If pollution continues to increase at the present rate, formation of aerosols (浮质) in the atmosphere will cause the onset of an ice age in about 50 years' time. This conclusion, reached by Dr. S.I. Rasool and Dr. S.H. Schneider, of the United States Goddard Space Flight Center, answers the apparently conflicting question of whether an increase in the carbon dioxide 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 meddling with the environment 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 media. 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 of the kind that plagued London in the 1950s and is an even greater problem in Los Angles today. These collections of aerosols reflect the sun's heat and thereby caused 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 on 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 radiation from below almost unimpeded. 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 50 years, if no steps are taken to curb 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. Rascool and Dr. Schneider that nuclear power may replace fossil fuels in time to prevent the aerosol content of the atmosphere from becoming critical. What is endangering our planet more urgently according to the passage?