填空题 Few of us realize how dust storms in the Sahara Desert can affect us directly in this country, but in fact, {{U}} {{U}} 1 {{/U}} {{/U}}can be felt many miles away. This has become more apparent in recent years as {{U}} {{U}} 2 {{/U}} {{/U}}with which dust storms occur has risen sharply. To understand how this situation {{U}} {{U}} 3 {{/U}} {{/U}}, you need to know something about the desert landscape. Under normal circumstances, there is {{U}} {{U}} 4 {{/U}} {{/U}}and lichen lying on top of the soft sand. The winds that {{U}} {{U}} 5 {{/U}} {{/U}}, although they may be powerful, don't blow away the sand because the crust {{U}} {{U}} 6 {{/U}} {{/U}}. But this protective covering, which has been in place for thousands of years, {{U}} {{U}} 7 {{/U}} {{/U}}, especially over the last decade or so. One reason for this is that people who {{U}} {{U}} 8 {{/U}} {{/U}}nowadays prefer jeeps to camels, and these vehicles are {{U}} {{U}} 9 {{/U}} {{/U}}. Of course, there are other contributing factors as well, the main two being {{U}} {{U}} 10 {{/U}} {{/U}}and deforestation on a large scale.
But how is it that these dust storms {{U}} {{U}} 11 {{/U}} {{/U}}? Well, storms in the Sahara send dust {{U}} {{U}} 12 {{/U}} {{/U}}, and it can descend many thousands of miles away. If you {{U}} {{U}} 13 {{/U}} {{/U}}, you can sometimes see red desert dust on the snow below! And this dust can {{U}} {{U}} 14 {{/U}} {{/U}}. It can, for example, carry cattle disease to distant places. In addition, {{U}} {{U}} 15 {{/U}} {{/U}}can fall on the sea, preventing the rays of the sun from {{U}} {{U}} 16 {{/U}} {{/U}}over large areas. Experts believe that the destruction of coral reefs in the Caribbean, {{U}} {{U}} 17 {{/U}} {{/U}}, can be traced back to this effect. Perhaps more worryingly, the dust can even {{U}} {{U}} 18 {{/U}} {{/U}}in Greenland. The ice, which is now dark in color, no longer reflects the sun's rays, so it {{U}} {{U}} 19 {{/U}} {{/U}}. So it appears that the increase in dust storms is yet one more of the many factors {{U}} {{U}} 20 {{/U}} {{/U}}.