| Quantum mechanics provides the basis for | | | | the difference between the infrared radiation that |
| computing the interactions between molecules and | | | | the surface would radiate to space if there were |
| radiation. Most of this interaction occurs when the | | | | no atmosphere and the actual infrared radiation |
| frequency of the radiation closely matches that of | | | | escaping to space. Then compute the percentage |
| the spectral lines of the molecule, determined by | | | | reduction in GE when a constituent is removed. |
| the quantization of the modes of vibration and | | | | The table below is computed by this method, |
| rotation of the molecule. (The electronic | | | | using a particular 1-dimensional model of the |
| excitations are generally not relevant for infrared | | | | atmosphere. More recent 3D computations lead to |
| radiation, as they require energy larger than that | | | | similar results. |
| in an infrared photon.) | | | | By this particular measure, water vapor can be |
| The width of a spectral line is an important | | | | thought of as providing 36% of the greenhouse |
| element in understanding its importance for the | | | | effect, and carbon dioxide 12%, but the effect of |
| absorption of radiation. In the Earth’s | | | | removal of both of these constituents will be |
| atmosphere these spectral widths are primarily | | | | greater than 48%. An additional proviso is that |
| determined by “pressure | | | | these numbers are computed holding the cloud |
| broadeningâ€, which is the distortion of | | | | distribution fixed. But removing water vapor from |
| the spectrum due to the collision with another | | | | the atmosphere while holding clouds fixed is not |
| molecule. Most of the infrared absorption in the | | | | likely to be physically relevant. In addition, the |
| atmosphere can be thought of as occurring while | | | | effects of a given gas are typically nonlinear in |
| two molecules are colliding. The absorption due to | | | | the amount of that gas, since the absorption by |
| a photon interacting with a lone molecule is | | | | the gas at one level in the atmosphere can |
| relatively small. This three-body aspect of the | | | | remove photons that would otherwise interact |
| problem, one photon and two molecules, makes | | | | with the gas at another altitude. The kinds of |
| direct quantum mechanical computation for | | | | estimates presented in the table, while often |
| molecules of interest more challenging. Careful | | | | encountered in the controversies surrounding |
| laboratory spectroscopic measurements, rather | | | | global warming, must be treated with caution. |
| than ab initio quantum mechanical computations, | | | | Different estimates found in different sources |
| provide the basis for most of the radiative | | | | typically result from different definitions and do |
| transfer calculations used in studies of the | | | | not reflect uncertainties in the underlying radiative |
| atmosphere. | | | | transfer. |
| The molecules/atoms that constitute the bulk of | | | | Positive feedback and runaway greenhouse effect |
| the atmosphere; oxygen (O2), nitrogen (N2) and | | | | When the concentration of a greenhouse gas (A) |
| argon; do not interact with infrared radiation | | | | is itself a function of temperature, there is a |
| significantly. While the oxygen and nitrogen | | | | positive feedback from the increase in another |
| molecules can vibrate, because of their symmetry | | | | greenhouse gas (B), whereby increase in B |
| these vibrations do not create any transient | | | | increases the temperature which, in turn, |
| charge separation that enhances the interaction | | | | increases the concentration of A, which increases |
| with radiation. In the Earth’s | | | | temperatures further, and so on. This feedback is |
| atmosphere, the dominant infrared absorbing | | | | bound to stop, since the overall supply of the gas |
| gases are water vapor, carbon dioxide, and ozone | | | | A must be finite. If this feedback ends after |
| (O3), these molecules being | | | | producing a major temperature increase, it is |
| “floppier†so that their rotation | | | | called a runaway greenhouse effect. |
| vibration modes are more easily excited. For | | | | According to some climate models (Clathrate gun |
| example, carbon dioxide is a linear molecule, but it | | | | hypothesis), such a runaway greenhouse effect, |
| has an important vibrational mode in which the | | | | involving liberation of methane gas from hydrates |
| molecule bends with the carbon in the middle | | | | by global warming, caused the Permian-Triassic |
| moving one way and the oxygens on the ends | | | | extinction event. It is also thought that large |
| moving the other way, creating some charge | | | | quantities of methane could be released from the |
| separation, a dipole moment. A substantial part of | | | | Siberian tundra as it begins to thaw, methane |
| the greenhouse effect due to carbon dioxide | | | | being 21-times more potent a greenhouse gas |
| exists because this vibration is easily excited by | | | | than carbon dioxide. |
| infrared radiation. Clouds are also very important | | | | A runaway greenhouse effect involving CO2 and |
| infrared absorbers. Therefore, water has multiple | | | | water vapor may have occurred on Venus. On |
| effects on infrared radiation, through its vapor | | | | Venus today there is little water vapor in the |
| phase and through its condensed phases. Other | | | | atmosphere. If water vapor did contribute to the |
| absorbers of significance include methane, nitrous | | | | warmth of Venus at one time, this water is |
| oxide and the chlorofluorocarbons. | | | | thought to have escaped to space. Venus is |
| Discussion of the relative importance of different | | | | sufficiently strongly heated by the Sun that water |
| infrared absorbers is confused by the overlap | | | | vapor can rise much higher in the atmosphere and |
| between the spectral lines due to different gases, | | | | is split into hydrogen and oxygen by ultraviolet |
| widened by pressure broadening. As a result, the | | | | light. The hydrogen can then escape from the |
| absorption due to one gas cannot be thought of | | | | atmosphere and the oxygen recombines. Carbon |
| as independent of the presence of other gases. | | | | dioxide, the dominant greenhouse gas in the |
| One convenient approach is to remove the | | | | current Venusian atmosphere, likely owes its |
| chosen constituent, leaving all other absorbers, | | | | larger concentration to the weakness of carbon |
| and the temperatures, untouched, and monitoring | | | | recycling as compared to Earth, where the carbon |
| the infrared radiation escaping to space. The | | | | dioxide emitted from volcanoes is efficiently |
| reduction in infrared absorbtion is then a measure | | | | subducted into the Earth by plate tectonics on |
| of the importance of that constituent. More | | | | geologic time scales |
| precisely, define the greenhouse effect (GE) to be | | | | |