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 changessuch as the destruction of the worlds tropical rain forests to create grazing land for cattle or the drying up of almost three-quarters of the Aral Sea, once the worlds fourth-largest freshwater lake, for irrigation purposeshave 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 environmentalistsprofessionals in various fields, as well as concerned citizens in the United States and other countriesare struggling to lessen the impact of human activity on the natural world.

　　Population Growth(人口增長)

　　Human population growth may be seen to be at the root of virtually all of the worlds 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 worlds 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

　　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 changessuch as the destruction of the worlds tropical rain forests to create grazing land for cattle or the drying up of almost three-quarters of the Aral Sea, once the worlds fourth-largest freshwater lake, for irrigation purposeshave 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 environmentalistsprofessionals in various fields, as well as concerned citizens in the United States and other countriesare struggling to lessen the impact of human activity on the natural world.

　　Population Growth(人口增長)

　　Human population growth may be seen to be at the root of virtually all of the worlds 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 worlds 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