Later Wet Seasons with More Intense Rainfall over Africa under Future Climate Change

What is it about?

Rainfall is projected to change as the planet warms in response to rising concentrations of atmospheric greenhouse gases. We use a novel method to investigate changes in the seasonal progression of rainfall across Africa. This methodology determines the start (‘onset’) and end (‘cessation’) of the wet season and is applicable across continental Africa. By applying this methodology to climate model simulations we produced projections of changes in wet season onset, cessation, length and rainfall totals under future climate change and for current trends. The projections show a delay in the onset of the wet season across much of western and southern Africa under future climate change. In regions where there is little change in the end date (‘cessation’, e.g. western Africa) or regions where the end of the wet season gets earlier (e.g. southern Africa) a later onset means the wet season shortens. Across much of central Africa projections suggest that the amount of rainfall occurring during the wet season will increase. However, over southern Africa projections show lower rainfall totals. The Horn of Africa (Somalia, southern Ethiopia, Kenya and Uganda) and equatorial regions experience two wet seasons per year; one in the Northern Hemisphere spring (known as the long rains) and one in the autumn (known as the short rains). Our results show the long rains ending earlier, and the short rains ending later. The most notable result, however, is the large increase in the amount of rainfall occurring during the short rains. We linked these changes in timing of the progression of the seasonal rainfall with changes in the summer heat low over the Saharan Desert. Another important finding is that within the wet season rainfall intensity will increase, whilst frequency will decrease.

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Why is it important?

Our research shows future changes in the timing and characteristics of the rainy seasons over Africa with important implications for impacts of climate change on vulnerable societies. A shorter wet season (e.g. southern Africa) may be problematic for crops, as a shorter wet season can lead to a shorter growing season and result in crops not reaching full maturity. In regions of reduced rainfall, agricultural adaptation (e.g. to alternative crop varieties) may be required. We also found projections for heavier rainfall in the future which is damaging to crops with delicate flowers such as coffee and cocoa. In addition, long dry periods can reduce soil moisture and harden the surface layer; thus, when heavy rainfall events do occur a smaller fraction infiltrates into the soil and increased runoff leads to soil erosion and flooding.