Increasing manmade air pollution likely to reduce rainfall in southern West Africa

Abstract. Southern West Africa has one of the fastest growing populations worldwide. This has led to a higher water demand and lower air quality. Over the last three decades, most of the region has experienced decreasing rainfall during the little dry season (LDS, mid-July to end of August) and more recently also during the second rainy season (SRS, September–October), while trends during the first rainy season (FRS, mid-May to mid-July) are insignificant. Here we use spatio-temporal variations of precipitation, radiation, cloud and visibility observations from surface stations and from space to investigate whether the increased pollution has contributed to suppressing rainfall by dimming incoming solar radiation. To isolate a potential aerosol influence, a multi-linear regression model based on sea-surface temperature (SST) indices is used. During both LDS and SRS weakly statistically significant but accelerating negative rainfall trends unrelated to known climatic factors are found. These are accompanied by a strong increase of pollution over the upstream tropical Atlantic caused by fire aerosol from Central Africa, particularly during the LDS. Over southern West Africa, where no long-term aerosol records are available, significant decreases in horizontal visibility and incoming surface solar radiation are consistent with the hypothesized pollution impact. The latter trend is further enhanced by an increase in low-level cloudiness. The larger spread of potentially aerosol-related effects during the LDS is consistent with the stronger monsoon flow and less wet deposition. Negligible aerosol impacts during the FRS are likely due to the high degree of convective organization, which makes rainfall generation less sensitive to surface radiation. The overall coherent picture and the accelerating trends – some of which concealed by SST effects – should alarm policymakers in West Africa to prevent a further increase in air pollution, as this could endanger water supply, and food and energy production for a large and growing population.