Rainfall is projected to change as the planet warms in response to rising concentrations of atmospheric greenhouse gases. New research indicates future changes in the timing and characteristics of the rainy seasons over Africa with important implications for impacts of climate change on vulnerable societies.
The fourth IPCC (Intergovernmental Panel on Climate Change) Assessment Report (AR4) states that “Africa is one of the most vulnerable continents to climate change and climate variability”1. This is partly related to low capacity for adaptation across much of the continent but is also related to the high dependence on climate, in particular wet season rainfall.
Most of Africa experiences one or two main wet seasons per year, when the majority of the annual rainfall occurs. For the large proportion of the population dependent upon rain-fed agriculture for their income and subsistence, the timing of this wet season and amount of rainfall is of high importance. Furthermore, wet season rainfall also impacts the recharge of reservoirs, the supply of electricity from hydro-power and the lifecycle of vectors (e.g. mosquitoes) responsible for the transmission of diseases such as malaria. It is therefore important to understand how future climate change will affect wet season rainfall across Africa.
Figure 1: Schematic showing changes in timing of the wet season (left) and changes in seasonal rainfall totals (right).
In a recently published paper, 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 (RCP 4.5 and RCP 8.5 scenarios), we produced projections of changes in wet season onset, cessation, length and rainfall totals under future climate change (Figure 1).
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. This 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.
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. In such regions agricultural adaptation (e.g. to alternative crop varieties) may be required.
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 (see paper for full details).
Another important finding is that within the wet season rainfall intensity will increase, whilst frequency will decrease; heavier rainfall 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.
Overall, we found that future climate change will lead to changes in the timing of wet seasons over Africa, with shorter wet seasons over southern Africa. Seasonal rainfall totals are projected to increase over central regions, but decrease over southern Africa. Suitable adaptation may be required; for example, using crop varieties that can cope with a shorter growing season or altering farming practices to minimise soil erosion. Other adaptation may be required to protect crops from increasing intensity of rainfall.