1．沙化问题日趋严重；
2．沙化问题成因；
3．解决办法。
[写作导航]可先用数字说明世界沙化的问题，然后指出每年沙漠面积还在扩大，并预计其严重后果；接着可分析造成沙漠扩大的原因，一是大自然本身造成的，如某些地方少雨；二是人为因素造成的，如过度放牧、耕作、砍伐、开采等；最后如建立绿化带等。
[范文]
There are vast areas of land on every continent where there is no rainfall or vegetation. It is estimated that more than 40 per cent of the land on the earth＇s surface is desert or desert-like. What is worse, these deserts are spreading at a surprising rate every year. If this trend continues, sand will bury almo
