West Africa’s climate is controlled by the interaction of two air masses, the influence of which varies throughout the year with the north-south movement of the Intertropical Convergence Zone (ITCZ). Hot, dry continental air masses originating from the high pressure system above the Sahara Desert give rise to dusty Harmattan winds over most of West Africa from November to February. In summer, moist equatorial air masses originating over the Atlantic Ocean bring annual monsoon rains (Nicholson, 2013).
As a result of these interacting air masses, West Africa’s precipitation regime is characterized by latitudinal belts of decreasing rainfall and wet season length. At the Gulf of Guinea, precipitation is abundant year-round without a marked dry season. At higher latitudes, precipitation decreases and is limited to a wet season of decreasing duration. This latitudinal pattern is somewhat modified by altitude, with higher mountain elevations, e.g. the Guinean Highlands and the Jos Plateau in central Nigeria, receiving more precipitation than lowlands of the same latitude. Along the south–north gradient of decreasing rainfall, Abidjan, Côte d’Ivoire (5° north latitude) records a mean annual rainfall of 1,600 mm; Ouagadougou, Burkina Faso (12° northern latitude) 700 mm within a 5-month rainy season; and Agadez, Niger (18° northern latitude) 165 mm annually in a short 2.5-month rainy season. Temperatures in the lowlands of West Africa are high throughout the year, with annual means usually above 18°C. In the Sahel, maximum temperatures can reach above 40°C.
Not only scarcity of rainfall, but also its variability and unpredictability become more significant with latitude. Thus, year-to-year rainfall variability ranges from 10 to 20 percent in the coastal areas to over 40 percent in the northern Sahel (FAO, 1983). Drought is a recurring phenomenon in semiarid West Africa, where average rainfall conditions seldom prevail, and rainfall is skewed to dryness, i.e., a few heavy rainfall years are balanced out by a larger number of below-average rainfall years. From the late 1960s through the 1980s, the Sahel zone experienced droughts of unprecedented spatial extent and duration (Hulme, 2001). These droughts followed a period of more favorable rainfall in the 1950s and early 1960s, which had encouraged government planners and farmers to expand agriculture northward (Glantz, 1994). The great Sahelian droughts forced the abandonment of agriculture at the arid margin, triggered a famine crisis that killed thousands of people and their livestock, and has been blamed for widespread environmental degradation in the region.
Average annual rainfall has recovered some from the low point of the early 1970s, however it has not been enough to erase the long-term drying trend since 1900 — the earliest available rainfall records (Nicholson, 2005). Moreover, for agro-pastoralists not only annual rainfall totals are important, but also the frequency and distribution of rainfall events throughout the wet season. Too much rain at once can damage crops or change pasture composition in unfavorable ways. Heavy rainstorms also cause severe soil erosion, particularly on cleared cultivated land.
The great Sahelian droughts provide the most dramatic worldwide example of multi-decadal climate variability that has been directly measured. However, for lack of an observational rainfall record before the 20th century, or sufficient proxy indicators, it remains unclear how unique these droughts have been at time scales of centuries and millennia (Hulme, 2001). Are they part of the normal variability of this semiarid climate, or harbingers of human-induced climate change? Understanding the climatological processes behind the droughts is a prerequisite for attributing them to natural or human causes and to eventually predicting the impacts of future climate change on rainfall in the region. The current understanding is that variations in sea surface temperatures in the global oceans play the largest role in Sahelian rainfall variability, amplified by land cover (Giannini, 2016). Thus, climate is not only driving land use and land cover change, but to some extent is also driven by it. Particularly at local scales, the effects of vegetated versus bare soil on temperatures and humidity are quite noticeable, as illustrated in the examples of farmer-managed natural regeneration (Reij Tappan, and Smale, 2009).
Temperatures over West Africa have increased over the last 50 years, in line with an increase in global temperatures (Niang and others, 2014). The impact of global warming on rainfall in West Africa, however, remains notoriously difficult to assess in a climate that is susceptible to significant variation at multiple time scales. Different climate models, which differ in their representation of atmospheric processes, show significant variation, and disagreement, in their projections of future rainfall in West Africa. While there is a high level of confidence that temperatures will continue to increase in West Africa (between 3°C and 6°C above the late 20th century baseline by 2100), some models project a drier future, others a wetter future, and yet others no significant change in rainfall totals (see bottom adjacent figure). An increase in the frequency of extreme rainfall events has been observed over the past 50 years and is likely to continue into the future. Future soil suitability for major crops is expected to be affected by climate change; in particular beans, maize and banana production might face declines and require cropping system transformations (Rippke and others, 2016). The coastal countries of West Africa are also vulnerable to sea level rise resulting from global warming, leading to flooding and coastal erosion.
Climate variability and change have impacted, and are continuing to impact, land cover in West Africa by changing the amount and timing of water availability to vegetation cover. Land use decision making responds to these changes in ways that further alter the land cover, from slight modifications of the quality of the land cover to outright transformations of the land cover type.
Models of projected changes in temperature and annual mean precipitation ofWest Africa were commissioned by the Intergovernmental Panel on climate Change (IPCC). The scenarios predict temperature and precipitation for both the mid-21st century (2046-2065) and the late 21st century (2081-2100) relative to the late 20th century (1986-2005), based on two alternative greenhouse gas emission scenarios, RCP2.6 and RCP8.5 (RCP stands for Relative Concentration Pathways). Model projections based on these two extreme pathways are contrasted here, with RCP2.6 assuming that global greenhouse gas emissions peak between 2010 and 2020 and decline substantially thereafter, and RCP8.5 assuming that emissions continue to increase throughout the 21st century. The projections shown are multi-model averages. The averages tend to level out the considerable variability and disagreement between the individual models.As the maps show, both scenarios predict a warming trend and predominantly positive changes in annual rainfall for most of West Africa. While most changes are small and insignificant, a wetter future is predicted for Niger and Chad, whereas the RCP8.5 scenario indicates a possible drying trend for the western part of West Africa.
These maps were reproduced for West Africa from data from the IPCC 5th Assessment Report (Niang and others, 2014).