Preface. This is a controversial theory that if true, “could help explain why, despite their distance from the oceans, the remote interiors of forested continents receive as much rain as the coasts—and why the interiors of unforested continents tend to be arid. It also implies that forests from the Russian taiga to the Amazon rainforest don’t just grow where the weather is right. They also make the weather.
This biotic pump theory has faced a head wind of criticism, especially from climate modelers, some of whom say its effects are negligible and dismiss the idea completely. The dispute has made Makarieva an outsider: a theoretical physicist in a world of modelers, a Russian in a field led by Western scientists, and a woman in a field dominated by men.”
Keep in mind that the idea forests could generate rain wasn’t accepted until 1979. This theory was first proposed in 2007, but it hasn’t been proven or disproved, and is hard to test.
Their theory may also explain why cyclones rarely form in the South Atlantic Ocean: The Amazon and Congo rainforests between them draw so much moisture away that there is too little left to fuel hurricanes.
Alice Friedemann www.energyskeptic.com author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer, Barriers to Making Algal Biofuels, and “Crunch! Whole Grain Artisan Chips and Crackers”. Podcasts: Collapse Chronicles, Derrick Jensen, Practical Prepping, KunstlerCast 253, KunstlerCast278, Peak Prosperity , XX2 report
***
Pearce F. 2020. Weather makers. Forests supply the world with rain. A controversial Russian theory claims they also make wind. Science 368: 1302-5.
For more than a decade, Makarieva has championed a theory, developed with Victor Gorshkov, her mentor and colleague at the Petersburg Nuclear Physics Institute (PNPI), on how Russia’s boreal forests, the largest expanse of trees on Earth, regulate the climate of northern Asia. It is simple physics with far-reaching consequences, describing how water vapor exhaled by trees drives winds: winds that cross the continent, taking moist air from Europe, through Siberia, and on into Mongolia and China; winds that deliver rains that keep the giant rivers of eastern Siberia flowing; winds that water China’s northern plain, the breadbasket of the most populous nation on Earth.
With their ability to soak up carbon dioxide and breathe out oxygen, the world’s great forests are often referred to as the planet’s lungs. But Makarieva and Gorshkov, who died last year, say they are its beating heart, too. “Forests are complex self-sustaining rainmaking systems, and the major driver of atmospheric circulation on Earth,” Makarieva says. They recycle vast amounts of moisture into the air and, in the process, also whip up winds that pump that water around the world. The first part of that idea—forests as rainmakers—originated with other scientists and is increasingly appreciated by water resource managers in a world of rampant deforestation. But the second part, a theory Makarieva calls the biotic pump, is far more controversial.
Many meteorology textbooks still teach a caricature of the water cycle, with ocean evaporation responsible for most of the atmospheric moisture that condenses in clouds and falls as rain. The picture ignores the role of vegetation and, in particular, trees, which act like giant water fountains. Their roots capture water from the soil for photosynthesis, and microscopic pores in leaves release unused water as vapor into the air. The process, the arboreal equivalent of sweating, is known as transpiration. In this way, a single mature tree can release hundreds of liters of water a day. With its foliage offering abundant surface area for the exchange, a forest can often deliver more moisture to the air than evaporation from a water body of the same size.
The importance of this recycled moisture for nourishing rains was largely disregarded until 1979, when Brazilian meteorologist Eneas Salati reported studies of the isotopic composition of rainwater sampled from the Amazon Basin. Water recycled by transpiration contains more molecules with the heavy oxygen-18 isotope than water evaporated from the ocean. Salati used this fact to show that half of the rainfall over the Amazon came from the transpiration of the forest itself.
Salati and others surmised the jet carried much of the transpired moisture, and dubbed it a “flying river.” The Amazon flying river is now reckoned to carry as much water as the giant terrestrial river below it, says Antonio Nobre, a climate researcher at Brazil’s National Institute for Space Research.
For some years, flying rivers were thought to be limited to the Amazon. In the 1990s, Hubert Savenije, a hydrologist at the Delft University of Technology, began to study moisture recycling in West Africa. Using a hydrological model based on weather data, he found that, as one moved inland from the coast, the proportion of the rainfall that came from forests grew, reaching 90% in the interior. The finding helped explain why the interior Sahel region became dryer as coastal forests disappeared over the past half-century.
