Introduction: The 'Greening' of the Sahel

摘要

This special issue of the Journal of Arid Environments contains papers presented by a panel of experts at a workshop on "Changes in the Sahel". The workshop was organized by the International Center for Remote Sensing of Environment on behalf of the United Nations Environment Programme, Food and Agriculture Organization of the United Nations, and United Nations Convention for Combatting Desertification. It was held in Nairobi, Kenya, October 14-16, 2003, prompted by a desire to clarify questions that had arisen regarding conditions in the Sahel. For the last four decades there has been sustained scientific interest in contemporary environmental change in the Sahel. It suffered several devastating droughts and famines between the late 1960s and early 1990s. Speculation about the climatology of these droughts is unresolved, as is speculation about the effects of land clearance on rainfall and about land degradation in this zone. Despite this history of presumed deterioration, recent findings suggest a consistent trend of increasing vegetation "greenness" in much of the region. Time series data from satellite have been analyzed for the African Sahel to study recent trends in vegetation greenness (see Olsson et al., Anyamba and Tucker, this issue). A strong increase in seasonal greenness was observed over large areas of the Sahel during the period 1982-1999. Preliminary studies indicate a continuation of the trend through 2003. Analyses of rainfall data indicate increasing rainfall during the same period, to levels that have not been experienced in some time (see Nicholson, this issue). However, the greening trend cannot be explained solely by rainfall. While extensive, the greening is not uniform, suggesting that factors other than rainfall may be contributing to greening of some areas and not others. In addition, the resolution of the satellite data set is coarse (8 km). So, the pattern of greening that might help explain its causes may be partly obscured by the resolution of the data.