The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

Abstract

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 3 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 analyse spatio-temporal variations in precipitation, aerosol, radiation, cloud, and visibility observations from surface stations and from space to find indications for a potential contribution of anthropogenic air pollution to these rainfall trends. The proposed mechanism is that the dimming of incoming solar radiation by aerosol extinction contributes to reducing vertical instability and thus convective precipitation. To separate a potential aerosol influence from large-scale climatic drivers, a multilinear-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 in pollution over the upstream tropical Atlantic caused by fire aerosol from Central Africa, particularly during the LDS. Over southern West Africa, no long-term aerosol records are available, inhibiting a direct quantification of the local man-made effect. However, significant decreases in horizontal visibility and incoming surface solar radiation are strong indicators for an increasing aerosol burden, in line with the hypothesized pollution impact on rainfall. The radiation trend is further enhanced by an increase in low-level cloudiness. The large spatial extent of potentially aerosol-related trends during the LDS is consistent with the stronger monsoon flow and less wet deposition during this season. Negligible aerosol impacts during the FRS are likely due to the high degree of convective organization, which makes rainfall less sensitive to surface radiation. The overall coherent picture and the accelerating trends – some of which are 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.

Sensitivity of low-level clouds and precipitation to anthropogenic aerosol emission in southern West Africa: a DACCIWA case study

“…Furthermore, the reduction of precipitation in southern West Africa due to the anthropogenic aerosol emitted along the coast, changes the liquid water transported towards the Sahel, and modifies the winds and soil humidity, and as a consequence dust emissions, as shown by Menut et al (2019). The present study adds evidence to the emerging hypothesis that, during the West African monsoon, increasing anthropogenic aerosol pollution in southern West Africa has already caused a precipitation reduction (Pante et al, 2021).…”

Climate models generally underrepresent the warming by Central Africa biomass-burning aerosols over the Southeast Atlantic

“…In both studies, atmospheric heating (BBA solar absorption) and SST anomalies (BBA surface dimming and cloud adjustments) are identified as key processes intrinsically linked to absorbing properties of smoke. These recent (25,26) results underline that insufficient absorption by smoke over the tropical ocean in global models would limit these interactions between BBA and the fast responses of precipitation (drying) over the southern West Africa. In addition, even if the SSA bias appears to be less pronounced over the African continent, the drying effect of absorbing smoke over central Africa (24) would not be fully represented for CMIP6 models with too scattering BBA over the continent (Fig.…”

“…We argue that the underrepresentation of BBAinduced heating in GCMs would lead to insufficient increases in low-level stratocumulus clouds, which are known to be extremely important for the radiation/climatic balance (22). Our findings make it difficult to have confidence in other fast responses linked to BBA solar absorption, such as precipitation feedbacks (34) over the tropics during the biomass-burning season including the drying effect of BBA recently highlighted by (25,26) over the densely populated southern West African region.…”

“Late-stage” deforestation enhances storm trends in coastal West Africa

“…The little dry season coincides with the peak of the West African monsoon, which brings heavy rain further north in West Africa. Interannual variability in rainfall is strongly influenced by sea surface temperatures (SSTs) in the Tropical Atlantic ( 25 ). In contrast to Amazonia, most of the rain comes from mesoscale convective systems (MCSs; ref.…”

“…Positive trends are also evident for extreme, daily rainfall totals in gauge-based, regional analyses over SWA ( 30 , 31 ). At the same time, there is a negative trend in total SWA rainfall in the little dry and second rainy seasons, which may be linked to rainfall suppression by anthropogenic aerosols ( 25 ). Here, however, we focus on spatial variations in the trends of convective systems on subregional scales and their links to local deforestation.…”