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Zhu Zhenda(Institute of Desert Research C A S, Lanzhou, 730000 Desertification Research Center N E P A, Nanjing, 210042)  
Desertification is not only one of the most serious environmental problems in the world but also a main content of global changes. Its importance is affirmed by the international convention to combat desertification signed in 1994. Based on definition given by the convention and on China′s situation, desertification as a process of land degradation is manifested itself by desert like landscape and is triggered by the interaction between irrational human activites and fragile ecosystems. Sandy desertification is one of the main types of desertification in North China, which is indicated by drought, frequent winds, exposed surface and loose sand composition of topsoil. Especially, continual aridization of North China in the recent times is responsible for the development of desertification. The sandy desertification in the arid and semi arid regions is caused mainly by human factors, i.e., 30 1% by overgrazing, 26 9% by over reclamation, 33 7% by excess woodcutting, 9 6% by misuse of water resources and 0 7% by industrial mining and communication activities. Based on aerial photos and landsat in different time, it is shown that desertification trends to spread, for example, the sandy desertified lands have already increased by 17838.3 km 2 from 1975~1987, especially, it is more distinct within the dry farmlands in the sandy steppe region.
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