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.

How Forests Create Rain: a New Study Demonstrates the Effects of Evapotranspiration

Image created by Dall-E

The idea that forests create rain has been known by peasants for hundreds, perhaps thousands, of years. The first scientific studies go back to Alexander von Humboldt (1769–1859), but the subject remains controversial. Nevertheless, we are starting to understand the deep and complex interactions between the atmosphere and the biosphere. They form a true "holobiont," a system of connected elements that affect each other in non-linear ways. A recent paper published by a research group led by Anastassia Makarieva shows how evapotranspiration, the evaporation of water by trees, modifies the water vapor dynamics and may generate high moisture content regimes that provide the rain needed by the land ecosystem. There is still much that we need to understand about these mechanisms, but one point is clear: forests are a crucial element of the stability of Earth's climate, and they must be preserved as much as possible (U.B.)

This is the press release about the new study. You can find the complete text here.

Forest transpiration and the terrestrial water cycle: A non-trivial relationship

Link

As water scarcity globally grows, and deforestation threatens the remaining natural forests, understanding how vegetation impacts the water cycle becomes increasingly important.  In their new paper, “The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence” published in Global Change Biology, an international and interdisciplinary team led by TUM demonstrated the existence of two potential moisture regimes – one drier, with additional moisture decreasing atmospheric moisture import, and one wetter, with additional moisture enhancing atmospheric moisture import. In the drier regime, water vapor behaves as a passive tracer following the air flow. In the wetter regime, it modifies atmospheric dynamics.

The team based their analysis on the previously established non-linear dependence of precipitation on atmospheric moisture content – increasing absolute humidity leads to a negligible precipitation increment if the atmosphere is dry, but to a large increment when the atmosphere is sufficiently wet. Combining this dependence with a full consideration of the water budget, the researchers showed that an increase in precipitation in humid conditions facilitated by increased evapotranspiration, should lead to enhanced moisture import. They illustrated these patterns with the data from the Amazon basin and the Loess Plateau in China.

Dr. Anja Rammig (TUM School of Life Sciences and study author) considers these results as having profound implications for the ongoing studies of the resilience of the Amazon forest in the face of the danger of deforestation and climate change. Dr. Scott Saleska (University of Arizona, study author) believes that the new results are in agreement with the profound role of leaf phenology in the Amazon forest for water cycle regulation. By forcing a decline in forest evapotranspiration, deforestation can dehumidify the atmosphere and thus drive the forest into the drier regime where transpiration of the re-growing vegetation would further aggravate aridity by decreasing moisture import. Getting out of this landscape trap could be impossible. Dr. Ruben Molina (University of Antioquia, Colombia, study author) hopes that the study findings will raise the awareness of the importance of tropical forest conservation.

Dr. Andrei Nefiodov (Petersburg Nuclear Physics Institute, Russia) participating in the study says that the new results corroborate the concept of the biotic pump of atmospheric moisture that emphasizes the dominant role of natural forests in transporting moisture inland. Dr. Antonio Nobre (INPE, Brazil, study author) compares this biotic moisture pumping to a beating heart, and highlights the good news: even in arid lands, by restoring the vegetation one should be able to enhance the atmospheric moisture convergence and streamflow. To achieve that, the ecological restoration strategy should be carefully designed to guide the ecosystem transition from the dry to wet regimes.

“I suspect that natural vegetation will be best for maintaining a moist and productive environment as these systems kept the world green and productive long before people got involved” – emphasizes Dr. Douglas Sheil (Wageningen University, author), collaborating on the research. “We do need to take into account the holobiontic relationships among all ecosystem elements that allow for an efficient regulation of the water cycle,” adds another author Dr. Ugo Bardi (Club of Rome, University of Florence).

Anastassia Makarieva (Institute for Advanced Study, TUM, lead author) emphasizes the need for a broad international cooperation in the studies of the ecology of the water cycle: “We have shown that the non-linear precipitation dependence on atmospheric moisture content, first noted by our co-author Dr. Mara Baudena (CNR-ISAC, Italy) and her colleagues, has widely ranging implications. The atmospheric water flows do not recognize international borders, thus deforestation disrupting evapotranspiration in one region could trigger a transition to the drier regime in another. Our results indicate that natural forests of the Earth, in both high and low latitudes, are our common legacy of pivotal global importance as they support the terrestrial water cycle. Their preservation should become a widely recognized priority for our civilization to solve the global water crisis.”

