|Reversing the Spiral - The Population, Agriculture, and Environment Nexus in Sub-Saharan Africa (WB, 1994, 320 p.)|
|2. Agricultural stagnation and environmental|
Over the past twenty-five years, agricultural production in Sub-Saharan Africa rose by only about 2.0 percent a year, while aggregate population growth averaged about 2.8 percent per year (Tables A-2 and A-9).¹ Per capita food production has declined in most countries of the continent (Table A-10). Cereal imports increased by 3 9 percept per year between 1974 and 1990, food aid by 7.0 percent per year. But the food gap (requirements minus production)filled by imports, or by many people going with less than what they needis widening. In the early 1980s, about 100 million people in Sub-Saharan Africa were unable to secure sufficient food to ensure an adequate level of nutrition for them-selves, and average food consumption per capita declined during the 1970s and 1980s in seventeen of the thirty-six SSA countries for which data are available (Table A-10).² In years of poor harvests the numbers affected have been much larger Severe food shortages were exceptional in the 1960s, but are no longer so. Famines in several countries in the 1980s were graphic indications of natural calamity, as well as of civil disruption, in the region. On average, officially estimated per capita food intake in Sub-Saharan Africa in the late 1980s, at 2,027 calories per day, was below the 1965 level and significantly lower than in other parts of the developing world. The average in India, for example, is 2,235 calories daily per person. The average African consumes only about ST percent of the calories needed for a healthy and productive life.
The available data show no acceleration of aggregate agricultural growth in the 1980s. It has, in fact, been slightly below the longer-term average of 2.0 percent a year recorded for the past three decades (Table A-9) (It was higher than 2.0 percent in the 1960s and much lower in the 1970s.) This poor performance is also evident in the decline of agricultural export earnings. Export volumes and values have declined for almost all SSA countries from 1980 to 1990 (Table A-13), with volume declining at 2.7 percent per year on average. There are notable exceptions. Exports of tea and horticultural products from Kenya' cocoa from Cd'Ivoire, and cotton from several West African countries have grown substantially in volume. But the success stories are few.
Projections, based on present trends, are disturbing. Aggregate population growth has accelerated to over 3.1 percent a year (Table A-2). Projections based on current trends in fertility and mortality rates (including the impact of AIDS) indicate only a slight deceleration in aggregate population growth through the year 2000. The total fertility rate (TFR) for Sub-Saharan Africa as a whole has declined only marginally from 6.6 from in 1965 to 6.4 at present (Table A-2). By contrast, the average TFR for all the world's low-income countries declined from 6.3 in 1965 to 4.0 in 1987. During the same period, the crude death rate in SubSaharan Africa. [elf from 23 to 16 (Table A-3). In countries with a nigh incidence of AIDS, death rates will rise, but nowhere is population growth expected to fall below 2 percent per annum by the year 2000, even under worst case AIDS scenarios currently considered plausible.³ Unless efforts to reduce TFRs succeed (or mortality rates rise dramatically due to currently unanticipated AIDS developments), population growth rates will decline very little.
Table 6.1 shows the implications of these trends for SubSaharan Africa's future food gap In 1990, Sub-Saharan Africa's 474 million people produced about 90 million metric tons of maize equivalent of food. With 100 million tons of aggregate consumption, there was a gap of 10 million tons met by imports. At currently projected growth rates, Sub-Saharan Africa's population will total about 1,184 million and its food production will reach about 163 million tons of maize equivalent in 2020. Even with no change in average per capita consumption, aggregate requirements will be about 250 million tons. The 87 million ton food gap would be almost nine times today's gap and equivalent to about onefourth of the present annual production of cereals in the United States. Food aid varied between 4 million and 7 million tons of cereals per year in the 1980s and could not conceivably increase sufficiently to fill this gap. Without significant per capita growth in agricultural production it is difficult to imagine sufficient overall economic growth that would generate the resources needed to finance food imports of this magnitudeor, for that matter, to maintain educational and health services and infrastructure facilities.
These disturbing trends will not continue indefinitely. What is at issue is how they will eventually be overcome. Will the strong synergies and the dynamics of these trends lead to human and environmental degradation and ultimately to widespread starvation? Or will these trends be overcome through voluntary, but determined, action to reduce population growth and promote sustainable agricultural development and growth?
