impacts of carbon dioxide warming on climate and man in the semi-arid tropics

I M P A C T S OF C A R B O N D I O X I D E W A R M I N G ON CLI MATE

A N D M A N IN T H E S E M I - A R I D T R O P I C S

J .A. M A B B U T T *

Emeritus Professor, University of New South Wales, Australia

Abstract. Tropical semi-arid climates occur between 10 and 35 deg latitude and are characterised by highly variable summer rainfall of between 300 and 750 mm in a rainy season of at least 4 months, generally adequate for rainfed cropping but with considerable drought risk. They support a mesic savanna vegetation. They have a land extent of 4.5 million km 2, mainly occupied by Third World nations with rapidly increasing populations which in the main are predominantly rural and largely agricultural with low per capita incomes, consequently vulnerable to climate change. A doubling of atmospheric CO 2 by the year 2030 is predicted to cause a rise in equilibrium mean temperature of 1-3 ~ however there is continuing uncertainty regarding the consequences for rainfall amount, variability and intensity, length of rainy season or the frequency of extreme rainfall events. Two scenarios are considered, with reduction and increase in rainfall respectively, involving corresponding latitudinal shifts in present climatic boundaries of about 200 km. Because of their variability, a clear signal of the greenhouse effect on these climates may be delayed, whilst, regional responses may differ. Vegetational and hydrological responses under the alternative scenarios are considered. The possible consequences for rainfed and irrigated agriculture, water and energy supplies and disease and pest ecology are discussed. Lands of the semi-arid tropics are already extensively desertified, with consequent lowered productivity and heightened vulnerability to drought, and the possible impacts of greenhouse warming on desertification processes and on measures for land rehabilition to the year 2030 are reviewed. Measures to conserve the biological diversity of savanna lands in face of greenhouse warming are discussed.

1. Nature and Extent of Semi-Arid Tropical Climates

'Semi-arid tropics' here describes those low-latitude regions in which a season of summer rainfall is limited by a longer and pronounced dry season. These seasonal contrasts are of dynamical origin, linked with the seasonal passage of the Inter- tropical Convergence Zone (ITCZ), when dry conditions associated with subsid- ence in the subtropical anticyclonic belt are interrupted by incursions of moist tropical air.

The world distribution of tropical semi-arid climates is shown in Figure 1, which indicates a land extent of around 4.5 million km e. They comprise several related climatic regimes: those of intertropical latitudes in which seasonal migration of the ITCZ lies within the annual march of solar declination, as in the Sahel-Sudan tran-

* Present address: RMB 218B Wattamolla Road, Berry, NSW 2535, Australia.

Climatic Change 15: 191-221, 1989. �9 1989 Kluwer Academic Publishers. Printed in the Netherlands.

192 J.A. Mabbutt

Fig. 1. The world distribution of tropical semi-arid climates as defined in the text, shown by dark shading. Based on the 'World Distribution of Arid Regions' (UNESCO, 1977).

sition in West Africa; the strongly monsoonal type where continental thermal regimes generate seasonally alternating wind systems and carry tropical airmasses into higher latitudes - beyond the 30th parallel in northwest India for example; an equatorial rain-shadow regime of two rainy seasons found in the Horn of Africa and Kenya; and variants due to altitude and relief aspect, as in Andean South America. The equatorward limit of these climates is set by increased rainfalls in tropical sub-humid and humid areas; over large areas the subtropical arid zones form a poleward boundary, supplemented in Asia by the Himalayan barrier. In North America and Australia, limits drawn in Figure 1 to exclude transitions to Mediterranean-type and warm-temperate summer rainfall regimes of continental interiors have been chosen with a view to delineating areas in which the climatic consequences of greenhouse warming might be broadly comparable.

Under these climates the main limitation on biological productivity is moisture availability and the effect of temperature is exercised mainly through its influence on evapotranspiration. UNESCO (1977) defines the semi-arid zones as those where ratios between mean annual rainfall and potential evapotranspiration (P/Etp) are in the range 0.2 to 0.5, which in the semi-arid tropics implies annual rainfalls of between 300 and 750 mm. In agro-climatic terms, 'semi-arid' implies average moisture conditions adequate for rainfed cropping, for which the UNESCO classification requires the critical monthly P/Etp value of 0.5 to be exceeded in at least four consecutive months. Surveys of the limit of millet cultivation in Sahelian Africa indicate however that rainfed cropping may here occur with rainfalls as low as 250-275 mm (Cochem6 and Franquin, 1967; Konate, 1984), with a minimal cropping season of 90 days (FAO, 1978).

Dry-season temperatures in the semi-arid tropics mostly range from warm (mean monthly temperatures 20-30 ~ in low-latitudes to temperate (mean

Impact of Carbon Dioxide Warming on Climate and Man 193

monthly temperatures 10-20 ~ under various combinations of higher latitude, altitude and continentality. Except at very high altitudes, temperatures are adequate for plant growth throughout the year although interior plateaux with more temperate dry-season regimes, as in southern Africa, may experience frost. Generally the dry season consists of a cooler lower-sun period followed by a hot period in the months preceding the rains, after which the cloudy, rainy conditions of the wet season bring some relief through a suppression of daytime temperatures.

Situated between humid areas of assured rainfall and the arid zones where lack of rain is the norm, the semi-arid tropics are characterised by highly variable rainfall with an interannual variability of between 25 and 40%. This variability, as with the seasonal alternations, is mainly dynamical in origin, linked with changes in seasonal movements of the subtropical high-pressure belts and the zonal airstreams that flank them and with variations in anticyclonicity within the zones. Rainfall variability generally increases towards the drier margin of the semi-arid zone and also tends to be higher in areas of steep rainfall gradient and pronounced anticyclonicity, as in West Africa.

In West Africa the records indicate a tendency for seasonal droughts to persist, with significant runs of 3 or 4 consecutive drought years and occasional catastroph- ic droughts lasting up to a decade (Walker and Rowntree, 1977). In the dry African tropics moreover, individual droughts can show a remarkable geographical coher- ence, affecting whole zonal areas (Nicholson, 1980). This impressive scale of Afri- can droughts, in time and in space, doubtless reflects the magnitude of the atmospheric controls involved and an absence of multiple sources of rainfall. Individual droughts in corresponding parts of India and Australia tend to be shorter and less extensive, but nonetheless still demonstrate a degree of persistence which appears to be lacking in midlatitude drylands (Gibbs and Maher, 1967; Hare, 1983).

Irregularities in the timing and geographical patterns of rainfall within the season are also characteristic and are particularly significant when they occur at crucial periods in crop development. The adverse impacts of such breaks in seasonal precipitation are heightened by an associated increase in solar radiation and resulting higher day temperatures and are particularly serious in the absence of seasonal carry-over of soil moisture.

2. Vegetation and Soils

Ecologically the semi-arid tropics correspond with mesic savanna (Johnson and Tothill, 1985), in which a mid-height grass layer, which becomes more open towards the arid zone, is associated with a varying cover of trees and shrubs, and locally with thorn scrub. The density of the upper vegetation storey ranges from woodland, shrubland or thicket through parkland to relatively open grassland. These variations are in part linked with rainfall amount and with moisture regimes linked with soil texture or site drainage, whilst in the wetter parts soil nutrient status and the occurrence of ironstone crust (plinthite) become increasingly signifi-

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cant. However dry-season fires, grazing and browsing are equally critical determinants. The woody shrub storey is diminished by fires, which become more frequent and intense in the wetter parts of the zone with the greater biomass of the grass layer, although this trend may be influenced by grazing pressure.

Climatic limitations on agriculture in the semi-arid tropics are reinforced by large areas of infertile soils subject to rapid loss of nutrients and minerals under cultivation, when they become liable to wind or water erosion (Young, 1976). They comprise ferruginous soils, including sandy ferruginous soils in old stabilised dunefields, and the weathered ferralitic soils of ancient planar land surfaces, commonly with plinthite.

3. Agricultural Production with Particular Reference to Climate

With the exception of Australia and the United States, countries included in the semi-arid tropics form part of the Third World, with per capita GNP's mainly below US $300 (1987 values).* The population in the semi-arid sectors of these countries is around 400 millions and is increasing at annual rates of 2 to 3%. In Africa and Asia it remains predominantly rural despite accelerating urbanisation, whilst in Latin America the rural component is between 30 and 40%. It can be estimated that about 200 million of these people are directly dependent on agriculture for their livelihood and that 90% of these are primarily crop-based.

Population densities among rural communities based on rainfed farming range from below 20 to more than 200 km -2. In areas of moderate population density, as in much of West Africa, some combination of crop-fallow rotations in outlying fields and more intensively farmed home gardens is typical; shifting agriculture with short-term cultivation of clearings and long bush fallows is found in more sparsely populated areas, particularly in the eastern and central African savannas; very closely settled areas, including those near large urban centres, tend to be under more intensive permanent cropping involving short fallows and heavier use of fer- tilisers or animal manures. Most of the farms are smaller than 5 ha.

Staple rainfed crops such as millet in India, and additionally sorghum and maize in Africa represent an adaptation to constraints of low rainfall and a short growing season. The unreliability of this rainfall is an equally important constraint; the climatic limit of regular cropping tends in fact to be set by the level of probability of inadequate seasonal rainfall rather than by any particular isohyet (cf. Perrin de Brichambaut and Wallen, 1963; Gregory, 1969). Crop failure through drought is an ever-present threat through the semi-arid tropics and a number of traditional risk - reducing agricultural strategies have developed in response to it, including crop mixing, rotation and mosaic planting, frequent clean tilling and stubble clearance

* Statistics in this and the following sections are based on a number of sources, including 'World Resources 1987' (International Institute for Environment and Development and World Resources Institute, 1987) and UN and FAO handbooks. They are often estimates only.


and manuring with animal and plant residues (UNESCO, 1979; Harris, 1980; Hadley, 1985). These in turn are supported by social arrangements, including food storage and the obligations established within extended family groups through food loans. Some of the recent technological, socio-economic and demographic changes within farming communities have tended to weaken the effectiveness of such strategies and so to increase drought hazard.

Seasonal aridity and drought are commonly countered by irrigation, including modern basin-irrigation projects based on reservoirs, small-scale supplementary irrigation from groundwater or local storages, as in India, and flood-recession farming and pasturing as along the inland Niger River in West Africa. In the Indian subcontinent and in Sudan and Mexico 20-30% of cropland is irrigated; elsewhere irrigation is locally important, but in tropical Africa generally the area under irrigation remains small and attempts to extend it through large-scale projects have met with significant environmental and social obstacles (Adams and Grove, n.d.).

