A Climate Change Threshold for Forest Dieback in the African Sahel

Increases in human greenhouse gas emissions to the atmosphere have increased global sea surface temperatures. Reinforced by a reduction in vegetation cover in the African Sahel, warmer sea surface temperatures have reduced rainfall in the Sahel by up to 30% in the 20th Century. In Senegal, annual precipitation fell to below one standard deviation of the 148 year mean for 5 years in the period 1968-1973. Although the region had experienced high historic variability in precipitation, the 1968-1973 drought crossed a climate threshold for agriculture that caused famine and human death. Sahel, Sudan, and Guinean ecosystems also crossed a climate threshold of aridity in an abrupt, nonlinear manner. The long-term decrease in precipitation caused extensive forest dieback and a latitudinal shift of the Sahel, Sudan, and Guinean ecological zones. The range of xeric forest species has expanded and mesic species have retracted southward towards areas of higher precipitation. Field inventories of tree species richness show declines in local biodiversity across the Sahel. Analyses of 1954 and 1989 aerial photographs and 2002 1-meter resolution IKONOS satellite images of three 200 km2 areas in Senegal and Mauritania also show declines in the density of trees of height > 3 m. Forest dieback fuels three positive feedback mechanisms: reduction of the evapotranspiration inputs necessary for the northward advance of the summer monsoon rains that sustain vegetation and forestall desertification, increases in the greenhouse gas emissions that cause the reduction in rainfall, and reduction of the forest biodiversity that strengthens ecosystem resilience to long-term drought. The interaction of climate change, desertification, and loss of biodiversity, as well as the complex social, economic, and political factors that lead to forest dieback and other ecological changes in the Sahel present difficulties in monitoring and foreseeing future threshold behavior. Nevertheless, any reduction in greenhouse gas emissions and any increase in vegetation cover will reduce the risk of future catastrophic ecological change in the Sahel.