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Outline the main controversies involved in the desertification debate.

Despite the fact that desertification is not a new concept, having occurred for millennia (Grainger, 1990) or since the Neolithic (Spooner, 1985), it has only in the last three or four decades been thrust into the limelight. It was conceptualised as a serious problem for the first time in the 1970s, upon the recognition of the varying patterns of spatial desert conditions, especially in the Sahel, where such changes coincided with a period of sustained drought. It was perhaps the first big environmental issue (Thomas + Middleton, 1994) which encompassed not only environmental issues, but also wider social, political and economic angles. It is thus no surprise that its study, interpretation and extent are clouded with controversy, as the complexity of the issue does not lead to any universally accepted and agreed explanations of the phenomenon. This complexity is further hampered by our practical inability to extrapolate individual causes and effects, from what is in reality, an interactive, multi-factorial system.

Desertification is fraught with confusions and contradictions, generalisations based on a lack of data, and uncertainties stated as facts (Thomas + Middleton, 1994)

Similarly, concern arises as to the spatial areas of vulnerability to desertification, their areal extent, and the temporal nature of the phenomena, which will set it apart from the naturally variable and stochastic nature of climate in drylands, and emphasise its irreversibility.

Semi-arid areas are more at risk to the supposed effects of desertification, as these form the desert (arid) borders, which are where desertification was initially believed to operate. The United Nations Environmental Programme (UNEP) believes that 35 % of the world is at risk to desertification, with especial risk for the 22 countries in the Sahel region, forming the southern border of the Sahara. It is here that desertification gained its current profile, in part as the result of the disastrous and well documented periods of famine that occurred sporadically throughout the 1960s, 70s and 80s, in which between 50 000 and half a million people are estimated to have died. This essay aims to assess the importance of the role played by desertification, outlining the various sides of the argument, from the definite recognition of desertification as an important and pressing environmental problem, to those who believe it is a myth , or that there is insufficient evidence to categorically deny either conclusion.

There is a large quantity of hysteria and ignorance surrounding the desertification question. Early images often portrayed unstoppable marching dunes, ready to ruthlessly take over productive lands, such that Stebbing (1935) wrote;

The desert is advancing . The end is obvious; total annihilation of vegetation and the disappearance of man and beast. (in Mortimore, 1987) Developments in our understanding have changed this ideology, as is noted by Lindqvist and Tengberg; the conception of the advancing desert edge is undoubtedly wrong. Given the need to discuss the topic to more academic lengths, in light of the problems being experienced by Sahelian countries, the United Nations Conference on Desertification (UNCOD) was convened in Nairobi in 1977, which began the modern era of discussion.

For the conference to get over the first hurdle, they had to initially tackle the controversial definitional issues, which in some instance, had led to gross overestimations of the extent of desertification by up to 66 %. (Thomas, in Ringrose et al 1995) The term desertification had, since its inception by Aubreville (1949) been adapted into popular parlance, resulting in it being generally overused and misused as a result. (Middleton, 1987) An element of social or interpretational relativism hampered attempts to clearly define desertification, as the differing views of each of the commentators resulted in differing, and often conflicting, aspects that each thought should be included in a definition. Generally, our increasing knowledge has allowed us to refine the definition over the years, such that it has become more specific over time. An early attempt was simply the spread of desert like conditions , which has since been replaced many times, up to the one in present usage, finally adopted by UNCED in 1992/3; desertification is land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and adverse human impacts”. By introducing land degradation it was implicitly understood that desertification leads to long lasting and possibly irreversible desert-like conditions. This definition also brings to play the relative contributions of human and natural factors, which themselves are causes of much controversy, as well as the spatial extent of desertification, which is now known not only to occur on desert borders, but in a number of dryland environments.

However, in spite of the effort put in, and continuing to be donated, to the resolution of all definitional controversies, there are still commentators who disagree with the need for a separate term to cover what they think, is covered by the term land degradation . Le Hou rou (1959) preferred to use desertisation as a term to describe the extension of the desert into areas previously not desert as a result of adverse human actions. El Baz (1983) said further that both the terms desertification and desertisation were vague and unnecessary , and recommended the universal use of the already defined term land degradation , which could be tailored to each individual user. (Verstraete, 1983). This would be one way, he said, to avoid the definitional controversies of desertification. However, this may lead to further confusion, as people are forced to disaggregate human and natural factors. I have regarded rainfall fluctuations as contributing to desertification, although I viewed the degradation as mainly man-made. (Rapp, 1987) This would then bring us back to the idea of contrasting individual interpretations. It is clear that the definition in use at present is by no means the last, and can t fully encompass the whole suite of interrelated causes and effects, mediated over numerous spatial and temporal scales. Nevertheless, even if one was able to find such an answer, we would still be left to debate the particulars of the term desert . Thus, desertification seems the best term to use as opposed to any other, for it is a more encompassing word to refer to the whole process. (Verstraete, 1986 in Grainger, 1990) One argument remains that its present global usage is strong justification for its future use. Nevertheless, it will remain a problem for all authors, that before one can talk about the causes and effects of desertification, they must first define how they interpret the term. The only way to eradicate definitional problems is to continue research, such that it would provide a firmer quantitative basis upon which to form a more objective judgement.

