Many people once thought that air pollution had a bad influence upon only the area immediately around large cities with factories and heavy automobile traffic. Today, we know that although these are the areas with the worst air pollution, the problem is nearly worldwide. On several occasions over the past decade, a heavy cloud of air pollution has covered the entire eastern half of the United States and led to health warnings even in rural areas away from any major concentration of factories 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 fossil fuels (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 opinion, less widely held, is that increasing tiny substances in the atmosphere are 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 farming 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 make up for each other and the world's temperature will stay about the same as it is now. As pointed out at the beginning of the passage, people used to think that air pollution   _  .
caused widespread damage in the countryside affected the entire eastern half of the United States almost brought worldwide effect existed merely in urban and industrial areas