Connected by Water: Deforestation and Hydrological Climate Change
Written by Brendan McNamara – 11/15/2020
When the general population talks about climate change, the issue focuses mainly around carbon emissions. The connection between greenhouse gasses released by humans and drastic change in average global temperature is not disputed among the vast majority of scientists that study the subject. Generally speaking, however, the history of anthropogenic climate change from CO2 emissions coincides with anti-ecological economic activities interfering with the continuity of the biosphere. The question is, how much of what we know of human-generated climate change is the direct result of emissions, and how much is the result of parallel destructive changes to the living surface of the Earth, primarily forests, facilitated by fossil-fuel-based industrialization?
There is little debate that plants play a role in regulation of greenhouse gasses, at least from the point of view of respiration. During the light-independent parts of photosynthesis they take in CO2 and convert it to sugars and biopolymers that they use for either energy or growth; in the process they literally convert the greenhouse gasses we are worried about into their bodies. (Lumen, n.d.) There is great hype in the environmental field about the promise of carbon sequestration through massive tree planting – with some startups even developing advanced AI-based remote sensing systems to monitor and calculate net carbon loss or gains in real-time as a way of verifying offset. (Pachama, 2020) However, even though the respiration aspect is the most obvious connection plants have to climate, even some mainstream climate scientists do not agree that the carbon capture aspect of forests is particularly significant in potential mitigation of global temperatures. It’s mostly seen as a short-term buffer to the effects of emissions, as plants will return much of what they sequester back into atmospheric CO2 upon death and decomposition. (Dean, 2019) Therefore, reforestation as a form of carbon sequestration doesn’t make much sense if the long-run plan for your plantation is to log it and turn it to biomass burning for electricity, as many are doing as an “answer” for fossil fuel consumption (Hillsdon, 2017), it only counts if you’re planting long-term multi-generational biodiverse sequestration engines, also known as standing forests. As we will see, however, carbon capture isn’t the only thing plants do for climate.
Trees and all plants provide essential and numerous ecological services besides just the sequestration of carbon, though that may be of particular interest to us now at the time being because it’s useful to clean up our atmospheric mess. Some of the other benefits of a given tree may include the water it holds in its body, which it moves between sky and earth in a daily and seasonal rhythm – which may have a specific local effect on climate, through transpiration. Transpiration is the biotic evaporation of moisture directly from leaves – all evaporation has a cooling effect, but biology allows for control of this. Flowering trees specifically may have evolved a dense arrangement of veins that might permit a profound control of evapotranspiration that may add up, in the form of the full forest, to a rainforest climate control system still being studied that is sometimes referred to as the “biotic pump”. These transpirative emissions of the trees’ leaves over the complicated vein systems create a pocket of evaporative pressure that isolates and increases the condensation and precipitation above the forest canopy, leading to “aerial rivers” of rain clouds running above the forest as the ground-based one runs beneath. (Bunyard, 2014) It should be noted that this theory is hotly debated by many climatologists who don’t fully recognize the role that forests may play in weather patterns, but evidence continues to mount for the strange and wonderful things life does with water at the top of a rainforest canopy.
It’s odd that something as universal to life as we know it as water can keep surprising us. Its properties of adhesion, surface tension, and a uniquely nebulous thermal capacity are all related in part to hydrogen bonding between molecules, caused by its polar shape. The behavior of water at the molecular level due to this polar configuration also explains why it makes such an excellent solution for the chemistry of life. As you pour energy into water in the form of heat, the time it takes for molecules to fly away and become vapor (aka boiling) is much higher than other liquids due in large part to these strong intermolecular forces. (Lower, 2017) These odd properties of water hold particular significance to its role in climate, as it seems the presence and ubiquity of liquid water on or in the land’s surface would by its very nature stabilize it thermodynamically. The presence of a large old-growth forest, its tree trunks full of water, therefore, will make a difference in the climate of that location versus the case were it the opposite, desiccated and barren.
Once water leaves that liquid state and becomes vapor, it can precipitate again to become liquid, but in the meantime it becomes a part of the problem. As water vapor is released into the air, it becomes a greenhouse gas itself – increasing the heat capacity of the air. “Vapor feedback” is a runaway cycle in which climate change brought on by greenhouse gasses and other human activities release the vapor, which work together to double the impact of carbon-based GHGs alone. In a study by Texas A&M on the issue, it was confirmed that the effect of this feedback was higher than originally anticipated and that it represents a real problem: “we now think the water vapor feedback is extraordinarily strong, capable of doubling the warming due to carbon dioxide alone.” (Hansen, 2008) So arguably, in addition to controlling carbon-based GHGs like CO2 and Methane, we need to be paying attention to anything we do to increase evaporation and release liquid water into the atmosphere as vapor.
