NEAR THE SOUTHERN EDGE of the Sahara desert, millions of people in some 20 West and Central African countries share a way of life. Year after year, they take part in the ancient practice of nomadic pastoralism, herding livestock across increasingly hot, arid lands of the Sahel region in search of fresh grasslands and accessible water as the seasons change. From the western coast of Senegal to the eastern shorelines of Sudan, herders follow different migratory routes that can cover hundreds of miles.