Much of Sub-Saharan Africa's natural resource base and ecological environment is deteriorating. If present trends continue, this deterioration will accelerate. The most pressing problem is the high rate of loss of vegetative covermainly the result of deforestation and the conversion of savanna to croplandwhich in turn leads to loss of soil fertility and soil erosion. Global and regional climatic changes and deviations from longerterm average conditions are also causal factorsbut human impact on the environment in Sub-Saharan Africa may itself be an important element contributing to these climatic changes.
In much of Sub-Saharan Africa, deforestation is a major problemwith significant local, national, and global consequences. Forests provide a multitude of products and serve many functions, including essential environmental ones. With deforestation, these are lost. Forests and woodlands are cleared for farming and logged for fuelwood, logs, and pulp wood. Data on forest resources and rates of extraction and clearing are imperfect, as are data on most of Africa's environmental resources, but information is continually improving and reliable enough to suggest the scale of the problem. In 1980, there were about 646 million hectares of forests and woodlands in Sub-Saharan Africa. A 1980 FAO/UNEP study estimated that 3.7 million hectares of tropical Africa's forests and open woodlands were being cleared each year by farmers and loggers (Lanly 1982). More recent estimates suggest that close to 2.9 million hectares were lost each year during the 1980s (Table A-19), mainly through conversion to farm land, but the rate of deforestation may be accelerating as the aggregate area still under forests continues to shrink. Reforestation during the 1980s amounted to 133,000 hectares per year, only about 5 percent of the area lost each year to deforestation (Table A-19).
Aggregate data obviously obscure important differences among regions and countries. Deforestation has been particularly rapid in West Africa, with East Africa and southern Africa also suffering substantial losses in forest cover. Large tracts of tropical forest still remain, especially in Zaire, Gabon, Congo, the Central African Republic, and Cameroon. It would take many years for Central Africa's forests to be completely destroyed, the process has started. In most of East Africa and southern Africa, as well as in the West African coastal countries, the process is far advanced.
Degradation and destruction of forests have a severe impact on wildlife habitat and biodiversity, with potentially irreversible losses of animal and plant life The World Conservation Union (IUCN) and the World Resources Institute (WRI) estimate that 64 percent of original wildlife habitat in SubSaharan Africa has already been lost (Table A-:22). The main causes are deforestation, conversion of wiIdlands to agricultural uses, and other human activity. Excessive harvesting, poaching, and illegal trade also take a heavy toll on many species. Degradation of tropical moist forests has a particularly negative impact on biodiversity by destroying plant and animal life that may exist nowhere else in the world.
As forests and woodlands are destroyed, people must walk farther or pay more for fuelwood, construction materials, and other forest products. Woodfuels are the staple source of household energy in Africa, and many agroprocessing and rural artisanal and semi-industrial activities also use woodfuels. Fuelwood deficits are severe in the Sahel, in the savanna regions of West, Central and East Africa and in the aria areas of southern Africa (Table A-21). They impose particular hardships on women who are usually responsible for household fuel provision. As fuelwood becomes scarce, women (and children) have to spend more time collecting it from more distant sources and eventually begin to substitute crop residues and manure which would otherwise be used to maintain soil fertility.
Box 2-1 The Threat to African Wildlife
The World Resources Institute (WRI) has compiled estimates of the number of threatened species In Africa (WRI 1992:304-309). Examples include the following (see also Tables A-23 and A-24):
· Eighteen of the 226 known species of mammals in Cs'Ivoire are threatened with extinction, and seventy of the 3,660 known plant species are rare and threatened;
··Zaire has the most known species of mammals in Africa, and twentytwo of these 409 are threatened;
· Fifty-three of Madagascar's 105 known mammal species and twentyeight of its 250 known species of birds are threatened;
· In Chad, eighteen of the 131 known species of mammals are threatened;
· In Kenya, fifteen of the 314 known mammal species are threatened, and 144 of the 6,500 known species of plants are rare and threatened; and
· Of South Africa's roughly 23,000 plant species, 1,145 are listed as rare and threatened.
