Recent historical climate change and its effect on land use in the eastern part of West Africa

There are indications that low rainfall, drought periods and famine become more frequent in West Africa. This may in part be a rather early expression of the effect of global warming but it is very likely that local factors such as drastic changes in land cover due to expanded cultivated area, as required by a growing population, play an important role. Before studying the causative mechanisms of climate change, it first needs to be established that climate did indeed change significantly, and if so, its nature, extent and magnitude have to be quantified. To this effect time series of annual rainfall (1950-1992) for 42 synoptic climate stations in eastern West Africa, covering 5 countries, were analyzed. The data were subjected to several statistical tests for the entire time series and parts thereof. The outcomes were interpolated in a GIS environment to assess the spatial pattern of change. The time series and spatial analysis reveals that climate change is indeed significant in the northern part of the study area and that the degree of change shows a spatial pattern that can be related to the weather system in combination with topography. A remarkable feature is that the change in rainfall is not a gradual one, but consists of a trend break with zero trend before and after the break. This is unexpected because it might imply that causal factors have to be sought under those that do not change gradually. The year of occurrence of the break is around 1970 but varies in time, again according to a geographic pattern. The reduction of rainfall shortens the length of the growing period (LGP) and has a considerable impact on potential crop yields and their variability. The paper shows the serious implications of recent historical climate change for land use in the semi-arid region of West Africa.