沙漠化
维库，知识与思想的自由文库
沙漠化指原由植物覆蓋的土地變成不毛之地的自然灾害現象。此處所指的「沙漠」多數強調土地不適合植物生長或發展農業，而非因為地域本身乾燥所造成的沙漠氣候。不過，沒有植物生長的土地由於不能蒸散分配水分，結果也可能反而導致乾燥氣候。 沙漠化現象可能是自然的。作為自然現象的沙漠化是因為地球乾燥帶移動，所產生的氣候變化導致局部地區沙漠化。不過，今日世界各地沙漠化原因，多數歸咎於人為原因；人口急速增長，所居土地被過份耕種以及牧畜，導致土地枯渴不適合耕種。 中東的美索不達米亞（今伊拉克）地區是世上最早發展農業的地域之一，從而發展成世上最早的文明發祥地之一。美索不達米亞的土壤本來甚為肥沃，不過由於過度的農業活動、人們不理會土地長期枯渴，更開發河段上流、採伐森林，上流土地從而不能吸收降雨，雨水一氣流入河中造成水土流失以及洪水。 根據中国国家林业局於2006年6月17日的公佈，中國沙漠化土地達到173萬9700平方公里，佔國土面積18%以上，影響全國30個一級行政區（省、自治區、直轄市）。[1] [编辑] 資料來源
[编辑] 起因
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. 人们普遍上认为干旱是造成沙漠化的元凶，这是一个错误的观念。 In most cases the causes are social and economic, having to do with tenure (access to resources), power and economics. Droughts are common in arid and semiarid lands. Well-managed lands can recover from drought when the rains return. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands has accelerated desertification. In some areas, nomads moving to less arid areas disrupt the local ecosystem and increase the rate of erosion of the land. Nomads are trying to escape the desert, but because of their land-use practices, they are bringing the desert with them. Some arid and semi-arid lands can support crops, but additional pressure from greater populations or decreases in rainfall can lead to the few plants present disappearing. The soil becomes exposed to wind, causing soil particles to be deposited elsewhere. The top layer becomes eroded. With the removal of shade, rates of evaporation increase and salts become drawn up to the surface. This is salinisation, and inhibits plant growth. The loss of plants causes less moisture to be retained in the area, which may change the climate pattern leading to lower rainfall. This degradation of formerly productive land is a complex process. It involves multiple causes, and it proceeds at varying rates in different climates. Desertification may intensify a general climatic trend toward greater aridity, or it may initiate a change in local climate. Desertification does not occur in linear, easily mappable patterns. Deserts advance erratically, forming patches on their borders. Areas far from natural deserts can degrade quickly to barren soil, rock, or sand through poor land management. The presence of a nearby desert has no direct relationship to desertification. Unfortunately, an area undergoing desertification is brought to public attention only after the process is well under way. Often little data are available to indicate the previous state of the ecosystem or the rate of degradation. |