The loss of future wood for the forest industry will be another important cost of continuing deforestation. In the period 1985-1987, the six largest African timber exporters (Cameroon, Congo, Cd'Ivoire, Gabon, Ghana, and Liberia) exported US$500-600 million worth of timber annually. Without significant afforestation, the potential for future export earnings will be lost as forests disappear.
Forests also provide a wide variety of nonwood products for local populations. Many are used particularly by women to meet subsistence needs or to generate cash income, and various wild plant and animal food sources are especially important in times of stress (FAO 1989;1990a). A recent FAO publication lists ninety-four different forest and farm tree foods as being commonly used in West Africa (FAO 1990a:102 - 103); thirty forest species are listed as being commonly used for fodder (FAO 1990:113). Women often possess much specialized knowledge in this regard (Molnar and Schreiber 1989); in Sierra Leone, women listed thirty-one different products they gather from bushes and trees near their villages (Hoskins 1989:43) Traditional medicine throughout Sub-Saharan Africa is highly dependent on a variety of forest plants.5 As forests are degraded and destroyed, these resources are no longer available and/or accessible to the local populations.
Deforestation also has a particularly severe impact on forest dwellers, such as the pygmies, threatening not only their traditional lifestyles, but their very survival (Bailey, Bahuchet and Hewlett 1992; Dyson 1992; Peterson 1992, Winterbottom 1992).
Box 2-2 Nonwood Forest Products Gathered by Women in Brakna, Mauritania
The wide variety of nonwood forest products utilized by women is illustrated by the arid region or Brakna in Mauritania:
· Foods and livestock feed: Gums, fruits, leaves and grasses, chemicals from plants for preserving butter, couscous seasonings, a wild (aze) used as animal feed;
· Medicines, cosmetics, dyes, etc: Medicinal plants, henna and pods for cosmetic purposes, incense plants;
· Utensils, handicrafts, etc.: Fronds, grasses, dyes, leather tannins, floor mats (Smale 1985).
Soil Degradation and Erosion
Much of Sub-Saharan Africa is highly vulnerable soil degradation and erosion. Such land degradation is often, more dramatically but some what loosely, referred to as "desertification": the process of sustained deterioration of the biological productivity of land. It is manifested in such phenomena as soil erosion, soil structure deterioration, compaction, reduction in organic matter and nutrient content, and salinization. The vulnerability of much of Sub-Saharan Africa to land degradation is due to factors such as soil characteristics, intense soil drying in the dry seasons, severely erosive seasonal rainfall in many areas, wind erosion in drier areas, and low-resource farming with inadequate soil conservation measures. The Soil Reference and Information Centre in Wageningen, Netherlands, has recently published estimates of the extent and severity of land degradation in Africa Its data indicate that about 321 million hectares (14.4 percent of the total vegetated land surface) are moderately, severely, or extremely degraded and a further 174 million hectares are lightly degraded (Oldeman and others 1990).6 Most of this is in the West African Sahelo-Sudanian Zone, in Sudan, Ethiopia, Somalia, and Kenya' as well as in southern Africa, but parts of many other countries (such as the northern areas of many West African coastal countries) are also affected.
Sizeable areas used for cropping in low-rainfall regions are subject to soil degradation and soil fertility loss. Topsoil losses even an gently sloping cropland have been reported to range from 25 tons to 250 tons per hectare annually from Niger to Madagascar and from Ethiopia to Zimbabwe (Table A-26). These rates translate into losses of between 2 mm and 2 cm of topsoil annually. Moreover, there is location-specific evidence that erosion is accelerating. A study of Tanzania's Shinyanga region, utilizing the fact that trees and bushes can tee dated to determine changes in ground surface height over tune, found that soil erosion during the first sixty years of the current century averaged about 1.4 t/ha/year; twenty to thirty years ago it was 10.5 t/ha/year; and during the past two decades it has averaged 22.4 t/ha/year (Stocking 1987:56-57).
