单项选择题
| There was one thought that air
pollution affected only the area immediately around large cities with factories
and heavy automobile traffic．At present, we realize that although these are the
areas with the worst air pollution, the problem is literally worldwide. (76)
On several occasions over the past decade, a heavy cloud of air pollution has
covered the east of the United States and brought health warnings in rural areas
away from any major concentration of manufacturing and automobile traffic．In
fact, the very climate of the entire earth may be infected by air pollution.
Some scientists consider that the increasing concentration of carbon dioxide in
the air resulting from the burning of fossil fuels (coal and oil) is creating a
"greenhouse effect"--conserving 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 in water. (77) Another view, less widely held, is that increasing particular 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 a new ice age, and would make agriculture difficult or impossible in many of our top fanning areas. Today we do not know for sure that either of these conditions will happen (though one recent government report drafted by exports in the field concluded that the greenhouse effect is very possible). Perhaps, if we are lucky enough, the two tendencies will offset each other and the world’s temperature will stay about the same as it is now. |