I have an argument with a climate change denier.

He says the rise of water vapor in the atmosphere causes a "much larger" cooling effect by taking vaporisation heat from the ground than the heat generated by the additional greenhouse effet.

I say the additional greenhouse effect integrates with time, so after a certain time T (which I suspect "small"), this heat will compensate the latent heat taken from the ground.

Does anyone have a reference establishing the order of magnitude of T ?

Note the question is NOT about the "normal" water cycle where latent heat is taken at vaporisation and released at condensation, it is about the (undisputed) global augmentation of water vapor in the troposphere.

  • $\begingroup$ There's no point in arguing with fools - they'll just drag you down to their level and beat you with experience. Ask him for his calculations. They'll fall apart on impact with the light of day. $\endgroup$
    – 410 gone
    Jan 12 '20 at 20:29
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    $\begingroup$ water vapor(clouds)do both cool earth during the day by reflecting sunlight and heat earth at night by trapping and reflecting heat back to earth,what effect wins depend on the situation so it will be wrong to say clouds do warm earth or to say that clouds cool earth it is a combination of the two effects. $\endgroup$ Jan 13 '20 at 5:55
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    $\begingroup$ This, and i still don't know what effect OP is actually after, so my remark aims at others stumbling over this question. Increasing water vapour in the atmopshere has an amplifying effect on warming, it does not cool. nasa.gov/topics/earth/features/vapor_warming.html and pnas.org/content/early/2014/07/23/… (sorry, paywalled). $\endgroup$
    – user18607
    Jan 13 '20 at 14:12
  • $\begingroup$ There is a flaw in your friends argument, water vapor does not increase on its own, It is controlled by temperature, so whatever effect it has is is a feedback effect not a cause. it is like arguing that because room gets a lot of heat given off by the stone in the fireplace that is heated by the fire , that then the fire IN the fireplace is not heating the room. $\endgroup$
    – John
    Mar 29 at 4:50

I am doing some calculations on this question myself. The atmospheric lapse rate is about 6.5K/1.000m. The adiabatic lapse rate is 9.8K/1.000 and the dry lapse rate of the naturally unstable atmosphere of Earth should even be 10.8K. So arguably the constant transport of latent heat by vapor from the surface into the atmosphere is reducing the lapse rate by some 4.3K/1.000m. Without a doubt the GHE would be much larger if it was not for the latent heat provided by vapor.

The GHE itself is a function of the emission temperature, which again is a function of the emission altitude. Vapor increases this emission altitude, which is why it is considered a GHG, the most significant one actually.

We need to ask what size GHE has (not 33K, but less), what bigger size it counterfactually had without latent heat and a lapse rate of 10.8K, determine the share of vapor in this case, and compare it to the (negative) effect of latent heat. Depending on what is bigger, vapor is either a GHG or an anti-GHG. From all the modelling so far I get definitely the latter result.


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