Box 2-3 Soil Erosion and Degradation: The Data Problem
Despite their pervasiveness, the extent and impact of the degradation, erosion, and desertification of Africa's soils are not easy to assess. Reliable data on which to base national, regional, and continental estimates are scarce. Soil erosion rates are difficult to calculate, and published data on degradation and erosion are highly location-specific and often of doubtful reliability, because of poor measurement techniques. They are also subject to considerable misinterpretation, especially when field data are extrapolated to develop aggregate estimates for entire watersheds, regions, or countries. Moreover, most research on the relationship between soil degradation and erosion and soil productivity has been carried out in ternperate zones (notably the United States), but there are vast differences in this relationship throughout the world as also in the resilience of land systems and the rate of new soil formation (Seckler 1987; Stocking 1987).
The agronomic relevance of such data is difficult to assess, however, without information on new soil formation and total topsoil remaining. Topsoil depth should be at least 15 cm for most annual plants, with an additional 35 cm of subsoil beneath to provide sufficient rooting depth (but optimal rooting depth obviously differs among crops). Intemperate climates the natural rate of soil formation on nonagricultural land is about 0.8 mm per year, but it maybe three times this much in the humid tropics (Seckler 1987:91); these rates are likely to be higher on wellrnanaged and lower on poorly managed farm land. Nevertheless, given the poor fertility characteristics of most African soils and the prevailing low-input farming practices, topsoil losses in the middle and upper ranges of the magnitudes reported will cause rapid productivity declines.
Soil erosion is usually accompanied by other aspects of soil degradation, such as deteriorating soil structure, reduced moisture retention capacity, soil nutrient depletion, and reduction in soil fauna and flora. A major study undertaken in the late 1970s estimated that, with unchecked soil degradation and erosion and no change in farming technology, the productivity of land in Africa would decline at an average rate of 1 percent per year between 1975 and 2000 (Higgins and others 1982:23-25). In Zimbabwe, nitrogen and phosphorus losses attributable to erosion on arable land were estimated to be about three times the amount of fertilizer used in the 1984/85 crop year; compensating for this nutrient loss through fertilizer applications would have cost US$1,500 millionor US$35 per hectare of arable land (FAO 1990b).
Much soil eroded from uplands and slopes is deposited in the bottomlands along river courses. But these deposits are deficient in organic material and poorly structured, require good tillage, and are usually too heavy for hoe cultivation or traditional plows Access to more efficient agricultural technology (machinery for land preparation, drainage to prevent waterlogging, etc.) has made it increasingly possible for land-hungry farmers to extend cultivation into these areasbut with often deleterious consequences for riverine ecosystems and for pastoralists (see below).
Box 2-4 Extent and Economic Cost of Soil Erosion in Mali
Soil erosion on cultivated land in Mali has been estimated to range from a low of 1 t/ha/year in the arid north to a high of 31 t/ha/year in parts of the more densely settled and intensively cultivated couth of the country. Given the enormous difficulties involved in quantifying the effect of soil degradation and soil loss on farm productivity, the researchers had to work with a range of values for the critical parameters that define this link The associated crop yield reductions were estimated to range between 2 and 10 percent per year for the country as a whole. The present value (using conservative parameters of a ten-year time horizon and a 10 percent discount rate) of current and future net farm income forgone as a result of one year's soil loss was estimated to fall between 4 and 16 percent of agricultural GOP (Bishop and Allen 1989).
Rangeland Degradation and Desertification
About 25 million of the world's estimated 40 million nomadic and transhumant pastoralists live in Africa (Bass 1990). Between 1963 and 1983, according to FAO estimates, the number of cattle increased by 74 percent in Sudano-Sahelian Africa, by 65 percent in humid and sub-humid West Africa, and by 61 percent in southern Africa (FAO 1986). At the same time, the extent and quality of the rangeland declined. Cultivators moved into the best grazing areas and converted them to cropland; the traditional use rights of pastoralists, and particularly of transhumant herders, were ignored or overridden, and their herds were increasingly forced to more marginal lend which is rapidly degraded by overgrazing. The increasing cultivation of valley bottoms has further compounded the problem: it restricts pastoralists' ability to move their herds there and to use these lands as migration routes during the dry season, thus forcing them to remain on degrading rangelands and around permanent water points. Restrictions on the movement of pastoralists across national boundaries have had similar effects.
