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Thursday, August 4, 2022

Forests: do they cool Earth, or do they warm it? A comment by Anastassia Makarieva


Anastassia Makarieva, giving a talk. Together with Viktor Gorshkov, she developed some fundamental concepts on the functioning of the ecosphere: the "biotic regulation of the environment" and the "biotic pump." Here, with her permission, I reproduce a message that she sent to a discussion forum on these matters. If you are interested in joining the forum, write me at "ugo.bardi(thingette)unifi.it"

by Anastassia Makarieva

Dear colleagues,

thank you very much for these fascinating discussions. I am learning so much from this group, just to mention a couple of more recent things, thanks Svet for reminding us of those important mice studies, thanks Mihail for the note about agroecology in North Korea and thank you, Christine, for sharing your experiences as a farmer. It is indeed a very hard job, I am no farmer but I lived in the wild where you have to care about most things that are vital, and this job leaves little time for doing science, especially in a cold climate. (you can find some photos here). And I am overwhelmed with more things discussed in the group, trying to catch up, and will write later.

Here I thought that I would share my understanding of whether the forests cool or warm the Earth, I did discuss it a few times so sorry if it is a repetition.

In the review article recently quoted by Ugo, as Mara rightly noted, there is nothing controversial or revolutionary. Everybody knows that when a certain part of solar energy is captured by evaporation, the surface gets locally cooler than in the absence of this process. Just because, by energy conservation, a certain part of solar energy, instead of heating the surface, is spent to extract water vapor molecules from the liquid phase by overcoming intermolecular attraction.

But, importantly, this energy remains in the biosphere -- unlike the part of solar radiation that is reflected back to space by a bright surface.

So, whether the Earth as a whole will get cooler or warmer in the presence of evaporation, will depend on how the biosphere dispenses this latent energy.

Take a look at this figure, above. It shows how condensation occurs in the rising air. The latent heat is released in the upper atmosphere and can radiate to space from those upper layers without interacting with greenhouse absorbers (that are mostly concentrated below). This will serve to cool the planet, by effectively making the planetary greenhouse effect smaller. Once again: a certain share of solar energy will leave the Earth with less interaction with the greenhouse absorbers. It is a cooling effect of evaporation.

Importantly, this effect will be stronger if the warm air spends more time in the upper atmosphere. If it descends shortly after condensation, all latent heat becomes sensible and just warms the surface. But if there is a large-scale circulation pattern with the air traveling thousands of kilometers, the effect will be more pronounced. So the biotic pump circulation will make this effect stronger (more than in a local shower). (*)

But, besides this cooling effect, there are warming effects. One of them is the mere presence of more water vapor in the atmosphere over moist surfaces. Since water vapor is a greenhouse substance, its presence over land increases the concentration of greenhouse absorbers. The share of energy leaving without interacting with them increases, but the total number of molecules increases as well. Which effect will win?

Furthermore, more water vapor and convection mean more clouds. And some of the cloud types warm the Earth. Others cool the Earth. Which will prevail?

These arguments show why the message about cooling by forests will never spring up from global climate models. They are not suitable to estimate whether it exists and how strong it might be.

My personal position is that focusing on cooling or warming is strategically harmful to the forest protection case. What natural forests definitely do is that they minimize the fluctuations of the water cycle, heat waves, droughts, and floods. While these extremes are currently officially attributed to CO2 emissions, it is well-known that this attribution suffers from many problems. I would recommend this short video by Dr. Sabine Hossenfelder https://www.youtube.com/watch?v=KqNHdY90StU on this topic.

So in fact to argue that a particular (LOCAL) heatwave has to do with forest destruction (which is known to severely change LOCAL temperatures) might be much easier and more productive than to argue about the role of forests in global warming or cooling -- where there is no simple argumentation.

So, think how it works now: we have a heatwave, and people are told it is due to CO2 emissions, to cut emissions people use "biofuel" by cutting more forests. With more natural forest loss, the water cycle is further disturbed and we have more heatwaves, which are again attributed to global warming, etc. It is a complex situation.


1 comment:

  1. The work of Winckler et al. 2019 https://doi.org/10.1029/2018GL080211 addresses how a large-scale deforestation (of historical magnitude) would affect the mean global temperature. Importantly, they separate the contributions from different processes. Without further ado, let us take a look at their Figure 2:

    Here "local" (red line) means the effect that we all know: when plants cease to transpire, the surface LOCALLY warms, even if it becomes brighter (e.g. change of a forest to a desert).
    However, as I mentioned in my original post, there are also non-local effects: as the planet captures less energy due to a brighter surface upon deforestation, it cools. This effect is NOT pronounced locally, because locally it is overcome by the warming caused by loss of transpiration.

    But globally it becomes pronounced. So, according to Figure 2b, in global models the net effect of the planet's brightening due to loss of vegetation cover is GREATER than whatever cooling transpiration can induce.

    The gray bar in Figure 2b is the warming due to deforestation-induced carbon emissions. So this picture essentially says that all influence of vegetation on climate is due to carbon emission/capture. The transpirational cooling is negated by albedo-related warming.

    Please note that to say that GCMs DO NOT account for the transpirational cooling is incorrect and will undermine the credibility of whoever is saying that. Scientists ARE very much concerned about evaluating this effect. What is possible to argue is that the models may be taking this effect into account INCORRECTLY. But to argue this, one must have at least some independent evidence.

    Now, please take a look at Figure 2c. It is the most interesting one. It shows separately how the planet COOLED (by almost 1 degree K) after the primary vegetation was historically destroyed and the planetary surface has become brighter. This albedo-based calculation is most straightforward and hence robust.

    Hence, the fact that the net effect is much smaller, about -0.05 K, means that in GCMs the GLOBAL transpirational cooling, first, does exist(*) and, second, is very substantial.
    The loss of transpirational cooling led to about 1 degree warming. (*)In my original note, I discussed how it is not possible on qualitative grounds to say whether transpiration will globally cool or warm the Earth.

    Now comes the main question. Albedo and transpiration are independent physical magnitudes. How has it come to be that these independent effects in current GCMs compensate each other so very precisely -- such that the net effect is more than an order of magnitude lower than both of the two?

    It is a very important question, because it affects our understanding of the historical changes of land cover on climate. Given that the transpiration process has a lot of unknowns(**) and thus heavily parameterized, my guess is that the main constraint governing its parameterization (when the models were tuned to historical changes) was precisely to largely offset the mutual influence of albedo and transpiration such that the climate signal could be attributed to CO2 alone.

    If so, it means that transpirational cooling is the ELEPHANT under disguise that is being masked in GCMs by parameterizations. This means that, provided there is political will (and hence funding), the models can be re-parameterized relatively easily to inform us that ALMOST all observed warming is due to the loss of transpirational cooling. The uncertainties MORE than accommodate such an opportunity.

    (**) To give you just one example about how poorly transpiration is known, I can refer you to this work (Teulling 2018 https://doi.org/10.2136/vzj2017.01.0031) which discusses the following puzzle: do forests transpire more than grasses? Even observations tell different stories.