Dear moderators,

I appreciate that you opened this topic pertaining to the role of water in Earth climate, although with a narrow (and in my opinion somewhat unfortunate) focus on anthropogenic “water vapour emissions”.

1) I put this framing in quotes because I think that reduction of anthropogenic interferences with global water cycle and especially with land hydrology to “anthropogenic water vapour emissions” is neither possible nor reasonable.

2) I am afraid that discussing ” water emissions” from fuel combustion, irrigation (or, e.g. laundry drying :-)) without considering possible (and in my opinion possibly quite significant) “negative emissions” may be seriously misleading. There may be, and likely were, many possible sources of a such “negative emissions”, for example: soil destruction, land deforestation, wetland drying, landscape drainage by agricultural meliorations, by river regulations and by building infrastructure that prevents soaking and speeds up runoff.

3) In view of the circumstance that these “negative emissions” were not mentioned in your post, I am somewhat afraid that they were not considered in the attribution of 4% of the “increase over background” (Do you mean an increase in average global absolute air humidity?) to “direct anthropogenic sources”. If so, the attribution would have been very questionable already for this single reason.

4) Moreover, I have a feeling that your post omitted the hint that changes in latent heat flux accompanying “positive” or “negative” “water vapour emissions” could have a significant influence on mean global surface temperature (GMST). As suggest results of a modelling study published by Lague et al

https://iopscience.iop.org/article/10.1088/1748-9326/acdbe1 ,

prevailing “positive” emissions might cool Earth surface and paradoxically result in global average absolute air humidity decrease, due to lowering water vapour supply from the ocean. Oppositely, in their “desert land” scenario, lowering water supply from land resulted in a GMST increase that caused global increase in the average absolute air humidity.

5) In view of the preceding remarks, I doubt that discussing “water vapour” merely as a possible source of greenhouse effect makes a sense. I am afraid that clouds and latent heat flux (both also playing a significant role in Earth surface temperature regulation) are so intricately combined with water vapour formation that all these mechanisms of Earth climate regulation cannot be assessed separately from each other.

6) For all the above reasons, I respectfully disagree with you opinion that (although it is not worth of the effort, because anthropogenic “direct water vapour emissions” are allegedly small in comparison with “water vapour feedback” to other anthropogenic climate forcings), “water vapour” could be, at least “in principle”, legally treated as an air pollutant, the same way as other anthropogenic air pollutants, including carbon dioxide.

7) I think that due to above mentioned complexity, water vapour is significantly different from all other pollutants. I am afraid that even a suggestion that it COULD be legally treated the same way as other pollutants is potentially dangerous, because it may be highly confusing for broad public and it might easily cause a huge legal turmoil.

Could you comment?

Best regards
Tomáš