Characteristically in these countries, the value of agricultural production ranges from more than 50% of GDP in the poorest economies to less than 20% where mining or oil production is important. The growing of export crops such as cotton, groundnuts and tobacco has been encouraged in the last few decades to help in meeting balance of payments problems in many neo-national economies, whilst growing urban markets also provide increasing opportunities for cash crops. Dis- tinction between commercial and subsistence agriculture is not clear-cut but the recognized cash crops still generally occupy less than 15% of the cropped area overall, although more prominent in irrigated lands and on the better soils. Food production by the peasant farmer for his own family remains a prime concern in most areas.

Livestock production varies in relative importance regionally. In tropical Aus- tralia and in corresponding 'outback' areas of South America, commercial ranching on native pastures dominates throughout the semi-arid zone. In tropical Africa, livestock - whether raised by pastoral communities on rangelands or as part of mixed farming - tend to be more important towards the dry margin of the semi- arid zone, where they may be part of systems of nomadic grazing extending into more arid areas; in the wetter African savannas they are still restricted by the tsetse- fly, despite some successes in fly control. In the Indian semi-arid tropics open rangelands have become very restricted through the spread of cultivation. Through- out the semi-arid tropics one finds a considerable integration of cropping with small-scale livestock-raising on farmlands and village commons, the animals providing the farmer with milk and meat, cash income and some insurance against crop failure. In general the quality of native pastures falls off rapidly during the dry season, and this - together with the seasonal incidence of tsetse-infestation in Africa - greatly influences livestock production and management.

The impacts of greenhouse warming on land use and primary production in the semi-arid tropics must be complicated by the present degraded state of land resources over large areas. The term 'desertification' has been widely used to de-

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scribe the reduction in biological productivity of land systems in the drylands through deterioration of vegetation cover and soils and their water relationships, constituting a shift towards more desert-like conditions irrespective of any con- temporary change in climate (UNCOD Secretariat, 1977), and the semi-arid tropics include the areas worst-hit by desertification in that sense of the term (Dregne, 1983; Mabbutt, 1984). Degraded croplands and rangelands are not only less productive but are less resilient under climatic stress, threatening an acclerating cycle of degradation, whereby the sustainability of dependent livelihood systems is put at risk. Possible effect of CO2 increase on the status and trends of desertification must accordingly be taken into account.

In tropical Africa as a whole, per capita food production has declined over the past two or three decades, together with a concomitant fall in the production of non-food crops, whilst the imports of food grains have increased. This is commonly held to be due in part to low rainfalls and in part to desertification, although individual successes in national food-production programmes - in Zimbabwe and Kenya for example - indicate that prevailing low levels of agricultural productivity cannot be wholly explained by climatic and other environmental limitations. Over the same period per capita food production has increased slowly in Asia and in Latin America overall - although probably least in the drier areas - but a continuing rise in populations maintains a concern for the future. Malnutrition is still common in the semi-arid tropics, among rural and urban populations alike, although to some extent reflecting inequality of incomes, ignorance about nutrition and distributional problems as well as low production, and it can intensify to critical levels in times of drought. Any climatic change affecting agricultural productivity under these conditions must have immediate consequences for living standards in rural and urban sectors and for the national economies concerned.

It will also have international repercussions, for agricultural production from these areas is important at the world scale. It is estimated that the semi-arid tropics contain 15% of the world's cropland and 10% of its irrigated area and maintain about 15% of its livestock. They account for 10% of cereal and 15% of non-cereal crop production and produce 10% of all agricultural non-food products.

4. Social and Economic Vulnerability

Whilst the tropical countries are generally underdeveloped and impoverished on the world scale, semi-arid regions within them, often distant from centres of political power, tend to be particularly disadvantaged economically. Combined with handicaps of geographical remoteness and an often harsh and unreliable climate, this commonly results in sub-standard social services. High levels of out-migration from the rural areas reflect this combination of unsatisfactory living conditions and lack of economic opportunity. They are conditions which leave local populations physically, economically and socially vulnerable to any adverse environmental impact on the availability of basic necessities such as food, water and energy sup-


plies and to any associated increase in the threat of disease. In 1985 less than half the rural populations of developing countries in the semi-

arid tropics had access to safe drinking water; the majority depended on open wells, village tanks and natural surface water replenished by seasonal rains, commonly unreliable in dry seasons as well as being subject to pollution by man and domestic animals. For the women particularly, fetching water for the household can be an onerous daily chore. Water supplies for village livestock, often from the same source, are equally x~alnerable to drought, whilst the dependability of pastoral communities on wells and natural water points is measured by their dicta- tion of seasonal movements between pastures. Even among the town-dwellers a substantial minority still lacks satisfactory water supplies, particularly in the shanty quarters of fast-growing cities. Where water supplies are primitive, sanitation is also generally unsatisfactory and the two combine to explain the high incidence of water-borne and sanitation-linked diseases. Any change of climate must significantly affect this situation.

The domestic consumption of energy in rural communities in developing countries of the semi-arid tropics is mainly for cooking and to a lesser extent for heating, the latter more particularly in winter and at higher altitudes. Its main sources are firewood and charcoal. Patterns of energy use and supply are more varied among town-dwellers, but wood still remains a main energy source. In sub-Saharan Africa fuelwood accounts for 80% of energy consumed. An annual per capita consumption of between 1 and 2 tonnes of fuelwood has been a main cause of destruction of woodland and removal of woody vegetation over large parts of the semi-arid tropics. In the villages, collecting firewood can take many hours of a woman's day, and the provision of firewood for the growing cities is a major industry with ever- widening demand areas as the sources of supply dwindle. With few exceptions, deforestation far outstrips replanting in the semi-arid tropics and the term 'fuelwood crisis' has been applied to the situation in much of the drier parts of sub- Saharan Africa and northern India, where fuelwood can no longer be thought of as a renewable resource (Eckholm, 1975).

The price of fuelwood has risen sharply in the face of scarcity, but alternatives such as kerosene, biogas and solar energy are generally still more expensive and fuelwood is likely to remain the prime energy source over much of the semi-arid tropics in the next few decades. Reliance on fuelwood as a main energy source in the semi-arid tropics, by rural and urban dwellers alike, is another aspect of environmental dependency relevant to any discussion of possible impacts of climatic change.

5. Postulated Effects of CO 2 Warming on Semi-Arid Tropical Climates

This paper adopts, as a working assumption, conclusions in the 'Villach Statement' (Bolin et al., 1986) that present trends of increase in greenhouse gases could result in the equivalent of a doubling of atmospheric COz concentrations by the 2030's,

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with a theoretical eventual equilibrium surface warming of between 1.5 and 4.5 ~ which could however be delayed for some decades because of heat absorption by surface layers in the ocean.

The consequences of such warming for other aspects of zonal climates - including those of the semi-arid tropics - remain uncertain, in part because of limitations in the numerical models on which predictions have been based (Tucker, 1988). These include inadequacies of input relating to many potential atmospheric feed- back processes - for example those linked with changes in cloud cover and type, soil moisture and vegetation cover - and also with respect to oceanic interactions, and corresponding inadequacies of output, for example information on secondary climatic effects such as the evaporation balance, and on changes in climatic variability and in the frequency and magnitude of extreme events which could be very significant for the semi-arid tropics. In addition, the coarse geographic scale of existing models does not allow the expression of regional factors which might lead to intrazonal differences in a climatic response to global warming. Accordingly, climatic changes projected for the semi-arid tropics must at this stage be seen only as possibilities to serve discussion about the sensitivity of the zonal environmental and social systems to such impacts, rather than as climatic forecasts of a given probability.

Furthermore, the relatively short records of semi-arid tropical climates show significant regional fluctuations in rainfall and less markedly in mean temperature, at scales ranging from a few years to several decades. To the extent that opposed trends have occurred in different regions within the zone, factors in addition to and independent of greenhouse warning may be involved. Against such a variable background an unequivocal climatic signal of global atmospheric warming may well be delayed.

Indications from modelling are that warming in the tropics is likely to be in the low part of the predicted global range, with a rise of mean temperatures of between 1 ~ and 3 ~ for CO2 doubling (Dickinson, 1986), resulting in decreased meridional temperature gradients. Such a pattern was apparent in an increase of annual mean temperatures of about 1 ~ established for midlatitudes of the southern hemisphere between 1913-45 and 1946-78 (Paltridge and Woodruffe, 1981, considered to be consistent with the theoretical consequences of greenhouse warming (Schlesinger, 1986; Pittock, 1988), when no significant temperature rise was recorded within 10 degrees of the equator. Other factors such as increases in atmospheric volcanic dust (Bryson and Goodman, 1980) must be invoked to explain cooling phases before 1913 and, particularly in northern midlatitudes, since the mid-1960's.

In the semi-arid tropics, overall warming could be expected to be most marked in a rise in dry season temperatures, including both daytime maxima and overnight minima. Frost seasons in continental interiors could in that event be shortened. Mean monthly temperatures in the rainy season are presently depressed by 1 ~ ~ through reflection above cloud cover and as a result of evaporation from vegetation and wet ground surfaces and soils: this effect would doubtless continue, but to a


degree that might be significantly affected by changes in amount and type of cloud and in precipitation linked with CO 2 warming and by related changes in ground moisture and vegetation cover.

Any change in the water balance must be of critical importance where availability of moisture is the main factor limiting biological production. Under existing relationships, a rise in mean monthly temperatures by 2 ~ should increase monthly pan evaporation by about 50 mm in these latitudes, suggesting that an increase in seasonal precipitation of between 40 and 100 mm, or somewhat less than 10%, could be needed to maintain present P/Etp ratios. These estimates do not however take into account biological and secondary climatic effects of CO2 increase which are relevant to the future water balance and which are discussed below.