The issue is not simply one of too many animals relative to the available grazing areas. Long periods of below-normal rainfall and severe droughts have accelerated the degradation of rangelands, and past efforts to address the problem of water supplies for pastoralists have often compounded, rather than ameliorated, the problems. Deep wells have been sunk to ensure water supplies during the dry season, but with free access to these wells, the number of animals congregating around them far exceeds the carrying capacity of the surrounding rangeland, causing rapid deterioration. Desertification has tended to spread outward from these areas of excessive and prolonged animal concentration
Water Resource Depletion and Degradation
In large parts of Sub-Saharan Africa, water is the critical limiting resource, nut merely in terms of agricultural production, but in the broader context of the population-agriculture-environment nexus as such. "Accelerating water scarcity may well influence the time of population stabilizationfor example, by significantly influencing birth rates, death rates, migration patterns, or all of these variables" (Falkenmark 1991:81). Unfortunately, many countries do not yet have adequate basic data to assess their water availability/but conflicts over competing demands on scarce resources are becoming increasingly evident. The potential for such conflicts rises rapidly with population growth and economic development. At the rates of population growth currently projected, ureter availability per capita will decline to half of its present levels HZ almost all SSA countries within twenty-five years. A recent macrolevel assessment suggests that ten countries in SubSaharan Africa will face severe water stress situations by the turn of the century: Mauritania, Niger, Somalia, Kenya, Burundi, Rwanda, Malawi, Zimbabwe, Namibia, and Lesotho. By the year 2025, eleven more will have joined this list Mauritania, Senegal, The Gambia, Burkina Faso, Togo, Benin, Nigeria, Ethiopia, Uganda, Tanzania, and Mozambique (Falkenmark 1991:83 85).
Rivers, streams, lakes, swamps, and coastal waters are important resources. They provide critical economic goods and services and perform vital ecological functions. They need. to be protected end prudently utilized, but many are seriously affected by sedimentation, siltation, agrochemical runoff' industrial pollution, and inefficient utilization. Pollution from domestic sources has become a concern around many large cities and in countless rural areas where lack of safe potable water is a major health issue. Such problems are increasingly serious in many parts of Sub-Saharan Africa, although quantitative information is particularly poor in this respect. The causes include soil erosion, deforestation, destruction of protective shoreline vegetation, indiscriminate drainage, encroachment for farming, poorly conceived irrigation development, and lack of environmental regulations and enforcement on industrial activities. Many irrigation and hydropower schemes that involve damming and diverting rivers have adversely affected the flora and fauna of the downstream floodplains, the wildlife and livestock carrying capacity of the floodplain grasslands, the extent and productivity of wetlands and riverine forests, and the productivity and sustainability of downstream fishing and of farming based on traditional recession irrigation. Large impoundments also imply large evaporation losses. Other problems include coastal erosion, saltwater intrusion into aquifers in coastal areas, and destruction of coastal wetlands critical for birds and marine life. Problems have also been encountered with the spread of water-related diseases around water unpoundments and irrigation schemes where water remains standing in fields and canals.
Groundwater resources have also come under pressure, especially in the arid and semiarid regions. In some areas, groundwater reserves are being drawn down for irrigation much faster than they can be replenished. Deforestation, soil degradation and erosion, and poor on-farm soil and water management all increase surface runoff (causing erosion) and reduce the amount of rainfall that infiltrates the soil and eventually percolates into underground aquifers. Prolonged periods of below-average rainfall and unusually frequent and severe droughts have, of course, greatly exacerbated this problem (Table A-25).
Drinking water in rural areas is the most pressing concern, but wafer scarcity is also a severe constraint on livestock and home garden pro auction in many parts of arid and semiarid Africa and even in many subhumid regions During the dry season, rivers, streams and springs in many areas of Sub-Saharan Africa run dry, and women often have to go very far to obtain meager quantities of water, which is often of very poor quality. As groundwater tables recede due to reduced rainfall and reduced rain infiltration into the soil and into subsurface aquifers, wells dry up and must be dug deeper or abandoned.
