| The molecules of carbon dioxide in the
Earth's atmosphere affect the heat balance of the Earth by acting as a one-way
screen.{{U}} (1) {{/U}}these molecules allow radiation at visible
wavelengths, where most of the energy of sunlight is concerned, to pass{{U}}
(2) {{/U}}, they absorb some of the longer-wave length, infrared
emissions radiated from the Earth's surface, radiation that would{{U}} (3)
{{/U}}be transmitted back into space. For the Earth to maintain a constant
average temperature, such emissions from the planet must{{U}} (4)
{{/U}}incoming solar radiation. If there were no carbon dioxide in the
atmosphere, heat would{{U}} (5) {{/U}}from the Earth much more
easily. Today,{{U}} (6) {{/U}}, the potential problem is too much carbon dioxide. Could the increase in carbon dioxide{{U}} (7) {{/U}}a global rise in average temperature, and could such a rise have serious {{U}}(8) {{/U}}for human society? Mathematical models that allow us to calculate the rise in temperature as a function of the increase{{U}} (9) {{/U}}that the{{U}} (10) {{/U}}is probably yes. One mathematical model{{U}} (11) {{/U}}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{{U}} (12) {{/U}}of 6.5℃ per kilometer. The assumption of constant relative humidity is important, because water vapor in the atmosphere is another{{U}} (13) {{/U}}absorber or radiation at infrared wavelengths. Because warm air can hold more{{U}} (14) {{/U}}than cool air, the relative humidity will be constant{{U}} (15) {{/U}}the amount of water vapor in the atmosphere increases{{U}} (16) {{/U}}the temperature rises.{{U}} (17) {{/U}}, more infrared radiation would be absorbed and reradiated{{U}} (18) {{/U}} to the Earth's surface. The resultant warming at the surface could be expected to melt snow and ice,{{U}} (19) {{/U}}the Earth's reflectivity. More solar radiation would be absorbed,{{U}} (20) {{/U}}to a further increase in temperature. |