Desert and Desertification
Desert and Desertification
Deserts are very dry regions that have seasonal high temperatures, low sporadic rainfall, and a high evaporation rate. Areas that receive less than 10 in (25.4 cm) of rain a year are generally classified as deserts. Dry (arid) regions are usually found in area of high pressure (subtropical highs, leeward sides of mountains, etc.) associated with descending divergent air masses that are common between 30 degrees N and 30 degrees S latitude. Arid lands cover 25% of Earth's land surface.
Desert temperatures and rainfall patterns are influenced by climate changes. Interrelated ocean-atmosphere systems are making deserts drier and reducing the diversity of their ecosystems. Wind and dry conditions in deserts create dust that can affect regions far removed from a desert.
Desertification—the degradation of productive land to desert—is an environmental crisis that is affecting the land where an estimated 100–200 million people live. It is a complex process that, like climate change, is being accelerated by human activities.
Historical Background and Scientific Foundations
Climatic processes driven by the interaction of ocean currents and the atmosphere create deserts. The intensity of solar radiation near the equator creates dry winds and deserts in subtropical regions, particularly in Africa. Most of the world's desert ecosystems are located in two belts near the tropics at 30 degrees north and 30 degrees south of the equator. At the coast, dry winds induce upwelling of deep cold ocean waters that are then driven by ocean coastal currents. Variations occur in rainfall in African deserts due to global weather cycles associated with changes in ocean-atmospheric conditions between the South Pacific and Indian Oceans.
The reduction of upwelling cold water in the Pacific along the coastline of North and South America, called El Niño, and the reverse process called La Niña, cause extremes in weather conditions that create desert regions near the coasts. Also, the leeward sides of coastal mountains are deserts because they do not receive ocean-generated moisture.
Normal variations in climate are being affected by climate change that has warmed deserts at an average rate of 0.4–1.4°F (0.2–0.8°C) per decade between 1976 and 2000—a rate that is higher than the average global temperature increase. Global warming is expected to increase rainfall in higher latitudes, but most deserts in the sub-tropical areas are expected to continue to get drier.
Impacts and Issues
As deserts become drier, the ecosystem changes. Higher temperatures result in higher evaporation rates that, in turn, increase soil acidity and reduce desert vegetation. Grasses are replaced with desert shrubs that provide lower nutrition. Biodiversity is impacted as animals move away and plant species start to disappear.
As a result of human activities, there has been a 36% increase in atmospheric carbon dioxide—a significant greenhouse gas—since the mid-1800s. Plants use carbon dioxide in photosynthesis, thus removing it from the atmosphere. The uptake of atmospheric carbon dioxide stored in desert plants is the lowest of any biomass on Earth, and getting lower.
The reduction of plant cover in deserts is increasing the albedo, the reflection of sunlight from Earth's surface back into the atmosphere. This is expected to actually cool areas away from the deserts, but to increase the degradation of lands adjacent to the deserts, a process called desertification. Desertification is also induced by such human activities as overgrazing, clearing woody vegetation, poor management of the land in farming, and water contamination from overpopulation of fragile semi-arid regions.
Desertification is spreading largely at the expense of grasslands and croplands. In Africa, deserts are expanding north and south from the Sahara. Desertification also is expanding deserts in the Middle East, Central Asia, and western and northern China.
Desertification produces deserts, and deserts produce dust which can travel great distances. Dust from the Asian deserts has been carried by winds over the North Pacific Ocean to North America. Dust from the Sahara in Africa has been carried to North and Central America. Climate change is increasing dryness in deserts and increasing wind speeds. The combination of dryness and increased wind speeds is increasing dust emissions to non-desert regions.
WORDS TO KNOW
ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.
BIODIVERSITY: Literally, “life diversity”: the number of different kinds of living things. The wide range of organisms—plants and animals—that exist within any given geographical region.
BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.
CONFERENCE OF THE PARTIES (COP) TO THE CLIMATECHANGE CONVENTION: Annual meeting of representatives from nations that are signatories of (parties to) the United Nations Framework Convention on Climate Change, a treaty drafted in 1992. The treaty entered into force in 1994, and COPs have been held ever since. It was at COPs that the Kyoto Protocol was drafted.
EL NIÑO: A warming of the surface waters of the eastern equatorial Pacific that occurs at irregular intervals of 2 to 7 years, usually lasting 1 to 2 years. Along the west coast of South America, southerly winds promote the upwelling of cold, nutrient-rich water that sustains large fish populations, that sustain abundant sea birds, whose droppings support the fertilizer industry. Near the end of each calendar year, a warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water. Because this condition often occurs around Christmas, it was named El Niño (Spanish for boy child, referring to the Christ child). In most years the warming lasts only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Niño conditions lastfor many months, more extensive ocean warming occurs and economic results can be disastrous. El Niño has been linked to wetter, colder winters in the United States; drier, hotter summers in South America and Europe; and drought in Africa.
LEEWARD: Downwind of an object: opposite of windward.
Wind blown dust particles—largely made of an aluminum-silicate mineral—are usually less than 2 micrometers in size. Significant amounts of wind-blown desert dust have settled onto coral reefs and contributed to their decline. Desert dust affects the atmosphere as it partly reflects and partly absorbs solar radiation. It affects rainfall, visibility, and human health in regions far from the deserts.
IN CONTEXT: U.N. EFFORTS TO COMBAT DESERTIFICATION
The United Nations first recognized desertification as a global problem in 1977. That year, representatives from several U.N. agencies, national governments, and international organizations met to devise and adopt the Plan of Action to Combat Desertification. This first plan focused on encouraging desertification prevention and water conservation work by national governments and international conservation agencies. Projects under the plan continued for over a decade, but a conference of the U.N. Environment Program in 1991 asserted that desertification was accelerating, severely affecting some of the most underdeveloped parts of the world. Furthermore, conference participants noted that issues of desertification, soil and freshwater salination, water shortages, agricultural land degradation, and political and economic turmoil often compound each other.
The following year, the U.N. Conference on Environment and Development proposed a new strategy of combating desertification at the local and community level. The new approach integrated economic and environmental concerns. For example, anti-desertification projects in sub-Saharan Africa targeted communities in need of improved access to freshwater and agricultural land-management assistance. Communities were taught agricultural and conservation practices that prevented soil erosion and nutrient depletion. New, deeper wells were dug to provide access to water.
In June 1994, the Convention to Combat Desertification was adopted. Nearly 180 signatory parties met two years later at the first Conference of the Parties. The eighth session of the Conference of the Parties took place in Madrid, Spain, in September 2007. Although the Convention to Combat Desertification still sponsors community-level programs to alleviate the effects of desertification, increasing attention is now paid to the effects of global climate change on desertification issues.
The U.N. designated June 17 for annual observance of World Day to Combat Desertification.
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