S ub -S aharan A frica : F ood P roduction at R isk
Schlenker and Lobell (2010) estimated the impacts of climate change on five key African crops, which are among the most important calorie, protein, and fat providers in Sub-Saharan Africa: maize, sorghum, millet, groundnuts, and cassava (rice and wheat are excluded from the study as they are usually irrigated). They estimated country-level yields for the 2050s (2046–65) by obtaining future temperature and precipitation changes from 16 GCMs for the A1B SRES scenario and by applying these future changes to two historical weather data series (1961 to 2000 and 2002, respectively) with regression analysis. In this study, for a 2050s global-mean warming of about 2.2°C above pre-industrial levels the median impacts across Sub-Saharan Africa on the yield of maize, sorghum, millet, groundnut, and cassava40 are projected to be negative, resulting in aggregate changes of –22 percent, –17 percent, –17 percent, –18 percent, and –8 percent. This important work also estimates the probability of yield reductions, which is useful for risk assessments looking at the tales of the probability distribution of likely future changes. It finds a 95-percent probability that the yield change will be greater than –7 percent for maize, sorghum, millet, and groundnut, with a 5-percent probability that damages will exceed 27 percent for these crops.41 The results further indicate that the changes in temperature appear likely to have a much stronger impact on crop yield than projected changes in precipitation. The negative results of this work for sorghum are reinforced by more recent work by Ramirez-Villegas, Jarvis, and Läderach (2011). They find significant negative impacts on sorghum suitability in the western Sahelian region and in Southern Africa in this timeframe, which corresponds to a warming of about 1.5°C above pre-industrial levels globally.42 In interpreting the significance and robustness of these results there are a number of important methodological caveats. It should be kept in mind that the methodological approach of Schlenker & Lobell (2010) does not consider the potential fertilization effect of increased CO2 concentration, which might improve projected results. However, maize, sorghum, and millet are C4 crops with a lower sensitivity to higher levels of CO2 than other crops. The authors also do not take into account any potential future developments in technology, shifts in the growing season as a potential adaptation measure, or potential changes in rainfall distribution within growing seasons (though temperature has been identified as the major driver of changes in crop yield in this study). Further, a potential disadvantage of the panel data used by Schlenker and Lobell (2010) is that responses to permanent changes in climatic conditions might be different compared to responses to weather shocks, which are measured by the observational data. The estimates presented should be assumed as conservative, but relevant as a comparison of predicted impacts on maize yields to previous studies (Schlenker and Lobell 2010). Further evidence of the potential for substantial yield declines in Sub-Saharan Africa comes from a different methodological
approach applied by Berg et al. (2012). Berg et al. assess the potential for impacts on the crop productivity on one of the most important staple foods, a C4 millet cultivar, in a tropical domain, including Africa and India, for the middle (2020–49) and end of the century (2070–99), compared to the 1970–99 baseline. Across both regions and for all climatic zones considered, the overall decline in productivity of millet was –6 percent (with a range of –29 to +11 percent) for the highest levels of warming by the 2080s. Changes in mean annual yield are consistently negative in the equatorial zones and, to a lesser extent, in the temperate zones under both climate change scenarios and both time horizons. A robust long-term decline in yield in the order of 16–19 percent is projected for the equatorial fully humid climate zone (which includes the Guinean region of West Africa, central Africa, and most parts of East Africa) under the SRESA1B scenario (3.6°C above pre-industrial levels globally) and the SRESA2 scenario (4.4°C), respectively, for 2100. Although projected changes for the mid-century are smaller, changes are evident and non-negligible, around 7 percent under the A1B – (2.1°C) and –6 percent under the A2 (1.8°C) scenario for the equatorial fully humid zone. The approach of Berg et al. (2012) accounts for the potential of an atmospheric CO2 effect on C4 crop productivity for the A2 scenario; the projections show that, across all models, the fertilization effect is limited (between 1.6 percent for the equatorial fully humid zone and 6.8 percent for the arid zone). This finding is consistent with the results of prior studies. The yield declines by Berg et al. (2012) are likely to be optimistic in the sense that the approach taken is to estimate effects based on assumptions that are not often achieved in practice: for example, optimal crop management is assumed as well as a positive CO2 fertilization effect. Berg et al. (2012) also point out that the potential to increase yields in Sub-Saharan Africa through improved agricultural practices is substantial and would more than compensate for the potential losses resulting from climate change. When considering annual productivity changes, higher temperatures may facilitate shorter but more frequent crop cycles within a year. If sufficient water is available, no changes in total annual yield would occur, as declining yields per crop cycle are compensated by an increasing number of cycles (Berg et al. 2012). As this much-needed progress has not been seen in past decades, it can be assumed that climate change will represent a serious additional burden for food security in the region.
40 Note that the model fit for cassava is poor because of its weakly defined grow-
ing season. 41 These are damages projected for the period 2045–2065, compared to the
period 1961–2006. 42 The authors use an empirical model (EcoCrop) and analyze the impact of the
SRESA1B scenario driven by 24 general circulation models in the 2030s for sorghum climate suitability.
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