Desertification

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R. L. HEATHCOTE

1 INTRODUCTION: ORIGINS OF CONCERNS

Desertification is commonly understood to mean the spread of desertlike conditions. The term came to international attention with the environmental devastation and loss of human and animal life accompanying extensive droughts in West Africa's Sahel region over the period 1968 to 1973, which were captured on televised documentary programs and in the print media. This media coverage likely played an important part in the subsequent extensive United Nations-sponsored international and national aid programs and associated scientific investigations into what appeared to be evidence of long-term environmental deterioration. These activities led to the United Nations Conference on Desertification (UNCOD) in Nairobi, Kenya, in 1977.

In fact, however, concern for evidence of adverse environmental change can be found in the accounts of European explorers in the deserts of Africa, Arabia, and Asia from the late eighteenth century onward. A Danish expedition to Arabia of 1761 to 1767 (Hansen, 1965) and the activities of explorers such as Charles Doughty (1843-1926), Sven Hedin (1865-1952), Baron von Richthofen (1833-1905), and Elsworth Huntington (1876-1947) uncovered evidence of past civilizations amid the desert sands. For example, Huntington's 1907 The Pulse of Asia drew upon his own and the earlier explorations to suggest that the evidence of climatic variation and associated environmental deterioration and reduced capacity to support the population in central Asia might have caused the folk migrations that led to the Mongol invasions of southwest Asia and Europe in the thirteenth century.

In the areas of nineteenth-century European colonization, droughts on the Great Plains of North America in the 1880s to 1990s (e.g., Brown, 1948), eastern Australia

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

1895 to 1902 (Heathcote, 1965), and southern Africa in 1918 and the early 1920s (Kokot, 1955) raised concerns for the long-term viability of human occupation. The 1930s saw further concerns for what appeared to be a combination of climatic variability and human mismanagement of the land's resources, with the evidence of the Dust Bowl on the southern Great Plains in 1935 and the first global study of soil erosion (Jacks and Whyte, 1938). In West Africa, Stebbing (1935, 1937) anticipated an advance of the southern edge of the Sahara as a result of overzealous clearance of the forest and savannah vegetation, and Ratcliffe (1938) studied the link between overgrazing and the expansion of the Australian deserts.

Post-World War II reconstruction efforts reflected continuing popular concerns about human use of the land in such writings as Sear's Deserts on the March (1949) for the Great Plains, Calder's Men against the Desert (1951), and Aubreville's Climats, Forêts et Desertification de l'Afrique Tropicale (1949) for African deserts. Pick and Aldis (1944) published concerns for Australia s Dying Heart. Lowdermilk (1953) undertook a global review of soil resource mismanagement and repeated earlier calls to reverse the loss of resources.

That such reversals were possible was the claim of the Israeli author, Reifenberg, whose book, The Struggle between the Desert and the Sown (1955), documented the efforts of the new nation of Israel to reclaim the deserts as the culmination of a long history of desert advances and retreats in this part of southwest Asia. In 1966 a retired British forester (Baker, 1966) with experience in Kenya and Nigeria and founder of the Men of the Trees Society, published his Sahara Conquest, yet another scenario to reclaim the desert, in this case by massive tree-planting schemes.

International scientific interest culminated in the United Nations Arid Zone Research Program, which ran from 1951 to 1971 (UNESCO, 1953). This program brought together scientists from around the world to study the globe's arid areas in an effort to understand both the physical characteristics and the past and present land uses in order to plan better for future management and possible desert reclamation. In effect this program laid the foundations for future United Nations interest in the desertification phenomenon.

This concern over the increasing evidence of environmental deterioration seems to have reflected in part not only the increased scientific evidence of that deterioration, but also a growing moral concern for the global environment and human relations with it. The "environmental movements" of the 1970s and their subsequent "green movements," by focusing upon evidences of resource mismanagement through human ignorance or greed, saw desertification as but one example of the adverse impact of human resource use upon the environment—an impact that had been of concern earlier (Glacken, 1967) and that gained an added ethical dimension (White, 1967; Passmore, 1980; Nash, 1990). Thus, by the 1990s desertification had been consolidated as an international "catch-cry," a political force and a global scientific problem for some of the reasons noted above, but also possibly as a result of the end of the Cold War removing political issues (Driver and Chapman, 1996) that had been competing with environmental ones.

