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Is water vapour really the most prolific green house gas?

Is it true that the most prolific green house gas is water vapour in the upper atmosphere? If it is, shouldn’t we be more worried about fluctuations in its concentration than levels of CO2?
Peter Warder from Surrey (Age 55+)

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2 Responses

  1. Water vapour is responsible for much of the greenhouse effect indeed. However, that does not happen because of its GWP (global warming potential) but because of its huge amount.

    Having said that the amount and concentration of water vapour in the atmosphere is barely possible to be controlled by humans and that’s why there is very little discussion about it. However, climate models are taking into account the potential increase of water vapour that could be cause by say the increase of temperature and heating of the oceans.

  2. Water vapour is a much more abundant greenhouse gas than carbon dioxide, and accounts for an estimated 36% to 66% of the greenhouse effect, with CO2 alone contributing between 9 and 26% (according to estimates by climate scientists published on the RealClimate website). However, water vapour has a very short residence time in the atmosphere, while CO2 builds up cumulatively.

    The warmer the atmosphere becomes, the more water vapour enters the atmosphere due to evaporation, and this extra water vapour contributes further to warming. However, if the atmosphere cools down, the water vapour content of the atmosphere drops again. The response is rapid – days to weeks. CO2 on the other hand stays in the atmosphere for decades to centuries – put more CO2 in the atmosphere and it stays there. So, increasing CO2 concentrations results in a cumulative heating, representing what is known as a forcing. Additional warming from water vapour is a consequence of the heating by CO2 and other long-lived greenhouse gases – water is seen not as a forcing, but as a feedback.

    The effects of water vapour are incorporated into modelling studies of climate. If we were ignoring the role of water vapour, estimates of potential future warming would be much too low, and the models wouldn’t work in any case.

    There is more on this issue on the RealClimate website at:

    http://www.realclimate.org/index.php?p=142

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