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世界上沙漠的面积每年都在扩大，许多地区曾经人口众多，五谷丰登，而今却成了沙漠。中国的敦煌曾经是一个大城市，而现在却深深陷入戈壁滩的腹地。

造成戈壁滩的原因之一是牲畜问题。年景好时，农民就增加牲畜饲养的头数。后来有一年天没有下雨，农民又不想把牲畜杀掉。因此，牲畜就把草和更加贫瘠的土地上所长的任何东西全都吃光。这种情况如果继续几年，那么，草就再也长不出来了，土地也就变成了沙漠。有人提出的解决办法是让农民限制饲养牲畜的数量。

另外一个问题就是沃土逐渐流失。树木被砍伐了，山坡上就没有什么东西能够把土壤保持住了。因此，下雨的时候，土壤从山上被冲刷下来，流到河里去了。大风也可能把田地表面的肥沃土壤刮走。

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