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Desertification

http://en.wikipedia.org/wiki/Desertification

From Wikipedia, the free encyclopedia

Global desertification vulnerability map

Lake Chad in a 2001 satellite image, with the actual lake in blue. The lake has shrunk by 94% since the 1960s.^[1]

Desertification is a type of land degradation in which a relatively dry land region becomes increasingly arid, typically losing its bodies of water as well as vegetation and wildlife.^[2] It is caused by a variety of factors, such as climate change and human activities. Desertification is a significant global ecological and environmental problem.^[3]

Contents

• 1 Definitions
• 2 History
• 3 Areas affected
• 4 Vegetation patterning
• 5 Causes
• 6 Poverty
• 7 Countermeasures and prevention
  • 7.1 Managed grazing

1. Definitions

Considerable controversy exists over the proper definition of the term “desertification” for which Helmut Geist (2005) has identified more than 100 formal definitions.The most widely accepted^[2] of these is that of the Princeton University Dictionary which defines it as “the process of fertile land transforming into desert typically as a result of deforestation, drought or improper/inappropriate agriculture”^[4]
Desertification has been neatly defined in the text of the United Nations Convention to Combat Desertification (UNCCD) as “land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities.”^[5]

The earliest known discussion of the topic arose soon after the French colonization of West Africa, when the Comitéd’Etudes commissioned a study on desséchementprogressif to explore the prehistoric expansion of the Sahara Desert.^[6]

2. History

The world’s most noted deserts have been formed by natural processes interacting over long intervals of time. During most of these times, deserts have grown and shrunk independent of human activities. Paleodeserts are large sand seas now inactive because they are stabilized by vegetation, some extending beyond the present margins of core deserts, such as the Sahara, the largest hot desert.^[7]

Desertification has played a significant role in human history, contributing to the collapse of several large empires, such as Carthage, Greece, and the Roman Empire, as well as causing displacement of local populations.^{[3][8][9][10][11]} Historical evidence shows that the serious and extensive land deterioration occurring several centuries ago in arid regions had three epicenters: the Mediterranean, the Mesopotamian Valley, and the loessial plateau of China, where population was dense.^[8][12]

3. Areas affected

Sun, Moon and Telescopes above the Desert^[13]

Drylands occupy approximately 40–41% of Earth’s land area^[14][15] and are home to more than 2 billion people.^[15] It has been estimated that some 10–20% of drylands are already degraded, the total area affected by desertification being between 6 and 12 million square kilometres, that about 1–6% of the inhabitants of drylands live in desertified areas, and that a billion people are under threat from further desertification.^[16][17]

As of 1998, the then-current degree of southward expansion of the Sahara was not well known, due to a lack of recent, measurable expansion of the desert into the Sahel at the time.^[18]

4. Vegetation patterning

As the desertification takes place, the landscape may progress through different stages and continuously transform in appearance. On gradually sloped terrain, desertification can create increasingly larger empty spaces over a large strip of land, a phenomenon known as “Broussetigrée“. A mathematical model of this phenomenon proposed by C. Klausmeier attributes this patterning to dynamics in plant-water interaction.^[19] One outcome of this observation suggests an optimal planting strategy for agriculture in arid environments.^[20]

5. Causes

A herd of goats in Norte Chico, Chile. Overgrazing of drylands is one of the primary causes of desertification.

A shepherd guiding his sheep through the high desert outside of Marrakech, Morocco

The immediate cause is the removal of most vegetation. This is driven by a number of factors, alone or in combination, such as drought, climatic shifts, tillage for agriculture, overgrazing and deforestation for fuel or construction materials. Vegetation plays a major role in determining the biological composition of the soil. Studies have shown that, in many environments, the rate of erosion and runoff decreases exponentially with increased vegetation cover.^[21] Unprotected, dry soil surfaces blow away with the wind or are washed away by flash floods, leaving infertile lower soil layers that bake in the sun and become an unproductive hardpan. Controversially, Allan Savory has claimed that the controlled movement of herds of livestock, mimicking herds of grazing wildlife, can reverse desertification.^{[22][23][24][25][26]}

6. Poverty

At least 90% of the inhabitants of drylands live in developing nations, where they also suffer from poor economic and social conditions.^[16] This situation is exacerbated by land degradation because of the reduction in productivity, the precariousness of living conditions and the difficulty of access to resources and opportunities.^[27]

A downward spiral is created in many underdeveloped countries by overgrazing, land exhaustion and overdrafting of groundwater in many of the marginally productive world regions due to overpopulation pressures to exploit marginal drylands for farming. Decision-makers are understandably averse to invest in arid zones with low potential. This absence of investment contributes to the marginalisation of these zones. When unfavourable agro-climatic conditions are combined with an absence of infrastructure and access to markets, as well as poorly adapted production techniques and an underfed and undereducated population, most such zones are excluded from development.^[28]

