Ecology

Desertification

June 14, 2013, 10:09 pm
Source: Green.facts
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Desertification due to overgrazing: Argentina. Source: C.Michael Hogan

Desertification is the persistent degradation of dryland ecosystems due to human activities and variations in climate. While climate oscillations have historically had pronounced effects upon desertification, activities of man during the Holocene have had the most pronounced impacts to induce desertification in the most recent 10,000 years. Home to a third of the human population in 2000, drylands occupy nearly half of Earth’s land area. Across the world, desertification affects the livelihoods of millions of people who rely on the benefits that dryland ecosystems provide.

Proximate chief drivers of desertification include overdrafting of groundwater, overgrazing and tillage practices in agriculture that place soils more vulnerable to wind and surface runoff scouring. In drylands, water scarcity limits the production of crops, forage, wood, and other services ecosystems provide to humans. Drylands are therefore highly vulnerable to increases in human pressures and climatic variability, especially sub-Saharan and Central Asian drylands.

caption Present-day Drylands and Their Categories (dry subhumid, semiarid, arid or hyper-arid, based on Aridity Index values. (Source: Millennium Ecosystem Assessment Desertification Synthesis Report (2005), based on data from UNEP Geo Data Portal, 2000)

Some 10 to 20% of drylands are already degraded, and ongoing desertification threatens many of the world’s ecosystems, including those inhabitated by some of the poorest human populations. Therefore, desertification is one of the greatest environmental challenges today and a major barrier to meeting ecological and human needs in drylands.

Impacts of desertification

Desertification affects the livelihoods of millions of people, since it occurs on all continents except Antarctica. caption Women play a key role in water
management in drylands, Mauritania.
Source: Millennium Ecosystem Assessment

Desertification takes place in drylands all over the world. Some 10 to 20% of all drylands may already be degraded, but the precise extent of desertification is difficult to estimate, because few comprehensive assessments have been made so far.

A large majority of dryland populations live in developing countries. Compared to the rest of the world, these populations lag far behind in terms of human well-being, per capita income, and infant mortality. The situation is worst in the drylands of Asia and Africa. Dryland populations are often marginalized and unable to play a role in decision making processes that affect their well-being, making them even more vulnerable.

caption Comparison of Infant Mortality and GNP per Person in Drylands. Source: Millennium Ecosystem Assessment

Desertification has environmental impacts that go beyond the areas directly affected. For instance, loss of vegetation can increase the formation of large dust clouds that can cause health problems in more densely populated areas, thousands of kilometers away. Moreover, the social and political impacts of desertification also reach non-dryland areas. For example, human migrations from drylands to cities and other countries can harm political and economic stability.

Drivers of desertification

The proximate causes of desertification are dominated by agricultural intensification. Unintended consequences of the so-called Green Revolution, begun in the 1970s has been massive overgrazing, monocropping, excessive tillage, deforestation, salt buildup of irrigated lands, overdrafting of groundwater and utilization of lands unsuitable for arable practises. Each of these practises has contributed to widespread desertification of drylands and even large amounts of previously  forested area.

Overgrazing typically occurs when a short term gain in productivity is sought at the expense of long term vegetation loss; moreover, the impact may be driven simply by a failure to move grazing livestock in a sustainable manner, avoiding the killing of vegetation in a local area. Overgrazing typically exposes more soil to erosion and creates an outcome of topsoil loss through wind and runoff erosion. Additionally the loss of native vegetation destabilises the ecosystem, locally extirpating arthropods, amphibians, birds and mammals that rely upon the original habitat. Where the overgrazing patterns are persistent over a widespread area, biodiversity loss may occur.

Monocropping and intensive tillage also tend to enhance exposure of soils to increased erosion, and also threaten bioversity by eliminating the full palette of plants that previously inhabited a given habitat. Recent agronomy research is encouraging cropping systems that retain certain elements of the original plant community as ground cover, interleaved vegetation and perimeter protection; the outcome of such cultivation is to promote persistence of native arthropods and other fauna. Correspondingly, erosion is discouraged by enhanced protection of the soil surface.