In 2010, van der Ent and his colleagues reported the model’s conclusion: Globally, 40% of all precipitation comes from the land rather than the ocean. Often it is more. The Amazon’s flying river provides 70% of the rain falling in the Río de la Plata Basin, which stretches across southeastern South America. Van der Ent was most surprised to find that China gets 80% of its water from the west, mostly Atlantic moisture recycled by the boreal forests of Scandinavia and Russia. The journey involves several stages—cycles of transpiration followed by downwind rain and subsequent transpiration—and takes 6 months or more. “It contradicted previous knowledge that you learn in high school,” he says. “China is next to an ocean, the Pacific, yet most of its rainfall is moisture recycled from land far to the west.”
If this theory is correct, forests supply not just the moisture, but the winds that carry it. In 2007, in Hydrology and Earth System Sciences, they first outlined their vision for the biotic pump. It was provocative from the outset because it contradicted a long-standing tenet of meteorology: that winds are driven largely by the differential heating of the atmosphere. When warm air rises, it lowers the air pressure below it, in effect creating space at the surface into which air moves. In summer, for example, land surfaces tend to heat faster and draw in moist breezes from the cooler ocean.
Makarieva and Gorshkov argued that a second process can sometimes dominate. When water vapor from forests condenses into clouds, a gas becomes a liquid that occupies less volume. That reduces air pressure, and draws in air horizontally from areas with less condensation. In practice, it means condensation above coastal forests turbocharges sea breezes, sucking moist air inland where it will eventually condense and fall as rain. If the forests continue inland, the cycle can continue, maintaining moist winds for thousands of kilometers.
The theory inverts traditional thinking: It is not atmospheric circulation that drives the hydrological cycle, but the hydrological cycle that drives the mass circulation of air.
Sheil, who became a supporter of the theory more than a decade ago, thinks of it as an embellishment of the flying river idea. “They are not mutually exclusive,” he says. “The pump offers an explanation of the power of the rivers.” He says the biotic pump could explain 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—the opposite of what would be expected if they resulted from differential heating.
Even those who doubt the theory agree that forest loss can have far-reaching climatic consequences. Many scientists have argued that deforestation thousands of years ago was to blame for desertification in the Australian Outback and West Africa. The fear is that future deforestation could dry up other regions, for example, tipping parts of the Amazon rainforest to savanna. Agricultural regions of China, the African Sahel, and the Argentine Pampas are also at risk.
In 2018, Keys and his colleagues used a model, similar to van der Ent’s, to track the sources of rainfall for 29 global megacities. He found that 19 were highly dependent on distant forests for much of their water supply, including Karachi, Pakistan; Wuhan and Shanghai, China; and New Delhi and Kolkata, India. “Even small changes in precipitation arising from upwind land-use change could have big impacts on the fragility of urban water supplies,” he says.
Some modeling even suggests that by removing a moisture source, deforestation could alter weather patterns beyond the paths of flying rivers. Just as El Niño, a shift in currents and winds in the tropical Pacific Ocean, is known to influence weather in faraway places through “teleconnections,” so, too, could Amazon deforestation diminish rainfall in the U.S. Midwest and snowpack in the Sierra Nevada.
Another example: a study showed that as much as 40% of the total rainfall in the Ethiopian highlands, the main source of the Nile, is provided by moisture recycled from the forests of the Congo Basin. Egypt, Sudan, and Ethiopia are negotiating a long-overdue deal on sharing the waters of the Nile. But such an agreement would be worthless if deforestation in the Congo Basin, far from those three nations, dries up the moisture source.
If true, water resource managers in the Midwest, Sierra Nevada, and Middle East need to care as much about the deforestation in the far away Amazon or Congo Basins as much as their local water.
The biotic pump would raise the stakes even further, with its suggestion that forest loss alters not just moisture sources, but also wind patterns. The theory, if correct, would have crucial implications for planetary air circulation patterns, especially those that take moist air inland to continental interiors.