Desertification often causes rural lands to become unable to support the same sized populations that previously lived there. This results in mass migrations out of rural areas and into urban areas, particularly in Africa. These migrations into the cities often cause large numbers of unemployed people, who end up living in slums.^[29][30]

7. Countermeasures and prevention

Anti-sand shields in north Sahara, Tunisia

Jojoba plantations, such as those shown, have played a role in combating edge effects of desertification in the Thar Desert, India.^{[citation needed]}

Techniques exist for mitigating or reversing the effects of desertification, however there are numerous barriers to their implementation. One of these is that the costs of adopting sustainable agricultural practices sometimes exceed the benefits for individual farmers, even while they are socially and environmentally beneficial.^{[citation needed]} Another issue is a lack of political will, and lack of funding to support land reclamation and anti-desertification programs.^[31]

Desertification is recognized as a major threat to biodiversity. Some countries have developed Biodiversity Action Plans to counter its effects, particularly in relation to the protection of endangered flora and fauna.^[32][33]

Reforestation gets at one of the root causes of desertification and is not just a treatment of the symptoms. Environmental organizations^[34] work in places where deforestation and desertification are contributing to extreme poverty. There they focus primarily on educating the local population about the dangers of deforestation and sometimes employ them to grow seedlings, which they transfer to severely deforested areas during the rainy season.^[35]

Techniques focus on two aspects: provisioning of water, and fixation and hyper-fertilizing soil.

Fixating the soil is often done through the use of shelter belts, woodlots and windbreaks. Windbreaks are made from trees and bushes and are used to reduce soil erosion and evapotranspiration. They were widely encouraged by development agencies from the middle of the 1980s in the Sahel area of Africa.

Some soils (for example, clay), due to lack of water can become consolidated rather than porous (as in the case of sandy soils). Some techniques as zaï or tillage are then used to still allow the planting of crops.^[36]

Another technique that is useful is contour trenching. This involves the digging of 150m long, 1m deep trenches in the soil. The trenches are made parallel to the height lines of the landscape, preventing the water from flowing within the trenches and causing erosion. Stone walls are placed around the trenches to prevent the trenches from closing up again. The method was invented by Peter Westerveld.^[37]

Enriching of the soil and restoration of its fertility is often done by plants. Of these, the Leguminous plants which extract nitrogen from the air and fixes it in the soil, and food crops/trees as grains, barley, beans and dates are the most important. Sand fences can also be used to control drifting of soil and sand erosion.^[38]

As there are many different types of deserts, there are also different types of desert reclamation methodologies. An example for this is the salt-flats in the Rub’ al Khali desert in Saudi-Arabia. These salt-flats are one of the most promising desert areas for seawater agriculture and could be revitalized without the use of freshwater or much energy.^[39]

Farmer-managed natural regeneration (FMNR) is another technique that has produced successful results for desert reclamation. Since 1980, this method to reforest degraded landscape has been applied with some success in Niger. This simple and low-cost method has enabled farmers to regenerate some 30,000 square kilometers in Niger. The process involves enabling native sprouting tree growth through selective pruning of shrub shoots. The residue from pruned trees can be used to provide mulching for fields thus increasing soil water retention and reducing evaporation. Additionally, properly spaced and pruned trees can increase crop yields. The Humbo Assisted Regeneration Project which uses FMNR techniques in Ethiopia has received money from The World Bank’s BioCarbon Fund, which supports projects that sequester or conserve carbon in forests or agricultural ecosystems.^[40]

Managed grazing methods are argued to be able to restore grasslands, thereby decreasing atmospheric CO₂ levels.^[41]

7.1 Managed grazing

Restoring grasslands store CO₂ from the air into plant material. Grazing livestock, usually not left to wander, would eat the grass and would minimize any grass growth while grass left alone would eventually grow to cover its own growing buds, preventing them from photosynthesizing and killing the plant.^[42] A method proposed to restore grasslands uses fences with many small paddocks and moving herds from one paddock to another after a day a two in order to mimick natural grazers and allowing the grass to grow optimally.^[42][43][44] It is estimated that increasing the carbon content of the soils in the world’s 3.5 billion hectares of agricultural grassland would offset nearly 12 years of CO₂ emissions.^[42]Allan Savory, as part of holistic management, claims that while large herds are often blamed for desertification, prehistoric lands used to support large or larger herds and areas where herds were removed in the United States are still desertifying.^[41]