Environmental Degradation and Climatic Change
The consequences of environmental degradation are profound. Most alarming is the possible negative impact on rainfall, although direct causality is difficult to establish. Extensive meteorological monitoring end research suggest increasing aridity throughout the Sahel during the 1970s and 1980s. Figure 2-1, which depicts annual rainfall deviation from the 19001987 long-term average, is telling. It shows that the Sahel has always experienced wide variations in annual rainfall, but also that rainfall has been consistently and significantly below the long-term average every year from 1970 to 1987 (Jayne, Day, and Dregne 1989). There have also been significant declines in average rainfall in the coastal countries along the Gulf of Guinea and in eastern Africa.8
Climatologists' hypotheses to explain rainfall decline in Sub-Saharan Africa's drier regions include long-term climatic cycles as well as changes in ocean surface temperatures and in wind patterns over Africa brought on by changes in global atmospheric temperatures. The causes of Sahelian drought may still be poorly understood, but abroad consensus is emerging that they are related to large-scale patterns of atmospheric circulation specifically to the reduced northward extension over Africa of the InterTropical Convergence Zone (ITCZ), the hand of wet weather that surrounds the globe where the trade winds from the southern and northern hemispheres converge (Odhiambo 1991:79)These ITCZ extensions, in turn, are affected by cyclical changes in ocean surface temperatures.
Figure 2-1 Departures in Rainfall the Long-Term Average for the Sahel Zone, 1900-1987 (percent)
There is increasing agreement that land surface changeswhich anclude changes in albedo, evapotranspiration, soil moisture, surface temperature and roughness, and dust generationcan prolong and intensify Sahelian drought by reinforcing the atmospheric conditions which initially reduce rainfall (Nicholson 1989:53-54). Evidence is accumulating which strongly suggests that the widespread and severe changes in land surface characteristics in West and Central Africa caused by human activity have disrupted the normal cycle of the ITCZ extension over Sahelian Africa, causing the prolonged decline in rainfall from the long-term average.
Box 2-5 Is the Sahara Expanding Southward?
Satellite imagery now allows scientists to monitor the latitudinal movement of the southern edges of the Sahara (and of the other North African deserts contiguous with it), running from southern Mauritania to mid-Sudan. These data are available only since 1980, so that longer-term trends cannot be inferred from them. The Sahara expanded southward in the first four years for which these data are available (1980 to 1984), when there was a serious drought As a result, it was about 1.3 million km² (or 15 percent) larger in 1984, when the drought was most severe, than in 1980 Although rainfall since 1984 has remained significantly below the mean for 1900-1987, it has not again been as low as in 1984 The Sahara has therefore receded in size from its 1984 peak, but not back to what it covered in 1980 (Tucker and others 1991).
Changes in the land surface are partly caused by reduced rainfall itself, but human activity, notably deforestation and removal of vegetative cover on rangeland and cropland, has a considerable impact. If the massive generation of smoke and atmospheric gases caused by biomass bunting is considered, as it should be, as an additional change in "surface" conditions over much of West Africa, it is difficult not to conclude that growing human populations have an impact on climatic change. Indeed, there is increasing concern about the effect of biomass burning of the enormous scale represented by African forest and grassland fires on the behavior and properties of clouds.9
Tropical forests are extremely important for recycling water between the Earth's surface and the atmosphere, and their disappearance has serious consequences for regional and global climate. They are highly efficient in returning rainwater to the atmosphere in the form of water vapor, which forms new clouds and leads to subsequent rainfall Rainforest regions thus store enormous quantities of water not only in the soil and biomass, but also in the atmosphere above them. When tropical forests disappear, water runs off quickly and much of it flows into the sea. This not only affects local and regional hydrological cycles, but also has potentially serious effects on climate An important mechanism for the redistribution of heat is the atmosphere's ability to store energy in the form of water vapor and to release this energy again when vapor condenses into cloud droplets. If less water is available for this process, heat absorbed at the ground has to be removed by other means such. as radiation and dry convection, leading to higher surface temperatures and to changes in the vertical distribution of heat (Andreae and Goldammer 1992:88).