Deforestation takes a variety of forms. In western countries, the chief syndrome is planting of monocultural forests followed by clearcutting, thence erosion. Scotland and New Zealand are prototypical instances of such practises. In developing countries, certain alien species plantation monocrops follow the same pattern, but vast areas are also subjected to slash-and-burn tactics, where rainforests are converted to marginal farmland and often overgrazed, with an end result of soil exposure and topsoil loss. This phenomenon is aggravated by the exploitation of forests for charcoal production in an energy-starved developing world.

When farmland is over-irrigated on a long term basis, build-up of salinity in soils is often attendant. This process is accentuated when the waters used for irrigation are return flow from fields that have excessive fertilizers applied; California's Central Valley is a prime example of such phenomena.

Aquifer overdrafting can create unsustainable effects, where groundwater levels steadily fall; eventually it is either too expensive or utterly infeasible to draft further water. In such cases, surface aridity increases, and once moist soils are turned into a dustbowl, subject to wind erosion. 

Root causes of desertification

caption Lands of the Himba people, Namib Desert. Precious dryland
grasses are sustained by nomadic herding practises
Source: C.Michael Hogan
Desertification is caused by a combination of social, political, economic, and natural factors which vary from region to region. Policies that can lead to an unsustainable use of resources and lack of infrastructures are major contributors to land degradation. Intensive agriculture and monocropping are significant causes of desertification. Policies favoring sedentary farming over nomadic herding in regions more suited to grazing can contribute to desertification.

The process of globalization can contribute to desertification. Certain studies have shown that trade liberalization, economic reforms, and export-oriented production in drylands can promote desertification. In other cases, enlarged markets outside of the drylands also contribute to agricultural improvements.

Historically, dryland livelihoods have been based on a mixture of hunting, gathering, farming, and herding. This mixture varied with time, place, and culture, since the harsh conditions forced people to be flexible in land use. Population growth has led to the extension of cultivated lands and the irrigation of these lands has brought about desertification, as well as attendant environmental problems.

Development paths and desertification

caption A farmer in semi-arid Burkina Faso works as a blacksmith during the dry season. Source: Millennium Ecosystem Assessment

Population growth and increased food demand are expected to drive the expansion and intensification of land cultivation in drylands. If no countermeasures are taken, desertification in drylands will threaten future improvements in human well-being and possibly reverse gains in some regions.

The Millennium Ecosystem Assessment developed four plausible scenarios to explore the future of desertification and human well-being until 2050 and beyond. The different scenarios are based on either increased globalization or increased regionalization, each combined with either a reactive or proactive way of addressing environmental issues.

In all four scenarios, the desertified area is expected to increase, though not at the same pace. Poverty and unsustainable land use practices will continue to be the main factors driving desertification in the near future, and climate change will also play a role.

Local adaptation and conservation practices can mitigate some losses of dryland services, but it will be difficult to reverse losses in terms of biodiversity and in the provision of food and water which is linked to biodiversity. Freshwater scarcity, which already affects 1-2 billion people globally, is expected to increase, causing greater stresses in drylands and ultimately a worsening of desertification.

Misconceptions of desertification

caption California's Delta Mendota Canal: Bringing diverted water for
farms, which ironically are desertifying due to soil salinization.
There are several common misconceptions that hinder the prevention of desertification:

  • Myth 1. Localised warming and drought create deserts. While increases in temperature and lack of moisture may abet desertification, they are not the proximate forceful drivers of dryland degradation compared to overgrazing, intensive cropping, monocropping and overtillage. While climate change can exacerbate the advance of deserts, it is rarely the fundamental driver of such change
  • Myth 2. Desertification is caused by advances in sand dunes. While advancing dunes are a symptom or outcome of desertification, it is only rarely that the simple sombination of sand dunes and winds increase the amount of degraded drylands.
  • Myth 3. Desertification can be rolled back by bringing in enough water. Restoring surface water is simplistically attractive, but will not fundamentally alter an imbalance being caused by unsustainable agriculture. Furthermore, the import of large quantities of water is expensive, and usually destabilises the region from which the water is diverted

By world region

caption Volubilis, Morocco, site of lush ancient Roman center,
degraded to near desert by about 500 AD due to aquifer mining
and climate change. Source: C.Michael Hogan
In Africa, about one fifth of the drylands of Sub-Saharan Africa are experiencing desertification, chiefly due to overgrazing and intensive cropping patterns. Some regions have been in extremely long term decline such as the margins of the Namib Desert, but in those areas the indigenous peoples have had considerable time to react to aridification. In Northern Africa, soil salinity as well as overgrazing are chief factors of desertification on plains and mountain slopes. As early as 1990, dryland grazing and crop productivity had already declined by 25 to 50 percent throughout Africa.

