The area of desert in the world is growing every year. Many parts of the world which once had large populations and produced plenty of crops, have become deserts. Dunhuang in China, deep in the heart of the Gebi Desert, was once a great city.

　　Cattle are one cause of the problem. In good years, famers increase the numbers of their cattle. Then one year, the rains fail to come. The farmers do not want to kill any of their cattle, so the cattle eat ail the grass and anything that grows on poorer land. If this continues for several years, the grass never grows again and the land becomes a desert. One suggested answer is for farmers to limit the numbers of their cattle.

　　Another problem is that good soil is gradually lost. When trees are cut down, there is nothing to hold the soil in place on the hillside. So when if rains, the soil is washed down the hills and into the rivers. Strong winds can also blow away the valuable soil that lies on the top of the fields.

　　世界上沙漠的面積每年都在擴(kuò)大，許多地區(qū)曾經(jīng)人口眾多，五谷豐登，而今卻成了沙漠。中國(guó)的敦煌曾經(jīng)是一個(gè)大城市，而現(xiàn)在卻深深陷入戈壁灘的腹地。

　　造成戈壁灘的原因之一是牲畜問(wèn)題。年景好時(shí)，農(nóng)民就增加牲畜飼養(yǎng)的頭數(shù)。后來(lái)有一年天沒(méi)有下雨，農(nóng)民又不想把牲畜殺掉。因此，牲畜就把草和更加貧瘠的土地上所長(zhǎng)的任何東西全都吃光。這種情況如果繼續(xù)幾年，那么，草就再也長(zhǎng)不出來(lái)了，土地也就變成了沙漠。有人提出的解決辦法是讓農(nóng)民限制飼養(yǎng)牲畜的數(shù)量。

　　另外一個(gè)問(wèn)題就是沃土逐漸流失。樹木被砍伐了，山坡上就沒(méi)有什么東西能夠把土壤保持住了。因此，下雨的時(shí)候，土壤從山上被沖刷下來(lái)，流到河里去了。大風(fēng)也可能把田地表面的肥沃土壤刮走。