Deforestation is also a major cause of the rapid increase in the accumuIation in the atmosphere of carbon dioxide (CO:) and nitrous oxide (N2O), two of the heat-trapping greenhouse gases that cause global warming (MacNeill and others 1991:11-13). Deforestation has been estimated to account for about one-quarter of worldwide net CO2 emissions into the atmosphere (Andreae 1991:276); the remainder comes from the combustion of fossil fuels (most of which occurs, of course, in the industrialized countries of the northern hemisphere, in the countries of the former Soviet Union, and in China).
Burning of forests and grasslands causes enormous atmospheric pollution with both regional and global implications. Burning of biomass (forests, grasslands, agricultural wastes, fuelwood, etc.) worldwide is responsible for about one-third of global emissions of carbon aerosols into the atmosphere. Africa accounts for over 42 percent of tropical and almost 37 percent of global biomass burning annually and contributes more to gas and smoke emissions from biomass burning than any other region of the world (Andreae 1991:272; Andreae and Goldammer 1992:82). The destruction of tropical rainforests through bunting directly contributes to the greenhouse effect, because the CO2 released (up to 600 tons of dry matter per hectare) is not recaptured rapidly enough by regrowth on the same site of grasses or crops (ranging from 5 tons to 50 tons of dry matter per hectare). About 31 percent of annual burning of tropical forest biomass worldwide occurs in Africa, 46 percent in South and Central America and 22 percent in Asia (Andreae 1991:272-273).
About 90 percent of biomass burning in Africa is accounted for by the annual dry-season burning of savanna and grasslandsto clear them for farming, to stimulate grass growth and control pests and shrub growth, or to facilitate hunting. About one-third of total worldwide emissions from biomass burning is due, thus, to savanna burning in Africa. Unlike deforestation, however, savanna burning does not contribute significantly to the greenhouse effect, because the CO2 released by the burning is recaptured into new savanna vegetation during the next annual growth cycle. But due to its geographic and temporal concentration, African biomass burning results in regional atmospheric pollution levels that are comparable to, and at times exceed, those in industrialized countries.
Acid deposition is higher in the Congo Basin and in Cd'Ivoire than in the Amazon Region or in the eastern United States and is largely caused by direct emissions from biomass burning and by subsequent photochemical reactions in the resulting smoke and gas plumes. High levels of acid deposition have a negative effect on plant health and on fish and other aquatic organisms Due to the longer average leaf life in the tropics, tropical forests are considerably more sensitive to foliar damage than temperate forests. Acid also poses a serious risk to amphibians and insects that have aquatic life cycle stages and depend on rain water collected in plants and mosses and between dead leaves. This risk extends further to the many plants that depend on such insects for pollination. There is also an effect of soil degradation through progressive acidification and associated problems such as leaching of aluminum, manganese and other cations, interference with nitrogen cycling, and the disturbance of microbial processes in the soil (Andreae and Goldammer 1992:88-89).
Environmental Degradation and Agricultural Stagnation
Soil degradation and erosion (excepting the often dramatic gully erosion that occurs where surface runoff is concentrated) are insidious processes, not readily apparent to farmers until the effects are severe and irreversible with the means traditionally available They deplete the soil of nutrients, diminish its moisture retention capacity, and reduce the depth of the rooting zone for annual crops. These effects exacerbate the impact of drought. Farmers and pastoralists in the semiarid regions of Sub-Saharan Africa have always had to cope with drought, and they relied on effective adjustment mechanisms. But when drought extends over several successive years, as was the case in the 1970s and early 1980s, the problems become extremely serious.
The problems were compounded by the fact that the main traditional coping and adjustement mechanisms-shifting cultivation with long-duration fallows, and pastoralists' mobilityhad become severely constrained. In the Sahel, for instance, rainfall during the 1950s and 1960s, when populations began to grow rapidly, was well above the long-term average almost every year As a result, cultivation had been expanded into traditional rangelands, making both cultivators and pastoralists more vulnerable to drought. Range and pasture areas were reduced in size and the mobility of transhumant pastoralists was increasingly restricted. At the same time, a growing share of total cropland was in marginal areas, and changes in farming practices (for example, shorter fallows. reduction of multivanety seeding and intercropping, displacement of traditional droughttolerant varieties) rendered farmers increasingly more vulnerable to climatic risk (as well as to plant pests and diseases).