It is important to stress the continuing uncertainty regarding the consequences of greenhouse warming for rainfalls in the semi-arid tropics. Numerical models have differed, both in their general and in their regional forecasts of zonal precipitation changes (see Schlesinger and Mitchell, 1987): predictions include a general increase in tropical summer rains (Manabe and Wetherald, 1980), little change for the Australian tropical region (Manabe and Wetherald, 1986), and effects likely to result in diminished summer rainfall in tropical Australia (Meehl and Washington, 1986). Some model results indicate an intensification of rainfalls within an existing rainy period (Mitchell, 1983; Mitchell et al., 1987), whilst others show a poleward displacement of rainfall belts (Manabe et al., 1981), implying shifts in the onset and duration of rains. Critical changes in the latitude, extent, continuity and intensity of the major subtropical anticyclonic belts and in the related migrations of the ITCZ - which constitute the major controls of seasonal rainfalls - cannot yet be predicted with confidence, nor possible feedbacks in the hydrological cycle linked with changes in cloudiness, vegetation cover and soil moisture.

Addressing this uncertainty, Pittock (1980, 1985) has noted an association between the southern hemisphere mid-latitude warming already noted and an increase in summer rainfalls in northern Australia between 1913-45 and 1946-78, linked with a poleward shift of the sub-tropical anticylonic belt and a more positive Southern Oscillation index (SOl). In what will here be termed the ~kustralian' scenario (Pittock, 1985; 1988), he has accordingly envisaged an increased latitudinal migration of the ITCZ, bringing summer rains into higher latitudes. On the basis of present isothermal gradients, he postulates a poleward extension of the area of summer rains by about 2 degrees of latitude in northern Australia, increases in mean annual rainfall of between 10 and 20%, with some increase in mean daily rainfalls and a lengthening of the rainy season by 2-4 weeks. The postulated rainfall changes appear to be consistent with evidence of increased rainfalls in northeastern Australia late in the Holocene Altithermal (de Deckker et al., 1988).

Whilst admitting individual deficiencies in these lines of evidence, Pittock (1988) suggests that their apparent convergence and consistency with dynamical reasoning give plausibility to projections of rainfall response based on them collec- tively. He admits to greater uncertainty regarding changes in rainfall variability and

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in secondary climatic effects such as the evaporation balance. An ~ustralian' scenario might well not apply on a global scale however, given the importance of El Nino-Southern Oscillation effects in the Australasian region.

A contrasting projection (Jaeger, 1988) gives weight to indications from a recent review of numerical modelling (Mitchell et al., 1987) of an enhancement of precipitation in tropical latitudes receiving heavy precipitation - presumably the equatorial climates. However it tentatively accepts the possibility of a negative rainfall response in at least part of the semi-arid tropics in both hemispheres, accentuated by warming to reduce soil - moisture availability, as predicted by Manabe and Wetherald (1986). This projection, contained in a report on a Workshop held in Villach in 1987, will be referred to as the 'Villach' scenario. The 'Villach' scenario notes the lower seasonal rainfalls and severe drought incidence in the African semi- arid tropics in recent decades (since the mid-1960's rather than the early 1950's as cited), not specifically in support of its rainfall projections but as evidence of the vulnerability of the semi-arid tropics to such changes as reflected in the record of desertification. Contrasting as it does in its rainfall predictions with the ~ustralian' scenario, it provides a useful alternative perspective for discussing possible impacts of greenhouse-induced changes in this critical climatic element in the semi-arid tropics.

The difference between the two scenarios underlines the continuing uncertainty about the further climatic consequences of greenhouse warming in the semi-arid tropics, identifies a prime need for further investigation and incidentally keeps to the fore the possibility - mentioned in both scenarios - of significant differences between regional responses.

If warming is seen as an enhancement of existing summer seasonal conditions, increased rainfall and intensification of the hydrological cycle might seem a logical consequence. On theoretical grounds, an increase in dewpoint temperatures by 2 *C should in itself increase the amount of precipitable water by a factor of 1.2 (Raudkivi, 1979). It can also be argued that a more rapid warming of land areas relative to the oceans, likely in the early phase of global warming at least, should strengthen monsoonal circulations and lead to increased rainfalls in areas such as northwestern India. In the geological past, moreover, higher rainfalls appear to have been associated with warming in the semi-arid tropics on the evidence of generally high lake levels in closed basins in tropical Africa and northwestern India during the Holocene Altithermal (Street and Grove, 1979; Singh et al., 1974). Palynologi- cal evidence from the Sudan (Ritchie and Haynes, 1987) also indicates a northward displacement of the savanna and desert steppe zones of about 450 km between 10,000 and 5000 BP, in which existing rainfall and vegetation gradients were maintained. Such comparative arguments however share the weakness of excluding the effect of the CO2 increase which is our present concern.

Evidence from African climatic records of the relationship between annual rainfall and the duration of the rainy season in the semi-arid tropics - a significant aspect of the agricultural impacts of rainfall fluctuations - is at present conflicting.


Nicholson and Chervin (1983) and Dennett et al. (1985) have claimed that drought in West Africa is linked rather with reduction in peak-season rains; for the dry semi-arid zone in central Sudan, however, Hulme (1987) shows a significant co- variance between annual rainfall and length of wet-season, with the termination date being particularly sensitive to rainfall change, possibly because the retreat of the ITCZ tends to be more rapid than its advance. The summer rainfall increase in northern Australia between 1913-45 and 1946-78 recorded by Pittock (1985) shows a similar pattern, and an extension of the rainy season is foreshadowed in his 7kustralian' scenario (Pittock, 1988). The 'Villach' scenario, whilst mentioning a possible decrease in 'rate of precipitation', makes no mention of change in time of onset or duration of rains.

Any change in the marked interannual variability of rainfall in the semi-arid tropics and in the frequency, intensity and persistence of drought must be of the first importance, given their present significance in limiting land use and in contributing to land degradation. Existing numercial models provide no predictions con- cerning such changes. Present-day rainfall variability within this zone tends to vary inversely with seasonal rainfall, and below-average rainfalls in the Sahel and in monsoonal India from the late 1960's to the present decade appear to be associated with heightened interannual rainfall variability (Hare, 1977, 1983). On these indications, the 'Villach' scenario should anticipate further increase in drought risk. Conversely, increased zonal rainfall as under the 7kustralian' scenario should bring with it increased reliability, although any associated displacement of the limits of semi-arid climates would still leave marginal high-risk zones. The question is whether relationships between amount and variability of rainfall in the semi-arid tropics would change on a CO2-warmed earth. The mid-troposphere warming predicted under the greenhouse effect could conceivably lead to a more stable tropical atmosphere and increased rainfall variability, compounding the adverse impacts of lower rainfalls, above all in the drier parts, whilst even in association with higher rainfalls it could extend the risk of crop failure geographically and increase the scale of drought loss.

There have also been no indications from modelling of change in the frequency of intense rainstorms. The warmer and generally wetter period 1913-45 to 1946- 78 in tropical Australia did not bring a significant increase in heavy rainfalls. Ex- treme rainfall events in the Australian region are commonly linked with decaying tropical cyclones late in the rainy season and are also associated with positive SOI values (Allan, 1985). On empirical grounds a rise in mean sea-surface temperatures (SST) should lead to a general increase in the frequency, intensity and latitudinal range of tropical cyclones in general (Holland et al., 1988), and the significance of this for wind intensities and rainfall in the humid tropics is noted in the 'Villach' scenario. However negative feedbacks could occur: warming of the middle troposphere under the greenhouse effect could limit the energy available for cyclo- genesis, and increased cyclone activity could lower sea-surface temperatures in the vicinity. Regional responses may well differ significantly: for example, even were

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tropical cyclones to become more frequent in the Pacific as a whole, higher SST's in the east Pacific, through their stabilising effect on the SOI, could reduce their frequency in the Australian region. Holland et al. (1988) have concluded that at present "no clear indication can be given of even the likely direction of change in Australian tropical cyclone numbers in a greenhouse scenario".

4. Effects on the Natural Vegetation

The most direct biological impacts of increased C O 2 concentrations are stimulation of photosynthesis and suppression of photorespiration in C3 plant species, and a general increase in the efficiency of water-use by plants through reduction in stomatal aperture and suppression of transpiration (Gifford, 1988). A growth response to CO2 increase has also been found experimentally in several C4 species in the absence of photosynthetic effects, apparently through morphological changes such as increases in leaf area and shoot/root ratios. Increase of biomass in natural vegetation will vary with species and with other external conditions including temperature and rainfall. The direct effect of CO2 concentrations in increasing biomass should accordingly be multiplied by the secondary effect of warming. The direct impact could be relatively more important under the higher ambient temperatures of the tropics despite the smaller warming there, and could benefit the dry tropics additionally in that suppression of transpiration can occur in plants under water stress.

The positive effect on evapotranspiration of increase in biomass due to higher ambient CO2 levels could be largely if not wholly balanced by the associated increase in stomatal resistance to transpiration. Verhoog (1987) suggests a net increase of 2% in evapotranspiration in the tropics, others (e.g. Aston, 1984) assume a small reduction, whilst Wigley and Jones (1985), stressing the range of variables, accept that evapotranspiration may increase or decrease in different circumstances.

The scale of these effects on vegetation will depend on the direction of rainfall change associated with CO 2 increase and warming. The lower rainfalls under the 'Villach' scenario would restrict any increase in biomass resulting from the photosynthetic response to CO2 increase, although the associated suppression of transpiration could still limit the rise in evapotranspiration due to higher temperatures. Lower rainfalls would bring an extension of desert steppe, shrubland or thorn scrub along the dry margins of the semi-arid tropics, depending on soil type, and favour more open dry savanna within the zone.

Increased rainfall, as under the ~ustralian' projection, would allow a fuller response by vegetation to the effects of CO2 increase and a corresponding increase in biomass in which the competitiveness of C3 shrubs could be enhanced relative to C4 grasses, although such increase could for example be opposed through concur- rent stimulation of termite activity. A longer and wetter rainy season would favour an expansion of mesic savanna woodland into the more open savanna shrublands in


the drier parts of the semi-arid zone, parallel with that indicated in the Sudan during the Holocene Altithermal (Ritchie and Haynes, 1987). This would involve replacement of a lower and more open Eragrostoid-Aristoid-Chloroid grass layer by mid-height Andropogonoid grasses on medium-textured soils, and in higher- rainfall areas an extension of tall wet savanna grasses in depressions with heavy soils. It would also favour a denser tree layer, with an extension of broad-leaved Fabaceae and Combretaceae at the expense of Acacias, although under present conditions the regeneration of trees and shrubs commonly follows extreme rainfall events, about which predictions cannot be made with confidence.

