Greenhouse Effect
A greenhouse is a building made of glass which is used for keeping plants warm when the outside temperature is low．In a similar way, there are several gases in the atmosphere which trap the heat generated by the sun and pr it from escaping．These gases are known as 'greenhouse gases', and the way in which they trap heat in the atmosphere is called the 'greenhouse effect'．This is not simply air pollution like photochemical smog (光化学烟雾), for example．Most of the main greenhouse gases occur naturally in small amounts in our atmosphere, and without them the earth would be thirty degrees colder and human life would not exist．In other words, the greenhouse effect is a natural process which is to some extent beneficial to us.
The problem is that in the last century and a half, we have been putting excessive amounts of these gases into the earth's atmosphere by burning large quantities of coal and oil and by cutting down forests．In 1850, there were 280 parts per million of carbon dioxide (二氧化碳) in the atmosphere．Now there are 360, and this figure is expected to rise to 460 by the year 2030．We now put 24 billion metric (公制的) tons of carbon dioxide into the atmosphere every year．We have also created a group of artificial greenhouse gases that are 20,000 times more effective than carbon dioxide in trapping heat in the atmosphere．These are the chlorofluorocarbon (氟氯碳) gases, which are used in refrigerators and air conditioning systems.
The rapid increase in greenhouse gases is the world warmer．The world's temperature has already gone up by half a degree this century, and the sea level has risen by ten centimeters．If the amount of carbon dioxide in the atmosphere doubles, there will probably be a rise in the earth's temperature of between 1° and 4°．This may seem a small increase, but it would be enough to cause major changes in geography and agriculture．Large areas of the world would be flooded, and some regions would become dry and unable to produce crops．It is important, too, to consider that there may be a delay of about thirty years in the greenhouse effect, This means that we are probably experiencing only now the effect of the gases put into the atmosphere up to the 1960s．Since then, our use of these gases has greatly increased.
According to the passage, a greenhouse is built to