As vegetative degradation and desertification proceed, the livestock carrying capacity of pastures and rangelands declines Crop yields decline as the result of soil degradation and erosion on cropland Available data on average cereal and root crop yields show decreases in many countriesdespite significant investments in agriculture (Tables A-11 and A-12). Site-specific information confirms the problem in many countries.10 The data suggest that environmental degradation, accelerated by population pressures, is part of the cause of Sub-Saharan Africa's slow rate of agricultural and economic developmentthrough its negative impact on soil fertility, rainfall, water availability, and the supply of fuelwood and other forest products. Exacerbating this are a frequently poor agricultural policy environment, low use of productivity-enhancing agricultural inputs, and the generally low productivity of rural laborattributable in large measure to low health and nutritional status and low educational attainment levels of the rural population.
1. Statistical information on agricultural performance, as on most other aspects of social and economic development, is difficult to obtain and tends to be of poor quality. This study craws on what is generally considered to tee the best available statistical information (see the Statistical Appendix for data and sources).
2. The data on which estimates of food avallability and consumption are based (such as crop acreage, yields, livestock production, processing and storage losses) are of poor quality in most African countries. Increasingly, it is also recognized that noncultivated plants and "bushmeat'' contribute far more to many Africans' diets, particularly in poor crop years, than has been captured in official statistics Nevertheless, few observers are as skeptical of the general picture of serious food deficits as Svedberg (1991).
3. Demographic modeling of the potential impact of AIDS m extremely difficult Some simulations suggest that AIDS may reduce the population growth rate of SSA as a whole by as much as 05 to 1.0 percentage points in the early decades of the 21st centurythrough drastically higher mortality rates. But higher mortality rates may delay fertility declines.
4. UNDP/World Bank (1992), Tables 14-8 rough 14-1, provides country-specific data on energy consumption, including consumption of fuelwood.
5. The InterAfrican Committee on Medicinal Plants and African Tropical Medicine and the Scientific, Technical and Research Commission of the OAU fave published a pharmacopeia of African medicinal plants of proven efficacy, African Pharmacopeia (1985), and several African countries }rave established research institutes focusing on traditional medicine and the sources and effects of the active ingredients in medicines administered by traditional healers (DeJong 1991).
6. WRI/IIED estimates are even higher, suggesting that more than 80 percent of SubSaharan Africa's productive drylands, some 660 million hectares, are affected by "desertification" (Table A-27).
7. The Sub-Saharan Africa Hydrological Assessment attempts to meet this need by assisting countries to develop a reliable hydrological data base (see Chapter 10, note 1).
8. In Cd'Ivoire, where deforestation has been the most rapid, mean annual rainfall declined significantly during the 1970s and 1980s (World Bank 1989a). Rainfall in Senegal decreased by 2.2 percent a year in the 1970s and 1980s, and there was a sharp decrease in rainfall in northern Nigeria and Cameroon (Lele 1989c; Lele and Stone 1989). Rainfall also declined dramatically throughout Ethiopia during that period (World Bank 1987a).
9. Cloud droplets form around aerosol particles, called cloud condensation nuclei (CCN). Biomass burning generates and releases into the atmosphere vast amounts of pyrogenic aerosol particles, which are very effective as CCN. The more CCN in the atmosphere, the more droplets form, resulting in smaller droplet size with a given amount of available water. Clouds composed of smaller droplets are lighter in color' reflect more sunlight bade into space, and are less likely to produce rain. Since clouds are a major regulatory and control mechanism for the Earth's heat balance, large scale modifications in cloud properties have a strong impact on global climate. The increasing abundance of CCN is, therefore, likely to have potentially critical impact on precipitation efficiencycompounding the changes in hydrological cycles in the tropics caused by land surface changes such as deforestation (Andreae and Goldammer 1992:87-88).
10 See, for example, Barnes 1990a and 1990b; Bishop and Allen 1989; Elliot 1986; Falloux and Mukendi 1988; Gorse and Steeds 1987; FAO/IBRD Cooperative Programme 1991; Lal and Okigbo 1990; Matlon 1990; de Montalembert and Clement 1983; Mortimore 1989a and 1989b; Nelson 1988; Stocking 1987.