- An expanded understanding of forests’ role in moisture transport and heat regulation raises the stakes on the health of the Amazon Rainforest and the need to stop cutting trees.
- The biotic pump theory, conceived by scientists Anastassia Makarieva and the late Victor Gorshkov, suggests that forests’ impact on hydrology and cooling exceeds the role of carbon embodied in trees.
- In an interview with Mongabay, Makarieva and Brazilian scientist Antonio Nobre explain how the theory makes the case for a more urgent approach by Brazilian President Luiz Inácio Lula da Silva to protect the Amazon.
Brazilian scientist Antonio Donato Nobre has long been a student of the Amazon Rainforest. An agronomist with a master’s degree in tropical biology and a Ph.D. in Earth science, Nobre has researched the region for nearly four decades with the National Institute for Amazonian Research (INPA) and the National Institute for Space Research (INPE). Like others who study the Amazon — a cohort that includes three of his brothers, Carlos, Paulo and Ismael — Antonio Nobre has watched in despair while stretches of untouched forest succumb to fire, machete and chainsaw, a scenario that’s been particularly acute under former president Jair Bolsonaro. In a 2019 interview with Mongabay, he called deforestation in the Amazon a “catastrophic situation” that threatens to disrupt Earth’s climate.
Now, a new documentary, Rivers Above the Canopy, by French filmmaker Pascal Cuissot, features Antonio Nobre and his investigations into the Amazon’s importance to the global climate. The film has been shown in France, Germany and Brazil.
Also featured is physicist Anastassia Makarieva of the Petersburg Nuclear Physics Institute in Russia, who, with the late Victor Gorshkov, first came up with the concept of the biotic pump, an active function of forests. Increasingly, Nobre’s work on the Amazon is informed by this theory. For example, in a 2014 report, “The Future Climate of Amazonia,” a tour-de-force of explanatory science, Nobre describes how this “green ocean” not only creates its own rain but moves moisture across the continent via aerial rivers. Lately, he has collaborated on scientific papers with Makarieva and other biotic pump researchers.
The biotic pump concept highlights the extent to which natural forests drive moisture-laden air currents, thereby governing wind and rain. This adds a level of urgency to the zero-deforestation pledge made by Brazil’s new president, Luiz Inácio Lula da Silva.
The theory calls attention to the dynamics of evaporation and condensation: water becoming gas (vapor) and then returning to liquid form. A forest dense with trees evaporates, or transpirates, prodigious quantities of moisture; Nobre likens the Amazon’s trees to geysers. This moisture rises from the canopy and eventually cools and condenses. When water changes from a gas to a liquid, it occupies less space. This creates a low-pressure zone so that moist air is pulled in, a kind of suction. The mechanism explains how regions far from the ocean get precipitation.
The biotic pump theory derives from fundamental principles of physics instead of the statistics-driven modeling that dominates climate science and meteorology. As such, some scientists view it with skepticism, even labeling it “controversial,” despite an absence of serious refutations.
It’s important, too, to note how this line of thinking underscores the need to preserve expanses of the forest like that of the Amazon: when stands are cleared, the impact is far greater than merely subtracting the moisture transpired by the downed trees. Rather, the entire mechanism shuts down. The effect is not gradual, but exponential. As Makarieva explains, there is a threshold beyond which there is at once too much sensible (surface) heat and not enough moisture to generate precipitation, leaving the would-be rain to be carried away by breezes.
Concern about deforestation is often framed around carbon emissions. For example, there’s concern that parts of the southeast Amazon have flipped from being a carbon sink to a carbon source. While this is certainly alarming, Nobre and Makarieva say the impacts on hydrology and cooling capacity are far more significant in terms of climate.
That said, insights from biotic pump research also point the way to strategic reforestation to reinstate the previously unrecognized processes by which forests sustain themselves as well as regulate wind, heat and moisture. This new lens on how water works drives home the pivotal role of healthy forests in staving off climate change — and it’s not just about the carbon.
What does this tell us about how forests like the Amazon cool the planet for us? For starters, evapotranspiration from forest vegetation dissipates solar radiation, so that heat energy becomes latent heat embodied in water vapor. This lowers local temperatures. Then there’s vertical cooling: via transpiration, trees harvest surface heat and release it higher in the atmosphere, where it can escape into space. Like CO2, water vapor is a greenhouse gas that traps heat. However, unlike CO2, water vapor condenses and releases heat, a process that summons wind and rain.
Also, ample daily condensation creates white clouds, which reflect heat back to space. And enhanced local water cycling brings late-afternoon rains, so there’s little cloud cover at night. Clear night skies mean heat flows back into space.
