RealClimate: Unforced variations: May 2023

Tomáš Kalisz says

25 May 2023 at 4:31 AM

Dear Piotr, and dear zebra (apology for replying to both of you at once),

I have a feeling that there are three central points in your answers:

1) water vapor is an important greenhouse gas (what is certainly true)

2) condensing water may form clouds that have very different influence on th eradiation balance, depending on cloud character, however, there is an evidence that when average tempeature rises, the greenhouse effect of increased water vapor concenbtration (absolute humidity) overweights the effect of cloud albedo (mostly because, due to basically constant relative humidity, cloud formation will not increase substantially)

It sounds quite reasonably.

3) Earth surface cooling by non-radiative heat transfer in form of latent heat cannot influence EEI because the condensation heat only warms troposphere and stays in the system:

"Your latent heat has a minor contribution at best – as Zebra already indicated it does not remove the heat into space, it just puts it higher in the atmosphere, so the only cooling effect would be if a bigger fraction of the IR was re-emitted at that height escaped into the space.But I doubt it would make a huge difference."

I think that in this third point, both of you may be wrong. As it appears that the view according 3) is still shared and spread by some scientists dealing with climate, I would be happy if the topics attracted the attention of moderators on this discussion site. As it has not happened yet, I will strive to do my best and try to explain my present reasoning as well as uncertainties linked thereto myself.

In my public orgpage (a dynamic interactive scheme in the web application OrgPad), accessible per link

https://orgpad.com/s/VhvfDd5uRIP ,

you may see the story behind my questions put on RealClimate. It might be perhaps characterized as an „unsettled discussion about role of water in Earth climate", which I translated into an idea of a „pilot scale geoengineering experiment". In this orgpage, I put also some references that may be relevant for the presently discussed topics.

First of all, you can consult the cell comprising a very basic, rough explanation of the greenhouse effect from a textbook (Physical climatology, Dennis Hartmann 2016).

Let us imagine Moon inside a glass sphere having a perfect transparency for sunlight and completely absorbing longwave infrared radiation. Assuming that the mean surface albedo of the Moon and Earth are the same, the sphere would have established a new steady state („equilibrium") with an average surface temperature ca 303 K (30 °C) and temperature of the sphere about 255 K (– 18 °C), which is equal the original average surface temperature of the Moon without atmosphere.

As soon as we fill the vacuum between the glass sphere with a gas, the situation changes due to an additional heat transfer enabled by thermal convection. The difference between the average surface temperature and average temperature of the glass sphere will decrease, because part of the energy coming from the Sun is now transported to the sphere by convection and the average radiative temperature of the surface decreases accordingly.

The original difference 48 K (between the average surface temperature and the average radiation temperature of a hypothetical „greenhouse cover" as described above) thus clearly represents a maximum (let me call it „greenhouse limit") of the greenhouse effect that may be achieved under given surface albedo / atmosphere transparency / insolation. Any non-radiative heat transfer mechanism will act as an additional „surface cooling" and will decrease the average surface temperature as well as the respective difference between the surface temperature and the average radiative temperature of the glassy „greenhouse cover".

You may note my uncertainty how to deal with the ambiguity of the term "greenhouse effect" as used in media and daily life. I think that it would be better using this term solely for the effect itself, in terms of the observed difference between the average surface temperature of a planet and its average steady state radiation temperature. The same term is, however, used also for a specific mechanism causing this effect in planetary atmospheres, namely for the "radiative forcing", resulting from the presence of "greenhouse gases" that absorb the longwave surface radiation of planetary bodies. Furthermore, the term "greenhouse effect" is sometimes used also for other mechanisms causing the observed temperature difference. An example of these mechanisms may be the back-reflectance of the longwave surface radiation by clouds.

