In the atmosphere, carbon dioxide acts rather like a one-way mirror-the glass in the roof of a greenhouse which allows the sun's rays to enter but prevents che heat from escaping.

　　According to a weather expert's prediction. the atmosphere will be 3C warmer m the year 2050 than it is today, if man continues to bum fuels at the present rate. If this warming up took place, the ice caps in the poles would begin to melt. thus raising sea level several meters and severely flooding coastal cities. Also, the increase in atmospheric temperature would lead to great changes in the climate of the northern hemisphere, possibly re9ulting in an alteration of the earth's chief food-growing zones.

　　In the past, concern about a man-made warming of the earth has concentrated on the Arctic because the Antarctic is much colder and has a much thicker ice sheet. But the weather experts are now paying more attention to West Antarctic, which may be affected by only a few degrees of warming: in other words, by a warming on the scale that will possibly take place in the next fifty years from the burning of fuels.

　　Satellite pictures show that large areas of Antarctic ice are already disappearing. The evidence available suggests that a warming has taken place. This fits the theory that carbon dioxide warms the earth.

　　However. most of the fuel is burnt in the northern Hemisphere, where temperatures seem to be falling. Scientists conclude. therefore. that up to now natural influences on the weather have exceeded those caused by man. The question is: Which natural cause has most effect on the weather?

　　One possibility is the variable behavior of the sun. Astronomers at one research station have studied the hot spots and "cold” spots (that is, the relatively less hot spots) on the sun. As the sun rotates, every 27.5 days. it presents hotter or "colder" faces to the earth, and different aspects to different parts of the earth. This seems to have a considerable effect on che distribution of the earth's atmospheric pressure, and consequently on wind circulation. The sun is also variable over a long term: its heat output goes up and down in cycles. che latest trend being, downward.

　　Scientists are now finding mutual relations between models of solar-weather interactions and the actual climate over many thousands of years, including the last Ice Age. The problem is that che models are predicting that the world should be entering a new Ice Age and it is not. One way of solving this theoretical difficulty is to assume a delay of thousands of years while the solar effects overcome the inertia (惯性) of the earth's climate. If this is right. the warming effect of carbon dioxide might thus be serving as a useful counterbalance to the sun’s diminishing heat.

　　1. It can be concluded that a concentration of carbon dioxide in the atmosphere would______.

　　A) prevent the sun's rays from reaching the earth's surface

　　B) mean a warming up in the Arctic

　　C) cause great climate changes in the northern hemisphere

　　D) raise the temperature of the earth's surface

　　2.The article was written io explain____________.

　　A) the greenhouse effect.

　　B) che sola:r effects on the earth.

　　C) the models of solar-weather interactions.

　　D) the causes affecting weather.

　　3.Why is the fuel consumption greater in the northern hemisphere. but temperatures there seem to be falling?

　　A) Mainly because the levels of carbon dioxide are rising.

　　B) Possibly because the ice caps in the poles are melting.

　　C) Because the inertia of the earth's climate take effect.

　　D) Partly because the output of solar energy varies.

　　4.On the basis of their models. scientists are of the opinion that_______.

　　A) the climate of the world should be becoming cooler

　　B) it will take thousands of years for the inertia of the earth's climate to take effect

　　C) the man-made warming effect helps to increase the solar effects

　　D) the new Ice Age will be delayed by the greenhouse effect

　　5.If the assumption about the delayof a new Ice Age is correct,___________.

　　A) the best way to overcome the cooling effect would be to burn more fuels

　　B) ice would soon cover the northern hemisphere

　　C) the increased levels of carbon dioxide could warm up the earth quickly.

　　D) the greenhouse effect could work to the advantage of the earth.