单项选择题
| It was once thought that air pollution
’affected only the area immediately around large cities with factories or
heavy automobile traffic. Today, we know that although these are the areas with
the worst air pollution, the problem is literally worldwide. On several
occasions over the past decade, a heavy cloud of air pollution has covered the
entire eastern halt’ of the United States and led to health warning even in
rural areas away from any major concentration of manufacturing and automobile
traffic. In fact, the very climate of the entire earth may be affected by air
pollution. Some scientists feel that the increasing concentration of carbon
dioxide in the air resulting from the burning of coal and oil is creating a
"greenhouse effect" holding in heat reflected from the earth and raising the
world’s average temperature. If this view is correct and the world’s temperature
is raised only a few degrees, much of the polar ice cap will melt and cities
such as New York, Boston, Miami, and New Orleans will be under water. Another view, less widely held, is that increasing particulate matter in the atmosphere is blocking sunlight and lowering the earth’s temperature — a result that would be equally disastrous. A drop of just a few degrees could create something close to new ice age and would make agriculture difficult or impossible in many of our top fanning areas. At present we do not know for sure that either of these conditions will happen ( though one recent government report prepared by experts in the field concluded that the greenhouse effect is very likely). Perhaps, if we are very lucky, the two tendencies will offset each other and the world’s temperature will stay about the same as it is now. |