Anyway, I believe that we can say that any non-radiative heat transfer mechanism „weakens" the greenhouse effect. In famous Trenberth's diagrams showing graphically the energy flows participating on the Earth energy „budget" made for a selected timespan, the arrow for the latent heat flow is approximately four times thicker than the arrow for the sensible heat flow. I think that this fact can be seen as a first hint that, contrary to your assumption cited above, Earth surface cooling by latent heat flow may play an important role in Earth climate. The second hint may be taken from the cited textbook which unequivocally asserts that the difference between average emission temperature computed for a hypothetical Earth with the same albedo but without atmosphere which is about 30 °C and the observed average surface temperature which is about ca 15 °C has to be ascribed to non-radiative heat transport from the surface.

One of the reasons why I posted my questions on the RealClimate site was just the circumstance that your view (that Earth surface cooling by latent heat flux has a negligible importance in Earth climate regulation) is still shared and actively promoted by some scientists. As an example could serve a fierce defence of this view by leaders of Czech Globe, an institute of the Academy of Sciences of the Czech Republic, published in October 2022 in a Czech newspaper as an answer to a public critique of their opinion Avex 4/2020 about climate change.

Their oponents criticized that Avex 4/2020 is completely silent about the role of the water cycle in climate regulation, and Czech Globe described them in their reply as people wishing to cool a closed room by opening the door of a fridge arranged in the room.

Personally, I see such „closed room" arguments as a very unfortunate kind of „climate advocacy", because spreading unfair arguments (If I understood correctly, even you admit that a certain part of the heat transported to the atmosphere must escape into space, because Earth can be no way considered as a closed thermodynamic system in a thermodynamic equilibrium) by people representing a scientific institution can, in my opinion, discredit the science as such.

I hoped that moderators of this discussion step up and make clear that the terms like „radiative equilibrium", „energy balance" etc., as usually used in the context of the climate science, in fact describe a mere steady state (which is still idealized, because the „Earth energy imbalance" may be quite likely no exception caused by human influence but rather a usual condition of the Earth system) and have nothing to do with thermodynamic. Unfotunately, the moderators stay virtually invisible on this site since I posted my first question in the end of March.

Although the above mentioned ratio of the latent and sensible heat flow certainly reflects the circumstance that majority of Earth surface is covered by water, I suppose that asking question whether or not we could somehow „manage" this ratio may be still relevant, at least because we do have technical means therefor. If mitigating the influence of rising concentration of non-condensing greenhouse gases this way was indeed possible, it might perhaps perhaps represent an alternative or additional way towards climate change mitigation. It could be quicker and/or cheaper than other proposed means like direct carbon dioxide capture from the ambient air (DAC) and might have less inpredictable effects than other "geoengineering" proposals like creating sulfate aerosol in upper atmosphere.

In this respect, I see quite unfortunate that even journals like Nature publish articles about DAC (which I personally see as a totally useless and potentially harmful idea, due to exorbitant costs that will be unavoidable if we really try to achieve the „decarbonisation" this way), while currently available options for an active water management attracted hardly any attention yet.

Back to my questions regarding the total annual rainfall. In view of the above explanations, I still believe that it does fit with the total latent heat annually transferred from earth surface into space. Furthermore, I do not see any reason yet, why its current value should be considered as an unchangeable parameter, or as a parameter dependent merely on the average Earth temperature. I can imagine that we need certain sensible heat flux because convection helps to move the water vapor from Earth surface into atmosphere, but I have not found any clear explanation yet that the current ratio of the latent heat flux and the sensible heat flux is already at a certain natural limit and cannot be increased by any kind of human intervence.

For these reasons, I am looking for further deeper discussion on this topics. As this site does not enable graphical presentations that may sometimes support and simplify the argumentation significantly, please do not hesitate to use the clone of the above mentioned orgpage that I designed especially for this discussion and put your arguments and comments therein. This public orgpage with the title „Discussion forum: Heat wave mitigation in urban environment, solar energy exploitation and global water cycle restoration" is accessible using the following link for commenting:

https://orgpad.com/s/6jf-rtG8wUP

I will be very pleased if the moderators of this discussion decide to contribute as well.

GreetingsTom