Rainy Season Failure in West Africa: Projected Changes and Sources of Uncertainty

While total rainfall and mean temperature are the main drivers of productivity for grains cultivated in the Sudanese and Sahelian regions of West Africa–with higher rainfall and cooler temperatures correlating with higher yield–the length of the rainy season provides a binary switch: if steady rains delay too long, plantings fail and the entire season is lost. Previous work has indicated that global warming is projected to reduce early rainfall in parts of West Africa and that rainfall might fall in more episodic downbursts. Both changes have the potential to increase the probability of rainy season failures.

This work investigates the sources of uncertainty in the projected changes in rainy season failures. We use the NCAR Large Ensemble (LENS) to estimate the uncertainty that comes from natural variability and the CMIP5 ensemble to estimate the role of structural (model) uncertainty. We also address uncertainty that derives from the choice of either using a precipitation-based definition of rain onset (as is typically done in the literature), or a definition that explicitly incorporates the effect of temperature and evapotranspiration in exacerbating the effect of dry spells on crops. Finally, we use high-resolution observational datasets of daily rainfall to investigate the spatial scale of the variability and change in rainy season failure and present climate projections in probabilistic terms that can be used by farmers and policy makers to inform their risk assessment and adaptation choices at the spatial scale of their interest.