Associated changes in fire regime will modify these responses however. Higher dry season temperatures could in themselves bring more frequent fires, as would any increase in the incidence of lightning. Fire intensities should be greater under the 'Australian' scenario because of more abundant grass fuel, leading to reduction in woody shrubs and low trees. Changes in grazing or browsing pressure by game animals or domestic livestock, possible under either scenario, could also have significant impacts on grass and tree layers. A combination of heavier grazing and increased seasonal rainfall together with the direct effects of increased atmospheric CO 2 could lead to invasion of savanna parkland by woody shrubs where fire frequencies had been reduced through man's intervention.

5. Hydrological Response and Consequences for Natural Erosion Rates

The hydrological response to CO2 warming in the semi-arid tropics must remain debatable as long as there is uncertainty about resulting changes in the amount, duration, intensity and interseasonal variability of rainfall, in the magnitude and frequency of extreme rainfall events and in precipitation-evaporation ratios. The partitioning of rainfall into infiltration and runoff will also depend on changes in vegetation cover, again linked with the uncertain factor of rainfall.

Any change in rainfall alone tends to be amplified in the runoff response by a factor approximately the inverse of the runoff coefficient, with a positive or negative shift of the runoff curve determined by change in evapotranspiration. With no change in evapotranspiration, a 10% increase in precipitation in an area with a runoff coefficient of 0.4 could bring a 25% increase in runoff (Wigley and Jones, 1985). In view of the prevailing low to moderate runoff coefficients in the semi-arid tropics, very significant changes in river discharges could follow any change in rainfall. Some indication of the hydrological consequences of rainfall decrease in the semi-arid tropics is given by the reduction in discharges and seasonal flooding in the Niger and Senegal Rivers and the drastic shrinkage of Lake Chad after two decades of low rainfalls in Sahelian Africa. Many presently perennial rivers could become seasonal or intermittent. Conversely, the possible scale of a positive hydrological response to higher rainfalls is indicated by the enlarged Lake Chad (Mega- chad) during the Altithermal (Servant and Servant-Vildary, 1980), which had an extent comparable with the present Caspian Sea. River inflows to balance the cor-

204 J.A. Mabbutt

respondingly greater evaporation under presumed higher temperatures must have been an order of magnitude greater than during the historical period.

The hydrologic response will be complicated in that many of the large rivers which traverse the semi-arid tropics, for example the Nile and the Niger, have sources in rainier uplands in adjoining humid areas, including lower latitudes where rainfall changes in response to greenhouse warming may differ from those in the semi-arid zone. In monsoonal India and Pakistan and in Andean Latin America these exogenic sources include montane glaciers and snowfields from which earlier and more rapid spring meltwater discharge is predicted in response to warming (Gleick, 1987).

Simulated flood-frequency curves for a selected river catchment in the summer- rainfall zone of north-eastern Australia on the assumptions of increases in seasonal rainfalls and various changes in evaporation, but neglecting possible changes in rainfall intensity-frequency-duration relationships, suggest that the greenhouse effect could have a greater impact on more frequent floods than on extreme events (Nathan et al., 1988). Such increases in peak discharges would predictably lead to changes in channel form as well as in extent and duration of flooding, with potential consequences for riverine landscapes and land use and for the design and per- formance of man-made structures including storage reservoirs and bridges.

Recharge to shallow groundwater in semi-arid regions tends to occur where runoff is naturally concentrated, as in alluvial depressions, and any change in runoff patterns and river discharges and in the extent and duration of flooding would bring a rapid response in watertables in unconfined alluvial aquifers, with significant changes in yield and reliability of water supplies dependent on them, together with changes in lakes and marshes linked with groundwater. Among associated problems could be the mobilisation of salts and increased salinization in groundwater and in lowland soils. The response of deeper confined aquifers to greenhouse climatic change could be delayed for centuries.

Linked with the hydrologic response to climatic change in the semi-arid tropics are potential changes in natural rates of erosion and sediment transport. With a modest change in seasonal rainfalls the denser vegetation cover resulting from increased ambient CO 2 should assist in limiting water erosion under natural conditions, but any significant increase in rainfall intensities and in the frequency of extreme rainfalls - irrespective of the direction of change in seasonal rainfalls - could lead to accelerated sheet erosion on sloping terrain, particularly on erodible tropical soils, to extension of gullying and increased stream sediment transport, resulting in shoaling and possible shifts of course in lower river sectors, and to increased siltation in reservoirs, navigable waterways and harbours. Acceleration of water erosion would naturally be most marked with a combination of increased rainfall and higher rainfall intensities. Any erosional response would be considerably greater on cleared or overgrazed land and must be considered among possible impacts on agriculture and land degradation.

Impact of Carbon Dioxide Warming on Climate and Man 2 0 5

6. Impacts of C02 Warming on Man and Land Use

Whatever the climate responses to C O 2 warming, the impacts on people and their livelihoods will differ widely, for example between populations in lower and higher rainfall areas, between rural and urban societies, between agricultural and non- agricultural communities, and above all between populations of the developed countries and those of the Third World who predominate by far.*

There is also uncertainty about the nature of the societies that will experience the effect of greenhouse warming, say in the year 2030, since many are now undergoing rapid demographic, social and economic changes. There is also a risk of over- emphasizing the climatic determinants of economic and social behaviour. Local responses will doubtless also be influenced by non-climatic environmental factors such as soils and terrain, and may differ markedly with levels of infrastructure and technology, with the world economic situation and commodity prices, with location and the availability of markets or storage facilities, with incomes and demand levels and with a wide range of cultural perspectives and social values.

In the light of recent experience of the vulnerability of many Third World communities in the semi-arid tropics to adverse climatic stress - particularly the rural communities most directly dependent on agriculture which still predominate in Africa and Asia - it is possible to identify basic needs in the light of which the consequences of greenhouse warming for such communities might be assessed. These include agricultural production and security of food supplies, water supplies for man and his livestock and for irrigation, fuelwood and other sources of energy and the incidence of pests and disease. The growing urban populations in those areas, although less directly exposed to the agricultural consequences, will be equally exposed to impacts on living conditions and essential supplies, whilst the economic effects on incomes and on regional and national economies will be shared by both sectors.

In semi-arid tropical regions within developed countries such as Australia, the agricultural consequences of greenhouse warming will directly affect only a small number of people although the economic repercussions could be more widely felt, to the level of the national economy. Its effects on living conditions and services will however be shared with the more numerous urban populations. Although it may be possible to maintain essential services and present living standards in face of predicted changes in mean climatic conditions, any change in the magnitude and frequency of extreme climatic events such as tropical cyclones remains an unknown threat at this stage.

A major concern shared by developing and developed countries is the possible impact of the greenhouse effect on land resources in the semi-arid tropics. Over large areas, vegetation and soils have already been degraded through land use, with

* The consequences of sea-level rise were treated separately in the workshop for which this paper was originally prepared and for that reason are not d iscussed here.

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consequent loss in productivity and an increased vulnerability to drought which have had economic and social repercussions. The impact of the greenhouse effect on the course of desertification and on progress in combating it in these areas is of great importance because of the need to preserve the natural resources as the bases of future productivity and to assist man's adaptation in the face of continuing climatic change.

6.1. Consequences for Rain-Fed Agriculture and Pastoralism

The direct and indirect effects of increased ambient CO2 concentrations on annual crops - including C4 crop plants in the semi-arid tropics - could to some extent cancel out in terms of agricultural production, in that the direct stimulus would be towards greater vegetative production, whilst the indirect effect of temperature increase, by shortening the plant life cycle, could be earlier maturation and lowered yields. The direct effect, being temperature-dependent, should be relatively more important in the tropics than in higher latitudes (Gifford, 1988). However both effects will be considerably influenced by associated changes in amount, duration and variability of rainfall and in humidity and soil moisture, together with changes in incoming solar radiation linked with cloudiness, and these constraints will be of overriding importance in the semi-arid tropics.

The serious consequences for rainfed agriculture and food production in Third World countries of a continuing reduction in rainfall in the semi-arid tropics, as envisaged in the 'Villach' scenario, particularly in association with higher temperatures and increased evaporation, can be gauged from recent experience of two decades of low rainfall and frequent drought over much of Africa, and from periodic failure of the Indian monsoon in the same period. Near the dry frontiers of agriculture especially, such a trend - particularly if linked with shorter and more unreliable rainy seasons - would result in declining average yields and more frequent failure of crops and in mounting livestock losses in severe droughts. With resulting food shortages and increased food prices together with loss of income from the sale of crops and livestock, unsatisfactory nutritional levels could fall further. Famine could occur in the worst-hit communities and among the economically and socially disadvantaged.

In some tropical countries, such as Nigeria, decline in production in semi-arid areas might be offset by agricultural developments in higher-rainfall areas less adversely affected by climate change, but in many others - for example Mexico and India - cultivable land in the wetter parts is already under intensive use. In countries which are entirely semi-arid or arid, including several Sahelian states, the agricultural consequences of decreased rainfalls could have serious repercussions on the national economy through necessary increases in food imports.

The increasing aridity of the 'Villach' scenario could bring a retreat of cropping limits of up to 200 km by the year 2030 in areas such as West Africa, involving enforced displacement of agricultural communities. Planned resettlement and


where possible reallocation of land in higher rainfall areas, as in less-populated parts of the Sudanian zone in West Africa, could be an eventual solution, but drought relief through food aid and other forms of assistance will almost certainly be required in many marginal areas during the early stages of adverse climatic impact. In higher rainfall areas, a necessary agricultural restructuring could include a switch to more drought-tolerant crops within the millet-sorghum-maize range, with selection of slower-maturing varieties to resist the ripening induced by higher temperatures. Dryland farmers will also need to adopt methods to trap and store water, make more efficient use of soil-moisture and reduce evaporation, not only to counter the rainfall trend but to benefit from the direct biological effects of CO2 increase. Such practices have already been developed and tested in several countries of the semi-arid tropics but have tended to remain on research station farms and demonstration plots, and their dissemination through extension services could now take on a new urgency.

Lower rainfalls in adjacent arid rangelands, by restricting the seasonal movements of nomadic pastoralists or displacing more sedentary systems, could bring increasing grazing pressure on what are presently semi-arid areas. At the same time, traditional supportive interactions between pastoralists and farmers, such as stubble-grazing of cropland and exchange of meat for grain, could be rendered less effective. An initial phase of hardship through livestock losses, with social disrup- tion and possible clashes of interest between farmers and pastoralists, would call for relief through provision of foodstuffs during drought and subsidies to assist the movement of livestock. Eventually, abandoned agricultural lands could be developed for pastoralism systems adjusted to the changing climate through changes in flock composition assisted by animal breeding and selection for productivity combined with heat and drought tolerance. This would doubtless require investment in a new infrastructure, including for example the provision of tubeweUs at watering points and facilities for growing forage crops under irrigation.

