Many climate change studies have already been performed, focussing on Africa in general and the Sahel region in particular. These studies use complex Global or Regional Climate Models to project climate change. Global Climate or Circulation Models (GCMs) predict a further warming of Africa in the 21st century, varying between 0.2 and 0.5°C per decade. The warming is expected to be even greater in the Sahel. Consequently, the temperature may rise another 2-7°C in the next 80 years. GCMs also provide projections of rainfall. However, rainfall projections are not heterogeneous over the African continent and are driven by complex interrelations between global atmospheric circulation patterns, sea surface temperatures, temperature gradients between land masses and oceans or tropical and subtropicl oceans etc.
The Potsdam Institute for Climate Impact Research (PIK) investigated recently climate change projections in the Upper Niger Basin (UNB), which is located in a region where projections, particularly for precipitation, are rather uncertain. They used four representative CMIP5 GCMs, representing a good range of divergent projections of climate change. To understand the sometimes contradicting projections in the UNB, these should be seen in the context of changes projected for the entire African continent.
Predicted relative change (%) in annual rainfall in Africa (2070-2099) according to four global climate models. Upper Niger Basin is indicated. Changes are relative to the reference period 1971–2000. Source Liersch et al. 2017.
All model projections show some comparable large-scale features, such as the drying trends in northern and southern Africa and increasing precipitation in the east, particularly in the Horn of Africa, over the Sahara and the northern Sahelian regions. Note that the changes are in relative terms and an increase of precipitation by 50% in extremely dry regions, like the Sahara, might not be very meaningful. Three of the four models (Fig. 1) show quite similar change patterns of rainfall but differ from the GFDL model, which shows particularly in western Africa and the Sahara and Sahelian regions some different patterns. The GFDL model projects wetter conditions in coastal west Africa including the western regions of the UNB. One reason for this contradiction is that the UNB is located in a climatic transition zone and has a share in the tropical savanna climate in the south-west, the warm semiarid climate in its centre, and the warm desert climate in the very north of the catchment. An adequate spatial representation of regional climate characteristics in such transition zones is extremely challenging for global climate models. Therefore, uncertainties of future climate projections are particularly high in this region.
The main message from these models is that the projected changes in annual rainfall are in the range of natural variability in the past. However, also air temperature influences the water cycle and its availability. All climate models agree on an increasing trend of air temperature and this has a direct impact on evapotranspiration and temperature stress on the vegetation. Therefore, wet or dry periods in the past are not automatically comparable with dry or wet periods in the future
The development of precipitation and temperature anomalies over the entire simulation period from 1971–2099 shows Fig. 2. The zero line represents the average of the reference period (1971-2000) and the polygons indicate the range of the model ensemble. The annual anomalies of the model ensemble were computed by subtracting the annual mean in the reference period from projected annual values.
In line with the contradicting global models, rainfall projections for the future are uncertain and highly dynamic, comparable to the situation in the past. But the global models show a high agreement in rising air temperature, as mentioned in the introduction of this Section.
Simulated annual rainfall (mm, left) and change in temperature between 1970 and 2100 according to two global climate models. Source Liersch et al. 2017.
Sources and more information :
Hulme M. 2001. Climatic perspectives on Sahelian desiccation: 1973-1998. Global Environmental Change 11:19-29.
Liersch, S, S. Fournet, H. Koch 2017. Assessment of Climate Change and Water Management Impacts on the Water Resources in the Upper Niger and Bani River basins. Potsdam Institute for Climate Impact Research, Potsdam. Draft, 9 June 2017.
Zwarts, L. 2010 Will the Inner Niger Delta shrivel up due to climate change and water use upstream? A&W rapport 1537 Altenburg & Wymenga ecologisch onderzoek, Feanwâlden.