So, we now understand what is meant by desertification, but does it exist, or is it simply a myth ? To analyse this, there is a need to understand the relative roles of natural variability, and adverse human action. A knowledge of the physical environment only illuminates half of any environmental issue, since an appreciation of factors in the human environment is also required before an issue can be fully understood. (Middleton, 1995) The interconnection of the human and physical environments are evidenced by a UNEP report, outlined by Rapp (1987) in which he notes that the phenomenon of desertification is linked to four factors.

1) The natural vulnerability of ecosystems in drylands

2) Population pressures leading to an overexploitation of resources

3) Economic considerations that hinder the establishment of appropriate land-use.

4) Political unrest that is not conducive to long term actions.

This shows the complex causal nature of desertification, encompassing not just environmental, but also social, political and economic considerations.

There is much disagreement as to whether desertification actually exists. Lamprey s (1975) study of the advancing Sahara boundary was perhaps the first to be widely accepted amongst the scientific community. His results over a seventeen year monitoring period, showed that the southern boundary of the Sahara had advanced southwards by an average of 5.5 km / yr, based on an absolute movement of 100 km over the period of study. Such an idea harped back to Stebbing s advancing front ideology, although Lamprey believed his evidence proved that the deserts were actually expanding, to the extent that his ideas were taken in by regional governments, development agencies and the scientific community alike.

However, it was found that Lamprey had made two fundamental mistakes. Firstly, a methodological mistake was made, as he compared the desert boundary as it was on an aerial photo, to a boundary given by a vegetation map. Secondly, the timing of his results meant that a period after a severe drought was compared with the same area after favourable rainfall. Thus as was later noticed, the evidence provided by Lamprey (1975) for desertification in the Kordofan Province of Sudan was flawed, and later examinations show what was seen was simply the natural fluctuation of drylands to

The prevailing conditions.

What lamprey observed was an expression of resilience of dry ecosystems, not the spread of desert conditions. (Olsson, 1993)

Tucker et al (1991) rubbished the idea of an advancing desert, with results of a ten-year study of the expansion and contraction of the Sahara desert . Simply taking the figures for 1980 and 1990, results would have shown that the southern boundary of the Sahara had expanded by 130 km. However, annual remotely sensed data showed that this would hide great interannual fluctuations of the southern boundary, which instead acts more as a tide; ebbing and flowing. Figures include a movement of 99 km south (1983-4), 110 km north (1984-5), 33 km north (1985-6), 55 km south (1986-7) and 100 km north (1987-8). This seems to show that over the 1980s, there was no progressive encroachment of desert, but rather a fluctuating pattern based primarily around the natural variability of rainfall in such areas. Nevertheless, Tucker et al (1991) note that we remain limited by our data, such that we are unable from this, to tell whether there is a long-term trend in the direction of movement of the Sahara s boundaries.

This lack of evidence for long-term desertification is furthered by Hellden (1991), who notes that three independent studies by Olsson, Ahlcrona and himself, show no evidence of the phenomenon. Each compared temporal changes of desert-like conditions on remote sensed images, from which Hellden (1991) is able to conclude; none of these studies verified the creation of long-lasting desert-like conditions in the Sudan during the 1932-84 period. He believed that there was no evidence for the creation or possible growth of desert patches around 103 examined villages and water holes. Similarly, no major shift in the northern cultivation limit was found, nor Sahara desert encroachment or dune transformations. Allied with no major change in vegetation in the area of study that could not be explained by rainfall variations, it was found that systematic Sahara desert encroachment trends were not identified.

Perhaps the most controversial area within the desertification debate is the attribution and detection of causation. Rapp s (1987) four factors outlined already show the involvement of both physical and human causes, although their individual extent and importance is hotly debated. As Middleton (1995) notes, it is difficult to distinguish between the adverse effects of human action, and the response of drylands to natural variations in the availability of moisture. However, his early view (Thomas + Middleton, 1994) states clearly; People cause desertification. This is in contrast to Hellden, who emphasises the climatic background to desert changes. What we do know, is that man plays a role to some degree, such that a UNEP (1991) definition stated, desertification results mainly from adverse human impact.

