单选题 A scorching sun, an endless sea of sand and a waterless, forbiddingly lonely land—that is the image most people have of deserts. But how true is this picture? Deserts are dry lands where rainfall is low. This is not to say rain never falls in deserts: it may fall once or twice a year in a fierce torrent that fades almost as soon as it has begun, or which evaporates in the hot air long before it has got anywhere near the earth. It may fall in a sudden sweeping flood that carries everything in its path. Rains may only come once in five or six years or not fall for a decade or more. The Mojave desert in the United States remained dry for twenty-five years.
Without water no living thing can survive, and one feature of the true desert land-scape is the absence of vegetation. With little rain and hardly any vegetation the land suffers under the sun. There are virtually no clouds or trees to protect the earth"s surface and it can be burning hot. Under the sun, soils break up and crack. Wind and torrential rain sweep away and erode the surface further. Eight million square kilometers of the world"s land surface is desert. Throughout history deserts have been expanding and retreating again. Cave paintings show that parts of the Sahara Desert were green and fertile about 10,000 years ago, and even animals like elephants and giraffes roamed the land. Fossil and dunes found in fertile and damp parts of the world show that these areas were once deserts. But now the creation of new desert areas is happening on a colossal scale. Twenty million square kilometers, an area twice the size of Canada, is at a high to very high risk of becoming desert. With a further 1.25 million square kilometers under moderate risk, an area covering 30% of the earth"s land surface is desert, becoming desert, or in danger of becoming desert. The rate of growth of deserts is alarming. The world"s dry lands which are under threat include some of the most important stock-rearing and wheat-growing areas and are the homes of 600～700 million people. These regions are becoming deserts at the rate of more than 58,000 square kilometers a year or 44 hectares a minute. In North Africa at least 100,000 hectares of cropland are lost each year. At this rate there is a high risk that we will be confined to living on only 50% of this planet"s land surface within one more century unless we are able to do something about it.