水污染的演讲稿怎么写？( 二 )

Scientists are working to understand the long-term
consequences that human actions have on ecosystems, while environmentalists-profeionals in various fields, as well as
concerned citizens in the United States and other countries-are struggling to leen the impact of human activity on the natural world.
Population Growth（人口增长）
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.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.
Global Warming（全球变暖）
This proce is referred to as the greenhouse effect.
These greenhouse gases, primarily carbon dioxide, methane, nitrous oxide, 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.
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.
Air pollution（空气污染）
A significant portion of industry and transportation is based on the burning of foil fuels, such as gasoline.As these fuels are burned, chemicals and particulate matter are released into the
atmosphere.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.
Water pollution（水污染）
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.
Point sources discharge pollutants at specific
locations-from, for example, factories, sewage treatment plants, or oil tankers.
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.