The molecules of carbon dioxide in the Earth's atmosphere affect the heat balance of the E

arth by acting as a one-way screen. (1)_____ these molecules allow radiation at visible wavelengths, where most of the energy of sunlight is concerned, to pass (2)_____, they absorb some of the longer-wave length, infrared emissions radiated from the Earth's surface, radiation that would (3)_____ be transmitted back into space. For the Earth to maintain a constant average temperature, such emissions from the planet must (4)_____ incoming solar radiation. If there were no carbon dioxide in the atmosphere, heat would (5)_____ from the Earth much more easily.

Today, (6)_____, the potential problem is too much carbon dioxide. Could the increase in carbon dioxide (7)_____ a global rise in average temperature, and could such a rise have serious (8)_____ for human society？ Mathematical models that allow us to calculate the rise in temperature as a function of the increase (9)_____ that the (10)_____ is probably yes.

One mathematical model (11)_____ 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 (12)_____ of 6.5℃ per kilometer. The assumption of constant relative humidity is important, because water vapor in the atmosphere is another (13)_____ absorber or radiation at infrared wavelengths. Because warm air can hold more (14)_____ than cool air, the relative humidity will be constant (15)_____ the amount of water vapor in the atmosphere increases (16)_____ the temperature rises. (17)_____, more infrared radiation would be absorbed and reradiated (18)_____ to the Earth's surface. The resultant warming at the surface could be expected to melt snow and ice, (19)_____ the Earth's reflectivity. More solar radiation would be absorbed, (20)_____ to a further increase in temperature.

A．Whereas

B．If

C．Although

D．Because