题目内容：
根据下列材料，回答题Effects of Environmental Pollution
If pollution continues to increase at the present rate, formation of aerosols (浮质) in the atmosphere will cause the onset (开始) of an ice age in about fifty years' time．This conclusion, reached by Dr．S.I．Rasool and Dr．S.H．Schneider of the United States Goddard Space Flight Centre, answers the apparently conflicting questions of whether an increase in the carbon dioxide (二氧化碳) content of the atmosphere will cause the Earth to warm up or increasing the aerosol content will cause it to cool down．The Americans have shown conclusively that the aerosol question is dominant.
Two specters haunting conservationists have been the prospect that environmental pollution might lead to the planet's becoming unbearably hot or colD．One of these ghosts has now been laiD．Because it seems that even an increase in the amount of carbon dioxide in the atmosphere to eight times its present value will produce an increase in temperature of only 2 ℃, which would take place over several thousand years．But the other problem now looms larger than ever.
Aerosols are collections of small liquid or solid particles dispersed in air or some other medium．The particles are all so tiny that each is composed of only a few hundred atoms．Because of this they can float in the air for a very long time．Perhaps the most commonly experienced aerosol is industrial smog (烟雾) of the kind that plagued London in the 1950s and is an even greater problem in Los Angeles today．These collections of aerosols reflect the Sun's heat and thereby cause the Earth to cool.
Dr．Rasool and Dr．Schneider have calculated the exact effect of a dust aerosol layer just above the Earth's surface in the temperature of the planet．As the layer builds up, the present delicate balance between the amount of heat absorbed from the Sun and the amount radiated from the Earth is disturbed．The aerosol layer not only reflects much of the Sun's light but also transmits the infrared (红外线的 ) radiation from below．So, while the heat input to the surface drops, the loss of heat remains high until the planet cools to a new balanced state.
Within fifty years, if no steps are taken to stop the spread of aerosols in the atmosphere, a cooling of the Earth by as much as 3.5~C seems inevitable.If that lasts for only a few years it would start another ice age, and because the growing ice caps at each pole would themselves reflect much of the Sun's radiation it would probably continue to develop even if the aerosol layer were destroyeD．
The only bright spot in this gloomy forecast lies in the hope expressed by Dr．Rasool and Dr．Schneider that nuclear power may replace fossil fuels in time to prevent the aerosol content of the atmosphere from becoming critical.
The author's main purpose in writing the article is to warn of________. A．warm weather
B．hot weather
C．a new ice age
D．a new iceberg
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