Unit5-课文
Unit 5
Text There is one gas prent in the air we breathe that is esntial to life. Unfortunately, one can have too much of a good thing and the growth in carbon dioxide threatens to warm up our planet to a dangerous extent. Isaac Asimov introduces us to this villain in the atmosphere, explaining how it works and what can be done about it.
The Villain in the Atmosphere
⑴The villain in the atmosphere is carbon dioxide.
⑵It does not em to be a villain. It is not very poisonous and it is prent in the atmosphere in so small a quantity —only 0.034 percent —that it does us no harm.
⑶What’s more, that small quantity of carbon dioxide in the air is esntial to life. Plants absorb carbon dioxide and convert it into their own tissue, which rve as the basic food su
pply for all of animal life (including human beings, of cour). In the process they liberate oxygen, which is also necessary for all animal life.
梦中的蝴蝶⑷But here is what this apparently harmless and certainly esntial gas is doing to us:
⑸The a level is rising very slowly from year to year. In all likelihood, it will continue to ri and do so at a greater rate in the cour of the next hundred years. Where there are low-lying coastal areas (where a large fraction of the world’s population lives) the water will advance steadily, forcing people to retreat inland.
⑹Eventually the a will reach two hundred feet above its prent level, and will be splashing against the windows along the twentieth floors of Manhattan’s skyscrapers. Florida will disappear beneath the waves, as will much of the British Isles, the crowded Nile valley, and the low-lying areas of China, India, and Russia.
⑺Not only will many cities be drowned, but much of the most productive farming areas of the world will be lost. As the food supply drops, starvation will be widespread and the structure of society may collap under the pressure.
搞笑古诗⑻And all becau of carbon dioxide. But how does that come about? What is the connection?
⑼It begins with sunlight, to which the various gas of the atmosphere (including carbon dioxide) are transparent. Sunlight, striking the top of the atmosphere, travels right through miles of it to warm the Earth’s surface. At night, the Earth cools by radiating heat into space in the form of infrared radiation.
⑽However, the atmosphere is not quite as transparent to infrared radiation as it is to visible light.
Carbon dioxide in particular tends to block such radiation. Less heat is lost at night, for that reason, than would be lost if carbon dioxide were not prent in the atmosphere. Without the small quantity of that gas prent, the Earth would be distinctly cooler, perhaps uncomfortably cool.
⑾We can be thankful that carbon dioxide is keeping us comfortably warm, but the concentration
of carbon dioxide in the atmosphere is going up steadily and that is where the villainy comes in. In 1958, carbon dioxide made up only 0.0316 percent of the atmosphere. Each year since, the concentration has crept upward and it now stands at 0.0340 percent. It is estimated that by 2020 the concentration will be nearly twice what it is now.
⑿This means that in the coming decades, Earth’s average temperature will go up slightly. As a失去与得到
result, the polar ice caps will begin to melt.
⒀Something like 90 percent of the ice in the world is to be found in the huge Antarctica ice cap,
and another 8 percent is in the Greenland ice cap. If the ice caps begin to melt, the a level will ri, with the result that I have already described.
俏黄蓉与黄河四鬼⒁But why is the concentration of carbon dioxide in the atmosphere steadily rising?
⒂To blame are two factors. First of all, in the last few centuries, first coal, then oil and natural gas,
have been burned for energy at a rapidly increasing rate. The carbon contained in the fuels, which has been safely buried underground for many millions of years, is now being burned to carbon dioxide and poured into the atmosphere at a rate of many tons per day.光纤是什么东西
⒃To make matters wor, Earth’s forests have been disappearing, slowly at first, but in the last
couple of centuries quite rapidly. Right now it is disappearing at the rate of sixty-four acres per minute.
⒄Whatever replaces the forest —grassland or farms or scrub —produces plants that do not
consume carbon dioxide at an equal rate. Thus, not only is more carbon dioxide being added to the atmosphere through burning of fuel, but as the forests disappear, less carbo苍耳图片
投向的意思n dioxide is being removed from the atmosphere by plants.慎终追远的意思
