The Untold Story of How Plants & Trees Cool Their Surroundings
Mysteriously, the most famous media outlets, the most authoritative governmental bodies and the most prestigious NGOs will not tell you how plants and trees impact the climate.
The Role of Vegetation in Climate Regulation
I’ve been a climate reporter since 2018, and I can tell you that the great untold story in climate coverage is the dramatic impact of vegetation (plants and trees) on our climate.
I have recently become acquainted with Dr. Poulomi Chakravarty, who shared this paper with me, in preparation for an upcoming webinar.
The Role of Vegetation in Climate Regulation
The document is by ...
Poulomi Chakravarty, PhD
Environment Scientist | Climate Educator
My pronouns: she, her, hers
Climate and Biodiversity Research Advisor (Volunteer) Biodiversity for Livable Climate
Director, Education and Sustainability, Planet Alba Biz
Founder, Global Climate Association
Executive Coordination Member and Digital Platform Manager, South Asian Meteorological Association
Chair, NSDC GCA Organisational Chapter
Website: cpoulomi.com
The article that follows is good for anyone who wants to go deeper in understanding how our climate works.
If you’ve ever walked under a shade tree, you have felt the cooling effect of vegetation. This cooling effect is neglected in climate science, and not for any good scientific reason.
I took the liberty to summarize some of the key concepts in the document The Role of Vegetation in Climate Regulation, as follows:
What is the Evaporative Fraction?
The Evaporative Fraction refers to the portion of the sun’s energy that is dedicated to evaporation or latent heat, as opposed to sensible heat, or heat you can feel.
What is evaporative cooling?
Just as humans perspire, exuding water from our pores and causing it to evaporate, plants transpire exuding water from their pores and causing it to evaporate. In both cases, the evaporating water cools its surroundings. Evaporation has a cooling effect.
In the case of plants, it is sunlight that gives them energy and causes transpiration to occur, which causes evaporation. So it makes a big difference whether sunlight falls on plants or bare ground. If it falls on bare ground, this causes a warming effect. Think of how hot the sand can be when walking on a beach. Think of how hot a sidewalk or pavement can be when walking on it with bare feet.
Even when the sunlight strikes a plant, some of that solar radiation creates a warming effect, but some creates a cooling effect. Scientists quantify the warming effect and the cooling effect. The cooling effect is called the evaporative fraction.
The Evaporative Fraction is one of the great untold stories in the climate conversation. In the mainstream climate conversation, almost the entire focus is on greenhouse gases. This is unfortunate, because vegetation tends to cool its surroundings, not just because of shade but because of transpiration.
What is albedo?
Albedo refers to how much of the sunlight is reflected by a body or absorbed. Light objects tend to reflect sunlight, while dark objects tend to absorb sunlight. White objects are said to have high albedo (high reflectance), while dark objects have low albedo (low reflectance).
Albedo is often discussed in the mainstream climate conversation. The assumption is that high albedo is good, because we want that light to be reflected back out into space. But this leads to the absurd conclusion that says it’s good to cut down trees because the trees tend to be darker than the white snow that they block. This ignores the benefits of trees in driving water cycles and causing water to vaporize, thus providing water vapor for subsequent rain events.
What is surface roughness?
Surface roughness is the property of vegetation that causes the vegetation to serve as a wind break and slow down the wind. Surface roughness serves to enhance vertical, up and down, flows of air that tend to create rainfall.
What is the radiation balance equation?
Section 3 of the outline in our document, The Role of Vegetation in Climate Regulation, introduces the radiation balance equation.
The radiation balance equation involves this concept: At any given time, there is incoming radiation and outgoing radiation. Whether there is more coming in or going out depends on these factors:
How much heat energy is radiating from surfaces, such as dirt and sand?
How much heat energy is converted to latent heat (i.e., hidden heat) through evaporation? Evaporation causes heat to disappear. That’s why it feels cool when we sweat..
How much heat disappears into the ground or emanates from the ground?
How much heat energy disappears into photosynthesis, causing it to be stored as chemical energy? Photosynthesis is an endothermic reaction, which means heat disappears into it and is stored in chemical bonds.
The net effect of all four of the above factors determines whether any piece of land is generating heat or absorbing heat.
What is the boundary layer height?
Section 4 asks the question: How high into the atmosphere does vegetation have an effect.
The boundary layer height refers to the fact that vegetation affects the lower part of the atmosphere, but not the upper part. The height of the boundary layer is lower if there is less vegetation and higher if there is more vegetation.
A higher boundary layer increases cloud formation. Also, a higher boundary layer tends to change pollution dispersion patterns, by slowing air flow and filtering out pollution.
Vegetation matters …
The moral of the story is that vegetation has many measurable effects on our climate.
Vegetation increases the frequency of short-duration rains. This is good.
Vegetation increases surface roughness and therefore rainfall.
Vegetation increases cloud formation and favorable dispersion of pollution.
Vegetation increases the evaporative fraction, i.e., the amount of solar radiation that “disappears” by becoming latent heat, the same as when sweat makes us feel cooler.
To me, this is the great untold story in the climate conversation: Does vegetation (plants and trees) have a substantial effect on our climate? The most famous and prestigious organizations among media, government and NGOs want us to believe that plants and trees are significant only in their capacity to absorb carbon dioxide.
It’s a lie of omission. Not that anyone is lying intentionally, but they are leaving out all of the ways that plants and trees have a cooling effect and favorable effects on rainfall.
It’s our job to demand that scientists and reporters do their job.