In Asia there are a multiplicity of causes at work. Overgrazing is a chief driver in central Asia and the Middle East , but salt buildup is also a culprit in Pakistan, Iraq, China and Russia. Overdrafting of groundwater and poor cropping practises on the North China Plain has taken that region from a very productive wheat growing region to an area whose output peaked about 1995. The salinization issues in China's Yellow River basin are centuries old, due to overpopulation in that region as a longstanding matter; in contrast, salinization of the Indus basin in Pakistan is a more recent creation.

Australia is a continent where desertification only began somewhat over a century ago, with overgrazing practises; however, the percentage of land damaged is small relative to other world regions due to land inhospitable to farming in many places and due to conversion of agricultural practises beginning as early as 1940. Greatest damage is in the saltbush-bluebush community of South Australia and New South Wales and overgrazed coastal valleys of the Ord, Victoria and Gascoyne Rivers.

In South America the greatest cause of desertification has been overgrazing by sheep, cattle, goats and other livestock; deforestation has been a very significant driver as well, both drivers being induced by the interest of short term economic gain from a fixed amount of real estate. North America experienced its greatest desertification with the dustbowl of the 1930s covering much of Oklahoma and west Texas; however, agricultural practises in the USA and Canada starting int the mid 20th century have greatly reduced desertification risks.

European issues of desertification focus chiefly on the Mediterranean, and relate more to anthropogenic changes in water use than in agricultural practises themselves. The inexorable population density increase of Europe over the last three thousand years, including urbanisation has led to a profile of over-appropriation of Mediterranean water supplies; hence the perception is one of drought or lack of available water, due to high human use and competing agricultural claims. For example, in Greece, 80 perecent of all water is appropriated for agricultural use, and there is simply no available water for marginal demand. In Spain there has been hostile inter-regional conflict, peaking with the debate over waters of the Ebro to supply neighboring regions' needs.

Impacts of desertification upon humans

In drylands, more people depend on ecosystem services for their basic needs than in any other ecosystem. Indeed, many of their resources, such as crops, livestock, fuelwood, and construction materials, depend on the growth of plants, which in turn depends on water availability and climate conditions.

caption Schematic Description of Development Pathways in Drylands. Drylands can be developed in response to changes in key human factors. Left side of the Figure shows developments that lead to a downward spiral of desertification. Right side shows developments that can help avoid or reduce desertification. Source: Desertification Synthesis Report, Millennium Ecosystem Assessment, 2005

Fluctuations in the services supplied by ecosystems are normal, especially in drylands, where water supply is irregular and scarce. However, when a dryland ecosystem is no longer capable to recover from previous pressures, a downward spiral of desertification may follow, though it is not inevitable. 

Desertification affects a wide range of services provided by ecosystems to humans: products such as food and water, natural processes such as climate regulation, but also non-material services such as recreation, and supporting services such as soil conservation. Changes can be quantified and methods are available to prevent, reduce, or reverse them.

caption Land Uses in Drylands. Source: Desertification Synthesis Report, Millennium Ecosystem Assessment, 2005

When faced with desertification, people often respond by making use of land that is even less productive, transforming pieces of rangeland into cultivated land, or moving towards cities or even to other countries. This can lead to unsustainable agricultural practices, further land degradation, exacerbated urban sprawl, and socio-political problems.

Prevention and mitigation of desertification

Effective prevention of desertification requires management and policy approaches that promote sustainable resource use. Prevention should be preferred to rehabilitation, which is difficult and costly. 

caption Terracing prevents further gully erosion and stores surface runoff for olive production (Tunisia). Source: Millennium Ecosystem Assessment

Major policy interventions and changes in management approaches, both at local and global levels, are needed in order to prevent, stop or reverse desertification. Prevention is a lot more cost-effective than rehabilitation, and this should be taken into account in policy decisions. Addressing desertification is critical and essential to meeting the Millennium Development Goals which aim to eradicate extreme poverty and ensure environmental sustainability amongst other objectives.