Any change in the variability of rainfall will be critical under the 'Villach' scenario, particularly to subsistence farming communities in marginal areas where failure of seasonal rainfalls is already a main cause of hardship and indebtedness. Neither climatic scenario includes predictions under this head and it is not certain that present inverse relationships between amount and variability of rainfall would be maintained under the greenhouse effect. Nevertheless the impacts of a failure of seasonal rains would be harsher with diminishing rainfalls, and the need for adaptive response more urgent.

With increase in rainfall and a longer rainy season, as under the 'Australian' scenario, and assuming that evaporation increase through warming is balanced by suppression of transpiration under higher ambient CO 2 levels, the greenhouse effect should on the whole benefit rainfed crops and pastures in the semi-arid tropics providing that there is some substitution of slower-maturing crop strains to suit higher temperatures and longer wet seasons. The possibility of introducing more productive staples, for example a replacement of millet by sorghum and maize, could al-

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leviate the present deficiency in food production in tropical Africa. It would however be mistaken to presume that this increased potential would

necessarily be realised, even in areas such as tropical Africa and northwestern India where present levels of food production are unsatisfactory. This assumes a willing- ness by farmers to change to new crops and farming routines, and to invest the additional labour and other inputs involved with higher-yielding crop varieties. The benefits of producing a local food surplus may not be evident where low incomes hold down demand or where there are no storage facilities. Similarly, an increase in production of cash crops such as cotton and groundnuts will depend on price incentives and hence on the state of external markets, and these would most certainly be profoundly affected by any large-scale change in the conditions of production.

Depending on market conditions, commercial growing of grain sorghum, grain legumes and oilseeds in northeastern Australia could extend into areas with soils of high moisture capacity (McKeon et aL, 1988), as it did in response to above-average rainfalls in 1955-75, and production of tropical forage crops in support of the pastoral industry might revive in other northern parts, but rainfed cropping in tropical Australia has a history of recurrent failure linked with economic conditions and soil factors rather than with limitations of climate. The main benefits from rainfall increase are likely to accrue to the extensive cattle ranching industry through increased production from native pastures. In the Nordeste of Brazil, higher rainfalls could allow an expansion of cash cropping of cotton and sisal from the Agreste into the western ranchlands, but such a response is likely to be hampered, as in the past, by existing land ownership.

The poleward shift of rainfall zones under the 'Australian' scenario could allow an expansion of agriculture on the dry margin of the semi-arid tropics of the order of 200 km by the year 2030 assuming a greenhouse equivalent of CO 2 doubling. In areas where pressure of population on land resources is high and where cropping has been pushed to existing climatic limits, as in parts of Sahelian Africa and northwestern India, this could result in further encroachment by peasant farmers on grazing lands; as evidenced by the northward advance of cropping of more than 100 km in Niger during the wetter years of the 1950's and early 1960's, despite legislation to stop it (Bernus, 1980). The Niger experience illustrates how the loss of better-watered pastures to farming could create hardship and hunger among pastoralists, contribute to overgrazing and deterioration of rangelands and lead to disputes among land-users. This suggests a need to identify such areas of potential agricultural advance, to plan ahead for changes in land use and settlement and to accommodate the needs of existing land users, supported by the establishment of titles to land where these are presently unclear.

Under the increased rainfalls of the 'Australian' scenario, areas once climatically marginal for cropping could become less at risk, but drought hazard might not be eliminated; it would in any case remain high at any new agricultural frontier should


cropping again advance to its climatic limit, whilst any ensuing farming changes that increased water requirements could maintain the level of risk elsewhere.

6.2. Consequences for Irrigated Agriculture

Irrigation agriculture is important in several countries of the semi-arid tropics as the basis for intensive production of food and industrial crops and on a smaller scale serves widely as a supplement to rainfed cropping, providing dry-season production and cash income and acting as an insurance against failure of the rains. Any diminution in production from rainfed cropping in response to lower rainfalls, as foreshadowed under the 'Villach' scenario, could increase dependence upon it when at the same time supplies of irrigation water from local rivers and surface storages and from shallow groundwater would be reduced. Schemes based on local supplies would face decreases in irrigated area and cropping ratios. Two decades of low rainfall in West Africa provide evidence of this, in that diminished wet-season discharges of the Senegal River, decreased flooding in the inland Niger delta and the shrinkage of Lake Chad have already had serious impacts on basin irrigation and flood-recession cropping and grazing.

Some of the largest irrigation developments in the semi-arid and arid tropics, for example in Pakistan, northwest India and the Sudan, draw their water from exogenic rivers rising in wetter lower latitudes - which could experience an increase in rainfall under the 'Villach' scenario - or in high mountain catchments, and their supplies should not be critically affected by reduction in local rainfalls, although the water requirements of irrigated crops would increase with higher temperatures. Some international water-sharing arrangements may come under pressure however. Mexico, a country very dependent on irrigation, could face a marked drop in local water supplies and severe problems in maintaining food production, and under such conditions, probably accompanied by increasing water scarcity in the southwestern United States, there could be pressure on both parties to seek a revision of treaty arrangements for sharing the waters of the lower Colorado River.

Under a drier climate, efficiency in transmission and application of irrigation water would be at a premium, and programmes to reclaim salinized and unproduc- tive irrigable lands, as in Pakistan, would take on a new urgency. Where groundwater is non-saline, pump discharges from tubewells in areas subject to water logging would assume greater importance. Improvements which could save water include lining irrigation canals, better land preparation, more flexible supply schedules to match the soil-moisture needs of crops, and the matching of the cultivated area with irrigation capacity to ensure adequate soil leaching under higher temperatures. Past experience indicates that improved efficiency at the field level to meet the new challenges will also depend on economic incentives and better support services, including greater availability of inputs and credit and better market-

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ing and transport facilities as well as farmer-training. A tendency for rapid maturation and significant lowering of yields of irrigated annual crops under higher temperatures and CO 2 concentrations (Gifford, 1988), together with likely water scarcity, would require the urgent development of slower-maturing crop varieties and possible changes in crop type to reduce water needs, with a shift of emphasis towards the criterion of production in relation to amount and cost of water used.

A combination of a drier climate in the semi-arid tropics and higher rainfalls in equatorial latitudes would raise the question of the feasibility of inter-basin trans- fers in support of new large irrigation projects. In Africa and South America, with the largest tracts of equatorial climate, this would be rendered difficult because the semi-arid areas are commonly plateaux above equatorial lowlands. In addition, apart from the deterrent of enormous costs, large-scale irrigation has not proved to be a panacea in the semi-arid tropics, particularly in Africa. Commonly established without due attention to their impacts on the environment and on local rainfed agriculture, large irrigation projects have all too often brought unforeseen and adverse social economic consequences (Adams and Grove, n.d.).

The rainfall increase of the 'Australian' scenario should increase irrigation water supplies from surface sources and from shallow groundwater in the semi-arid tropics and generally increase the efficiency of natural and man-made storages. Aras such as northwest India and East Africa, where there is a need for further irrigation but where it is presently limited by water shortage, could benefit at the cost of investment to harness the increased potential. Any associated changes in the frequency of very heavy rainfalls and large flood discharges might necessitate modification or even reconstruction of existing dams and spillways. Another concern is a possible increase in erosion rates. Many reservoirs in the dry tropics - for example the Khashm el Girba Dam storage in the Sudan - are already undergoing rapid siltation, and there could be need for greater efforts in soil conservation in the catchments. In the many parts of the semi-arid tropics which already have undeveloped irrigation potential, such as Andean South America and northern Australia, the economic and other reasons for this are likely to remain valid despite any favourable climatic change.

Waterlogging and secondary soil salinization in irrigated lands in the semi- arid tropics could be worsened if higher rainfalls and a resulting increase in water supplies were to result in over-irrigation, the effects of which could be compounded under higher temperatures. To benefit from increased irrigation and cropping ratios, present efforts to lower and control watertables through improving land drainage and pumping would need to be intensified. The importance of improved engineering operation down to the level of field irrigation by the farmer would be no less than in the case of a reduction in water supplies, and would call for similar incentives and support measures for farmers, whilst there would be a similar need to select slower-maturing crop varieties adapted to the warmer climate.


6.3. Consequences for Drinking Water Supplies for Man and Livestock

Local supplies of drinking water in rural areas in developing countries, commonly already unsatisfactory, could deteriorate further as a result of greenhouse warming. A reduction in annual rainfalls combined with higher temperatures could diminish the yield and reliability as well as the purity of water supplied from shallow wells and natural and man-made storages, increase the daily task of fetching water and watering animals, and possibly lead in the end to the abandonment of settlements and pastures. A large number of town-dwellers in these countries face similar problems. Even where piped domestic supplies exist they are commonly already barely adequate in the face of growing populations and rising consumption in the towns, and existing supply schemes could need to be substantially enlarged with declining rainfalls. Any associated increase in rainfall variability would add to the unreliability of local rural supplies, whilst larger storages and new and possibly more distant sources of water could be needed to maintain piped supplies.

In conditions of growing water scarcity, conflicts of need could arise - at the village level between farmers and pastoralists for example. These already exist on a larger scale, notably in the developed world. In the southwestern United States, domestic and industrial uses have taken water supplies from irrigation, and comparable situations could arise in the semi-arid tropics should industrial development in Third World countries proceed against a background of declining rainfalls. The limited water resources in the semi-arid tropics would be a dominant factor in planning any necessary resettlement of rural and urban populations from the driest areas and any restructuring of primary production in response to climatic change, and in the longer term an increasing handicap to areas already disadvantaged in terms of economic development. Exploitation of groundwater resources could help relieve the problem and in the longer term there could also be pressures to create large inter-basin water transfer schemes linking with lower latitudes, on the scale of those in California and Arizona, but for geographical reasons these are likely to be costly.