Overcultivation, whether it concerns the intensification of farming, or its spread into unsuitable areas, or an unsuitable change in crop or technology, is one frequently cited cause. Deforestation and tree cover damage is another causal factor, as it reduces protection for the soil, and can indirectly lead to its impoverishment by diverting away sources of fertiliser. Similarly fuelwood shortages have resulted in large treeless plains around many Sahelian cities. Desertification can supposedly result from salinisation, alkalisation, and waterlogging, caused by the human mismanagement of irrigation projects.

Whilst less relevant to the Sahel example, Khalaf (1989) adds sand / gravel quarrying and off-road driving to his inventory of desertification causes. With the rapid rise in population from 150 000 (1946) to 1.7 m (1985), humans have put a much greater pressure of the land. Overgrazing beyond the natural carrying capacity has led to the deterioration and removal of the protective vegetation layer, quarrying to support increasing urbanisation has exposed sands previously protected by overlying gravels, off-road traffic leads to the compaction and sterilisation of the sub-soil which leads to the loss of vegetation, and finally the application of irrational control measures has worsened the whole problem. The involvement of adverse human actions has led to an increase in dust and sandstorms that plague the region. Khalaf (1989) seems in agreement with Middleton (1987) that the increases in dust storm activity are particularly marked in areas of degradation where man had played an identifiable role. Whilst it is recognised that mobile sand is a natural phenomena (Khalaf, 1989) one is left to talk about the enhanced effect of man. The question remains, however, as to the permanence of such changes, although only time can reveal the answer.

Mortimore (1987) poses a similar human environment interaction, in which the regional climate is varied by four feedback mechanisms. The first, or dust theory , says that increasing dust in rain-bearing clouds modifies the air temperature gradient, causing a stabilisation of the atmosphere, a suppression of convention, and thus less rain. The albedo theory shows how decreasing vegetation leads to an increasing reflectivity of the land, which increases subsidence and inhibits rain. The moisture theory outlines how a diminution of rainfall is caused, as a result of part of this moisture having previously been evaporated into the atmosphere from the vegetation. Finally, the CO2 theory implies that globally increasing CO2 levels affect rainfall distribution by modifying tropical heating. Such feedbacks represent the combined role of man and climate, which remains a confuse issue.

The feedback mechanisms that are supposed to influence rainfall behaviour show the difficulty of separating dependent from independent variables in the process of desertification . even if it can be defined without ambiguity. (Mortimore, 1987)

The anthropogenic activity posed as a causal factor, that has been the subject of most controversy is overgrazing. Conventional thinking has been that having too many animals, or animals of the wrong type, has degraded the land. This has been justified by two reasons; firstly, predator / prey models of herbivory illustrate how high grazing pressure can hold a system below its maximum productivity. Secondly, herders are individuals likely to push the common rangelend to an overgrazed state due to self-interest (Hardin, 1968). Due to overgrazing, edible plant species and removed, and replaced if at all with woody, inedible plants. A further impact of animals, is through trampling of the soil, which ruins its structure, and increases its susceptibility to erosion.

The traditional view of overgrazing has come under much scrutiny in recent decades. A major problem with the old view is that the carrying capacity of the land is very difficult to meaningfully calculate. This is a result of an annually variable productivity in line with rainfall. Mace (1991) says that one cannot assign a non-variable carrying capacity in such areas as a result of the inherently variable nature of the climate and conditions. A further definitional / perceptional confusion is added to the argument, as for example, range scientists have adapted the word for their use from its initial use by population biologists. Thus any attempt to calculate a carrying capacity will suffer from variability, error and subjectivity (Mace, 1991) in a similar way to desertification. Mace does admit, by use of an example in the Gourma region of Mali, that a 15 km grazing orbit around water sources can be found post dry season, although its evidence is short lived, as the resilience of the environment ensures a return to productivity upon the next rains. Returning to the notion of cattle destroying their resource base, Mace (1991) says;

If rangelend degradation is occurring on a large-scale, then livestock numbers should be in long-term decline. There is no evidence for this.

