Like all other living beings, humans have clearly changed their environment, but they have done so generally on a grander scale than have other species. Some of these changes-such as the destruction of the world's tropical rain forests to create grazing land for cattle or the drying up of almost three-quarters of the Aral Sea, once the world's fourth-largest freshwater lake, for irrigation purposes-have led to altered climate patterns, which in turn have changed the distribution of species of animals and plants.
Scientists are working to understand the long-term consequences that human actions have on ecosystems, while environmentalists-professionals in various fields, as well as concerned citizens in the United States and other countries-are struggling to lessen the impact of human activity on the natural world.
Human population growth may be seen to be at the root of virtually all of the world's environmental problems. Increasingly large numbers of people are being added to the world every year. As the number of people increases, more pollution is generated, more habitats are destroyed, and more natural resources are used up. Even if new technological advances were able to cut in half the environmental impact that each person had, as soon as the world's population size doubled, the earth would be no better off than before.
The Population Division of the United Nations predicts that the 5.63 billion humans alive in 1994 will increase to 6.23 billion in the year 2000, 8.47 billion in 2025, and 10.02 billion in 2050. The UN's estimate assumes that population will peak and stabilize at 11.6 billion in 2200.
Although it is true that 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 the population growth problem is primarily a problem of developing countries. In fact, because larger amounts of resources per person are used in the developed nations, each citizen from the developed world has a much greater environmental impact than does a citizen from a developing country.
Evidence now exists suggesting that the most important factors necessary to lower population growth rates in the developing world are democracy and social justice. Studies show that population growth rates have fallen in areas where several social conditions have been met. In these areas, literacy rates have increased, and women are given economic status equal to that of men and thus are able to hold jobs and own property; also, birth control information is more widely available, and women are free to make their own reproductive decisions.
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. 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. 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 over the next century. 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.
Depletion of the ozone layer（臭氧层变薄）
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.
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.
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. With many of the world's fastest growing countries in the process of industrializing and modernizing, there is reason to believe that destruction will continue to increase well beyond that year.
A significant portion of industry and transportation is based on the burning of fossil fuels, such as gasoline. As these fuels are burned, chemicals and particulate matter are released into the atmosphere. Although a vast number of substances contribute to air pollution, the most common contain carbon, sulfur, and nitrogen. These chemicals interact with one another and with ultraviolet radiation in sunlight in various dangerous ways. Smog, usually found in urban areas with large numbers of automobiles, can cause serious health problems. Acid rain is a serious global problem because few species are capable of surviving in the face of such acidic conditions. Acid rain has made numerous lakes so acidic that they no longer support fish populations. Acid rain is also thought to be responsible for the decline of many forest ecosystems worldwide.
Estimates suggest that nearly 1.5 billion people 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 or nonpoint sources. Point sources discharge pollutants at specific locations-from, for example, factories, sewage treatment plants, or oil tankers. The technology exists for point sources of pollution to be monitored and regulated, although political factors may complicate matters. Nonpoint sources-runoff water containing pesticides and fertilizers from acres of agricultural land, for example-are much more difficult to control. Pollution arising from nonpoint sources accounts for a majority of the contaminants in streams and lakes.
With almost 80 percent of the planet covered by oceans, people have long acted as if those bodies of water could serve as a limitless dumping ground for wastes. Raw sewage, garbage, and oil spills have begun to overwhelm the diluting capabilities of the oceans, and most coastal waters are now polluted. Beaches around the world are closed regularly, often because of high amounts of bacteria from sewage disposal, and marine wildlife is beginning to suffer.
Water that seeps through porous rocks and is stored beneath the ground is called groundwater. Although groundwater is a renewable resource, reserves are replenished relatively slowly. When groundwater is depleted in coastal regions, oceanic salt water commonly intrudes into freshwater supplies. Saltwater intrusion is threatening the drinking water of many areas along the Gulf and Atlantic coasts.
The EPA has estimated that, on average, 25 percent of usable groundwater is contaminated, although in some areas as much as 75 percent is contaminated. Contamination arises from leaking underground storage tanks, poorly designed industrial waste ponds, and seepage from the deep-well injection of hazardous wastes into underground geologic formations. Because groundwater is recharged and flows so slowly, once polluted it will remain contaminated for extended periods.
Habitat Destruction and Species Extinction（居住环境破坏与物种灭绝）
It is difficult to estimate the rate at which humans are driving species extinct because scientists believe that only a small percentage of the earth's species have been described. What is clear is that species are dying out at an unprecedented rate; minimum estimates are at least 4000 species per year, although some scientists believe the number may be as high as 50,000 per year. The leading cause of extinction is habitat destruction, particularly of the world's richest ecosystems-tropical rain forests and coral reefs. At the current rate at which the world's rain forests are being cut down, they may completely disappear by the year 2030. If growing population size puts even more pressure on these habitats, they might well be destroyed sooner.
This loss is critical from several perspectives. The economic value of species lost and of natural products and drugs that never will be discovered or produced is incalculable. Similarly, it is impossible to place either a moral or an aesthetic value on our growing list of extinct species. As habitats are destroyed and species lost, the world is increasingly losing threads from the interconnected fabric of life.
Pesticide residues on crops and mercury in fish are examples of toxic substances that may be encountered in daily life. Many industrially produced chemicals may cause cancer, birth defects, genetic mutations, or death. Although a growing list of chemicals has been found to pose serious health risks to humans, the vast majority of substances have never been fully tested.
Studies have shown that not all individuals are equally exposed to pollution. Three of the five largest commercial hazardous waste landfills in America are in predominantly black or Hispanic neighborhoods, and three out of every five black and Hispanic Americans live in the vicinity of an uncontrolled toxic waste site. The fact that the wealth of a community is not nearly as good a predictor of hazardous-waste locations as is the ethnic background of the residents reinforces the conclusion that racism is involved in the selection of sites for hazardous-waste disposal.
Environmental racism takes international forms as well. Dangerous chemicals banned in the United States often continue to be produced and shipped to developing countries. Additionally, the developed world has shipped large amounts of toxic waste to developing countries for less-than-safe disposal.
The world cannot continue to rely on the burning of fossil fuels for much of its industrial production and transportation. Fossil fuels are in limited supply; in addition, when burned they contribute to global warming, air pollution, and acid rain.
Nuclear energy as an alternative is opposed by many because of the massive devastation an accident can cause. The accident at the Chernobyl' nuclear power plant in 1986 scattered radioactive contamination over a large part of Europe. Approximately 135,000 people were evacuated, and human health has been dramatically affected. The World Health Organization released a report in late 1995 attributing the "explosive increase" in childhood thyroid cancer in Belarus, Ukraine, and Russia directly to the accident.
One reasonable solution is to combine conservation strategies with the increased use of solar energy. The price of solar energy relative to traditional fuels has been dropping steadily, and if environmental concerns were factored into the cost, solar power would already be significantly cheaper. Although it is desirable to have a wider range of energy options, other alternative sources of power （such as wind, geothermal, or hydroelectric） are not likely to provide large-scale solutions in the forseeable future.
Global environmental collapse is not inevitable. But the developed world must work with the developing world to ensure that new industrialized economies do not add to the world's environmental problems. Politicians must think of sustainable development rather than economic expansion. Conservation strategies have to become more widely accepted, and people must learn that energy use can be dramatically diminished without sacrificing comfort. In short, with the technology that currently exists, the years of global environmental mistreatment can begin to be reversed.