Increased seasonal rainfalls on the scale envisaged under the ~ustralian' scenario could ease water-supply shortages to the extent that resulting increases in river discharges, in inflow to surface storages and in recharge to shallow groundwater outweighed the increased evaporation losses resulting from higher temperatures. The extent of the benefits would depend on the reliability of the increased rainfall and could also be offset if any associated increase in the incidence of extreme rainfalls resulted in more frequent severe flooding and increased sediment transport - for example through damage to installations, siltation in reservoirs and increased turbidity in water supplies. Considerable investment could be needed to modify existing structures and provide new schemes to benefit from or cope with increased discharges and flood frequencies.

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6.4. Consequences for Renewable Energy Supplies

In Third World countries the main impact of greenhouse warming on renewable energy supplies will be through its influence on the supply and consumption of fuelwood. The 'fuelwood crisis' is at its worst in the semi-arid tropics, above all in Africa and India where climatic constraints on regrowth are combined with considerable population pressure and where - despite afforestation programmes - the accelerating destruction of woodland far outstrips its replacement. Any change in climate must be significant for this serious imbalance.

In these communities greenhouse warming will reduce energy needs for heating in areas with cool winters and low night temperatures, but over large parts of the semi-arid tropics this accounts for a minor part of fuelwood use. A more important consideration must be the impact of changes in rainfall on fuelwood production and on the replenishment of fuelwood resources through afforestation, not only to meet present shortages and slow down the depletion of reserves, but as a desirable alternative to an increasing use of fossil fuel in face of rising atmospheric concentrations of CO>

A decline in rainfalls would undoubtedly exacerbate the fuelwood problem in the immediate future since most counter-measures must be long-term. On the demand side, the introduction of more efficient wood stoves has already had some success, for example in West Africa. In Iran, government assistance with substi- tuting kerosene for fuelwood has helped to reduce the destruction of woody vegetation in particular areas, but this is unpalatable as a general solution in face of the greenhouse problem. Large-scale afforestation must be the general answer to the supply problem, at the same time answering a need for environmental improvement and opposing the greenhouse effect whilst exploiting it biologically through the stimulation of tree growth by increased CO2 concentrations. It remains a feasible solution even with the prospect of diminishing rainfalls, for there has been significant progress over the last 25 years in selecting trees for arid and semi-arid areas and in developing methods of growing them. The problems, as in the past, are likely to be social and economic rather than environmental.

Increased zonal rainfalls under the 'Australian' scenario would assist afforestation, with the bonus of countering any associated increase in potential erosion rates.

Much of the energy used in homes and business and public premises in those parts of developed countries within the semi-arid tropics, for example northern Australia and the southernmost U.S.A., is for air conditioning and refrigeration powered by electricity, as it is increasingly by higher income minorities and corresponding buildings in Third World cities. Greenhouse warming, particularly with any increase in humidity, will increase levels of human discomfort where indoor climates already cause strain on an average of more than 100 days in the year and will accelerate the growth of this energy expenditure. The effect is likely to be smaller in the Third World countries and would in any case be less apparent against


the background of a predictable surge in energy consumption as industrial development proceeds there. The most effective counter-measures will be improvements in insulation, design, layout and shading of buildings and the introduction of man- datory conservational standards in energy use similar to those which reduced energy expenditure per employee in the commercial sector in the U.S.A. by 24% between 1970 and 1982.

Present trends indicate that accelerating energy consumption for Third World industrialisation - in the semi-arid tropics as elsewhere - will be met mainly from fossil fuels, but it is presumed that programmes to minimise the greenhouse effect will emphasize renewable energy, including hydropower and solar power. The developing world has by far the greater share of unexploited hydropower capacity, much of it in the humid tropics but with considerable potential in drier areas also. The potential for hydropower development will obviously be affected by rainfall change in the semi-arid tropics, although less so for installations on exogenic rivers. A combination of declining rainfalls in the semi-arid zone and rainfall increase in equatorial latitudes, as projected in the 'Villach' scenario, could encourage the establishment of continent-wide electricity grids for interzonal sales of electricity, for example in West and Central Africa and in South America.

Photovoltaic modules which can provide power to settlements and equipment without access to power grids are particularly appropriate for sparsely-settled parts of the semi-arid tropics, and their use is extending as their relative costs fall. Low-cost solar cookers and seed driers for individual households have also been developed, although not as yet widely adopted. The sunny climate of the semi-arid tropics must favour considerable growth in the use of solar power: the impact of greenhouse warming on this will be mainly through changes in cloudiness during the rainy season as they affect hours of sunshine, and is unlikely to be drastic.

6.5. Changes in Disease and Pest Ecology

An increase in rainfall sufficient to cause more extensive flooding over longer periods and the growth of lakes and swamps could, in combination with warming, bring a resurgence of water-related and insect-borne human and livestock diseases that are widely endemic in the semi-arid tropics. The extension of breeding grounds for mosquitos particularly would threaten an increased incidence of malaria, dengue, yellow fever, filariasis, Australian encephalitis and epidemic polyarthritis. Anti-malaria programmes in Third World areas in the tropics - and in Africa especially - have made only partial headway, their progress hampered by new resistance of mosquitos and parasites to larvicides, insecticides and drugs. With 200-400 million new cases and 5 million deaths annually, malaria could well readvance. In the semi-arid tropics, where mosquito-breeding is checked by dry seasonal conditions, a wetter and longer rainy season could have a particularly drastic impact, and many areas from which malaria has been eradicated, such as northern Australia, could again experience epidemics. Among other waterborne

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diseases, bilharzia - still very widespread in Africa - could increase, whilst oncho- cerciasis, although now largely controlled, could re-emerge.

With the primitive water supplies and sanitation commonly found, an increase in flooding and waterlogging due to rainfall change could through their effects on drinking water and sanitary conditions lead to an increase in diarrheal diseases - a major factor in infant mortality - typhoid fever, dracunculiasis (guinea worm disease) and cholera.

The expansion of wet savanna into presently drier areas of tropical Africa, particularly if accompanied by closer shrub and tree cover and increasing numbers of game animals, could carry with it the tsetse-fly infestation and trypanosomias is presently confined to the wetter margin of the semi-arid tropics, with serious consequences for man and his domestic animals.

Many of the fungal leaf diseases affecting crops in the semi-arid tropics, already more troublesome in abnormally wet seasons, could become still more prevalent with a combination of higher temperatures and increased humidity. Post-harvest food losses through fungal diseases are already serious in the semi-arid tropics, and problems of food storage could also increase under such conditions, particulary since sorghum, which does not keep well, is likely to become more widely grown under higher rainfalls.

Invasion of crops and pastures by weeds - commonly C3 species - could increase with higher ambient CO2 levels, whilst associated climatic and biological changes could lead to increase in insect, bird and rodent pests on standing crops and stored food and forage.

Any significant change in rainfall and savanna zonation and related shift of the desert margin must also affect patterns of locust invasion in West and East Africa and in the Indian semi-arid zone. Drier conditions could bring the present recession area equatorwards, and with it the depth of invasion of savanna and cultivated lands by feeding swarms. With higher rainfalls an increase in the extent of swampy breeding grounds near the desert margin could lead to more frequent invasions by the Red and African Migratory locusts, whilst at the same time bringing cultivation closer to existing centres of outbreak. Any change in the wind systems in the wake of the ITCZ which influence the migrations of locust swarms could also be significant. New control programmes will be needed to combat possible increases in the incidence of diseases and pests, and existing arrangements, as for locust control, may need to be reshaped in view of projected changes.

6.6. Effects on Land Degradation and Land Rehabilitation Programmes

The incidence of land degradation in the semi-arid tropics indicates its close con- nection with man's use of land and water resources, and in particular the growing pressures on dryland ecosystems arising from a number of causes, including increase in human populations and livestock numbers, and economic social, technological, and political trends. These have interacted with contributory environ-


mental factors, some of which could be changed by greenhouse warming, including a generally low and unreliable rainfall, periodic drought stress, and the inherent sensitivity of dryland ecosystems under pressure of land use. The degraded land systems in the semi-arid tropics, through their lowered productivity and heightened vulnerability to drought stress, have been seen as contributing to unsatisfactory social and economic conditions in Third World countries of the semi-arid tropics, especially among rural communities (UNCOD Secretariat, 1977). It is accordingly relevant to consider the possible impacts of greenhouse warming on the status and trends of land degradation and on the course of land rehabilitation measures to combat desertiflcation which are planned to extend into the next century (United Nations, 1978).

Major forms of land degradation include: - soil deterioration and accelerated wind and water erosion in rainfed croplands,

with enlarging areas of sand drifting and increase in atmospheric dust; - pasture degeneration in rangelands, together with accelerated erosion in de-

nuded areas; - destruction and deterioration of woodlands and of woody vegetation generally;

ly; - waterlogging and secondary salinization of soils in irrigated lands; - adverse changes in river regimes, with increased flash flooding and seasonality of

discharges and deterioration in water quality through increased salinity and sediment concentrations;

- deterioration in yield, quality and reliability of ground-water supplies. Any substantial decrease in rainfall, as under the 'Villach' scenario, would lower

the carrying capacity of rangelands in the semi-arid tropics through reduction in biomass, particularly of annual grasses, despite the direct benefit from CO2 increase. The increased pressure on available forage could be heightened further if the grazing range of animals were reduced as a result of higher temperatures or through any decrease in water supplies. Range management could be made even more difficult if rainfall variability and drought incidence were to increase at the same time. There would be a relative decline in the more palatable pasture species and an increase in less desirable plants, particularly in C4 species including woody shrubs. Diminished ground cover and trampling by stock could accelerate wind erosion and drifting of sandy soils, with localised mobilisation of the extensive fossil dunefields in drier parts of the semi-arid tropics, particularly around watering points, stock routes, stock yards and settlements. The severity of wind erosion could be influenced by changes in wind regime finked with the greenhouse effect - not as yet predictable - as much as by the reduction in vegetation cover induced by diminished rainfall (Ash and Wasson, 1983). The source areas for the atmospheric dust clouds so prevalent in Sahelian Africa during the dry Harmattan season lie to the north of the semi-arid zone, and changes in atmospheric stability and wind regime over larger areas will be critical for any changes in dust conditions.

On rainfed croplands, accelerated mineralization of humus under drier and

216 J.A. Mabbutt

warmer conditions together with a reduction in crop residues could break down soil structure and leave it vulnerable to wind erosion, particularly on bare fallows, with further winnowing out of organic matter and mineral nutrients. Despite lower rainfalls, sheet and gulley erosion of degraded soils could accelerate under intense tropical downpours. Drier and warmer conditions would favour secondary salinization of irrigated soils in waterlogged areas and wherever shortage of irrigation water results in inadequate leaching. Since the need for fuelwood in most Third World countries would not be greatly reduced by warmer and drier conditions, whilst regrowth of woody vegetation could be slowed, the present unsatisfactory situation could worsen.

