The molecules of carbon dioxide in the Earth’s atmosphere affect the heat balance of the Earth by acting as a one-way screen. Although these molecules allow radiation at visible wavelengths, where most of the energy of sunlight is concentrated, to pass through, they absorb some of the longer-wavelength, infrared emissions radiated from the Earth’s surface, radiation that would otherwise be transmitted back into space. For the Earth to maintain a constant average temperature, such emissions from the planet must balance incoming solar radiation. If there were no carbon dioxide in the atmosphere, heat would escape from the Earth much more easily. The surface temperature would be so much lower that the oceans might be a solid mass of ice.
　　Today, however, the potential problem is too much carbon dioxide. The burning of fossil fuels and the clearing of forests have increased atmospheric carbon dioxide by about 15 percent in the last hundred years, and we continue to add carbon dioxide to the atmosphere. Could the increase in carbon dioxide cause a global rise in average temperature, and could such a rise have serious consequences for human society? Mathematical models that allow us to calculate the rise in temperature as a function of the increase indicate that the answer is probably yes.
　　Under present conditions a temperature of-18℃ can be observed at an altitude of 5 to 6 kilometers above the Earth. Below this altitude (called the radiating level), the temperature increases by about 6℃ per kilometer approaching the Earth’s surface, where the average temperature is about 15℃. An increase in the amount of carbon dioxide means that there are more molecules of carbon dioxide to absorb infrared radiation. As the capacity of the atmosphere to absorb infrared radiation increases, the radiating level and the temperature of the surface must rise.
　　One mathematical model predicts that doubling the atmospheric carbon dioxide would raise the global mean surface temperature by 2.5℃. This model assumes that the atmosphere’s relative humidity remains constant and the temperature decreases with altitude at a rate of 6.5℃ per kilometer. The assumption of constant relative humidity is important, because water vapor in the atmosphere is another efficient absorber of radiation at infrared wavelengths. Because warm air can hold more moisture than cool air, the relative humidity will be constant only if the amount of water vapor in the atmosphere increases as the temperature rises. Therefore, more infrared radiation would be absorbed and reradiated back to the Earth’s surface. The resultant warming at the surface could be expected to melt snow and ice, reducing the Earth’s reflectivity. More solar radiation would then be absorbed, leading to a further increase in temperature.
　　17. The primary purpose of the passage is to
　　(A)warn of the dangers of continued burning of fossil fuels
　　(B)discuss the significance of increasing the amount of carbon dioxide in the atmosphere
　　(C)explain how a constant temperature is maintained on the Earth’s surface
　　(D)describe the ways in which various atmospheric and climatic conditions contribute to the Earth’s weather
　　(E)demonstrate the usefulness of mathematical models in predicting long-range climatic change
　　18. According to the passage, the greatest part of the solar energy that reaches the Earth is
　　(A)concentrated in the infrared spectrum
　　(B)concentrated at visible wavelengths
　　(C)absorbed by carbon dioxide molecules
　　(D)absorbed by atmospheric water vapor
　　(E)reflected back to space by snow and ice
　　19. According to the passage, atmospheric carbon dioxide performs all of the following functions EXCEPT:
　　(A)absorbing radiation at visible wavelengths
　　(B)absorbing infrared radiation
　　(C)absorbing outgoing radiation from the Earth
　　(D)helping to retain heat near the Earth’s surface
　　(E)helping to maintain a constant average temperature on the Earth’s surface
　　20. Which of the following best describes the author’s attitude toward the increasing amount of carbon dioxide in the atmosphere and its consequences?
　　(A)Incredulous
　　(B)Completely detached
　　(C)Interested but skeptical
　　(D)Angry yet resigned
　　(E)Objective yet concerned
　　21. It can be concluded from information contained in the passage that the average temperature at an altitude of 1 kilometer above the Earth is about
　　(A)15℃
　　(B)9℃
　　(C)2.5℃
　　(D)-12℃
　　(E)-18℃
　　22. It can be inferred from the passage that the construction of the mathematical model mentioned in the passage involved the formulation of which of the following?
　　(A)An assumption that the amount of carbon dioxide added to the atmosphere would in reality steadily increase
　　(B)An assumption that human activities are the only agencies by which carbon dioxide is added to the atmosphere
　　(C)Assumptions about the social and political consequences of any curtailment of the use of fossil fuels
　　(D)Assumptions about the physical conditions that are likely to prevail during the period for which the model was made
　　(E)Assumptions about the differential behavior of carbon dioxide molecules at the various levels of temperature calculated in the model
　　23. According to the passage, which of the following is true of the last hundred years?
　　(A)Fossil fuels were burned for the first time.
　　(B)Greater amounts of land were cleared than at any time before.
　　(C)The average temperature at the Earth’s surface has become 2℃ cooler.
　　(D)The amount of carbon dioxide in the atmosphere has increased measurably.
　　(E)The amount of farmland worldwide has doubled.
　　B B A E B D D

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