RealClimate: Unforced variations: July 2023 – GWC Mag

In Re to

Unforced variations: July 2023

Dear Piotr,

Many thanks for your reply, as well as for your reaction

Unforced variations: July 2023

to my post of July 19.

Let me comment on your objections.

1) Gaps in understanding and unwillingness to read and/or accept arguments

I apologize for the gaps in my understanding. They are the reason why I am asking my questions. As regards the objected ignorance, I can only say that I strive to read all replies posted by my opponents thoroughly and think about them. As far as I find time, I strive to read the relevant literature as well.

On March 30, in my very first post on this site

https://www.realclimate.org/index.php/archives/2023/03/unforced-variations-march-2023/comment-page-2/#comment-810452 ,

I referred to two articles – one that deals with modelling of the water cycle intensity influence on global climate, and another one asking questions if the present models are suitable for such task. Thus, there might be a persisting gap in understanding generally, not only on my side. Unfortunately, no one of the participants in the discussion following this first post and continuing till now addressed these original questions yet.

Instead, there formed a group of opponents, asserting either that

(i) latent heat transport cannot play any role in global mean Earth surface regulation at all (because a modification of mean Earth surface temperature due to latent heat flow would allegedly violate the energy conservation law, or, in another formulation, because the latent heat must „stay in the system“ due to alleged impermeability of upper Earth atmosphere layers with respect to heat transported in the troposphere by LE), or (including you) that

(ii) a small surface cooling effect of latent heat flux may exist, however, it is negligible in comparison with (and effectively overturned by) the greenhouse effect of water vapour, so that it (globally and any time) applies that „Earth is a water warmed planet“.

I admit that although I scrutinize the arguments presented by each opponent quite carefully and that I any time strived to explain the reasons why I do not see specific arguments consistent and convincing, I often struggle with finding the right way how to explain my point of view. Therefore, I am thankful for any feedback that helps me to recognize mistakes or possible weak points in my argumentation.

As regards my ability to accept arguments of my opponent, I am sorry that I still see discrepancies on your side. I believe, however, that as soon as I do not see a reasonable doubt anymore, I am willing to correct my opinion.

2) Water cycle intensity and global annual average of latent heat flow

To become more specific, in previous discussion about the role of latent heat flux in Earth climate regulation, I referred to the second edition (2016) of a standard textbook Physical climatology written by prof. Dennis Hartmann. I pointed to his explanation that the mean Earth surface temperature is about 15 °C (instead of about 30 °C calculated from the simplest model of the greenhouse effect) due to “vertical transport of energy by atmospheric motions” (page 33).

I understood this explanation the way that the difference between the mean Earth surface temperature and the mean emission temperature is regulated not only by surface albedo and by content of greenhouse gases in the atmosphere, but also by proportion of radiative and non-radiative heat transport from the surface to the atmosphere.

I further pointed to diagrams explaining the global energy balance (GEB), such as that on page 34 of Hartmann’s Physical climatology textbook or such that is accessible under

https://en.wikipedia.org/wiki/File:The-NASA-Earth%27s-Energy-Budget-Poster-Radiant-Energy-System-satellite-infrared-radiation-fluxes.jpg

In these diagrams, the value of the averaged latent heat flow (LE) is about 80 W/m2. I pointed to the fact that this value is consistent with the heat consumed for evaporation of about 500 000 km3 water annually from Earth surface (which is about 510 000 000 km2), and that this amount of annually evaporated water is consistent with global mean precipitation which is about 990 mm.

Nevertheless, the relationship between LE flux and mean global annual precipitation is no way my invention, I only cannot remember the source anymore from that I became aware thereof. I think that the equivalence between the average latent heat flow and the sum of annual global precipitation is seen so obvious by authors of the GEB diagrams that they desist from detailed explanations in this respect.

3) Earth surface cooling by latent heat flow

I still do not fully understand why you so fiercely strive to disprove the simple observation that the latent heat flow is an important factor in Earth climate regulation, because it decreases the difference between the mean surface temperature of Earth and its average emission temperature in comparison with the case that this difference would have been caused solely by the greenhouse effect, without any modification by non-radiative heat transport.

