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Forests Regulate Climate and Produce Wind - Biotic Pump Theory
Forests Regulate Climate and Produce Wind - Biotic Pump Theory

Anastasia Makarieva, a nuclear physicist from the St. Petersburg Institute of Nuclear Physics, has been defending the theory that the taiga forests of Russia regulate the climate of northern regions of Asia for more than ten years. Many Western meteorologists disagree with her, but the government and scientists in Russia are interested in this theory.

Every summer, when the days are getting longer, Anastasia Makarieva leaves her laboratory in St. Petersburg and goes on vacation to the endless forests of the Russian North. A nuclear physicist pitches a tent on the shores of the White Sea, among firs and pines, swims in a kayak on the endless rivers of the region and takes notes about nature and weather. “Forests are a big part of my personal life,” she says. For 25 years of annual pilgrimage to the north, they have become an important part of her professional life.

For more than ten years, Makarieva has been defending the theory, which she developed together with Viktor Gorshkov, her mentor and colleague from the Petersburg Institute of Nuclear Physics (PNPI), about how the boreal (taiga) forests of Russia, the largest forest on Earth, regulate the climate of northern Asia. This simple but far-reaching physical theory describes how the water vapor exhaled by trees creates winds - these winds cross the continent, carrying humid air from Europe across Siberia and further to Mongolia and China; these winds carry the rains that feed the giant rivers of Eastern Siberia; these winds water the northern plain of China, the granary of the most populous country on the planet.

Because of its ability to absorb carbon dioxide and breathe out oxygen, great forests are often referred to as the lungs of the planet. But Makarieva and Gorshkov (he died last year) are convinced that they are also her heart. “Forests are complex, self-sustaining rain systems and a major factor in the circulation of the atmosphere on Earth,” says Makarieva. They recirculate huge amounts of moisture into the air and in the process create winds that pump this water around the world. The first part of this theory - that forests make it rain - is consistent with research by other scientists and is increasingly being remembered when managing water resources amid rampant deforestation. But the second part, the theory that Makarieva calls the biotic pump, is much more controversial.

The theoretical background of the work was published - albeit in lesser-known journals - and Makarieva was supported by a small group of colleagues. But the biotic pump theory has received a flurry of criticism - especially from climate modelers. Some believe that the impact of the pump is insignificant, while others deny it altogether. As a result, Makarieva found herself in the role of an outsider: a theoretical physicist among model developers, a Russian among Western scientists, and a woman in an area ruled by men.

However, if her theory is correct, it will be able to explain why, despite the considerable distance from the oceans, in the interior of wooded continents there is as much rainfall as on the coast, and why the interior of treeless continents, on the contrary, is usually arid. It also implies that forests - from the Russian taiga to the rainforests of the Amazon - do not just grow where the weather is right. They make it themselves. “From what I read, I have concluded that the biotic pump is working,” says Douglas Sheil, a forest ecologist at the Norwegian University of Life Sciences. With the fate of the world's forests in question, he says, "Even if there is the slightest chance that this theory is correct, it is imperative to find out for sure."

Many textbooks on meteorology still provide a diagram of the water cycle in nature, where the main cause of atmospheric moisture, which condenses in clouds and falls in the form of rain, is the evaporation of the ocean. This scheme completely ignores the role of vegetation and especially trees, which act like giant fountains. Their roots draw water from the soil for photosynthesis, and microscopic pores in the leaves evaporate unused water into the air. This process - a kind of perspiration, only in trees - is called transpiration. Thus, one mature tree releases hundreds of liters of water per day. Due to the large area of ​​foliage, the forest often releases more moisture into the air than a body of water of the same size.

Rain parade

The so-called "flying rivers" are the prevailing winds that absorb water vapor emitted from forests and deliver rain to distant water bodies. A controversial theory suggests that the forests themselves govern the winds.

According to the biotic pump theory, forests not only cause rain, but also wind. When water vapor condenses over coastal forests, air pressure decreases and winds are created that suck in moist ocean air. Cycles of transpiration and condensation create winds that carry rain thousands of kilometers inland.

So, about 80% of precipitation in China comes from the west thanks to the Trans-Siberian flying river. And the flying Amazon River provides 70% of rainfall in the southeastern part of South America.

The role of this secondary moisture in the formation of nutrient rains was largely overlooked until 1979, when Brazilian meteorologist Eneas Salati examined the isotopic composition of rainwater from the Amazon Basin. It turned out that the water returned by transpiration contains more molecules with the heavy isotope oxygen-18 than water evaporated from the ocean. So Salati showed that half of the rainfall over the Amazon fell as a result of forest evaporation.

Meteorologists tracked the atmospheric jet over the forest at an altitude of about 1.5 kilometers. These winds - collectively referred to as the South American lower jet stream - blow from west to east across the Amazon at the speed of a racing bike, after which the Andes mountains drag them south. Salati and others suggested that it was they who carried the bulk of the moisture released, and dubbed them "the flying river." According to climatologist Antonio Nope at the Brazilian National Space Research Institute, the flying Amazon River carries as much water today as the giant Earth's river below it.