Forests and History: A Tale of the Great Earth Holobiont

 

France seems to be the only area of the world for which "Our World in Data" has a complete dataset for forest cover for a long timespan -- it goes back to the year 1000 AD. Similar but less extensive data are available for a few other countries but, in most cases, the data cover only the past 30 years. 

I can't say how reliable these data are, but the curve for France makes a lot of sense if compared with the historical record. Note how the late Medieval expansion corresponds to a decline in forest cover. The crusades (1095 -1291) see deforestation continue. Then, the whole economic system collapses: the crusaders go back to Europe to find the land devastated first by famines, and then by the Black Death. It is said that some 30% of the European population disappeared. Forests, then, had a chance to recover and attain levels similar to those of the early Medieval period. 

Then, with the discovery of the New World, another cycle of expansion started. Population boomed, but the economic prosperity had to be paid, and in part was fueled, by another cycle of deforestation. It ended in mid 19th century with the start of a coal-based, industrial economy. Coal provided the same services as wood but at a lower cost, and that allowed the population to continue increasing without having to deforest the land. The trend continued with another cycle that started during the 20th century: the oil-based economy. The population rapidly shot upward, but deforestation didn't restart. 

In the 21st century, we see the trend continuing, at least in rich countries such as Europe. Forests are still growing, while the population has now plateaued, and it is starting to decline. Globally, deforestation is continuing, but the reversal is clear in several countries (source)

So, there may be ground for optimism: forests may be regrowing as the world goes through its demographic transition. It is an especially welcome trend, now that the link between forest cover and rainfall is being recognized (see a recent paper in Nature). 

But don't forget that the destruction of forests is always around the corner. During the Middle Ages, France had little more than ten million inhabitants, and yet it could raze its forests to the point of destroying its economy and causing some of the greatest famines in European history. We have been able to avoid this destiny, today, only because we have cheap energy from fossil fuels. Now that the fossil supply is dwindling, and with climate change looming as an even worse threat, we could see a new assault on forests, this time with the green label of "sustainable biofuels." 

We need to understand that we need forests not because we can use wood to power our SUVs, but because they are part of the great planetary holobiont that connects everything to everything else. They generate the "biotic pump" mechanism that brings rain to the land (see this recent paper by Makarieva et al.). If we lose the forests, we lose rain. And if we lose rain, we lose everything.  

 

Rain is Life, and Holobionts Create it

 

Buying vegetables at a stall in Florence on a rainy day. My wife, Grazia, is the one with the pink umbrella. 

By Ugo Bardi

After three months of drought during the summer, Florence is now drenched in rain. It has been raining for two months and people are complaining that it is too much! But it is how things have to be: I recently discovered that rain is an "autocatalytic" phenomenon. A more humid atmosphere creates more rain, and rain creates a more humid atmosphere. And that creates long periods of static weather -- too little rain and then too much. 

I learned that from the work of Anastassia Makarieva, Mara Baudena, and several others who have studied the relationship between atmospheric humidity and rain. Look at this figure, from a paper by Makarieva et al., to be published.

The y-axis is the amount of rain, in mm/hour. The x-axis is the amount of water vapor in the "air column." Note the strongly non-linear relationship. It is a typical power law: a small increase in atmospheric humidity causes a large change in rainfall. The three red points indicate the boundary of the "abiotic regime" (no rain) and the power law region.  

As I said, it is an autocatalytic phenomenon, Rain tends to generate more rain, at least as long as it wets the land and it generates moisture transpiration, which increases the water vapor content in the atmosphere. This has very practical consequences in many senses. One is the role of forests in weather and climate. Forests generate strong evapotranspiration, that is they pump water from the soil to the atmosphere. And, also, forests tend to keep water in the soil, slowing down the runoff.

So, not only do forests generate rain, but they also tend to maintain the rain pattern. Without forests, and with the land covered with buildings, you have the typical desert climate: dry most of the time, then with short periods of heavy rain. Disasters ensue, now a common pattern in areas such as California or Italy, where deforestation has taken place. 

So, we need our fellow holobionts, the trees. Onward, fellow holobionts!

(below, some rain-loving holobionts pictured together)