I have read, many times, that water molecules are far more effective than CO2 molecules at trapping heat...

But, the 'relative G.W.P.' value of water vapor (or its 'radiative forcing') is not listed anywhere I can find, and the stated reason (if one is stated) is that water vapor has always been in the atmosphere in substantial amounts, of course, and if it has increased in prevalence recently it is because of increased CO2 levels, not the other way around....

So what is the amount of solar energy trapped/re-radiated by a water vapor molecule, over a very short time period, in direct comparison to a CO2 molecule?

I am very curious....

  • $\begingroup$ Generally compounds are assigned a global warming potential over a 100 year period, probably because the lengthy averaging facilitates understanding and simulation (and likely increases accuracy/precision). It may not make quantitative sense to lower that period to a single day, in which case nobody would bother assigning such a value. Apparently there is already criticism of even the 20-year GWP being too short a timespan to consider. $\endgroup$ Aug 26, 2020 at 22:54
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    $\begingroup$ @NicolauSakerNeto These 20 or 100 year averages do afaik simply take into account the lifetime of the compound in the atmosphere. That is imo not what OP was asking for. (If I'm right, OP should make that more clear, i.e. strike "over a day".) $\endgroup$
    – Karl
    Aug 27, 2020 at 0:04
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    $\begingroup$ I think what you’re asking is the relative value of the integrals of the IR absorbance spectra at the bottom of this article? acs.org/content/acs/en/climatescience/greenhousegases/… $\endgroup$
    – Andrew
    Aug 27, 2020 at 0:17
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    $\begingroup$ Generally is said, water makes 75% of the GH baseline and CO2 25%. Considering lifetime of H2O in atmosphere does not make much sense in GH effect context, as the water content is dynamically kept about the same in global long term view, slowly increasing with the global warming. $\endgroup$
    – Poutnik
    Aug 27, 2020 at 6:41
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    $\begingroup$ @BuckThorn This may have been a mistake; now that I've read everything carefully this may have been a good question for Earth Science SE but it's not really about Chemistyr, is it. The OP was "Last seen 11 hours ago" so they may not have given up entirely... $\endgroup$
    – uhoh
    Sep 13, 2020 at 15:37


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