For a time it was believed that the flying rivers were limited to the Amazon basin. But in the 1990s, hydrologist Hubert Savenije at Deltfe University of Technology began studying moisture recirculation in West Africa. Using a hydrological model on weather data, he found that the further inland from the coast, the higher the proportion of rainfall falling from forests - up to 90% in the interior. This discovery explains why the inner Sahel is becoming drier: coastal forests have disappeared over the past half century.

One of Savenier's students, Ruud van der Ent, developed his idea by creating a global model of moisture airflow. He brought together observations of precipitation, humidity, wind speed and temperature and theoretical estimates of evaporation and transpiration, and created the first model of moisture transport at scales beyond river basins.

In 2010, Van der Ent and his colleagues unveiled their finding that globally, 40% of all rainfall occurs on land, not on the ocean. Often even more. The flying Amazon River provides 70% of the rainfall in the Rio de la Plata Basin, which stretches across southeastern South America. Van der Ent was quite surprised to find that China receives 80% of its water from the west - moreover, it is mainly Atlantic moisture, which is processed by the taiga forests of Scandinavia and Russia.The journey has several stages - transpiration cycles with associated rain - and takes six months or more. “This contradicts the previous information that everyone learns in high school,” he says. "China is close to the ocean, the Pacific Ocean, but most of its rainfall is moisture from land in the far west."

If Makarieva is right, forests provide not only moisture, but also create the wind that carries it.

She worked with Gorshkov for a quarter of a century. She started as a student at PNPI, a subdivision of the Kurchatov Institute, the largest Russian nuclear research institute, both civil and military. From the very beginning, they worked in the field and were engaged in ecology at the institute, where physicists study materials using nuclear reactors and neutron beams. As theoreticians, she recalls, they had "exceptional freedom of research and thought," - they were engaged in atmospheric physics, wherever it took them. “Victor taught me: fear nothing,” she says.

In 2007, they first presented their theory of the biotic pump in the journal Hydrology and Earth Sciences. It was considered provocative from the outset, because it contradicted a long-standing principle of meteorology: winds are caused mainly by differential heating of the atmosphere. As the warm air rises, it lowers the pressure of the layers below, essentially creating new space for itself on the surface. In summer, for example, the land surface heats up faster and attracts moist breezes from the cooler ocean.

Makarieva and Gorshkov argue that sometimes a different process prevails. When water vapor from the forest condenses into clouds, the gas becomes a liquid - and it takes up less volume. This reduces air pressure and draws in air horizontally from areas with less condensation. In practice, this means that condensation over coastal forests forges a sea breeze, pushing moist air into the interior, where it eventually condenses and falls as rain. If forests stretch inland, the cycle continues, maintaining moist winds for thousands of kilometers.

This theory overturns the traditional view: it is not the atmospheric circulation that controls the hydrological cycle, but, on the contrary, the hydrological cycle regulates the mass circulation of air.

Sheel, and he became a supporter of the theory more than ten years ago, considers it to be a development of the idea of ​​flying rivers. “They are not mutually exclusive,” he says. "The pump explains the strength of the rivers." He believes that the biotic pump explains the "cold Amazon paradox." From January to June, when the Amazon basin is colder than the ocean, strong winds blow from the Atlantic to the Amazon - although differential heating theory would suggest otherwise. Nobre, another longtime proponent, enthusiastically explains, "They don't come from data, but from underlying principles."

Even those who doubt the theory agree that the loss of forests has far-reaching consequences for the climate. Many scientists argue that deforestation thousands of years ago led to the desertification of inland Australian lands and West Africa. There is a risk that deforestation in the future will lead to droughts in other regions, for example, part of the Amazon rainforest will turn into savannah. China's agricultural regions, the African Sahel and the Argentine pampas are also at risk, says Patrick Keys, an atmospheric chemist at the University of Colorado, Fort Collins.

In 2018, Kees and colleagues used a model similar to van der Ent's to track precipitation sources for 29 global metropolitan areas. He found that most of the water supply of 19 of them depends on remote forests, including Karachi (Pakistan), Wuhan and Shanghai (China), New Delhi and Kolkata (India).“Even small changes in precipitation caused by land-use changes downwind can have a large impact on the fragility of urban water supplies,” he says.

Some models even suggest that deforestation, by destroying the moisture source, threatens to change weather conditions far beyond the floating rivers. As you know, El Niño - fluctuations in wind temperature and currents in the tropical Pacific Ocean - indirectly affects the weather in remote places. Likewise, deforestation in the Amazon could reduce rainfall in the US Midwest and snow cover in the Sierra Nevada, says University of Miami climatologist Roni Avissar, who is modeling such connections. Far-fetched? “Not at all,” he replies. “We know that El Niño is capable of this, because, unlike deforestation, this phenomenon repeats itself, and we observe a pattern. Both are caused by small changes in temperature and moisture that is released into the atmosphere."