For many conservation scientists, the end of the Bolsonaro administration represents a possible turning point in the fate of the Amazon Rainforest. Mongabay interviewed Antonio Nobre and Anastassia Makarieva by video call and email in Nov. 2022 and Jan. 2023 to discuss what to look out for, what the latest science tells us about the unforeseen perils of forest loss, and opportunities for restoration so that forests can continue cycling, drawing in moisture, and promoting natural cooling.
Mongabay: The Bolsonaro years saw science budgets slashed and scientists attacked by the president and his allies. While Lula says he aims to restore several policies, and investment in science seems like an obvious one, can he generate the funding and commitment to fulfill promises necessary for science to advance, or even keep up?
Antonio Nobre: Let’s be optimistic. Brazil’s federal budget is large and science has historically represented a small fraction. We saw tens of billions criminally wasted on political corruption, many times greater than what is typically spent in support of science. Furthermore, Lula is a believer in the philosophy of abundance rather than scarcity. In other words, he conveys confidence that there will be enough, effectively setting the economic wheel in a positive direction. Plus, Ricardo Galvão, the INPE director-general famously fired by Bolsonaro in 2019 in a dispute over deforestation numbers, is a member of Lula’s team. Finally, Brazil’s scientific community has endured more than six years under right-wing power and continued to produce important science, including monitoring the Amazon.
Mongabay: Your Amazon research now focuses on the biotic pump. What is the biotic pump, and how might it explain extreme weather events? Let me toss this over to Makarieva, who developed the concept.
Anastassia Makarieva: The biotic pump describes how forest ecosystems influence the delivery of water from ocean to land. For rain to fall over land, first, horizontal winds must carry water vapor to land; and second, the wind must lift this water vapor to the cooler atmospheric layer so it can condense and form precipitation. Natural forests evolved to regulate these air circulation patterns to sustain their own water needs. Remember: land is continuously losing water as rivers flow to the ocean.
When a forest or natural wetland loses vegetation, such as by logging, the mechanism of moisture delivery is disrupted. This leads to fluctuations of the water cycle, including droughts and floods.
Mongabay: Given this explanation, it makes sense that deforestation would lead to changes in precipitation. Does the biotic pump have other climate impacts, such as on temperature?
Anastassia Makarieva: First, we need to distinguish between local and cooling effects of transpiration. The local effect is simple: if solar energy is spent on evaporation, the surface is cooler than if solar energy directly becomes heat. The broader impact depends on what happens next. Let’s say air laden with evaporated moisture rises, water vapor condenses, and latent heat is released in the upper troposphere, where it remains for a long time. This heat radiates into space without being absorbed by the greenhouse gases beneath. This effectively reduces the greenhouse effect and cools the Earth as a whole.
If, by contrast, the air descends quickly back to the surface, all released heat remains at the surface and radiates to space only after having interacted with the greenhouse gases. The effect is just as if the solar energy converted to heat at the surface: the global cooling is near zero. Thus, a determining factor in global cooling from transpiration is how long air travels in the upper atmosphere — which depends on the characteristics of the long-distance moisture transport.
Research is finding global cooling from transpiration to be significant, exceeding the effect of carbon emissions from historic deforestation. However, this is usually reported as just a small addition to the carbon effect. Why? Because in modern climate models, global transpiration cooling is canceled out by the increase in albedo (reflectivity) upon deforestation: a deforested surface is brighter, and so reflects more sunlight to space, a cooling effect. However, transpirational cooling and albedo change are complex effects with different physics, and the legacy of historical devegetation is poorly understood. As a result, the coincidence of neatly canceling each other may be an artifact of internal model tuning. If so, transpirational cooling, and its loss upon deforestation, is like an elephant in the china shop hidden under a thin curtain of albedo change.
Mongabay: Wait a second: Transpirational cooling and albedo could have been swapped out for the sake of convenience? As someone who’s written about water’s effect on climate, I’ve wondered how the scientific/environmental climate community so underplays the climate impacts of devegetation. Thoughts?
Anastassia Makarieva: Historically, global climate modeling has focused on CO2, and this has determined the trajectory of model development. In order to investigate the role of plant life, it is not enough to remove or add vegetation into models developed to assess CO2-related climate change. It would require the entire set of model parameters to be reconsidered. Second, it is known that the Amazon moisture transport is underestimated in global climate models by 50% or more. This is equivalent to 20% of global river runoff and, hence, to a considerable share of ocean-to-land long-distance moisture transport (the biotic pump).
The price of not performing such a stress test — or crash test — for global climate modeling could be very high: for if natural forests are key to climate stability due to their role in moisture transport, using them for biomass to lower CO2 emissions will only aggravate the climate situation.
