Climatic and environmental change in Africa during the last two centuries

Climatic and environmental changes in Africa during the last 2 centuries ha ve been examined, using both systematic rainfall records and proxy informat ion concerning lakes and rivers and the occurrence of famine and drought. T he rainfall records provide excellent detail for the 20th century. The prox y data have been used to produce a semi-quantitative data set spanning most of the continent and having an annual time resolution. These provide an ov erview of conditions during the 19th century. Various issues related to the causes of these 2 centuries of variability are also considered: atmospheri c and oceanic processes, desertification, surface albedo, mineral dust and hydrological feedbacks. The most significant climatic change that has occur red has been a long-term reduction in rainfall in the semi-arid regions of West Africa. This was on the order of 20 to 40 % in parts of the Sahel, The re have been 3 decades of protracted aridity. Nearly all of Africa has been affected by increased aridity, particularly since the 1980s. Few changes i n temperature have been demonstrated. These have occurred on a much smaller scale and are of considerably lower magnitude than those over the continen ts. The rainfall conditions over Africa during the last 2 to 3 decades are not unprecedented. A similar dry episode prevailed during most of the first half of the 19th century. By mid-century, conditions more typical of the ' normal' for the current century again prevailed. Thus, the 3 decades of dry conditions evidenced in the Sahel are not in themselves evidence of irreve rsible global change. On the other hand, the processes controlling rainfall over most of the continent are now reasonably well understood. One of the most important factors, particularly in the Sahel, is sea-surface temperatu res. It has been hypothesized that anthropogenic changes in the land surfac e, particularly land use change and desertification, have contributed signi ficantly to the decline in rainfall. Current evidence suggests that if chan ges in the land surface (e.g., vegetation cover, surface albedo, soil moist ure) significantly impact climate, they are much more strongly controlled b y natural climate variations, such as the recent decline in rainfall, than by human-induced land-use change or degradation. Unfortunately, we still do not have any accurate large-scale assessments of the extent, nature and de gree of such changes. The dreaded 'desertification' process appears to be c onfined to relatively small scales. However, there is still cause for conce rn because the net effect of the various feedback processes involved in lan d degradation appears to be destabilization of ecosystems. Thus, a priority must be large-scale monitoring of the land surface and estimates of the de gree of change.