2 DEFINING THE PHENOMENON

Specific definitions of desertification have been many and varied, partly reflecting the fact that it has been seen on the one hand as a process of environmental deterioration and on the other as the product of environmental deterioration—the devastated environment itself (Glantz and Orlovsky, 1986).

All definitions agree, however, upon certain basic criteria:

• Explicit or implicit evidence of changes in the characteristics of the present environment by comparison with previous characteristics.

• Those changes have resulted in a reduced capacity of the environment to support human life.

• The resultant degraded environment has a desertlike appearance.

• Unless rectified, those changes may continue to further reduce the capacity of the environment to support human life in the future.

To adequately and convincingly identify desertification in those terms, scientists need compatible objective data for a considerable time period for specific areas of the world. Identifying either the processes at work or the end product—the deserti-fied landscape—has proved to be extremely difficult.

The first problem has been the lack of the requisite historical data sets and the assumption of linear trends between those data sets that do exist. The degradation of the soil resource capacity to sustain vegetation or crops (by removal from wind or water erosion, by modification of chemical content through salinization, alkalization, or acidification, or by modification of the soil texture and thus moisture retention capacity by compaction) is recognized to be a significant indicator of desertification. Global descriptions of soil characteristics are still sparse and of varying quality, although formulas for estimates of soil erosion rates are available and have been partly used in the Global Assessment of Human-Induced Soil Degradation (GLASOD). GLASOD was commissioned by the United Nations Environment Programme and was incorporated in the World Atlas of Desertification as the most scientifically acceptable measure of desertification (UNEP, 1992).

The other main indicator of desertification is the change in vegetation cover, whether of natural or domesticated plants. Change here may be indicated by reduced vegetation quantity and/or quality (in terms of reduced biodiversity, and/or reduced biomass, and/or productivity), but these indicators are much more difficult to measure. The problem here is the interseasonal and interannual fluctuations in that cover and the extent to which the cover at any given time reflects a linear trend toward increasing or decreasing density or whether it reflects merely cyclical (natural) changes or fluctuations.

In West Africa, where the contemporary concern for desertification originated, there have been serious disagreements among scientists as to the extent of recent vegetation change (Table 1). The differences reflect contrasting assumptions on the

TABLE 1 Conflicting Views of West African Desertification: Forest Loss during the Twentieth Century

Orthodox View" Revisionist View"

Forest Cover* Forest Cover4

TABLE 1 Conflicting Views of West African Desertification: Forest Loss during the Twentieth Century

Country

1900

1990s

Loss/Gain (±)

1900

1990s

Loss/Gain (±)

Ghana

9.9

1.6

-8.3

2.5

1.6

-0.9

Cote d'Ivoire

14.5

2.7

-11.8

6.0

2.7

-3.3

Benin

1.1

0.4

-0.7

0.5

0.4

-0.1

Sierra Leone

5.0

0.5

-4.5

0.1

0.5

+0.4

Liberia

6.5

2.0

-4.5

5.5

4.8

-0.7

" Sources noted in Fairhead and Leach (1996, p. 189). b Forest cover areas in millions of hectares. Source: Fairhead and Leach (1996).

" Sources noted in Fairhead and Leach (1996, p. 189). b Forest cover areas in millions of hectares. Source: Fairhead and Leach (1996).

historical extent of natural vegetation and the relationships between climate and natural vegetation, differences in the classification of "natural" vegetation, and a failure to recognize historical human-induced deliberate or accidental revegetation of previously sparsely vegetated areas.

The global satellite coverage of the early 1970s onward has improved upon the earlier estimates of vegetation conditions, which had been based upon subjective assessments by individual explorers and other observers. Satellite coverage has provided the base for a Global Vegetation Index—the global vegetation cover averaged over the 1983 to 1990 period as a baseline for subsequent documentation of trends (UNEP, 1992). This of course does not help the debates on changes prior to the 1970s.

A second problem has been the interpretation of the data itself. Faced with what appears to be a desertified landscape in terms of apparently degraded vegetation and/or soils, the scientist must assess whether the landscape is in decline from a more productive past or in process of rehabilitation to a more productive future.