Despite the outlined adverse human impacts on desertification, controversy still remains as to their extent if at all, as Hellden (1991) notes, there is a lack of data to substantiate the man-made hypothesis. He believes that the driving force behind desertification, if it is proved not to be a myth , is rainfall variability. Most of the negative and positive annual deviation from the mean annual production of natural and agricultural systems can be explained by climatic variations, without the need to consider possible adverse effects on the environment by man. (Hellden, 1991) That many of the deserts are older than mankind land credence to this view. Like Tucker et al s (1991) work on fluctuating desert boundaries, Hellden notes the large spatial and temporal variations in green biomass productivity, assigning the cause to normal natural phenomena , resulting in no permanent shifts in vegetation. Their similar conclusions add weight to his argument; it is not yet possible to confirm the prevailing concept of the status and cause of desertification in the Sahel. (Hellden)

Grainger (1990) likens the role of climate to that of a catalyst in a chemical reaction. Thus drought creates the conditions whereby human impact on the land increases, and the capacity of the land to tolerate it decreases. In this instance, drought is the indirect cause, whilst poor land use is the direct cause. However, poor land use can also be the result of poverty, ignorance, greed, or socio-economic change. So whilst natural and man-made factors catalyse each other, similarly social change, upset, or an unjust market system can exacerbate the problem. Of major concern, is the role of desertification in famine. Conventional wisdom states that desertification does directly lead to famine.

Mortimore (1987) believes that endemic societies have developed forms of insurance to reduce the risk of famine, which cope adequately on the whole, but which have been steadily eroded by societal and economic changes. Olsson (1993) believes that the link between famine and desertification is weak, being superseded by a greater importance played by the malfunctioning Sahelian market system and unjust credit and shiel systems. Thus a severe rainfall deficit of 1984 triggered speculation in food, which pushed prices out of the reach of rural budgets. Mortimore (1987) recognises the harsh role played by economics during such times. At the village level, the price of grain is as exogenous as the rainfall, and the mechanisms of its variability as impossible to control . Thus where food was available nationally, the rural dwellers couldn t afford it, and the internal redistributive mechanisms were poor. Such an idea alludes to Sen s notion of inadequate entitlements to resources

A brief, final note of controversy is that of the permanence of damage to drylands. Earlier commentators such as Stebbing believed in the complete irreversibility of land which had been lost to the desert. Much trouble in the present day, stems from our inability to show long-term impacts, that would confirm the time-scale over which desertification could be mediated. As both Hellden and Tucker et al noted from their studies of 10 to 30 years, such time-scales are insufficient to substantiate the desertification phenomenon. Olsson (1993) believes that we are ignorant to the inherent resilience of dryland ecosystems, which has been gravely underestimated . Therefore, what we may see as desertification, may be a temporary condition resulting from the annual climate variation. In many of the above examples, recovery of the ecosystem has not been long to follow dry years, emphasising this resilience. However, in the case of Oursi in northern Burkina Faso (Lindqvist + Tengberg, 1993), spatial discontinuities are shown between areas of recovery, and those without. Despite the increased rainfall (1985+), the lack of recovery suggests a deterioration of this ground resilience. Nevertheless, the high variability of productivity in time and space demonstrated by many other authors may still prove to mitigate the role of desertification.

It is evident that there is no issue within the framework of desertification that is free from controversy. An inadequacy of detailed, in-depth data and long-term observations continues to hamper the attribution and detection of causal factors in any greater detail than we have at present. The only way to eradicate these problems is to continue research and debate, and to this end, present controversy may be good for our understanding of desertification, as it forces us to examine aspects in greater detail. Without this increased knowledge, we are unable to substantiate either side of the debate; desertification reality or myth, human or physical? Accompanying the changing definitions, have been changing methodological approaches to the way in which such degradation might be managed. These seem to have changed from an environmental to a social point of view, alongside the growing recognition of the impacts of humans, and their social, political, and economic environment. However, even given an unambiguous definition, distinct causality and long-lasting effects, one is still left to ponder the different, and highly varying, temporal and spatial scales on which these factors operate.


Grainger, S. 1990 The Threatening Desert

Hellden, U. 1991 Desertification time for an assessment?

Ambio, 20, 372-383

Khalaf, F. 1989 Desertification and aeolian processes in the Kuwait desert.

Journal of Arid Environments, 16, 125-145

Lindqvist + 1993 New evidence of desertification – northern Burkina Faso Tengberg Geografiska Annalar, 75A, 127-135

Mace, R. 1991 Overgrazing overstated?

Nature, 349, 280-281

Middleton, N. 1987 Desertification and wind erosion Sahel Mauritania

Oxford SOG Research paper

Mortimore, M. 1987 Shifting sands and Human sorrows. Response to

Desertification Control Bulletin, 14, 1-14

Olsson, L. 1993 On the causes of famine drought, desertification

Ambio, 22, 395-403

Rapp, A. 1987 Reflections on desertification 1977-87, probs and prospects

Desertification Control Bulletin, 15, 27-33

Ringrose et al 1995 Progress towards the evaluation of desertification Botswana desertification Control Bulletin, 27, 62-68

Thomas + 1994 Desertification exploding the myth.


Tucker et al 1991 Expansion and contraction of the Sahara Desert 1980-90

Science, 253, 299-301

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