| 英文摘要: | The climate of West Africa is characterized by a sensitive monsoon system that is associated with marked natural precipitation variability. This region has been and is projected to be subject to substantial global and regional-scale changes including greenhouse-gas-induced warming and sea-level rise, land-use and land-cover change, and substantial biomass burning. We argue that more attention should be paid to rapidly increasing air pollution over the explosively growing cities of West Africa, as experiences from other regions suggest that this can alter regional climate through the influences of aerosols on clouds and radiation, and will also affect human health and food security. We need better observations and models to quantify the magnitude and characteristics of these impacts.
The West African monsoon is one of the most important large-scale atmospheric circulation systems in the tropics. It controls winds, temperature, clouds and most importantly precipitation over a land area of about 6 × 106 km2 (~5–25° N, 15° W to 15° E) and has remote impacts, for example through hurricane genesis. Through water resources, agriculture and power generation, the health and livelihoods of hundreds of millions of people depend on monsoonal rainfall. The West African monsoon is a sensitive system that can be perturbed by different factors across a wide range of scales. A prominent example is the devastating drought in the 1970s and 1980s1 that most severely affected the Sahel, one of the regions with the largest precipitation variability worldwide. A large fraction of decadal-scale rainfall variability in the West African monsoon area is explained by variations in Atlantic sea-surface temperatures, which have been linked to natural oscillations but also to changes in anthopogenic aerosol emissions during the twentieth century, predominantly from industrialized areas in the mid-latitudes2, 3. It is anticipated that the West Africa regional climate will change because of the effects of global-scale warming, implying an increased likelihood of unprecedented heat waves and a threat to low-lying, densely populated coastal areas from sea-level rise4, and also because of land-use and land-cover change, as the increasing transformation of rain and savannah forests into agricultural land creates changes in the surface energy and water balance through effects on albedo, evapotranspiration, water transport and storage as well as surface roughness5, 6. Studies on the Indian and East Asian monsoons suggest that anthropogenic emissions of aerosols and aerosol precursor gases from these densely populated and increasingly industrialized areas can affect the amount and seasonality of rainfall. Earlier studies concentrated on scattering aerosols such as sulphates, which reduce monsoonal circulation and precipitation through a reduction of short-wave radiation reaching the surface, sometimes termed 'solar dimming'7. The inclusion of absorbing aerosols such as black carbon creates a more complicated response in models that, among other things, depends on whether a coupling to the ocean is taken into account8. According to the 'elevated heat pump' concept, aerosol heating over the Tibetan Plateau causes large-scale circulation changes over South and East Asia9, but this idea is difficult to prove from observations10. Recent studies are increasingly including effects of aerosols on clouds and typically find a reduction of monsoon-season precipitation through the combined effects of clouds and radiation changes11, 12. In West Africa anthropogenic emissions of aerosols and aerosol precursor gases have increased rapidly in recent years and are projected to keep increasing13, 14. This is particularly the case for the explosively growing cities along the Guinea Coast, as illustrated by high aerosol optical thickness along the coastal strip in the satellite image shown in Fig. 1, particularly in the area of Lagos. In this Perspective we will discuss the question of whether this increasing pollution can be expected to perturb the sensitive West African monsoon system and thereby contribute to regional climate change in addition to the more established long-term factors global warming and regional land-use and land-cover change. In contrast to the Indian and East Asian monsoon, this emerging research topic has not received much attention and therefore the relative magnitude of this problem and possible interactions of different factors are unclear. Undoubtedly urban air pollution has already become a significant threat for human and ecosystem health across West African cities such that any regulatory actions could have multiple benefits. We will begin with a short overview of the meteorological conditions over West Africa followed by a discussion of anthropogenic aerosols and aerosol-climate interactions. Concrete steps needed to improve our understanding of the role of air pollution for the West African climate are given in the concluding section.
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