A third problem is the time scale chosen for the analysis of the significance of the environmental changes. While significance on a human scale may be set on scales ranging from interseasonal variations to trends over decades or even a generation (usually seen as 20 to 30 years), significance in ecological terms may range from decades to centuries or millennia, and while these latter scales may be seen as irrelevant for human planning purposes, they may be relevant to any attempt to explain what appears to us to be desertification processes. The basic difficulty remains, however, for the vegetation cover, as for the soil degradation observations, that the implied trend will depend upon the length of time between observations. Oscillations in conditions from whatever cause, which may occur between those observations in time, may not be noticed.

A fourth problem facing definitions of desertification is the fact that there are several interested parties concerned with the phenomenon, some of whom may have an interest in stressing the extent of and dangers from desertification, while others may be more interested in playing down its significance. For scientists seeking research funding for projects of personal interest, for self-identified victims of the process and their political leaders, for political groups who have identified particular organizations or ideologies as contributing to the process, there may be an incentive to exaggerate the significance of the phenomenon, or at least its natural as opposed to the human causes (Glantz, 1977; Heathcote, 1986). For scientists seeking funding for other research areas, for resource managers accused of exacerbating the desertification processes or seeking to gain time to complete exploitive management strategies, and for political groups anxious to defend specific resource management orthodoxies, there may be an incentive to play down the dangers. There seems to be evidence of both approaches to the phenomenon in the literature (Heathcote, 1980; Garcia, 1981; Watts, 1983; Beinart, 1996; Chapman and Driver, 1996).

A fifth and final problem in defining and specifically in accounting for desertification is that the phenomena are of interest to both natural and social scientists. As a result, the investigations of one group may underestimate the significance of factors of interest to the other. This division has been most evident in the debates about whether desertification stems from natural fluctuations; for example, from climatic variations such as drought periods and the associated vegetation death and soil desiccation and erosion, or from the effects of human activities producing environmental stresses that cannot be sustained in particular locations (Rhodes, 1991; Thomas, 1993).

Bearing in mind such difficulties, the variability of definitions is not surprising. However, the consensus view as adopted by the United Nations has evolved from the definition adopted by the UNCOD in 1977:

Desertification: The intensification or extension of desert conditions; it is a process leading to reduced biological productivity, with consequent reduction in plant biomass, in the land's carrying capacity for livestock, in crop yields and human wellbeing. (UNCOD, 1977, p. 3)

to that used in the World Atlas of Desertification:

desertification/land degradation is defined as: land degradation in arid, semiarid and dry subhumid areas resulting mainly from adverse human impact. (UNEP, 1992, p. vii)

and further modified as:

Desertification is land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors including climatic variations and human activities. (UN Convention to Combat Desertification in Countries Experiencing Serious Drought and/or Desertification, Particularly in Africa, June 1994)

Thus, consensus has defined the phenomenon as limited to the drier areas of the world, where highly variable climatic conditions, particularly drought, combined with the adverse impacts of human activities appear to constitute a major threat to the long-term sustainability of support for human occupation in those areas.

3 DOCUMENTING DESERTIFICATION

Since UNCOD in 1977, there have been various estimates of the area desertified. The World Map of Desertification, showing areas at risk and prepared for the conference, identified 4.56 billion hectares as "affected or likely to be affected" (UNCOD, 1977, Annex 1). If the extreme deserts were excluded, on the grounds that their condition could not worsen, the area was reduced to 3762 million hectares. In 1984, when the progress of the UN Plan of Action to Combat Desertification was reviewed, the area was reduced to 3.48 billion hectares (Tolba, 1984). Dregne and colleagues estimated 3.56 billion hectares in 1991 (Dregne et al, 1991), while the latest UN estimate is 1.04 billion hectares (UNEP, 1992).

In fact, these varying figures are not strictly compatible, since they refer to drylands defined in different ways at the different times and to the inclusion of areas suffering vegetation degradation but that may not have been suffering soil degradation. At least the most recent UN estimate (UNEP, 1992) does include estimates of soil degradation based upon GLASOD.

The 1992 estimates (Table 2) provide basic information on soil rather than vegetation degradation and while, therefore, marginally more acceptable, must still be viewed with caution. The message is that over 15% of the global soils appear to be degraded, over half of that area (53% of the 1.04 billion hectares) is located within the global drylands. The largest degraded areas are in Asia and Africa, with 38 and 25%, respectively, of the global total, and the proportions of the global degraded drylands are similar—36 and 31%, respectively. Interestingly, the largest proportion of regional drylands desertified is in Europe (33%), but all regions have at least 10% of their drylands showing desertification.