It is my feeling that you somehow mix “Earth surface cooling” with “Earth cooling” – that may be the reason why you so many times repeated that a significant part of latent heat flux returns to Earth surface in downwelling longwave infrared radiation (DLR).

In my opinion, there is in fact no discrepancy between surface cooling by latent heat flux on one hand and the DLR value resulting from greenhouse effect on the other hand. I only emphasized that in the above mentioned GEB diagrams, the returned part of LE is already included in the depicted DLR, whereas you insisted in your opinion that only a small part of the depicted LE actually cools the surface, because more than two thirds thereof “return back”.

In my last post, I tried to find another way how to support my opinion that any attempt to recalculate the LE with the aim to obtain the hypothetical “genuine” value thereof will destroy the entire GEB. If you take the GEB diagrams as they are and accept the LE value given therein as true, there will be only a small Earth energy imbalance (EEI) up to 1 W/m2, and no catastrophic heating or cooling of the atmosphere or any other part of Earth.

4) Decoupling between water cycle intensity and absolute air humidity

Another persisting objection from your side (against the opinion that water is not ONLY a greenhouse gas but, in fact, it plays also other comparably important roles in Earth climate regulation that may, under specific circumstances, also decrease the difference between the mean Earth surface temperature and the Earth radiative temperature resulting from the radiative greenhouse effect which is caused by greenhouse gases in the atmosphere) is that any artificial water cycle intensification must miss its intended goal (reducing the Earth surface warming), because it will be unavoidably accompanied by a commensurate absolute humidity increase.

If I understood you correctly, you assume that this absolute humidity increase will overturn the intended surface cooling (that we strived to achieve by intensified LE) due to enhancement of the radiative greenhouse effect caused by increased water vapour concentration.

I strived to show that for the contemplated water cycle intensity increase over Sahara from ca 75 mm annual evaporation to ca 1300 mm, a sufficient water atmospheric vapour pool may already exist not only globally, but also locally.

I think that the provided example gives a hint that even over hot arid areas such as Sahara (that might be perhaps quite prominent candidates for most effective artificial surface cooling by enhanced water supply), current absolute air humidity may be high enough and no substantial increase thereof must necessarily result if we arrange an additional water supply enabling the discussed artificially increased evaporation. In other words, it appears that (contrary to your assumption), even if we desist from horizontal water transport and simplify the model situantion by an assumption that all evaporated water stays localized in the Sahara region, no substantial increase of the mean regional absolute humidity above existing level may necessarily result from the assumed, heavily artificially enhanced water cycle in the region.

In this respect, I would like to remind you that I introduced the example with Sahara desert (assuming an artificial supply of about 13 000 km3 sea water annually for evaporation therein) to show that a comparable global water cycle weakening in the past might be considered as an alternative (with respect to greenhouse effect of the increasing concentration of greenhouse gases in the atmosphere) cause of the global warming (about 1 K) observed during the last few decades.

5) Concluding remark

I am aware that you can take the preceding paragraph as a further evidence that I am an agent of fossil fuel industry striving to deflect public attention from increasing CO2 concentration in the atmosphere. Nevertheless, putting aside the circumstance that broad public ignores websites like RC, I think that all my posts can be read also as a mere expression of a concern that mechanism of the observed global climate change may be more complicated as it is presented in mainstream media, and that policies based on that simplified view may very easily fail.

I still think that stepwise weakening of global water cycle in the past might be well possible. To me, arguments therefor presented by some hydrologists and biologists pointing to

(i) human caused continent deforestation,
(ii) soil destruction by improper agricultural practices, and
(iii) improper water management including wetland draining and supporting runoff from the land to sea

do not sound implausibly. Moreover, I have a concern that even in case that the global water cycle intensity might have been in fact still relatively unaffected by human activities, possible regional changes in water cycle intensity over the globe may be still very important and potentially destructive.

That is why I still think that the questions and doubts raised by Makarieva et al in their article

https://arxiv.org/abs/2301.09998

(which is cited by me in my first post mentioned above) may be indeed relevant and might deserve much more attention than they have attracted so far.

Greetings

Tomáš