Direct and indirect effects of CO2 increase on crop yield in West Africa

Climate change directly threatens food security in West Africa through a negative impact on productivity of the main staple food crops. However, providing consistent future crop yield projections in the region remain challenging because of uncertainty in the response of the regional climate to the CO 2 increase and in the response of the cultivated crop to this altered climate with more CO 2 in the atmosphere. Here, we analyse a set of idealised climate simulations to investigate the effect of CO 2 concentration increase on the West African monsoon and potential impacts on crop yields of maize. On the one hand, simulations with prescribed SST and quadrupled CO 2 concentration are analysed to study the atmospheric response to direct radiative forcing induced by increasing CO 2 concentration, not mediated by ocean heat capacity. On the other hand, simulations with prescribed SST augmented by 4 K are analysed to study the atmospheric response to the global ocean warming expected as a consequence of the increasing CO 2 radiative forcing. We show that if CO 2 concentration increase has a positive impact on crop yield due to the fertilisation effect, it also has a direct effect on the monsoon which acts to increase (decrease) rainfall in the Eastern (Western) part of the Sahel and increase (decrease) crop yields consequently. Finally, we show that SST warming acts to reduce rainfall and increase local temperatures leading to strong reduction of crop yield. The reduction of crop yield is more important in the Eastern part of the Sahel where the warming is more intense than in the Western part of the Sahel. Overall, positive effects are weaker and more uncertain than the negative effects in the analysed simulations.