envious of the farmers' lands--became Khartoum's staunchest stalwarts.

So what caused the rains to fail? When climate scientists studied the drought, they discovered that rising temperatures in the tropical and southern oceans had combined with cooling in the North Atlantic to disrupt the African monsoons. The roots of the drying in Darfur lay in changes to the global climate.

The crisis has since spilled over into Chad and the Central African Republic. Nomads from Sudan are pushing deep into the Congolese rainforest. Other regions are worried they will be next. When the United Nations Security Council held its first-ever debate on the impact of climate change last year, the Ghanaian representative stood up to declare that he hoped the "repeated alarm" about the threats posed by global warming would "lead to action that is timely, concerted and sustainable." In his country, climate change had expanded the Sahara desert, forcing the country's nomadic cattle herdsmen into agricultural lands. The pastoralists, he said, were buying high-powered assault rifles to defend their animals against angry local farmers.