TABLE 2 Global Soil Degradation, c. 1992

Soil Degradation (million hectares)

In Susceptible In Other Areas Total Degraded

In Susceptible In Other Areas Total Degraded

Africa

319.4

24.8

174.8

10.4

494.2

25.2

Asia

370.3

22.1

376.6

14.6

746.9

38.0

Australasia

87.5

13.2

15.4

7.0

102.9

5.2

Europe

99.4

33.1

119.4

18.3

218.8

11.1

North America

79.5

10.9

78.7

5.4

158.2

8.1

South America

79.1

15.3

164.3

13.1

243.4

12.4

Total global

1035.2

20.0

929.2

11.9

1964.4

100.0

" Percentage of total area of regional susceptible drylands. h Percentage of total of regional other areas. ' Percentage of global total degraded area.

Total degraded area of 1964.4 million hectares is 15.1% of global land area. Source: UNEP (1992, p. 25).

" Percentage of total area of regional susceptible drylands. h Percentage of total of regional other areas. ' Percentage of global total degraded area.

Total degraded area of 1964.4 million hectares is 15.1% of global land area. Source: UNEP (1992, p. 25).

TABLE 3 The Most Serious Soil Degradation Areas in Drylands, c. 1992

Region

Soil Degradation Areas (strong and extreme) (million hectares)

Percentage of Global Total Areas

(strong and extreme)

(%)

Africa

74.0

53.8

Asia

43.7

31.8

Australasia

1.6

1.1

Europe

4.9

3.5

North America

7.1

5.2

South America

6.3

4.6

Total

137.6

100.0

The most serious dryland soil degradation and implied desertification is identified in Table 3. Using the worst two categories of soil degradation identified (strong and extreme degradation), Asia and Africa again dominate the scene, although it is Africa's turn to lead with almost 54% of the world's seriously degraded dryland areas compared to Asia's 32%. By comparison, the other regions each contain less than 6% of the global areas.

Maps of the desertified areas of the world, regardless of the definitions used, all show the most intensive areas of desertification to lie on the humid edge of the drylands (UNCOD, 1997; Dregne et al„ 1991; UNEP, 1992). This is usually explained as the result of the encroachment of agriculture, with its implied soil disturbance, into areas traditionally given over to purely livestock grazing, together with increasing pressure on vegetation resources for fuel and building materials from increased human populations. This somewhat simplistic view, however, has been criticised as noted below.

4 EXPLAINING DESERTIFICATION

There is an extensive literature devoted to alternative explanations for the desertification phenomenon. Essentially, the arguments fall into three camps. First are those that claim natural processes, specifically climate change or variability—mainly through drought, and feedback linkages between the characteristics of the ground surface and air temperatures, are the cause (Bryson and Murray, 1977); second are those that place the blame squarely on human mismanagement of the environment, mainly through human resource demands that exceed the capacity of the environment to supply in the long term (Sinclair and Fryxell, 1985); and third are those that see a mix of both natural processes and human activities combining and interacting to create an unstable environment (Hare et al., 1977).

In the World Atlas of Desertification (UNEP, 1992) the causes of desertification were listed as entirely human derived (Table 4) and no specific listing of natural causes was provided. Implicit, however, were links between human activities and the natural ecosystems. As identified, deforestation and overgrazing imply the reduction of vegetative cover resulting in less protection for the soils from solar insolation and increased wind speeds, leading to increased evaporation and potential wind and water erosion, along with possible increases in groundwater levels and soil waterlogging from reduced vegetation soil moisture requirements. Agricultural activities include the plowing up of fragile soils, thus baring them for wind erosion; attempts to grow crops in areas with insufficient soil moisture leading to crop failure and soil exposure to further erosion; and the excessive application of irrigation water leading to the buildup of salts in the soil (salinization).

Overexploitation implies excessive use of vegetation for fuel or building materials that reduces the capacity of the vegetation to reproduce or recycle essential nutrients to maintain soil fertility. Bioindustrial impacts imply contamination from pollution sources and are usually associated with intensive land uses outside the drylands, hence the relatively small area shown.