Mongabay: We are seeing the folly of cutting trees for biomass in the name of addressing climate change. But while we can’t uncut trees, we can reforest. Can we reinstate the biotic pump?
Antonio Nobre: The concept of the biotic pump opens up a series of possibilities for slowing down climate change, possibilities previously insufficiently considered. The theory suggests that once certain conditions are met, reforestation will become self-perpetuating. Restoration should always start at the coast, covering a strip of land at least 150 kilometers [90 miles] wide — a coastal “green edge” that can start the biotic pump to import moist air from the ocean. Then, each year, the coastal buffer can grow inland, expanding the atmospheric conveyor belt. As we have seen in the Amazon and other large forests, this extension of forest cover to regions far from the shores can propel moist air masses thousands of kilometers inland, generating atmospheric, or “flying,” rivers.
We are confident that this effect could even reverse aridity in desert lands, countering the landlocked dry circulation with a new ocean breeze driven by the biotic pump. One tool for amassing vegetation before the biotic pump kicks in is syntropic cultivation, which accelerates ecological succession. Our intent is to show that vegetation is not the passive target of adverse weather, but rather the main factor in creating and maintaining a friendly climate.
Mongabay: From a biotic pump perspective, what other large forests should we be paying attention to?
Anastassia Makarieva: The boreal forests of North America and Eurasia are important. In Eurasia, forests stretch over 4,200 miles [6,800 km] from the Atlantic to the Pacific Ocean. These forests are active in the warmer season when transpiration is possible. They play an enormous role in moisture transport across the continent. Here the main stressors are logging and the fires that often follow logging due to resulting ecosystem disruption. Russia still has vast forested areas that retain self-sustainability. It is urgent to protect them from exploitation. Trees in plantations do not function the same way.
Mongabay: What does the biotic pump tell us about healing degraded forests?
Antonio Nobre: More than planting trees, we need to “grow” forests. Nature everywhere teaches us that life is a highly complex and rich phenomenon that requires us to respect nature’s rhythm and laws. Soil cover is essential to start correcting the hydrological cycle on land. Then come the shrubs in several layers and only later come the trees. Nature, left to its own devices, will green unfrozen continents everywhere, as it has done many times through geological time. But it can take thousands or millions of years to turn continents green. Our best hope today is to reinvent technology to mimic nature.
We also have the opportunity to learn from native cultures how to live in harmony with nature. Indigenous knowledge has a powerful advantage over modern technology and engineering because reverence and care for nature have evolved over millennia of harmonious cohabitation.
Mongabay: While many advocate tree planting, some say additional trees can deplete moisture due to transpiration. What is the story?
Anastassia Makarieva: In a study now under review, we found it depends on whether the surrounding air is wet or dry. Imagine a very dry atmosphere and a small tree plantation transpiring at the expense of limited soil moisture. It becomes more humid, but far from the saturation needed for condensation. If there is no condensation, this transpired moisture is likely blown away by horizontal winds. Here, tree planting would only worsen the aridity, with competition for limited water among local stakeholders. This scenario is widely discussed, and in China it is seen as setting a limit on eco-restoration.
On the other hand, if a native forest evolved in the context of local climate conditions, trees “wisely” transpire when the atmosphere is sufficiently moist. This added transpired moisture leads to more vigorous heat transfer, precipitation and moisture transport. The forest receives more water than it has expended for transpiration — and all life benefits from greater water availability.
Mongabay: Back to the Amazon. What does the defeat of Bolsonaro mean for the biotic pump in the Amazon?
Antonio Nobre: Bolsonaro set in motion a massive fanatical campaign aimed at the demise of the Amazon Rainforest and the annihilation of its native people. He, his ministers, and many ideologically motivated officials and businessmen unleashed a tsunami of disruptive administrative and legislative attacks on the national deforestation control system, which the new government will have the challenge of rebuilding.
We need two things to keep Amazon biotic pump viable. The first is the immediate and forceful interruption of deforestation and suppressing the fires that generate smoke and soot — two powerful agents that reduce rainfall. The second, equally important, is the restoration of forest ecosystems, especially forests degraded by logging, fires, and criminal land-grabbing activities. If the attack against the forest ceases, and if the destroyed forest is allowed to return, it is very likely that the weakening of the Amazon biotic pump will be halted. Unfortunately, Bolsonaro has cultivated a swarm of criminals, who are unlikely to give up crimes of their own free will. A challenge now is to restore law and order. International collaboration with Brazil’s new government, associated with economic pressures, will likely be necessary to tame crime and conflict in the Amazon.
Banner image: A blackwater oxbow lake in the Peruvian Amazon. Image by Rhett A. Butler/Mongabay.
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