The level of carbon dioxide, CO2 , in the atmosphere has an important effect on climatic change. Most of the Earth’s incoming energy is short-wavelength radiation, which tends to pass through atmospheric CO2 easily. The Earth, however, reradiates much of the received energy as long-wavelength radiation, which CO2 absorbs and then remits toward the Earth. This phenomenon, known as the greenhouse effect, can result in an increase in the surface temperature of a planet. An extreme example of the effect is shown by Venus, a planet covered by heavy clouds composed mostly of CO2 whose surface temperatures have been measured at 430℃. If the CO2 content of the atmosphere is reduced, the temperature falls. According to one respectable theory, if the atmospheric CO2 concentration were halved, the Earth would become completely covered with ice. Another equally respectable theory, however, states that a halving of the CO2 concentration would lead only to a reduction in global temperatures of 3℃。

　　If, because of an increase in forest fires or volcanic activity, the CO2 content of the atmosphere increased. A warmer climate would be produced. Plant growth, which relies on both the warmth and the availability of CO2 would probably increase. As a consequence, plants would use more and more CO2 . Eventually CO2 levels would diminish and the climate, in turn, would become cooler. With reduced temperatures many plants would die; CO2 would thereby be returned to the atmosphere and gradually the temperature would rise again. Thus, if this process occurred there might be a long-term oscillation in the amount of CO2 present in the atmosphere, with regular temperature in creases and decreases of a set magnitude。

　　Some climatologists argue that the burning of fossil fuels has raised the level of CO2 in the atmosphere and has caused a global temperature increase of at least 1℃.But a supposed global temperature rise of 1℃ may in reality be only several regional temperature increases restricted to areas where there are many meteorological stations and caused simply by shifts in the pattern of atmospheric circulation. Other areas for example the Southern Hemisphere oceanic zone may be experiencing an equivalent temperature decrease that is unrecognized because of the shortage of meteorological recording stations。

　　6. The passage supplies information for answering which of the following questions?

　　Why are projections of the effects of changes in water vapor levels on the climate so    inaccurate?

　　What are the steps in the process that takes place as CO2 absorbs long-wavelength radiation?

　　How might our understanding of the green house effect be improved if the burning of fossil fuels were decreased?

　　D. What might cause a series of regular increases and decreases in the amount of CO2 in the atmosphere?

　　7. The author is primarily concerned with

　　A. explaining the effects that the burning of fossil fuels might have on climate

　　B. illustrating the effects of CO2 on atmospheric radiation

　　C. discussing effects that changes in the CO2 level in the atmosphere might have on climate

　　D. challenging hypotheses about the effects of water vapor and CO2 on climate

　　8. The passage suggests that a large decrease in the amount of CO2 in the atmosphere would result in

　　A. at least a slight decrease in global temperatures

　　B. at the most a slight increase in short-wavelength radiation reaching the Earth

　　C. a slight long-term increase in global temperatures

　　D. a large long-term increase in the amount of volcanic activity

　　9. The author refers to Venus primarily in order to

　　A. show the inherent weakness of the greenhouse effect theory

　　B. show that the greenhouse effect works on other planets but not on Earth

　　C. show the extent to which Earth’s atmosphere differs from that of Venus

　　D. support the argument that the CO2 level in the atmosphere has a significant effect on climate

　　10. The passage suggests that if there were a slight global warming at the present time, it would be

　　A. easy to measure the exact increase in temperature because of the abundance of temperature recording stations throughout the world

　　B. difficult to measure the increase of CO2 in the atmosphere because of local variations in amounts

　　C. easy to demonstrate the effects of the warming on the water vapor in the atmosphere

　　D. difficult to prove that the warming was caused by the burning of fossil fuels

　　11. The discussion of climate in the passage suggests which of the following conclusions?

　　Ⅰ. Climate is not perfectly stable, and slight regional temperature variations can be considered a normal feature of the environment。

　　Ⅱ. We are unable at present to measure global temperature changes precisely

　　Ⅲ. The most important cause of regional climatic fluctuations is the change in CO2 levels in the atmosphere。

　　A. Ⅰonly               

　　B. Ⅲ. Only   

　　C. ⅠandⅡ only  

　　D. Ⅱand Ⅲ.only

　　12.  All of the following can be found in the author’s discussion of climate Except

　　A. a statement about the effects of increased volcanic activity on the Earth’s temperatures

　　B. an indication of the effect of an increase in water vapor in the atmosphere

　　C. a contrast between two theories about the effects of a lowering of CO2 levels in the atmosphere