There is no doubt that climate variability, particularly in terms of precipitation trends over decades, has had measurable impacts upon the success of human occupation of the drylands of the globe. Some of the most telling evidence comes from the Sahel (Kates, 1981; Hare, 1983; Nicholson, 1994). In such locations seasonal climates resemble desert climates (low precipitation, high evaporation, and high solar insolation) so that any human resource management involving reduction of protective vegetation cover at such time would run the risk of permanently damaging the environment (Aubreville, 1949; Bullock and Le Houerou, 1996).

Such damage might be interpreted as the result of longer-term climate change, such as a trend toward increasing aridity, and labeled desertification. However, dryland ecosystems demonstrate considerable ability to recover from periods of desiccation so that whether or not desertification is identified could depend upon the time period chosen for the study, as suggested earlier. Indeed, the edge of the Sahara Desert defined in terms of vegetation cover has been identified from satellite imagery to have shifted 240 km south between 1980 and 1984, but after several annual retreats and advances was by 1990 only 130 km south of the 1980 location (Tucker et al., 1991). Such variation suggests the importance of short-term fluctuations rather than long-term climatic changes.

Certainly, the clearance of vegetation for cropping or through excessive livestock grazing may change the albedo (reflectivity) of the ground surface and, thus, increase the air temperatures, thereby increasing evaporation and creating droughtlike conditions that encourage soil erosion as a result of increased wind speeds. Significantly, most desert reclamation techniques involve attempts to establish new vegetation cover to directly protect the soil and to act in part as wind breaks (Baker, 1966; Zhu and Liu, 1981).

Table 4, however, omits mention of some of the less obvious but possibly equally important factors, such as the history of colonialism and recent economic development both of which have been identified as relevant to the spread of desertification. Most colonial powers in Africa, for example, introduced new tax systems that forced a monetary economy upon a previously subsistence pastoral or agricultural commu-

TABLE 4 Causes of Desertification

Attributed Cause

Area Affected (million hectares)

Percentage of Total Desertified

Deforestation

578.6

29.4

Overgrazing

678.7

34.5

Agricultural

551.6

28.1

Overexploitation

132.8

6.8

Bioindus tri al

22.7

1.2

Total

1964.4 m.ha.

100.0%

nity, requiring extra numbers of livestock to be grazed for cash sale to pay taxes, or food crops on the better soils to be replaced by cash crops and necessary food production to be pushed into areas climatically more vulnerable or possessing poorer soils more prone to erosion (Franke and Chasin, 1980; Garcia, 1981; Watts, 1983; Baker, 1984; Macdonald, 1986; Morgan and Solarz, 1994). More recently, improved veterinary services and permanent water supplies developed with foreign aid, which replaced traditionally limited seasonal supplies, allowed larger herds to be carried all year, and this often led to overgrazing of the ranges (Glantz, 1977).

In addition, the increase of human populations in the drylands threatened by desertification, from 57 millions in 1977 to 135 millions by 1984 (UNEP, 1992, p. iv), and particularly the rising populations in Africa (Caldwell and Caldwell, 1990) have brought increased pressure on the environment and, in association with the periodic devastation from civil strife and warfare, have no doubt contributed to the desertification process (Glantz, 1987).

5 FUTURE OF DESERTIFICATION

Despite over 20 years of international efforts, the complexity of the factors involved in desertification has meant that those efforts to reverse the trends have had limited success. Reviews of the results of the 1977 UNCOD initiative were not particularly impressive (Mabbutt, 1987; Odingo, 1992; Rapp, 1987; Spooner, 1987). And despite ongoing research and publications by the United Nations Environment Programme through its Desertification Control Bulletin, which documents attempts to halt desertification, the debate about definitions cannot hide the fact that the phenomenon continues to be extensive and locally increasing in the area it affects.

A new UN "Convention to Combat Desertification in those Countries Experiencing Serious Drought and/or Desertification, Particularly in Africa," was agreed to in 1994 and came into force in December 1996. The new emphasis is on support for local schemes to rehabilitate desertified areas or to reduce the potential for their expansion. This shift in scale holds better promise for the future since the specific causes of desertification are more often related to local economic, social, and poli tical events in the context of the seasonal weather rather than to broad changes in climatic patterns.

Having said that, however, global warming climate scenarios (in essence forecasts) could result in increased aridity in the drylands, with associated increases in natural soil erosion, even without human interference (Bullock and Le Houerou, 1996). Such a scenario does not offer much hope for reversing desertification in the drylands in the future, unless the pressure imposed by human resource uses can, itself, be reduced.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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