Conserving old growth forests is key to stabilising the Earth’s climate

From the Blog of the Club of Rome




© Greatandaman | Dreamstime
By Ugo Bardi, member of The Club of Rome


02 May 2023 – Do forests create rain? It is a question that has been debated for a long time. We know that trees produce huge amounts of water vapor that is pumped from humidity in the ground and condensed into clouds that generate rain, but the mechanisms that govern condensation and vapor water movements are still not completely clear.

In our new paper, a group of researchers led by Anastassia Makarieva of the Theoretical Physics Division of Petersburg Nuclear Physics Institute (PNPI) and the Institute for Advanced Study of the Technical University of München (TUM) highlight how evapotranspiration – the evaporation of water by trees, modifies water vapor dynamics to generate high moisture content regimes that provide the rain needed by land ecosystems. The research is a significant step forward in understanding the critical need to conserve old-growth forests to stabilise the Earth’s climate and maintain the biodiversity needed for the ecosystem to survive.

The study titled “The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence” shows that two potential moisture regimes exist: one is drier, with additional moisture decreasing atmospheric moisture import, and one is wetter, with additional moisture enhancing atmospheric moisture import. In the drier regime, that may be caused by deforestation, water vapor behaves as a passive tracer following the air flow. In the wetter regime, it modifies atmospheric dynamics and amplifies the atmospheric moisture import, creating rain.

There is still much that we need to understand about these mechanisms, but we are starting to understand how forests and the atmosphere form a system of connected elements that affect each other. One thing is clear: forests are crucial to the stability of the Earth’s climate.

Not only do trees store carbon in a form that does not cause greenhouse warming, but they actively cool the planet, due to how moisture condensation is managed. Forests also control the water cycle on land, pumping water vapor from the oceans inland by a mechanism called the biotic pump. Old growth forests generate giant flows of water known as “flying rivers” that fertilise entire continents. Our study shows that the non-linear precipitation dependence on atmospheric moisture content has wide-ranging implications. Atmospheric water flows do not recognize international borders, meaning deforestation disrupting evapotranspiration in one region could trigger a transition to a drier regime in another.

Our results indicate that the Earth’s natural forests, in both high and low latitudes, are our common legacy of pivotal global importance, as they support the terrestrial water cycle. Their preservation should be a recognised priority for our civilisation to solve the global water crisis. Important on-going work calls for re-appraisal of the forest’s role in the global temperature regime.

The study was performed by an international research team that included scientists from North and South America, and Eastern and Western Europe.

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.

Anastassia