It’s a purely logical conclusion that the clear-cutting of an old-growth forest would cause in a very short timeframe the evaporation of whatever water was being stored in liquid form in the land and in the bodies of the trees and plants. In a standing forest, temperatures start to drop just below the top canopy, and so the on-the-ground effect of plant cover is substantially improved over bare soil. In the most basic terms we can see how the presence, versus lack, of a forest affects the local climate directly as well as (by keeping vapor out of the atmosphere) the global climate indirectly. Any given tree left standing therefore has an impact, even just as a shade and a home for liquid water, on the climate – possibly just as powerful as its value as a carbon sink (if not moreso).
The reduction of plant cover to bare ground has obvious thermodynamic implications that can’t be overlooked as a source of anthropogenic climate change. Deforestation can contribute directly to the way temperature is distributed on the ground, as well as rainfall patterns. By decreasing evapotranspiration, increasing albedo (surface reflection) of the land, and decreasing ground texture, the removal of forests forces temperatures to pool at the surface and heat pockets to emerge. Though the increase in surface reflection causes cooling, the other effects cause a breakdown of the latent heat flux of the land, creating net warming. In some cases, pooled thermal energy can release unevenly, causing whatever evaporation occurs to move swiftly up the air column, producing a desiccating effect. (Chen et al., 2019) We can see then how the destruction of a given forest has a massive impact on climate, at least locally, well beyond its loss as a potential carbon sponge. Add these cumulative local effects together into the level of deforestation we are currently observing globally – 17 percent in the Amazon alone in the last 50 years – and we can begin to tie these local thermodynamic alterations with a global phenomenon. (Nunez, 2019)
I hope I’ve shown how the issue of these other avenues of anthropogenic climate change are nuanced and important. In my opinion, this topic begs further consideration and research. For something as complicated and critical to the future of our species as the climate, can we risk having one dominant narrative push out others that may be just as important? The relationship between forests and climate is still being researched and our understanding continues to grow. At the same time, everything we already know about these systems says how critical they are to the thermodynamic equilibrium of the surface of our planet – and thus worth protecting, restoring and enhancing in order to better our odds at our own survival, and that of the biosphere itself.
WORKS CITED
Hillsdon, M. (2017, May 24). Biomass a burning question for climate | Reuters Events | Sustainable Business. https://www.reutersevents.com/sustainability/biomass-burning-question-climate
Dean, A. (2019, August 21). Deforestation and Climate Change. Climate Council. https://www.climatecouncil.org.au/deforestation/
Nunez, C. (2019, February 7). Deforestation and Its Effect on the Planet. National Geographic. https://www.nationalgeographic.com/environment/global-warming/deforestation/
Bunyard, P. P. (2014). How the Biotic Pump links the hydrological cycle and the rainforest to climate: Is it for real? How can we prove it? Fondo de publicaciones Universidad Sergio Arboleda. https://doi.org/10.22518/9789588745886
Hansen, K. (2008, November 17). NASA – Water Vapor Confirmed as Major Player in Climate Change [Feature]. Brian Dunbar. https://www.nasa.gov/topics/earth/features/vapor_warming.html
The Light-Independent Reactions of Photosynthesis | Boundless Biology. (n.d.). Retrieved November 15, 2020, from https://courses.lumenlearning.com/boundless-biology/chapter/the-light-independent-reactions-of-photosynthesis/
Chen, C.-C., Lo, M.-H., Im, E.-S., Yu, J.-Y., Liang, Y.-C., Chen, W.-T., Tang, I., Lan, C.-W., Wu, R.-J., & Chien, R.-Y. (2019). Thermodynamic and Dynamic Responses to Deforestation in the Maritime Continent: A Modeling Study. Journal of Climate, 32(12), 3505–3527. https://doi.org/10.1175/JCLI-D-18-0310.1
Lower, S. (n.d.). Water and hydrogen bonding. Retrieved November 15, 2020, from http://www.chem1.com/acad/webtext/states/water.html
Pachama. (n.d.). Retrieved October 20, 2020, from https://pachama.com/
