The continent of Africa is characterized by several climatic regimes and ecological zones. All parts of the continent, except the Republic of South Africa, Lesotho, and the Mediterranean countries north of the Sahara, have tropical climates. These tropical climates may be divided into three distinct climatic zones: wet tropical climates, dry tropical climates, and alternating wet and dry climates (Huq et al., 1996).
Several comprehensive descriptions of the climates of Africa exist, most notably
those of Thomson (1965) and Griffiths (1972). Surveys of African rainfall have
been carried out by Newell et al. (1972), Kraus (1977), Klaus (1978), Tyson
(1986), and Nicholson (1994b). These researchers agree that summer rainfall
maxima, which are dominant over most of Africa, are controlled primarily by
the Inter-Tropical Convergence Zone (ITCZ). Over land, the ITCZ tends to follow
the seasonal march of the sun and oscillates between the fringes of the Sahara
in boreal summer and the northern Kalahari desert in the austral summer. The
latitude zones of these arid and semi-arid deserts demarcate the tropics from
the subtropics. Rainfall in the subtropics is modulated by mid-latitude storms,
which may be displaced Equator-ward in winter. Further modification of these
broad patterns is provided by natural features such as lakes and mountains,
and by the influence of ocean currents. The poleward extremes of the continent
have extratropical influences associated with mid-latitude synoptic disturbances,
resulting in significant winter rainfall (Griffiths, 1972).
|Figure 2-2: Observed annual temperature changes in Africa (see Annex A).|
In general, surface air temperatures over most of Africa display a high degree
of thermal uniformity, spatially and seasonally (Riehl, 1979). The extreme north
and south of the continent, however, experience cold frontal systems that quasi-regularly
introduce abrupt air mass changes. Temperatures there are more variable in response
to a large annual cycle of insolation and the effects of seasonally varying
air masses and winds. Mean temperature trends over the past 100 years, averaged
over continental Africa, are shown in Figure 2-2. The
highland areas of eastern and southern Africa are substantially cooler than
lowland regions, and there is evidence that recent warming trends may have been
exaggerated in these mountain areas (Hulme, 1996a).
Box 2-2. Africa's Natural Resources,
African economies have made relatively low levels of investment in infrastructure and directly productive capital goods-and hence continue to rely heavily on natural capital (natural resources). This natural capital is at risk because poverty and high population growth often induce land degradation and deforestation-which in turn lead to growing food insecurity and loss of biodiversity. This pattern contributes to migration into rural areas that often are less suitable for agricultural expansion and to urban areas with inappropriate physical, social, and economic infrastructure. The process also contributes to population growth by creating an incentive for large families: Adding family labor becomes one way of coping with the increasing time costs of gathering fuel and water and clearing new land. The severity of this population-agriculture-environment nexus is compounded by low investment in human capital (human resource development), which often restricts individuals to continued reliance on unskilled labor and short-term exploitation of natural resources (the land) as the only feasible survival options.
Although Africa's youthful population represents future social capital, a great deal of investment in terms of education, training, and skills development will be required to ensure its full productivity in the next decade and beyond. Particular attention will need to be paid to capacity-building in information technology to better prepare African societies for efficient, sustainable management of the continent's fragile resources. In addition, the region's political structures, which determine decision making in resource allocation and consumption, will have to be stabilized.
|Figure 2-3: Rainfall fluctuations for select areas of Africa, expressed as a regionally averaged standard departure (Nicholson, 1993).|
Rainfall over Africa exhibits high spatial and temporal variability (see Figures 2-3 and 2-4). Mean annual rainfall ranges from as low as 10 mm in the innermost core of the Sahara to more than 2,000 mm in parts of the equatorial region and other parts of west Africa (Figure 2-4). The rainfall gradient is largest along the southern margins of the Sahara-the region known as the Sahel-where mean annual rainfall varies by more than 1,000 mm over about 750 km. This tight rainfall gradient means that relatively small changes in the position of the ITCZ can have large consequences for rainfall in the Sahel; thus, this region is a sensitive indicator of climate change in Africa. Coefficients of rainfall variability in Africa exceed 200% in the deserts; they are about 40% in most semi-arid regions, and between 5% and 20% in the wettest areas (Figure 2-3).
|Figure 2-4: Mean annual precipitation (mm) in Africa (Martyn, 1992).|
Most policymakers now recognize drought as a normal feature of Africa's climate and acknowledge that its recurrence is inevitable. Widespread occurrences of severe drought during the past three decades have repeatedly underscored the vulnerability of developed and developing societies to its ravages in Africa. Although the variability of African climate is inevitable, loss of human life and economic disruption associated with extreme climatic fluctuation can be lessened by advance warning. Recent scientific advances in understanding the climate system and breakthroughs in predictive capabilities on seasonal time scales provide an opportunity to reduce the vulnerability of human societies by planning for previously unexpected variations from mean climatic conditions. Current forecasting capabilities, although by no means perfect, provide a better indication of climatic conditions that are expected to prevail during the next season or two than simply assuming that rainfall and temperature will be normal (Bonkoungou, 1996).
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