Despite a decrease in mean rainfalls, runoff ratios could be higher from less- vegetated and degraded soil surfaces and the streamflow response more rapid, with flash flooding, more intermittent regimes and generally higher sediment loads. Bank erosion and shoaling in mid-courses and silting in lower stream sectors are predictable. In general, recharge to alluvial aquifers would diminish and watertables fall.

Under the 'Australian' scenario of increased rainfalls with greenhouse warming, one possible adverse effect could be increased rates of water erosion on cropland and degraded rangeland. On the evidence of a relationship between latitude and erosion rates in the Eastern Highlands of Australia, Wasson et al. (1988) predict that a poleward displacement of summer rainfall isohyets by 200 km could increase average annual erosion rates on cropland by between 12 and 26%, quite apart from the possible effects of changes in rainfall intensity and storm frequency.

On irrigated lands, increase in rainfall could lead to an extension of waterlogging and secondary salinization in areas with inadequate land drainage. Higher river discharges and more frequent floods could require modifications to storage dams and spillways and any increase in sediment loads could lead to more rapid siltation of reservoirs. Resulting changes in channel form, in levels, extent and frequency of flooding and in siltation in estuaries and river mouths, as already described under the hydrological impacts of greenhouse warming, could bring additional problems. A rise in watertables in alluvial aquifers could in some areas bring adverse consequences for health, land use, settlement and communications.

An assessment of the UN Plan of Action to Combat Desertification (PACD) seven years after its inception (Mabbutt, 1984; Dregne, 1984) indicated little progress in the semi-arid tropics, particularly in the developing countries that were generally worst-affected. Among stated reasons was the persistence of low rainfalls into the 1980's, particularly in Africa. The climatic effects of greenhouse warming must clearly have a major influence on the further progress of anti-desertification programmes and should now be taken into account in programme and project planning.

A reduction in rainfall in the semi-arid tropics, as under the 'Villach' scenario, will give added urgency to revegetation programmes generally, to water conservation in agriculture, to reclamation of irrigated lands and where feasible the exten-


sion of irrigation, and to the protection of water supplies for settlements, livestock and agriculture. Large-scale planning will be required to bring about necessary changes in land use and the resettlement of agricultural populations. Any change in rainfall variability could be critical for this response. An increase in rainfalls, as under the Australian scenario, could clearly assist progress with reforestation, revegetation of rangelands and the improvement of agricultural soils. Planning supported by political and financial commitment to combat desertification would be no less necessary if the benefits of rainfall increase were to be realised however. Without appropriate management, it could merely result in more intensive exploitation and an accelerated degradation of land resources.

7. Conservation of Biological Resources

The savannas of the semi-arid tropics have a rich biological diversity reflecting a long evolution through the Cainozoic, free from the glacial interruptions of higher latitudes. During that long period the savannas have extended and contracted periodically with climatic changes at least as drastic as those to be expected from the greenhouse effect. The wide range in physiognomy and floral and faunal diversity reflects a variety of adaptive strategies in the face of change, including species evolution and migration. Man has played an important part in the late stages of this long history and the present savannas are by no means natural climax forms. On the other hand their extensive survival is partly explained through the repulsion of agricultural colonisation by infertile and often crusted soils and the exclusion of pastoralists by livestock diseases.

The stability of savanna ecosystems under a variable climate depends on a diversity which is expressed structurally in the range of physiological adaptations to seasonal stress, spatially in a characteristic mosaic of local variations, and tem- porally in the phenological range of its components. The essential duality of the herbaceous and woody components of savanna vegetation is maintained through complex interactions and feedbacks, including plant competition, fire and grazing and browsing by large ruminants adapted to a coarse forage low in nutrients (Walker, 1984). Ecological response to greenhouse warming and related climate change will be complicated moreover by man-made constraints, including fragmen- tation of savanna areas through land use and modifications in fire regime. In Australia the already significant impact of introduced feral animals could be altered through climatic change: a rise in temperatures could reduce the northward range of the rabbit whilst any change in rainfall must affect the competitiveness of species introduced from humid environments. Because of these additional constraints the natural adaptation of savanna ecosystems to greenhouse warming will need to be supported by conservational management.

The tropical savannas are presently protected by a combination of reserves and legislation. In east and southern Africa in particular the reserves include large national parks with the large herbivores and their predators for which a role as

218 J.A. Mabbutt

game animals has largely been supplanted by their economically significant interest for tourists. The very extent of these 'natural' landscapes adds to their attraction for visitors from urbanised developed countries. Management of parks and reserva- tions and protective legislation alike are in some areas subject to political pressures arising from land shortage, and presently face problems arising from encroaching land uses, poaching, illegal burning and mining developments.

Some of these problems could increase with greenhouse warming, when at the same time the maintenance of the biological diversity of the savannas will be vital for their adaptation to climate change. From a practical viewpoint the preservation of genotypes such as wild maize and tropical legumes will be important in the search for new crop varieties to assist change in tropical agriculture, whilst the store of plants rich in alkaloids will be needed for future medical research. Protective management of the fauna will be equally important in this adaptation to the extent that their seasonal migrations and essential range of habitats have become limited by the advance of cropping and pastoralism.

Existing reserves and ecological baselines, generally established on the assumption of climatic stability, will probably need to be extended and supplemented in face of the CO2-related climate change, with attention to securing sufficient con- tiguous areas to preserve the range of interdependent ecosystems and to facilitate migration. In such new layouts it may be prudent to establish reserves across rainfall gradients to accommodate the predicted changes (Main, 1988). Management practices within existing reserves may need to be changed, for example in timing and frequency of burning, with appropriate shifts between the early-dry season light burning suited to drier savanna and the later more intense burning more bene- ficial to primary production in moister savanna conditions (Hadley, 1985). Protec- tive legislation may also require amendment, not only in general support of changes in reserve management but also in the interests of newly-threatened species such as migratory birds.

Acknowledgements

The author thanks the Beijer Institute for the opportunity to present an early draft of this paper to a Workshop on Developing Policies for Responding to Future Climatic Change at Villach, Austria in September-October 1987, and acknowl- edges the help of authors of background papers prepared for that Workshop. He is grateful to Dr. A.B. Pittock and two anonymous referees for their guidance in its subsequent revision and to Kevin Maynard for drawing the text figure.

References

Adams, W. M. and Grove, A.T.: n.d., Irrigation in Tropical Africa. Problems and Problem Solving, Cambridge African Monographs 3, Centre for African Studies, Cambridge, 148 pp.

Allen, R. J.: 1985, The Australasian Summer Monsoon, Teleconnections, and Flooding in the Lake Eyre Basin, South Australian Geographical Papers No. 2, Royal Geographical Society of Australasia (S. A. Branch), Adelaide, 47 pp.


Ash, J. E. and Wasson, R. J.: 1983, 'Vegetation and Sand Mobility in the Australian Desert Dunefield', Z. Geom. Suppl. Bd. 45, 7-25.

Aston, A.R.: 1984, 'The Effect of Doubling Atmospheric CO 2 on Streamflow: A Simulation', J. Hydrol. 67, 273-280.

Bernus, E.: 1980, 'Desertification in the Eghazer and Azawak Region', in Mabbutt, J. A. and Floret, C. (eds.), Case Studies in Desertification, Natural Resources Research XVIII, UNESCO, Paris, pp. 115-146.

Bolin, B., Do6s, B. R., J~iger, J., and Warrick, R.A. (eds.): 1986, The Greenhouse Effect, Climatic Change, and Ecosystems, SCOPE 29, John Wiley, Chichester, 541 pp.

Bryson, R. A. and Goodman, B.M.: 1980, 'Volcanic Activity and Climatic Changes', Science 207, 1040-1044.

Cochem~, J. and Franquin, P.: 1967, A n Agroclimato logy Survey of a Semi-A rid Area in Africa South of the Sahara, WMO Technical Note No. 86, WMO, Geneva, 136 pp.

De Deckker, P., Kershaw, A. P., and Williams, M. A. J.: 1988, 'Past Environmental Analogues', in Pear- man, G. I. (ed.), Greenhouse. Planning for Climate Change, CSIRO, Melbourne, pp. 473-488.

Dennett, M. D., Elston, J. and Rodgers, J. A.: 1985, 'A Reappraisal of Rainfall Trends in the Sahel', J. Clim. 5, 353-361.

Dickinson, R.E.: 1986, 'How Will Climate Change? The Climate System and Modelling of Future Climate', in Bolin, B., Do6s, B. R., J~iger, J. and Warrick, R. A. (eds.), The Greenhouse Effect, Climat- ic Change, and Ecosystems, SCOPE 29, John Wiley, Chichester, pp. 207-270.

Dregne, H. E.: 1983, Desertification of Arid Lands, Advances in Desert and Arid Land Technology and Development Vol. 3, Harwood Academic Publishers, New York, 242 pp.

Dregne, H. E.: 1984, 'Combating Desertification; Evaluation of Progress', Environmental Conservation 11, 115-121.

Eckholm, E.: 1975, The Other Energy Crisis; Firewood, Worldwatch Paper 1, Worldwatch Institute, Washington, 22 pp.

FAO: 1978, Report on the Agro-Ecological Zones Project. Vol. 1. Methodology and Results for Africa, World Soil Resources Report No. 48, FAO, Rome, 158 pp.

Gibbs, W. J.: 1975, 'Drought - its Definition, Delineation and Effects', in Drought, Special Environ- mental Report No. 5, WMO, Geneva, pp. 1-39.

Gibbs, W. J. and Maher, J. V.: 1967, Rainfall Deciles as Drought Indicators, Bulletin 48, Common- wealth Bureau of Meteorology, Melbourne, 33 pp.

Gifford, R.M.: 1988, 'Direct Effects of Higher Carbon Dioxide Concentrations on Vegetation', in Pearman, G. 1. (ed.), Greenhouse. Planning for Climate Change, CSIRO, Melbourne, pp. 506-519.

Gleick, P. N.: 1987, 'Regional Hydrologic Consequences of Increases in Atmospheric CO 2 and Other Trace Gases', Climatic Change 10, 137-160.

