Expansion and intensification of agriculture in dryland environments can lead to land degradation. When drought occurs in such environments, further land degradation is so severe that the ecosystem cannot recover fully after the rains return, and the region becomes a desert. Should drought frequency increase in the future, vulnerability to desertification will also increase.

Desertification is a process of land degradation in arid and semiarid areas resulting from climatic variations and human activities. Degradation results from pressure from expansion of agriculture and livestock numbers, which make the land increasingly vulnerable to the impact of drought. Also, rising human populations have led people to farm on increasingly marginal land, which is even more at risk. The pressure on the land is such that, when drought occurs, the land degrades to the point that it is unable fully to recover.

Manifestations of desertification include a breakdown of soil structure, accelerated soil loss to wind and water erosion, an increase in atmospheric dust, a reduction in soil moisture-holding capacity, an increase in surface-water runoff and streamflow variability, salinization of soils and groundwater, and reductions in species diversity and plant biomass. The net result is a reduction in the overall productivity of dry-land ecosystems. This reduction leads to the impoverishment of human communities that are dependent on the land for survival.

The best examples of desertification are to be found in the Sahel region of Africa and the Rajasthan state of India, vulnerable areas on the borders of the Sahara and Thar Deserts, respectively. The underlying problem is poverty, which means few resources are available for managing the environment. Most of the people are subsistence farmers whose food supply is dependent on an adequate harvest each year. Farmers rely on summer monsoon rains. If one rainy season fails, people have very little in the way of stored food or money to see them through. The most vulnerable are the pastoralists, whose animals rapidly weaken and perish when there is nothing left to graze. Those animals that do survive will have stripped the land of vegetation so intensively that it may fail fully to recover when the rains eventually return.

The impact of drought is in some cases linked to feedback processes between the atmosphere and a land surface that is modified and used by the very population that is at risk. Stress on the land is not the sole cause of desertification, but it weakens an ecosystem's ability to withstand drought. It is an important part of the feedback chain. In wet years, there is often an expansion and intensification of grazing and cultivation of land that is otherwise marginal. Following a relatively dry year, excessive demands may be placed on the water stored in the soil. The soil will then dry, become susceptible to wind erosion, and eventually blow away. Even if the rains were to return, what soil remains would be washed away by sheet erosion and gullying. Most important, if such changes affect a large area, positive feedback processes to which localized climate is highly sensitive are set in motion, which accentuates existing anomalies in climate.

A key factor in the role of climate is stability of air. When the atmosphere is stable, upward motion of air is suppressed. Even in humid airstreams, rainfall will not occur unless stability is overcome. Subtropical high-pressure belts at about 30œ latitude on either side of the equator are associated with masses of stable air. This air accounts for most of Earth's large areas of arid and semiarid climate. Rain occurs when the air in these high-pressure belts is displaced by the advance of unstable, rain-bearing oceanic air moving with the Inter-Tropical Convergence Zone (ITCZ). This movement of rain-bearing air is known as the summer monsoon. The Sahel and Rajasthan are at the northern edge of an area that experiences drought when the summer monsoon does not reach far enough north.

Earth's atmospheric pressure systems migrate with the seasons. Should the ITCZ advance north only a few degrees of latitude less than normal, large areas of the desert borderlands will experience a reduction in rainfall. If climate changes so that the norm is redefined and the northward movement of the rain-bearing air is frequently below the previous norm, the desert would expand southward. There are various theories as to how this expansion could come to pass.

One popular theory asserts that overgrazing of the land by livestock leads to loss of vegetation, resulting in bare soil being exposed to wind erosion. Large quantities of windblown dust in the atmosphere reduce the amount of solar radiation reaching and heating the land surface. Overgrazing also increases the land surface's albedo--that is, it causes the surface to reflect more incoming energy from the Sun compared with a vegetated surface.

This additional reflection of solar energy results in land cooling. Thus, overgrazing has multiple consequences that increase cooling and air stability, which effectively adds to the extent of the stable air of the subtropical high-pressure belt and erodes the northern edge of the advancing rain-bearing air of the summer monsoon. The dry conditions result in a further loss of vegetation, causing increased reflectivity and subsidence of dry air even further to the south, and so on in a cycle of positive feedbacks.

The Intergovernmental Panel on Climate Change (IPCC) has commented on the possibility of increased frequency of droughts in certain areas. If the vulnerable desert borderlands are among the regions affected, then desertification may be intensified. However, connections between anthropogenic increases in greenhouse gases (GHGs) in the atmosphere and changed drought frequency and intensity are only speculative, as climate models that are used to assess these connections have not been shown to be reliable. The level of scientific uncertainty and the existence of conflicting results are such that reliable predictions of future climate are not possible at this time.

Owing to the uncertainty surrounding scientists' understanding of the global climate, neither the trends in drought occurrence nor the interannual variability of droughts can be simulated reliably in global climate models. Despite this, projections have been made about future trends in precipitation extremes linked to increases in GHGs. Vulnerability will decline if drought frequency and intensity are reduced. The salient point is that there is no clear answer to the question of what will happen to trends in drought occurrence.

References:

1)         Delville, Philippe L. Societies and Nature in the Sahel. London: Taylor & Francis, 2007.

2)         Geist, Helmut. The Causes and Progression of Desertification. Burlington, Vt.: Ashgate, 2005.

3)         Middleton, N. Global Desertification: Do Humans Cause Deserts? Amsterdam, the Netherlands: Elsevier, 2004.

4)         Williams, M. A. J., and Robert C. Balling, Jr. Interactions of Desertification and Climate. London: Arnold, 1996.

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