荒漠化Desertification(急要)

聯合國糧食及農業組織
>(http://www.fao.org/)

聯合國對抗荒漠化條約秘書處
>(http://www.unccd.int/main.php/)

聯合國環境項目
>(http://www.unep.org/)

Desertification is the degradation of land in arid, semi arid and dry sub-humid areas resulting from various climatic variations, but primarily human activities. Current desertification is taking place much faster worldwide than historically and usually arises from the demands of increased populations that settle on the land in order to grow crops and graze animals.

A major impact is biodiversity loss, and loss of productive capacity, such as the transition from grassland dominated by perennial grasses to one dominated by perennial shrubs. In the southwestern deserts of the United States, semiarid ecosystems dominated by perennial bunchgrasses, including blue grama and black grama, have been replaced by shrublands dominated by creosotebush since the early 1900s. The change in vegetation is thought to have induced desertification in this region. In the Madagascar central highland plateau, ten percent of the entire country has been lost to desertification due to slash and burn agriculture by indigenous peoples. In these areas on earth, the landscape resembles the moon or Mars in its barrenness more than our historical concept of Earth.

Deserts may be separated from surrounding, less arid areas by mountains and other contrasting landforms that reflect basic structural differences in the terrain. In other areas, desert fringes form a gradual transition from a dry to a more humid environment, making it more difficult to determine the desert border. These transition zones have very fragile, delicately balanced ecosystems. Desert fringes often are a mosaic of microclimates. Small hollows support vegetation that picks up heat from the hot winds and protects the land from the prevailing winds. After rainfall the vegetated areas are distinctly cooler than the surroundings.

In these marginal areas human activity may stress the ecosystem beyond its tolerance limit, resulting in degradation of the land. By pounding the soil with their hooves, livestock compact the substrate, increase the proportion of fine material, and reduce the percolation rate of the soil, thus encouraging erosion by wind and water. Grazing and collection of firewood reduce or eliminate plants that help to bind the soil.

In large desert areas, sand dunes can encroach on human habitats. Sand dunes move through a few different means, all of them helped by wind. One way that dunes can move is through saltation, where sand particles skip along the ground like a rock thrown across a pond might skip across the water's surface. When these skipping particles land, they may knock into other particles and cause them to skip as well. With slightly stronger winds, particles collide in mid-air, causing sheet flows. In a major dust storm, dunes may move tens of meters through such sheet flows. And like snow, sand avalanches, falling down the steep slopes of the dunes that face away from the winds, also moving the dunes forward.

To halt desertification the number of animals on the land must be reduced, allowing plants to regrow. Soil conditions must be made favourable for plant growth by, for example, mulching. Mulch (a layer of straw, leaves or sawdust covering the soil) reduces evaporation, suppresses weed growth, enriches soil as it rots, and prevents runoff and hence erosion. Reseeding may be necessary in badly degraded areas. Mulching and reseeding are expensive practices.

However, the only realistic large-scale approach is to prevent desertification through good land management in semi-arid areas.

Desertification often occurs over many generations, on a very large scale and so it is difficult for individuals to take action. Some ideas for combatting this problem include:

* Take part in the activities of conservation groups.

* Bring overgrazing and land mismanagement to the attention of the Directorate of Resource Conservation