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英语四级听力美文第93篇:Hotter or Colder ?
It was once thought that air pollution affected only the area immediately around large citieswith factories and/or heavy automobile traffic.
Today, we know that although these are the areas with the worst air pollution, the problem isliterally worldwide.
On several occasions over the past decade, a heavy cloud of air pollution has covered theentire eastern half of the United States and led to health warnings even in rural areas awayfrom any major concentration of manufacturing and automobile traffic.
In fact, the climate of the entire earth may be affected by air pollution.
Some scientists feel that the increasing concentration of carbon dioxide in the air resultingfrom the burning of fossil fuels (coal and oil) is creating a “greenhouse effect” — holding inheat 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 thepolar ice cap will melt and cities such as New York, Boston, Miami, and New Orleans will beunder water.
Another view, less widely held, is that increasing particulate matter in the atmosphere isblocking sunlight and lowering the earth's temperature — a result that would be equallydisastrous.
A drop of just a few degrees could create something close to new ice age and would makeagriculture difficult or impossible in many of our top farming areas.
At present we do not know for sure that either of these conditions will happen (though onerecent government report prepared by experts in the field concluded that the greenhouseeffect is very likely).
Perhaps, if we are very lucky, the two tendencies will offset each other and the world'stemperature will stay about the same as it is now.