Stockholm University researcher Lan Wang-Erlandsson, who is researching the interaction of land, water and climate, says it’s time to switch from water and subsurface use within a particular river basin to land-use change beyond. “New international hydrological agreements are needed to maintain forests in areas where air masses form,” she says.

Two years ago, at a meeting of the UN Forum on Forests, where governments of all countries participate, land researcher from the University of Bern David Ellison presented a case study. He demonstrated that up to 40% of the total rainfall in the Ethiopian Highlands, the main source of the Nile, comes from moisture that returns from the forests of the Congo Basin. Egypt, Sudan and Ethiopia are negotiating a long-overdue deal to share Nile waters. But such an agreement would be meaningless if deforestation in the Congo Basin, far from the three countries, dries up the moisture source, Ellison suggested. "The relationship between forests and water in managing the world's freshwater is almost completely ignored."

The biotic pump theory will raise the stakes even further, as forest loss is expected to affect not only moisture sources, but also wind patterns. Ellison warns that the theory, if confirmed, will be "critical to planetary air circulation models" - especially those that transport humid air inland.

But so far, the supporters of the theory are in the minority. In 2010, Makarieva, Gorshkov, Shil, Nobre, and Bai-Liang Li, an ecologist at the University of California, Riverside, submitted their historical description of the biotic pump in Atmospheric Chemistry and Physics, a major subject journal with open peer review. But the article "Where Do the Winds Come From?" was criticized on the Internet, and it took the magazine many months to find only two scientists to review it. Isaac Held, a meteorologist at the Geophysical Fluid Dynamics Laboratory at Princeton University, volunteered - and recommended that the publication be turned down. “This is not a mysterious effect,” he says. "It is generally insignificant and, moreover, is already taken into account in a number of atmospheric models." Critics say the expansion of air from the heat generated by condensation of water vapor counteracts the spatial effect of condensation. But Makarieva says that these two effects are spatially separated: warming occurs at altitude, and the drop in condensation pressure occurs closer to the surface, where biotic wind is created.

Another reviewer was Judith Curry, an atmospheric physicist at the Georgia Institute of Technology.She has long worried about the state of the atmosphere and felt that the article should be published, because "the confrontation has a bad effect on climatology, and she needs blood from her nose for physicists." After three years of debate, the editor of the magazine rejected Held's recommendation and published the article. But at the same time, he noted that the publication cannot be considered approval, but it will serve as a scientific dialogue on a controversial theory - to confirm it or refute it.

Since then, no confirmation or refutation has come out - the confrontation continued. Columbia University climate simulator Gavin Schmidt says, "This is just nonsense." The authors respond to criticism like this: "In fact, because of mathematics, they are not sure whether it is worth continuing the dialogue." Brazilian meteorologist and head of the National Center for Monitoring and Disaster Prevention, Jose Marengo, says: “I think the pump exists, but it's all theoretical now. Experts on climate models did not accept it, but the Russians are the best theorists in the world, so appropriate field experiments must be carried out to test everything. " But so far no one, not even Makarieva herself, has proposed such experiments.

For her part, Makarieva relies on theory, arguing in a series of recent works that the same mechanism can affect tropical cyclones - they are driven by the heat released when moisture condenses over the ocean. In the 2017 Atmospheric Research newspaper, she and her colleagues suggested that forest-shaped biotic pumps draw moisture-rich air from cyclone origins. This, she says, explains why cyclones rarely form in the South Atlantic Ocean: the rainforests of the Amazon and Congo drain so much moisture that there is too little left for hurricanes.

Lead hurricane researcher at MIT, Kerry Emanuel, says the proposed effects are "significant, but negligible." He prefers other explanations to the absence of hurricanes in the South Atlantic, for example, the cool waters of the region release less moisture into the air, and its strong winds prevent the formation of cyclones. Makarieva, for her part, is equally dismissive of traditionalists, believing that some of the existing theories about the intensity of hurricanes "contradict the laws of thermodynamics." She has another article in the Journal of Atmospheric Sciences - pending review. “We are worried that, despite the editor's support, our work will be rejected again,” she says.

Although in the West Makaryeva's ideas are considered marginal, in Russia they are gradually taking root. Last year, the government launched a public dialogue on the revision of forestry laws. With the exception of the old protected areas, Russian forests are open for commercial exploitation, but the government and the Federal Forestry Agency are considering a new category - climate protection forests. “Some in our forestry department are impressed with the idea of ​​the biotic pump and want to introduce a new category,” she says. The idea was also supported by the Russian Academy of Sciences. Makarieva says that being part of the consensus, and not an eternal outsider, is new and unusual.

This summer, her trip to the northern forests was disrupted by the coronavirus epidemic and quarantine. At home in St. Petersburg, she sat down for yet another round of objections from anonymous reviewers. She is convinced that the pump theory will prevail sooner or later. “There is natural inertia in science,” she says. With dark Russian humor, she recalls the words of the legendary German physicist Max Planck, who gave the famous description of the progress of science: "a series of funerals."

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