Global climate change, acting through warmer sea surface temperatures, and desertification, acting through a reduction in vegetation cover, have reduced rainfall over the West African Sahel by up to 30% in the 20th Century. In addition, mean annual temperature in the Sahel increased up to 1ºC in the 20th Century. During the 1968-1973 drought, rainfall fell more than one standard deviation below the long-term mean for up to five consecutive years. The drought reduced agricultural yields so significantly that it contributed to the death of up to a quarter of a million people. The drought also caused forest dieback across West Africa. Field inventories of 135 1-ha plots across 7600 km2 in Senegal and at 14 sites in Burkina Faso, Chad, Mali, Mauritania, and Niger showed decreases in species richness of up to 57% in the period 1945-2000. Analyses of 1954 and 1989 aerial photos in Senegal showed a statistically significant (p < 0.001) decline in tree density of 23 ± 5%. Field inventories of 12 1-ha plots and analyses of 1954 aerial photographs and 2002 1-meter resolution IKONOS satellite images of three 200 km2 areas in Senegal and Mauritania also show forest dieback. Species from Mimosaceae and other xeric families expanded in the north, tracking a concomitant retraction of species from Anacardiaceae, Meliaceae, and other mesic families to the south. Climate change has shifted the Sahel, Sudan, and Guinea ecological zones south 25-30 km towards areas of higher rainfall. The vegetation shift has altered ecosystems, reduced the provision of ecosystem services, and rendered local people more vulnerable to climate change and desertification.