Throughout Africa, the staple crops are cereals-maize in particular. Millet and sorghum are widely grown as well; wheat and teff are common in some regions. Almost all agriculture is rain-fed, although irrigation is important in some regions. The absence of irrigation (less than 10% of the cultivated area is irrigated) increases the sensitivity of crop yields to climatic variations. Cash crops are important in every country but vary in their distribution and profitability. Coffee, tea, groundnuts, cocoa, tobacco, and palm oil are grown as cash crops. Other significant crops (at least in terms of household consumption) include cassava, yams, legumes, and horticultural crops. Agro-pastoralism and extensive nomadic pastoralism are common in semi-arid regions. Relying on grass and browse, pastoralism is particularly sensitive to long periods of drought when grazing resources are depleted by livestock and not renewed.
The regions of Africa have distinct characteristics. North Africa and the Indian Ocean islands rely on irrigated agriculture. In west Africa, the gradient of climates from the Sahara to the humid coast determines the potential for agriculture. Subsistence agriculture and pastoralism dominate the Sudan and Sahelian regions; plantation agriculture is found along the Guinea coast. The highlands of east Africa are well known for productive agriculture that takes advantage of the two rainy seasons. The lowlands, however, are subject to erratic rainfall and poor soils. Coffee and tea are major cash crops in the highlands. The humid and sub-humid zones of central Africa, where drought is seldom a problem, are conducive to roots and tubers.
Most rural households engage in subsistence agriculture, although large commercial estates are found throughout Africa. Moreover, regionally diverse values, cultures, and practices in agriculture make for a truly unique region. In many African cultures, identity and the measure of personal wealth and value are determined by the amount of land one owns, the number of cattle in one's herd, or the amount of food produced for the community, rather than monetary wealth. These nuances make African agriculture a particularly important sector within the climate change debate.
Prolonged drought-lasting a season or longer over a widespread area-is the most serious climatic hazard affecting African agriculture, water supplies, and ecosystems. If droughts become more common, widespread, and persistent, many subhumid and semi-arid regions will have difficulty sustaining viable agricultural systems. Drought-prone environments already have been settled and land converted from extensive grazing and long-fallow cultivation to permanent cropping. Box 2-6 reviews the frequency of drought in Africa and its impacts.
The effects of climatic variations on African agriculture have been well established through decades of field experiments, statistical analyses of observed yields, and monitoring of agricultural production. The most important climatic element is precipitation, particularly seasonal drought and the length of the growing season. The distribution of rainfall within the growing season also may affect yields. Local flooding and storms are minor problems. Low temperatures and radiation limit production in some high-elevation regions; frost is a hazard in South Africa. High temperatures can affect yields and yield quality in semi-arid and arid regions, although water is more important. Sea-level rise and coastal erosion will affect groundwater, irrigated agriculture, and low-lying coastal land in some areas.
The direct effects of CO2 enrichment on plants tend to increase yields and reduce water use. Increased CO2 concentrations increase the rate of photosynthesis and increase water-use efficiency (the efficiency with which plants use water to produce a unit of biomass or yield). The direct effects are strongest for plants with C3 pathways, such as wheat, compared with C4 plants like maize, sorghum, millet, and sugarcane-which are staples for much of sub-Saharan Africa. CO2 enrichment also affects weeds, many of which are C4 plants (Ringius et al., 1996). According to the IPCC Second Assessment Report, the effect of a doubling in CO2 concentrations (from the present) varies from a 10% increase to almost a 300% increase in biomass; WUE may increase by up to 50% (or more) (IPCC, 1996). Thus, the beneficial effects of increased concentrations of CO2 are likely to offset some of the effects of decreased precipitation. However, the effect of CO2 on crops in Africa-where nutrients often are a limiting factor and leaf temperatures are high-remains highly uncertain.
Unfortunately, regional projections of precipitation change diverge quite strongly in Africa. For example, scenarios of summer precipitation in the Sahel show a range of ±20% for nine atmosphere-ocean GCMs reported in Annex B. However, present trends in precipitation in Africa show a decrease in some regions. Recent transient scenarios report lower temperature changes, globally as well as for Africa. Thus, for agriculture, there is little confidence in present scenarios for precipitation-the most important aspect of climate change for African agriculture. However, a combination of the potential effects of increased CO2 concentrations and lower temperatures (at time of doubling) using transient scenarios suggests that the impacts of these scenarios may be less severe than those of earlier equilibrium GCM model experiments. Nevertheless, even a small decrease in precipitation can be significant. Furthermore, few scenarios of drought risk and the distribution of rainfall within the growing season have been developed.
