RealClimate: Area-based global hydro-climatological indicators – GWC Mag

In Re to

Area-based global hydro-climatological indicators

Dear Piotr,

Many thanks for your reply to Nigel. I especially appreciate that you replied my question raised in point 6 of my reply

Area-based global hydro-climatological indicators

to your earlier post

https://www.realclimate.org/index.php/archives/2023/07/area-based-global-hydro-climatological-indicators/#comment-813732 .

Let me repeat your explanation first, and add a few comments.

Piotr:

I would not say though that “increased evaporation has no net cooling effect on the atmosphere as a whole” the latent heat moves heat in the atmosphere. The higher you can get the heat in the atmosphere, the bigger the portion of it could escape into space. That said, your implied conclusions of zero escape – is still closer to the truth than the opposite assumption by Tomas and Shurly that 100% of LH escapes the Earth atm. system.

Yes, the extra water vapour both warms (as a greenhouse gas) and cools (by increasing the shortwave albedo by the clouds) and the next outcome – positive or negative – is condition dependent – where the increase in humidity and clouds happens, how high are the clouds, are they made from liquid droplets or ice crystals, nut even it is the net cooling – it would not be as strong as considering only the cooling part would suggest.

Water cycle is coupled with global temperature so – if we warm the Earth with GHGs, the resulting changes in the water cycle would amplify the warming; if we cool the Earth by reducing GHGs the resulting changes in water cycle would amplify the cooling.

And as I said: “the only practical way to use water cycle to help us with GW, is NOT through some absurd schemes of directly altering global evaporation, because these are far too small to matter, but by reducing GHG concentrations in the atmosphere, with water cycle amplifying the consequences of our actions on GHG conc. – if we cool the Earth with dropping GHGs – water cycle changes would make the cooling bigger, if we keep adding GHGs – the water cycle would amplify warming.

TK:

1) Latent heat flow as regulation of Earth average surface temperature

In my recent reply

Area-based global hydro-climatological indicators

to your post

https://www.realclimate.org/index.php/archives/2023/07/area-based-global-hydro-climatological-indicators/#comment-813762 ,

I strived to review the previous discussion on this point and summarize my view. Therefore, only a small remark here. I never suggested or assumed that “all latent heat escapes in the space”, this is your interpretation of my view that is:

Although it may look paradoxical at a first glance, a change in average non-radiative heat flow from Earth surface should be equal to an opposite change in the upwelling infrared radiation from the surface – DESPITE the distribution of the energy absorbed in the atmosphere (to the outgoing infrared radiation and downwelling “backradiation”) works exactly as you repeatedly (and in my opinion correctly) note. Please look on my reasoning in the cited post and check it.

It appears, however, that we mutually agree that the latent heat flow plays a crucial role in present Earth climate at least in one point: Its present value about 80 W per square meter secures relatively comfortable mean Earth surface temperature about 15 °C, because in case of solely radiative heat transport from the surface, average Earth surface temperature would have been about 30 °C.

2) Positive (warming) feedback of water vapour to rising concentration of non-condensing GHG in Earth atmosphere, due to greenhouse effect of water vapour

I must admit that (although it is very likely that somebody already presented reference to theoretical models describing this feedback during the previous discussion about water vapour and water cycle role in Earth climate), I have not paid a due attention and cannot cite these references.

I have, however, a feeling that a contribution of enhanced water vapour greenhouse effect was either not mentioned at all in the discussion about factors contributing to the Earth energy imbalance (EEI) derived from satellite observations and its development during the last two decades, or was very small – although there perhaps is an evidence from satellite spectroscopic observations that average absolute humidity of Earth atmosphere increased during this time span, in accordance with models that predict such absolute humidity increase with increasing Earth mean surface temperature. Please correct me, if I am wrong.

3) Negative (cooling) feedback of rising water cycle intensity, due to higher cloudiness / increased Earth albedo

First of all, it appears that observations confirm a certain increase in global annual precipitation during the last two decades, what is indeed a measure of increasing water cycle intensity, in accordance with rising global mean surface temperature.

Nevertheless, I have a feeling that state-of-art models predict that although the average absolute humidity shall slightly rise with average Earth temperature, an opposite should apply to the relative humidity. If the cloudiness depends rather on the relative than on the absolute humidity (because water condensation to form clouds requires cooling the humid air below the respective dew point), one could expect rather an opposite effect of the global warming on the cloudiness and albedo than you suggest.

It appears that a DECREASE in cloudiness and a corresponding decrease in Earth albedo (thus practically enhancing the “greenhouse” global Earth warming caused directly by back-radiation enhancement due to higher GHG concentration) was indeed observed during the last two decades (macias’ references to CERES data). Nevertheless, there is another interpretation of these observations by James Hansen, who ascribes the lower Earth albedo to decreasing atmosphere pollution with sulfate aerosol.

4) Enhancement of the GHG effects by water cycle

In view of the doubts and uncertainties described above, I do not see your opinion about water cycle as an enhancing factor for effects caused by changing GHG level. Oppositely, I still see as well possible that the water cycle intensity plays rather a stabilizing role in Earth climate.

Greetings

Tomáš

P.S.
Track of the previous discussion is publicly accassible on my orgpage to this topics,
https://orgpad.com/s/7zfynb_y5o7