Gregory, S.: 1969, 'Rainfall Reliability', in Thomas, M. E and Whittington, G. W. (eds.), Environment and Land Use in Africa, Methuen, London, pp. 56-82.

Hadley, M. E.: 1985, 'Comparative Aspects of Land Use and Resource Management in Savanna En- vironments', in Tothill, J. C. and Mott, J. J. (eds.), Ecology and Management of the World's Savannas, Australian Academy of Science, Canberra, pp. 142-158.

Hare, E K.: 1977, 'Climate and Desertification', in UNCOD Secretariat (eds.), Desertification: Its Causes and Consequences, Pergamon Press, Oxford, pp. 63-167.

Hare, E K.: 1983, Climate and Desertification: A Revised Analysis, World Climate Programme No. 44, WMO/UNEP, Geneva, 149 pp.

Harris, D. R.: 1980, 'Commentary: Human Occupation and Exploitation of Savanna Environments', in Harris, D. R. (ed.), Human Ecology in Savanna Environments, Academic Press, London, pp. 31-39.

Holland, G. J., McBride, J. L. and Nicholls, N.: 1988, 'Australian Region Tropical Cyclones and the Greenhouse Effect', in Pearman,-G. I. (ed.), Greenhouse. Planning for Climate Change, CSIRO, Mel- bourne, pp. 438-455.

Hulme, M.: 1987, 'Secular Changes in Wet Season Structure in Central Sudan', Z Arid Environments 13, 31-46.

Jaeger, J.: 1988, Developing Policies for Responding to Climatic" Change, WCIP-1, WMO/UNEP, Geneva, 53 pp.

Johnson, R. W. and Tothill, J. C.: 1985, 'Definition and Broad Geographic Outline of Savanna Lands, in Tothill, J. C. and Mott, J. J. (eds.), Ecology and Management of World's Savannas, Australian Academy of Science, Canberra, pp. 1-13,

220 J.A. Mabbutt

Konate, M.: 1984, 'Climate of the Sorghum and Millet Cultivation Zones of the Semi-arid Tropical Regions of West Africa', in Virmani, S. M. and Sivakumar, M. V. K. (eds.), Agro-Meteorology of Sorghum and Millet, ICRISAT, Patancheru, India, pp. 101-113.

Lamb, H. H.: 1982, Climate, History and the Modern World, Methuen, London, 387 pp. Mabbutt, J.A.: 1984, 'A New Global Assessment of the Status and Trends of Desertification', En-

vironmental Conservation 11, 103-113. McKeon, G. M., Howden, S. M., Silburn, D. M., Carter, J. O., Clewett, J. E, Hammer, G. L., Johnston,

G. W., Lloyd, E L., Mott, J. J., Walker, B., Weston, E. J., and Willcocks, J. L.: 1988, 'The Effect of Climate Change on Crop and Pastoral Production in Queensland', in Pearman, G. I. (ed.), Green- house. Planning for Climate Change, CSIRO, Melbourne, pp. 546-563.

Main, A. R.: 1988, 'Climate Change and Its Impact on Nature Conservation in Australia', in Pearman, G. I. (ed,), Greenhouse. Planning for Climate Change, CSIRO, Melbourne, pp. 361-374.

Manabe, S. and Wetherald, R.T.: 1980, 'On the Distribution of Climate Change Resulting from an Increase in CO 2 Content of the Atmosphere', J. Atmos. Sci. 37, 99-118.

Manabe, S, and Wetherald, R. T.: 1986, Reduction in Summer Soil Wetness Induced by an Increase in Carbon Dioxide; Science 232,626-628.

Manabe, S, Wetherald, S. T. and Stouffer, R. J.: 1981, 'Summer Dryness due to an Increase of Atmos- pheric CO2 Concentration', Climatic Change 3,347-386.

Meehl, G. A. and Washington, W. M.: 1986, 'Tropical Response to Increased CO2 in a GCM with a Simple Mixed Layer Ocean: Similarities to an Observed Pacific Warm Event, Monthly Weather Rev. 114,667-674.

Mitchell, J. EB.: 1983, 'The Seasonal Response of a General Circulation Model to Changes in CO2 and Sea Temperatures', Quart. J. Roy. Met. Soc. 109, 113-152.

Mitchell, J, E B., Wilson, C. A., and Cunnington, W. M.: 1987, 'On CO 2 Climate Sensitivity and Model Dependence of Results', Quart, J. Roy. Met. Soc. 113, 293-322.

Nathan, R. J., McMahon, T. A., and Finlayson, B. L.: 1988, 'The Impact of the Greenhouse Effect on Catchment Hydrology and Storage-Yield Relationships in Both Winter and Summer Rainfall Zones, in Pearman, G. I. (ed.), Greenhouse. PlanningJbr Climate Change, CSIRO, Melbourne, pp. 273-295.

Nicholson, S. E.: 1980, 'The Nature of Rainfall Fluctuations in Subtropical West Africa', Monthly Wea- ther Rev. 109,473-487.

Nicholson, S. E. and Chervin, R. M.: 1983, 'Recent Rainfall Fluctuations in Africa - Interhemispheric Teleconnections', in Street-Perrott, A. and Beran, M. (eds.), Variations in the Global Water Budget, D. Reidel, Dordrecht, Holland, pp. 221-238.

Paltridge, G. and Woodruff, S.: 1981, 'Changes in Global Surface Temperature from 1880 to 1977 Derived from Historical Records of Sea Surface Temperatures', Monthly Weather Rev. 109, 2427- 2434.

Perrin de Brichambault, G. and Wallen, C. C.: 1963, A Study of Agroclimatology in Semi-Arid and Arid Zones of the Near East, Tech. Note 56, WMO, Geneva.

Pittock, A, B.: 1975, 'Climatic Change and Patterns of Variation in Australian Rainfall', Search 6, 498- 504.

Pittock, A. B.: 1980. 'Towards a Warm Earth Scenario for Australia', in Pearman, G. I. (ed.), Carbon Dioxode and Climate: Australian Research, Australian Academy of Science, Canberra, pp. 197- 209.

Pittock, A. B.: 1985, 'Recent Climatic Change in Australia: Implications for a CO2-Warmed Earth; Climatic Change 5,321-340.

Pittock, A. B.: 1988, 'Actual and Anticipated Changes in Australia's Climate', in Pearman, G. I. (ed.), Greenhouse. Planning for Climate Change, CSIRO, Melbourne, pp. 35-51.

Raudkivi, A. J.: 1979, Hydrology, Pergamon Press, Oxford, 478 pp. Ritchie, J. C. and Haynes, C. V.: 1987, 'Holocene Vegetation Zonation in the Eastern Sahara', Nature

330, 645-647. Schlesinger, M .: 1986, 'Equilibrium and Transient Climatic Warming Induced by Increased Atmos-

pheric CO2' , Climate Dynamics 1, 35-51. Schlesinger, M. E. and Mitchell, J. E B.: 1987, 'Climate Model Simulations of the Equilibrium Climatic

Response to Increased Carbon Dioxide', Rev. Geophys. 25, 76(I-798.

Impact of Carbon Dioxide Warming on Climate and Man 2 21

Servant, M. and Servant-Vildary, S.: 198(/, 'L'environment Quaternaire du Bassin du Tchad; in Williams, M. A. J. and Faur6, H. (eds.), The Sahara and the Nile, Balkema, Rotterdam, The Nether- lands, pp. 133-162.

Singh, G., Joshi, R. D., Chopra, S. K., and Singh, A. B.: 1974, 'Late Quaternary History of Vegetation and Climate of Rajasthan Desert, India', Phil Trans. Roy. Soc. London, B., Biological Sciences 267, 467-501.

Street, E A. and Grove, A. T.: 1979, 'Global Maps of Lake-Level Fluctuations since 30,000 yr B.E', Quaternary Research 12, 83-118.

Tucker, G. B.: 1988, 'Climatic Modelling: How Does it Work?', in Pearman, G. 1. (ed.), Greenhouse. Planning for Climate Change, CSIRO, Melbourne, pp. 22-34.

UNCOD Secretariat (eds.): 1977, Desertification: Its Causes and Consequences, Pergamon Press, Oxford, 448 pp.

UNESCO: 1977, Map of the World Distribution of Arid Regions, MAB Tech. Note 7, UNESCO, Paris, 54 pp.

UNESCO: 1979, Tropical Grazing Land Ecosystems. A state-of-knowledge report prepared by UNESCO/UNEP/FAO, Natural Resources Research XVI, UNESCO, Paris.

United Nations: 1978, United Nations Conference on Desertification. Round-Up, Plan of Action and Resolutions, United Nations, New York, 43 pp.

US Dept. of Energy: 1985. Direct Effects of lncreasing Carbon Dioxide on Vegetation, US Department of Energy, Washington D.C., DOE/ER-0138,286 pp.

Verhoog, E H.: 1987, 'Impact of Climate Change on the Morphology of River Basins', in The Influence of Climate Change and Climatic Variability on the Hydrologic' Regime and Water Resources, Publica- tion No. 168, IAHS, Washington, pp. 315-326.

Walker, B. H.: 1985, 'Structure and Function of Savannas: An Overview, in Tothill, J. C. and Mott, J. J. (eds.), Ecology and Management of the World's Savannas, Australian Academy of Science, Canber- ra, pp. 83-91.

Walker, J. and Rowntree, E R.: 1977. 'The Effect of Soil Moisture on Circulation and Rainfall in a Tro- pical Model, Quart. J. Roy. Met. Soc.103, 29-46.

Wasson, R. J., Fleming, P. M., and Srikanthan, R.: 1988, 'Sediment Delivery and Stream Behaviour, with a Note on Wind Erosion', in Pearman, G. I. (ed.), Greenhouse. Planning for Climate Change, CSIRO, Melbourne, pp. 231-237.

Wigley, T. M. L.: 1983, 'The Pre-lndustrial Carbon Dioxide Level', Climatic Change 5, 315-320. Wigley, T. M. L. and Jones, E D.: 1985, 'Influences of Precipitation Changes and Direct CO 2 Effects

on Streamflow', Nature 314,149-152. Young, A.: 1976. TropicalSoils and Soil Survey, Cambridge University Press, 468 pp.

(Received 8 March, 1988; in revised form 22 February, 1989)

impacts of carbon dioxide warming on climate and man in the semi-arid tropics

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