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  3. Factors Threatening the Environment The problems facing the environment are vast and diverse. Global warming, the depletion

Factors Threatening the Environment The problems facing the environment are vast and diverse. Global warming, the depletion

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问题                              Factors Threatening the Environment
    The problems facing the environment are vast and diverse. Global warming, the depletion of the ozone layer in the atmosphere, and destruction of the world’s rain forests are just some of the Problems that many scientists believe will reach critical proportions in the coming decades. All of these problems will be directly affected by the size of the human population.
A. Population Growth
    Human population growth is at the root of virtually all of the world’s environmental problems. Although the growth rate of the world’s population has slowed slightly since the 1990s, the world’s population increases by about 77 million human beings each year. As the number of people increases, crowding generates pollution, destroys more habitats, and uses up additional natural resources.
    The Population Division of the United Nations (UN) predicts that the world’s population will increase from 6.23 billion people in 2000 to 9.3 billion people in 2050. The UN estimates that the population will stabilize at more than 11 billion in 2200. Other experts predict that numbers will continue to rise into the foreseeable future, to as many as 19 billion people by the year 2200.
    Although rates of population increase are now much slower in the developed world than in the developing world, it would be a mistake to assume that population growth is primarily a problem of developing countries. In fact, because larger amounts of resources per person are used in developed nations, each individual from the developed world has a much greater environmental impact than does a person from a developing country. Conservation strategies that would not significantly alter lifestyles but that would greatly lessen environmental impact are essential in the developed world.
    In the developing world, meanwhile, the most important factors necessary to lower population growth rates are democracy and social justice. Studies show that population growth rates have fallen in developing areas where several social conditions exist. In these areas, literacy rates have increased and women receive economic status equal to that of men, enabling women to hold jobs and own property. In addition, birth control information in these areas is more widely available, and women are free to make their own reproductive decisions.
B. Global Warming
    Like the glass panes in a greenhouse, certain gases in the Earth’s atmosphere permit the Sun’s radiation to heat Earth. At the same time, these gases retard the escape into space of the infrared energy radiated back out by Earth. This process is referred to as the greenhouse effect. These gases, primarily carbon dioxide, methane, nitrous oxide, and water vapor, insulate Earth’s surface, helping to maintain warm temperatures. Without these gases, Earth would be a frozen planet with an average temperature of about 18℃ instead of a comfortable 15℃. If the concentration of these gases rises, they trap more heat within the atmosphere, causing worldwide temperatures to rise.
    Within the last century, the amount of carbon dioxide in the atmosphere has increased dramatically, largely because people bum vast amounts of fossil fuels--coal and petroleum and its derivatives. Average global temperature also has increased--by about 0.6 Celsius degree within the past century. Atmospheric scientists have found that at least half of that temperature increase can be attributed to human activity. They predict that unless dramatic action is taken, global temperature will continue to rise by 1.4 to 5.8 Celsius degrees over the next century. Although such an increase may not seem like a great difference, during the last ice age the global temperature was only 2.2 Celsius degrees cooler than it is presently.
    The consequences of such a modest increase in temperature may be devastating (破坏性的). Already scientists have detected a 40 percent reduction in the average thickness of Arctic ice. Other problems that may develop include a rise in sea levels that will completely inundate a number of low-lying island nations and flood many coastal cities, such as New York and Miami. Many plant and animal species will probably be driven into extinction, agriculture will be severely disrupted in many regions, and the frequency of severe hurricanes and droughts will likely increase.
