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A. Distribution of desertification phenomena
B. Views on the desertification process
C. Desertification caused by natural factors
D. Desertification caused by artificial factors
4.1 Publications with desertification issues have identified the occurrence of various desertification phenomena in different geographical areas of China. These are deflation and mobilization of aeolian dunes, sandstorms or duststorms, soil erosion, degradation of grazing land and cropping land and salinisation (cf. bibl. entry 26).
A. Distribution of desertification phenomena
4.2 The formation, deflation and mobilization of dunes under dry and windy climatic conditions are common desertification phenomena in arid areas. Wind erosion occurs easily and frequently on the loose, surface layer with limited or no vegetation coverage. Wind erosion causes severe deflation through loss of top soil and sand cover on grazing and cultivated land. This phenomena can be widely seen in the desert areas of the northwestern provinces, such as Xindiang, Gansu, Ningxia and Inner Mongolia of bibl. entries 8, 33, 46, 79, 98, 140).
4.3 The sandstorm or duststorm is one of the most catastrophic phenomenon to occur in northwest China. The sandstorm that occurred between 1400 to 2000 on 5 May 1993 in the desert areas of Gansu province had a dust cloud height of 300 to 400 m, frontal dust cloud mean velocity of 50 to 80 km/hr and in stannous wind speed of 20 m/s, causing great damage to industrial and agricultural production in the Hexi Corridor of the province (cf. bibl. entry 133). Catastrophic sandstorm events occur in the arid areas with varying degrees of intensity (cf. bibl. entries 24, 90, 133).
4.4 Soil erosion is wide spread in upland areas of arid, semi-arid and dry sub-humid regions. Many publications which deal with desertification issues on the Loess Plateau discuss soil erosion under the influence of both climate change and human activities. It is estimated that 80 percent of the total 623.7 million ha plateau area is eroded (cf. bibl. entries 51, 63, 81, 125, 136). Soil erosion closely related to desertification is discussed for the Tibet Plateau and coastal regions (cf. bibl. entries 19, 39, 102, 125, 146).
4.5 Degradation of grazing land and cropping land from inappropriate vegetation and land resource utilization is generally expressed in terms of lost productivity, rather than direct observation in the field, since overgrazing and over cultivation usually lead to gradual desertification, particularly near oases (cf. bibl. entries 19, 27, 64, 78, 88, 125, 129, 140).
4.6 Soil salinisation commonly occurs in interior river basins, coastal areas and irrigated areas in arid, semi-arid or dry sub-humid regions. Land salinisation processes can be seen where water conditions are changing from moist to drier (cf. bibl. entries 39, 46, 123, 125).
B. Views on the desertification process
4.7 The formation and development of deserts in both geological and modern times is discussed in many publications. There is general agreement that the desertification process started and expanded following the change from a cold-humid climate to a warm-dry climate during the late Tertiary or early Quaternary periods. Various geological, geochemical and lithological methods used to study the geological ages of different desert areas in China indicate that desertification coincided with global climatic change, although estimates as to when desertification started in northwest China vary widely: early Tertiary, late Tertiary to early Pleistocene, late Pleistocene and early Holocene (cf. bibl. entries 14, 28, 42, 57, 60, 79. 100, 107, 122, 123, 127).
4.8 In comparison to the gradual and slow desertification process in geological periods, seasonal or periodic dry climates alternating with humid conditions over a period of ten years or longer, are responsible for relatively drastic and rapid desertification in some regions, such as the coastal areas of Yellow and Bohai Seas (cf. bibl. entries 39, 43, 127).
4.9 Numerous publications deal with the issue of acceleration of natural desertification processes in the modern period due to human activities (cf. bibl. entries 21, 27. 31. 43. 46, 51, 64. 78, 94, 98, 100, 102, 123, 125, 129. 136, 146). The greenhouse effect created by modern industrialization and urbanization also contributes to desertification (cf. bibl. entries 26. 98).
4.10 The literature attributes desertification to two factors (cf. bibl. entries 26, 31, 98). Natural or climatic factors include climatic change in geological periods and climatic variation or periodical alternations in the modern period. Artificial or biological factors include negative influences of human activities on the natural environment, such as over cultivation, overgrazing, large-scale irrigation of farmland, overcutting of fuelwood and overuse of water resource. Although natural factors were originally responsible for desertification, artificial factors now speed up the progress under certain dry and windy climatic conditions (cf. bibl. entries 26, 31, 98).
C. Desertification caused by natural factors
4.11 Many studies address the development of desert areas or aeolian landforms in the geological period. The aeolian sand landform process existed in the late Tertiary period in the Tengger Desert when the climate was dry and hot of bibl. entry 107): it has been proven that the modern desert environment started during the early Tertiary and continued into the Quaternary to form the Taklimakan Desert (cf. bibl. entry 79). The northwest desert areas, Mu Us Sandy Land, Loess Plateau, Tibet Plateau, formed during the Tertiary and Quaternary: climate change was the dominate factor (cf. bibl. entries 19, 28, 57, 60, 64, 79, 100. 107, 358).
4.12 Periodical climate variations from humid to dry and windy, as well as seasonal change in coastal climates, promote regional desertification. In the Mu Us Sandy Land region, the climate changed periodically from drier to wetter over the last 40 years, i.e. wetter in 1960s, drier in 1970s, wetter in 1980s and now drier in 1990s (cf bibl. entry 82). In coastal areas of the Yellow and Bohai Seas, a dry climate combined with strong winds in winter and spring has caused seasonal desertification (cf. bibl. entry 39).
D. Desertification caused by artificial factors
4.14 Population increases and associated human activities in regions with arid, semi-arid or dry sub-humid climate put pressure on natural ecosystems. This exploitation of natural resources, including the natural vegetation resources and land and water resources, causes serious soil erosion that has lead to desertification of these regions.
4.15 Overgrazing on the steppe is a wide spread problem in rural areas of north China. The quality of plant species, vegetation coverage and community structure of the grasslands are greatly reduced; grassland steppe is more vulnerable to conversion into desert, steppe or hare land (cf. bibl. entries 19, 27, 64, 125, 129).
4.16 Over cultivation and loss of original irrigation conditions on farmland due to insufficient water supply encourages land salinisation or desertification in areas with a dry and windy climate. Strong winds and water shortages cause farmland to be decertified and converted into desert, even in oases (cf. bibl. entries 27, 125, 140).
4.17 Overcutting of fuelwood from natural woodlands has caused serious destruction of vegetation and soil erosion in the upland areas of the arid or semi-arid regions. Extensive over cultivation of woodland or grassland has led to soil erosion or desertification of the cultivated area (cf. bibl. entries 110, 102, 140).