Although African agriculture clearly is sensitive to climatic variations, perhaps
equally important is the gap between present and potential agricultural yields
in Africa. For example, a serious impact of climate change might be a decrease
of 20% in maize yields. Yet present yields among smallholders often are only
half (or even one-tenth) of potential yields. The evaluation of potential impacts
of climate change should not mask the enormous potential for more-productive
agricultural systems in Africa (see Section 184.108.40.206.3).
|Figure 2-13: Maize yields in Africa (Hulme, 1996b), based on FAO data.|
At the national level, Figure 2-13 shows the variability in national maize yields for selected countries (Hulme, 1996a). The effects of the 1984-85 and 1991-92 droughts are clear (see Box 2-6). The coefficient of variation for annual maize yields varies from about 10% in central Africa to almost 50% in drier countries such as Botswana and Swaziland. A significant component of the variability is likely to be related to rainfall, although prices and market policies are influential. The role of precipitation in agricultural productivity was demonstrated dramatically in the Sahel and eastern and southern Africa during the drought period 1970-95 (Buckland, 1992). Water scarcity revealed widespread dependence on rain-fed agriculture and the lack of infrastructural development for supplemental irrigation and water resources. For example, Zimbabwe in 1991 and 1992 imported 800,000 tons of maize, 250,000 tons of wheat, and 200,000 tons of refined and semirefined sugar to make up the shortfall associated with reduced agricultural production as a result of rainfall shortages (Makarau, 1992). Studies of the role of climatic variability in African food security have a long tradition. At the local level, agroclimatic studies such as Akong'a et al. (1988), Downing et al. (1990), Mortimore (1989), and Sivakumar (1991, 1993) considered the effects of climatic variability on agriculture, with an emphasis on coping with drought. Back-to-back drought episodes in subhumid and semi-arid zones led to the failure of crop production and dependency on other sources of income to buy food, or on famine relief.
National crop modeling studies that specifically address climate change now have been carried out for many countries for a variety of purposes (see Sivakumar, 1991, 1993; Eid, 1994; Muchena, 1994; Fischer and van Velthuizen, 1996; Makadho, 1996; Matarira et al., 1996; Sivakumar et al., 1996; USCSP, 1996). Recent studies sponsored by the United Nations Environment Programme (UNEP), the Global Environment Facility (GEF), the USCSP, and others are to be published soon. A few regional studies have been conducted (e.g., Hulme et al., 1995; Hulme, 1996a; Ringius et al., 1996), although an authoritative continental assessment has not been compiled. Global studies have included Africa-often, however, using poor data or inadequate spatial coverage and ignoring many critical issues of vulnerability and food security.
The general conclusion is that climate change will affect some parts of Africa negatively, although it will enhance prospects for crop production in other areas (see Downing, 1992, for case studies of agriculture in Kenya, Zimbabwe, and Senegal). Warmer climates will alter the distribution of agroecological zones. Highlands may become more suitable for annual cropping as a result of increased temperatures (and radiation) and reduced frost hazards. Although C3 crops exhibit a positive response to increased CO2 (as much as 30% with 2xCO2), the optimal productive temperature range is quite narrow. Some regions could experience temperature stress at certain growing periods-necessitating shifting of planting dates to minimize this risk. Expansion of agriculture is important in the east African highlands. For example, agroecological suitability in the highlands of Kenya would increase by perhaps 20% with warming of 2.5°C based on an index of potential food production (Downing, 1992). In contrast, semi-arid areas are likely to be worse off. In eastern Kenya, 2.5°C of warming results in a 20% decrease in calorie production. In some lowlands, high-temperature events may affect some crops. Growth is hindered by high temperatures, and plant metabolism for many cereal crops begins to break down above 40°C. Burke et al. (1988) found that many crops manage heat stress (with ample water supply) through increased transpiration to maintain foliage temperatures at their optimal range. Because a large portion of African agriculture is rain-fed, however, heat-related plant stress may reduce yields in several key crops-such as wheat, rice, maize, and potatoes. At the other extreme of the C3 temperature spectrum, several crops (such as wheat and several fruit trees) require chilling periods (vernalization). Warmer night temperatures could impede vernalization in plants that require chilling, such as apples, peaches, and nectarines. Locations suitable for grapes and citrus fruit would shift to higher elevations. C4 crops are more tolerant, in general, to climate variations involving temperature ranges between 25°C and 35°C. These crops most often are located in warmer, dryer climates; they are quite susceptible to water stress.
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