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