C. Depletion of the Ozone Layer
    The ozone (臭氧) layer, a thin band in the stratosphere (layer of the upper atmosphere), serves to shield Earth from the Sun’s harmful ultraviolet rays. In the 1970s, scientists discovered that chlorofluorocarbons (CFCs)-- chemicals used in refrigeration, air-conditioning systems, cleaning solvents, and aerosol sprays--destroy the ozone layer. CFCs release chlorine into the atmosphere; chlorine, in turn, breaks down ozone molecules. 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 continued depletion of the ozone layer would be dramatic. Increased ultraviolet radiation would lead to a growing number of skin cancers and cataracts (白内障) and also reduce the ability of immune systems to respond to infection. Additionally, growth of the world’s oceanic plankton, the base of most marine food chains, would decline. Plankton (浮游生物) contains photosynthetic organisms that break down carbon dioxide. If plankton populations decline, it may lead to increased carbon dioxide levels in the atmosphere and thus to global warming. Recent studies suggest that global warming, in turn, may increase the amount of ozone destroyed. Even if the manufacture of CFCs is immediately banned, the chlorine already released into the atmosphere will continue to destroy the ozone layer for many decades.
    In 1987 an international pact called the Montréal Protocol on Substances that Deplete the Ozone Layer set specific targets for all nations to achieve in order to reduce emissions of chemicals responsible for the destruction of the ozone layer. Many people had hoped that this treaty would cause ozone loss to peak and begin to decline by the year 2000. In fact, in the fall of 2000, the hole in the ozone layer over Antarctica was the largest ever recorded. The hole the following year was slightly smaller, leading some to believe that the depletion of ozone had stabilized.
D. Habitat Destruction and Species Extinction
    Plant and animal species are dying out at an unprecedented rate. Estimates range that from 4,000 to as many as 50,000 species per year become extinct. The leading cause of extinction is habitat destruction, particularly of the world’s richest ecosystems--tropical rain forests and coral reefs. If the world’s rain forests continue to be cut down at the current rate, they may completely disappear by the year 2030. In addition, if the world’s population continues to grow at its present rate and puts even more pressure on these habitats, they might well be destroyed sooner.
E. Air Pollution
    A significant portion of industry and transportation burns fossil fuels, such as gasoline. When these fuels burn, chemicals and particulate matter are released into the atmosphere. Although a vast number of substances contribute to air pollution, the most common air pollutants contain carbon, sulfur, and nitrogen. These chemicals interact with one another and with ultraviolet radiation in sunlight in dangerous ways. Smog, usually found in urban areas with large numbers of automobiles, forms when nitrogen oxides react with hydrocarbons in the air to produce aldehydes (乙醛) and ketones (酮). Smog can cause serious health problems.
    Acid rain forms when sulfur dioxide and nitrous oxide transform into sulfuric acid and nitric acid in the atmosphere and come back to Earth in precipitation. Acid rain has made numerous lakes so acidic that they no longer support fish populations. Acid rain is also responsible for the decline of many forest ecosystems worldwide, including Germany’s Black Forest and forests throughout the eastern United States.
F. Water Pollution
    Estimates suggest that nearly 1.5 billion people worldwide lack safe drinking water and that at least 5 million deaths per year can be attributed to waterborne diseases. Water pollution may come from point sources or nonpoint sources (非点污染源). Point sources discharge pollutants from specific locations, such as factories, sewage treatment plants, and oil tankers. The technology exists to monitor and regulate point sources of pollution, although in some areas this occurs only sporadically. Pollution from nonpoint sources occurs when rainfall or snowmelt moves over and through the ground. As the runoff moves, it picks up and carries away pollutants, such as pesticides and fertilizers, depositing the pollutants into lakes, rivers, wetlands, coastal waters, and even underground sources of drinking water. Pollution arising from nonpoint sources accounts for a majority of the contaminants in streams and lakes.
The layer that serves to shield Earth from the Sun’s harmful ultraviolet rays is called ______.
选项
答案the stratosphere
解析 参见第3个小标题C. Depletion of the Ozone Layer 下面的第1段：The ozone layer, a thin band in the stratosphere (layer of the upper atmosphere)，serves to shield Earth from the Sun’s harmful ultraviolet rays．由此可知，阻挡太阳的有害紫外线对地球起保护作用的层叫做同温层。
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