The Role of Forests as Rainmakers

A new model of Forests as Rainmakers

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According to environmental journalist Katrina Marland, “If you follow environmental science at all, you already know that there’s a lot more we don’t know about how nature works than what we actually do know.”  When a new theory or model is introduced, we should see if it will explain some mystery people have been wondering about for ages — or upset an existing theory that, while already widely believed, may not be true.

The Biotic Pump Model

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The Biotic Pump model claims to be one of the latter. It holds that precipitation occurs over land, not because of differences in temperature, but because of the condensation that forests produce. The traditional belief is that winds bringing precipitation over land are caused by differences in temperature and pressure. The Biotic Pump model holds that those winds are actually created and controlled by forests and the condensation that they produce. Wind is created when air moves from high-pressure to low-pressure areas. The Biotic Pump model shows that as forest condensation pushes vapor into the air and the vapor condenses from a gas to a liquid, it creates a low-pressure area, pulling the wind in.

The Physics behind the Biotic Pump Model

The Biotic Pump model, published in journals such as Bioscience; Atmospheric Chemistry and Physics; and Forest News could revolutionize the way we understand local climates, and their vulnerability, with many major implications.  New studies support the physics behind this new climate model indicating that forests play a significant role in determining rainfall, and in creating atmospheric winds that pump moisture across continents. 

How Deforestation diminishes Heat Absorption and Rainfall

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Forests (especially tropical rainforests) play a vital role in local climate regulation by their interaction with water cycles. However, rainforests also have a significant effect on global weather. Rainforests, like all forms of vegetation, affect the reflectivity of a surface by absorbing more heat than bare soil. In turn, this warm air carries moisture from forest trees into the atmosphere, where it condenses as rain. In other words, tropical forests cool local climate and help generate rainfall. Conversely, the loss of forest vegetative cover means less heat absorption, translating to less moisture being taken up into the atmosphere. 

How Forest Fires, in themselves, diminish Rainfall

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Rainfall is affected when forest fires (whether intended or unintended) create air pollution and release tiny particles, known as aerosols, into the atmosphere. While aerosols can both heat and cool the air, depending on their size, shape, and color, high concentrations of biomass-burning aerosols directly impact local climate by increasing cloud formation but decreasing rainfall.  In areas with lots of smoke, cloud droplets form around the aerosol particles, but may never grow large enough to fall as rain, say researchers with NASA's Goddard Space Flight Center who studied the effect. Thus large forest fires have the effect of further reducing rainfall, leaving burned areas more prone to dryness and future fires.   In the long run, these changes explain why deforested regions may experience a decline in rainfall.  If our forests continue to be destroyed, global weather patterns (including wind currents, ocean currents, and rainfall distribution), will become even more unstable and extreme. 

The Link between Deforestation and Rainfall

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If the Biotic Pump model is proven to be correct, it has a number of implications.  For one thing, it means that messing with our forests is an even worse idea than we ever realized because we would be getting rid of a vital part of the system that brings us rain.

The Biotic Pump model is supported by recent findings that link deforestation to the catastrophic drought that essentially ended ancient Mayan civilization, as well as by recent research linking deforestation to drought in today’s Amazon rainforest.  To the extent that deforestation leads to drought, we will be relying on our forests and woodlands more than ever before.

Impacts upon Reforestation Efforts and upon the Protection of Existing Forest Cover

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Significant forest loss can transform lush temperate and tropical regions into arid landscapes.  However, the Biotic Pump model suggests that by strategically replanting forests we can attract rainfall into desert and arid regions such as the American Southwest and the African Sahel, where drought has ravaged crops and induced famine.  This model provides us with yet another reason to protect and conserve forest cover, for once you accept this idea that forest cover determines rainfall, there is a huge amount of policy that would need to be created to recognize that value. It also opens up a lot of potential to improve rainfall in dry areas through reforestation. 

Impacts upon Farmers, Ranchers, Industrial Water Users, and Householders

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Farmers and ranchers are, by far, the largest users of water in the USA and in most countries.  Farmers rely on rainfall for their crops; ranchers rely on rainwater for their livestock; and their fates are closely linked to rainfall quantity and regularity.  Knowledge of the Biotic Pump model should cause farmers and ranchers to become ardent supporters of the preservation of trees and forests, and advocates of tree planting and reforestation.

Industries such as electric power, pulp and paper, food processing, chemicals, public and private water supply utilities, and metal extraction and refining companies are all heavily water dependent.  Given the great extent to which aquifers throughout the USA have been depleted in recent decades, these industries will become more dependent than ever upon rainfall.  Leaders of these industries should therefore become vocal advocates for the preservation of trees and forests, as well as for robust tree planting and reforestation efforts.  Household water users should become tree preservationists, as well.    

How Local Forest Loss can lead to Regional Rainfall Decline

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The Biotic Pump model indicates that forest cover plays a much greater role in determining rainfall than previously recognized, and it explains how forested regions generate large-scale flows in atmospheric water vapor from one area to another.  The underlying mechanism emphasizes the role of evaporation and condensation in generating atmospheric pressure differences, and accounts for several phenomena neglected by current and past climate models. It suggests that even localized forest loss can flip a wet continent to arid conditions.  A 2005 study by NASA found that deforestation in the Amazon region of South America influences rainfall from Mexico to Texas and the Gulf of Mexico, while forest loss in Central Africa affects precipitation patterns all the way to the upper and lower U.S Midwest.  So, deforestation in South America and Africa can be felt in the USA.

Conclusion

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If the Biotic Pump model survives scrutiny, it will transform how we view forest loss, climate change, hydrology, and environmental services. It offers new lines of investigation in macroecology, landscape ecology, and forest restoration.  It also provides compelling new motivation for forest conservation.  For physicists Victor Gorshkov and Anastasia Makarieva, the originators of the Biotic Pump model, the lesson for forestry policy from their research is clear.  “Policy makers must react promptly to this new knowledge as they are managing the systems that create rain and that manage the circulation of water and moisture.”  Per Gorshkov and Makarieva, the message is unambiguous: “Spend your life working to completely stop deforestation and start recovering what can still be recovered.”

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—Mark Latour, Fall 2019