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 almost everything in the near future.


Two reasons account for the fact that deserts are spreading. The first reason comes from nature. These areas are usually located in the far inner land where rainfall can never reach. The second reason comes from man himself. Many people regard immediate profit as a supreme value, which results in over-grazing, poor farming, tree cutting, and strip mining. All these activities leave land unprotected. Consequently, rain water washes away topsoil into rivers and oceans, wind blows away loose soil.


The expansion of deserts must be stopped if we are to survive on this planet. Man can make deserts, he can also prevent them from spreading. Stopping over grazing, poor farming, tree cutting, and strip-mining is the first step. Planting a green belt of trees along the edge of deserts is the second step. Covering the deserts with vegetation is another step. We hope man can make great efforts to develop better ways to bring the spreading deserts under control