Before humans came on the scene, the world changed only slowly. The climate warmed and cooled, new plants and animals evolved (进化) and became extinct in their turn, and sea levels rose and fell over periods of thousands, if not millions, of years. But during the last two thousand years there have been very great changes. Forests have disappeared, river courses have been changed, and large areas of natural vegetation have turned into farmland and cities. There are serious problems for the survival of the human race.
The Greenhouse Effect
It may be cold outside, but on a sunny day it can be hot in a greenhouse. Some of the gases in the Earth’s atmosphere act like the glass in a greenhouse. Radiant heat from the Sun can pass through them to warm the Earth below. But the ground also loses heat by radiation (辐射). The "’greenhouse gases" send some of this heat back towards the Earth’s suce and help to keep it warm. However, by burning fuels and forests, we are putting larger and larger amounts of these greenhouse gases into the atmosphere. As a result, the Earth is slowly warming up. This is called the greenhouse effect.
The greenhouse effect could cause dry weather in some parts of the world. If the polar icecaps melt and sea levels rise, many areas of the world will be flooded. Scientists believe that the only way to slow the greenhouse effect is for us to produce less of the gases which cause it.
Air Pollution and Acid Rain
Pollution is caused when what we do damages our surroundings. Factories, power stations and motor vehicles send waste gases and soot (烤烟) into the air. The polluted air damages people’s lungs. Some petrol had lead in it. The lead comes out in car exhaust fumes and it can cause brain damage in children.
The waste gases coming from burning coal, oil and petrol include sulphur (硫黄) dioxide and nitrogen (氮) dioxide. These gases may dissolve (溶解) in water in the atmosphere to form weak acids. They later fall to the Earth as acid rain, sometimes hundreds of kilometers from where they were formed. Much of the acid rain in Canada is caused by smoke from factories and power stations in the USA; the acid rain in Scandinavia may come from Britain.
Acid rain attacks trees and other plants, and kills the fish and water animals living in lakes and rivers. Acid rain and polluted air also damage the bricks and stonework of buildings, and corrode (Neck) the metalwork of steel bridges and railings.
Acid rain is difficult to control because it spreads so far. And the ways of avoiding acid rain are all expensive.
The Ozone Hole
Ozone (臭氧) is a gas which forms a layer around the planet at about 20 to 50 km above the Earth’s suce. The ozone layer prs the Sun’s erous ultraviolet radiation reaching the Earth where it would damage our skin and cause cancers. Many scientists are worried that the ozone layer is being destroyed by the CFCs which are used in refrigerators and freezers. These chemicals are also important greenhouse gases. At certain times of the year the ozone layer becomes extremely thin near the north and south poles. Already skin cancers are increasing in Australia.
Many countries are trying to stop the production of CFCs and to find other chemicals to do the same thing.
Radioactive Waste
Nuclear power stations produce waste which is radioactive. Some of this waste is released by the power stations into the air or water; some is stored. It can be carried long distances by wind or by water. Many scientists worry about the long-term effects of this type of pollution on humans and wildlife.
The Greenhouse EffectAir Pollution and Acid RainThe Ozone HoleRadioactive WasteIf the ultraviolet radiation from the sun arrives at the Earth, it would ______.