Greenhouse effect

The greenhouse effect, first discovered by Joseph Fourier in 1824, is the process by which an atmosphere warms a planet.

Effects of various gases

It is hard to disentangle the percentage contributions to the greenhouse effect by different gases, because their respective infrared spectrums overlap. However, one can calculate the percentage of trapped radiation remaining, and discover:

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(Source: Ramanathan and Coakley, Rev. Geophys and Space Phys., 16 465 (1978))

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Water vapor effects

Water vapor is the major contributor to Earth's greenhouse effect. Its effects vary due to localized concentrations, mixture with other gases, frequencies of light, different behavior in different levels of the atmosphere, and whether positive or negative feedback takes place. High humidity also affects cloud formation, which has major effects upon temperature but is distinct from water vapor gas.

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The IPCC TAR (2001; section 2.5.3) reports that, despite non-uniform effects and difficulties in assessing the quality of the data, water vapor has generally increased over the 20th Century.

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Estimates of the percentage of Earth's greenhouse effect due to water vapor:

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• 36% (table above)
• 60-70% Nova. Greenhouse - Green Planet http://www.pbs.org/wgbh/nova/ice/greenhouse.html

Including clouds, the table above would suggest 50%. For the cloudless case, IPCC 1990, p 47-48 estimate water vapor at 60-70% whereas Baliunas & Soon estimate 88% http://www.greeningearthsociety.org/climate/previous_issues/vol4/v4n19/cutting1.htm considering only H2O and CO2.

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Water vapor in the troposphere, unlike the better-known greenhouse gases such as CO2, is essentially passive in terms of climate: the residence time for water vapor in the atmosphere is short (about a week) so perturbations to water vapor rapidly re-equilibriate. In contrast, the lifetimes of CO2, methane, etc, are long (hundreds of years) and hence perturbations remain. Thus, in response to a temperature perturbation caused by enhanced CO2, water vapor would increase, resulting in a (limited) positive feedback and higher temperatures. In response to a perturbation from enhanced water vapor, the atmosphere would re-equilibriate due to clouds causing reflective cooling and water-removing rain. The contrails of high-flying aircraft sometimes form high clouds which seem to slightly alter the local weather.

Related Topics:
Troposphere - Contrail - Aircraft

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