by Buwen Dong
The Sahel, a band of semi-arid region south of the Sahara Desert that stretches from the Atlantic Ocean to the Red Sea, is one of Africa’s most productive crop regions. Mean annual rainfall in the region is on the order of 100 to 200 mm in the north and 500 to 600 mm at its southern limit, with about 90% of annual rainfall falling between July and September, associated with the West African Monsoon. The monsoon rainfall is fundamental for local agricultural production and economy. In the 1970s and 1980s a large part of North Africa suffered from persistent drought, which had devastating impacts on local populations. Between the 1950s and 1980s Sahelian summer rainfall declined by around 40%. Since then rainfall levels have recovered significantly, associated with a spatially coherent pattern of rainfall change across most of North Africa (Figure 1, see animation, Evan et al. 2015, Dong and Sutton 2015). The increase between the recent period 1996-2010 and the drought period 1964-1993 was 0.26 mm day-1, about one third of the decrease that occurred between the 1950s and the drought period. Figure 1. Observed changes in summer rainfall (mm day-1) in the Sahel region of North Africa (JAS = July-August-September). (a) Time series of Sahel rainfall (average over region indicated by red box in b), and (b) spatial pattern of difference between the recent period of 1996-2010 and the drought period of 1964-1993.
Since the drought, several potential drivers of Sahel rainfall have changed. Sea surface temperatures (SSTs) have warmed, particularly in the North Atlantic and Indian Oceans. Greenhouse gas concentrations have increased and there have been significant changes in anthropogenic aerosol precursor emissions, including a significant decline in sulphur dioxide emissions from Europe and North America and a significant increase from Asia. In order to understand the relative importance of these forcing factors on the recent Sahel rainfall recovery, numerical experiments with the atmospheric component of a state of the art global climate model have been performed in a study by Dong and Sutton (2015).
When all these three factors are changed, the model simulates an increase in Sahel rainfall of 0.23 mm day-1, similar to the observed change. The pattern of change (sum of Figure 2a and 2b) shows a coherent anomaly stretching across the Sahel, and is also similar to that observed. When individual forcing factors are changed separately we find, perhaps surprisingly, that the substantial changes in SST have almost no impact on Sahel rainfall (Figure 2a). Most of the signal – three quarters of the change in Sahel rainfall (Figure 2c) – is simulated in response to the increase in greenhouse gas forcing alone, the additional one quarter of the change being a response to the change in anthropogenic aerosol precursor emissions. Thus our results clearly indicate that greenhouse gas forcing has been the dominant factor in the recent recovery of Sahel rainfall. These results suggest that in the short term climate change has had some beneficial effects for this part of Africa. But this certainly doesn’t mean that the long term impacts will be beneficial.
Figure 2. Simulated changes in summer rainfall in the Sahel region of North Africa (JAS = July-August-September). (a) Response to changes in sea surface temperatures (SST), (b) Responses to the changes in greenhouse gas (GHG) concentrations and anthropogenic aerosols (AA), and (c) responses to the changes in greenhouse gas concentrations alone. Thick lines highlight regions where the differences are statistically significant.
Looking forward to the next few decades, greenhouse gas concentrations will continue to rise. Our results suggest that this rise is favourable for sustaining, and potentially amplifying, the recovery of Sahel rainfall. It is also expected that anthropogenic aerosol precursor emissions will decline globally. Large regional variations in emissions mean the impact of this decline is hard to anticipate in detail, but our results suggest that the impact on Sahel rainfall may be less important than that caused by the sustained increase in greenhouse gases.
Evan, A. T. et al., 2015. Water vapor–forced greenhouse warming over the Sahara desert and the recent recovery from the Sahelian drought. J. Climate, 28, 108–123. doi: http://dx.doi.org/10.1175/JCLI-D-14-00039.1