The creation of a “culture of prevention” that promotes alternative livelihoods and conservation strategies can go a long way toward protecting drylands both when desertification is just beginning and when it is ongoing. It requires a change in governments’ and peoples’ attitudes. Building on long-term experience and active innovation, dryland populations can prevent desertification by improving agricultural and grazing practices in a sustainable way.

Even once land has been degraded, rehabilitation and restoration measures can help restore lost ecosystem services. The success of rehabilitation practices depends on the availability of human resources, funds, and infrastructures. It requires a combination of policies and technologies and the close involvement of local communities.

Links to biodiversity loss and climate change

Desertification diminishes biological diversity, a diversity which contributes to many of the services provided to humans by dryland ecosystems. Vegetation and its diversity are key for soil conservation and for the regulation of surface water and local climate. Desertification also contributes to global climate change by releasing to the atmosphere carbon stored in dryland vegetation and soils.

caption Linkages between Desertification, Global Climate Change, and Biodiversity Loss. Source: Millennium Ecosystem Assessment

The effect of global climate change on desertification is complex and not yet sufficiently understood. On the one hand, higher temperatures resulting from increased methane and carbon dioxide levels can have a negative impact through increased loss of water from soil and reduced rainfall in drylands. On the other hand, for certain species, an increase in carbon dioxide in the atmosphere boosts plant growth.

Environmental management approaches for combating desertification, conserving biodiversity, and mitigating climate change are linked in many ways, thus a joint implementation of the U.N. Conventions to Combat Desertification, on Biological Diversity, and on Climate Change can yield multiple benefits.

Increasing awareness of desertification

caption Erroneous plowing techniques can often cause serious erosion. Source: Millennium Ecosystem Assessment

Scientifically robust and consistent information about the extent of land degradation is important when it comes to identifying priorities and monitoring the consequences of actions. Previous assessments had diverse shortcomings that made them unreliable. Remote sensing and long term monitoring are needed to better understand desertification processes and determine the extent of desertification. Additionally, to better comprehend the impacts of desertification on human well-being we need to improve our knowledge of the interactions between socioeconomic factors and changing ecosystem conditions.

Uncertainties remain about the way various biological, physical, social, and economic factors interact, which limits our ability to assess the actual effect of policies on desertification. Among other things, the impact of poverty reduction strategies on ecosystem services and desertification has not yet been fully explored. The impact of cities in dryland areas also has to be evaluated, since they may both increase and relieve pressures on desertified areas.

Conclusion

caption West China near Tibet: water diversions for human use
has effected aridity. Source: C.Michael Hogan
Desertification poses one of the greatest environmental threats today and constitutes a major barrier to meeting basic human needs in drylands.

Desertification is land degradation that affects biological productivity as well as the livelihoods of millions of people. It is caused by a combination of human and natural factors that contribute to an unsustainable use of scarce natural resources.

Some 10 to 20% of drylands are already degraded, and the ongoing desertification threatens the world’s poorest populations. Various scenarios that explore the future of desertification and human well-being in drylands show that global desertified area is likely to increase. Prevention is the most effective way to cope with desertification, because later attempts to rehabilitate desertified areas are costly and tend to deliver limited results. Combating desertification yields multiple local and global benefits and helps fight biodiversity loss and global climate change.

Efforts to reduce pressures on dryland ecosystems need to go hand in hand with efforts to reduce poverty as both are closely linked. Effectively fighting desertification will help reduce global poverty and will contribute to meeting the Millennium Development Goals.

References

  • Millenium Ecosystem Assessment. 2005. Ecosystems and Human Well-being: Desertification Synthesis: Key Questions on Desertification in the Millennium Ecosystem Assessment. Topic editor: Ed.-in-chief, Cutler Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC
  • Farouk El-Baz and M.H.A.Hassan. 1986. Physics of desertification. International Centre for Theoretical Physics. Springer. 473 pages
  • Nichola Geeson, C.Jane Brandt and John B.Thornes. 2002. Mediterranean desertification: a mosaic of processes and responses. John Wiley and Sons. 440 pages
  • A. S. Alsharhan. 2000. Desertification in the third millennium. Dubai. Taylor & Francis. 489 pages.

Disclaimer: This article contains information that was originally published by GreenFacts. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the GreenFacts should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

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Citation

, G. (2013). Desertification. Retrieved from http://www.eoearth.org/view/article/151708

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