Climate Science

Warming Vapors

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Science  07 Nov 2008:
Vol. 322, Issue 5903, pp. 825
DOI: 10.1126/science.322.5903.825b

Water vapor is the atmospheric gas that collectively has the greatest greenhouse effect on climate, although it does not directly instigate warming or cooling trends, because the amount of water vapor in the atmosphere varies only in response to temperature change. Instead, water vapor only amplifies temperature trends being caused by other factors such as atmospheric CO2 concentration or Earth's albedo. The extent to which humidity changes in response to temperature variation is therefore a key parameter in global climate models, because that quantity determines the strength of the associated warming or cooling. Dessler et al. present satellite data from 2003 to 2008 which show that models have gotten that relationship correct, and that relative humidity is effectively constant at any given temperature. Thus, the temperature increases predicted by global models are virtually guaranteed to be several degrees Celsius by the year 2100. Knowing the water vapor content of a warmer atmosphere is also important for predicting rainfall and storminess. — HJS

Geophys. Res. Lett. 35, L20704 (2008).

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