Global Warming Like the glass panes in a greenhouse, certain gases in the earth's atmosphere permit the sun's radiation to heat the earth but retard the escape into space of the infrared energy radiated back out by the earth. This process is referred to as the greenhouse effect. These gases, primarily carbon dioxide, methane, nitrous oxide, and water vapor, insulate the earth's surface, helping to maintain warm temperatures. Without these gases, the earth would be a frozen planet with an average temperature of about -18° C (about 0° F) instead of a comfortable 15° C (59° F). If the concentration of these gases were higher, more heat would be trapped within the atmosphere, and worldwide temperatures would rise. Within the last century, the amount of carbon dioxide in the atmosphere has increased dramatically, largely because of the practice of burning fossil fuels-coal and petroleum and its derivatives. Global temperature has also increased 1° C (about 1.8° F) within the past century. Atmospheric scientists have now concluded that at least half of that increase can be attributed to human activity, and they have predicted that unless dramatic action is taken, temperature will continue to rise by between 1° and 3.5° C (between 1.8° and 6.3° F) over the next century. Although this may not seem like a great difference, global temperature was only 2.2° C (4° F) cooler during the last ice age than it is presently. The consequences of such a modest increase in temperature may well be devastating. Sea levels will rise, completely inundating a number of low-lying island nations and flooding many coastal cities such as New York and Miami. Many plant and animal species will probably be driven into extinction, agricultural regions will be disrupted, and the frequency of severe hurricanes and droughts is likely to increase. CDepletion of the Ozone Layer The ozone layer, a thin band in the stratosphere (a layer in the upper atmosphere), serves to shield the earth from the sun's harmful ultraviolet rays. In the 1970s, scientists discovered that the layer was being attacked by chlorofluorocarbons (CFCs), chemicals used in refrigeration, air-conditioning systems, cleaning solvents, and aerosol sprays. CFCs release chlorine into the atmosphere; chlorine, in turn, breaks ozone down into its constituent parts of oxygen. Because chlorine is not affected by its interaction with ozone, each chlorine molecule has the ability to destroy a large amount of ozone for an extended period of time.The consequences of the depletion of the ozone layer are dramatic. Increased ultraviolet radiation will lead to a growing number of skin cancers and cataracts and also reduce the ability of people's immune systems to respond to infection. Additionally, the growth rates of the world's oceanic plankton, the base of most marine food chains, will be negatively affected, perhaps leading to increased atmospheric carbon dioxide and thus to global warming. Even if the manufacture of CFCs was immediately banned, the chlorine already released into the atmosphere would continue to destroy the ozone layer for many decades. Additionally, the latest studies suggest that global warming may increase the amount of ozone destroyed. Predicting the rate of ozone depletion is difficult. Optimists claim that if international agreements for the phasing out of ozone-depleting chemicals agreed to in Montréal in 1987 are followed, ozone loss will peak in the year 2000.
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