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Annual CO2 emissions have risen at a growth rate (geometric mean) of 2.5% in the last 9 decades (1930-2019). Humans produced about 940% more carbon emissions in 2020 than they did in 1920. While there is a fervent push towards going green and moving the economy away from fossil fuels and more towards renewables, such a shift, due to practical reasons, would take time (Ritchie & Roser, 2020).
The global infrastructure, unfortunately, is not just reliant on fossil fuels, but this dependence, arguably, is entrenched. In 2020, 84% of world energy still came from fossil fuels, and pragmatically, it will take decades to fully recover from the insidious 'oil addiction' which can irreversibly damage the planet and fragile ecosystems (Ritchie & Roser, 2020).
A common 'natural' solution that is often inquired/presented at a personal level to counter CO2 emissions is planting trees. Of course, trees absorb CO2, and planting trees, i.e., a global combined effort to increase the tree plantation substantially, can be carried out to counter carbon emissions. This report examines how many trees must be planted to achieve carbon neutrality and the costs associated with such an endeavor.
How many trees do we need to plant for global carbon neutrality?
The first important issue is the CO2 absorption rate per tree. The carbon absorption per tree varies by species and by the age of the tree; broadly, we can state that a mature tree absorbs about 48 lbs. or 21.8 kg of CO2 per year (Stancil, 2019). Nonetheless, there are other tree species that can absorb substantially more; for example, an adult Paulownia tree (also known as the Empress tree & Kiri tree) is reported to capture 450 kg C02 per year (Chasan, 2019).
Therefore, the answer depends not just on how many trees, but on which species of trees to plant as well. To achieve carbon neutrality by planting a mix of different species of trees, we would need to plant about 154 billion trees today to offset the carbon emissions produced when these trees would be fully grown (approximately 20 years), as maximum carbon absorption occurs when a tree is fully grown. If carbon emissions grow at a 1% rate from 2020 onwards, global yearly carbon emissions will stand at about 254 billion tons p.a. If annual carbon emissions are controlled, the number of trees required would be reduced, of course.
On the other hand, if we plant a high carbon-absorbing tree species, such as the Paulownia tree, we would need to plant approximately 433 to 557 million trees of this species to offset the yearly carbon emissions produced globally in 12 years, when these trees mature, with as assumption that carbon emissions would increase at an annual rate of 1.5% in the next 12 years.
With about 400 mixed variety trees planted per acre, we would need approximately 385 million acres of land for a variety of conventional trees for this exercise; that means an area equaling about 23% of the Amazon Forest.
For Paulownia trees, which are ideally planted at a space of 16 X 16 (170 trees per acre), we would require about 3 million acres of land or 1214057 hectares (Graves, 1989).
How much would such an endeavor cost?
The two main costs would be the plantation of trees, and the cost (if any) of land, i.e., the opportunity cost of capital or any outlay costs.
Many charity projects plant trees for $1; however, for strategic analysis, we cannot rely on such numbers, as the undertaking analyzed here is beyond the capability of a single organization.
While planting a mixed variety of trees by utilizing tech such as drones would cost approximately $1 per tree, planting the Paulownia species for enhanced carbon absorption would cost more:
Thus, broadly it can be claimed that planting 3 million acres of Paulownia trees would cost about $530 million. Planting 154 billion mixed variety of conventional and native trees using drone technology would cost approximately $150 billion.
The estimates here do not include an outlay cost, or opportunity cost of land. Nonetheless, if drone tech is used to reseed illegally logged land/forest areas suffering from deforestation, or national parks across the world, no opportunity of outlay cost would be included, and the cost of the plantation would be the total cost.
However, for a species such as Paulownia, such a method would not yield desired results, as it requires a specific amount of sunlight, and unsystematic plantation would result in high mortality.
Which method should be implemented?
For risk management concerns and environmental concerns regarding some species of Paulownia considered an invasive species, it would be unrealistic to plant just one type of tree around the world to accomplish this objective.
Furthermore, a pest or related attack may emerge that may be very damaging for one particular species. Disease(s) may emerge as well that may be dangerous for one specific species. Planting one plant type across the world wouldn't be possible due to other factors, such as climate and other environmental considerations making other species more suitable for a specific region.
Nevertheless, if one plant type absorbs 10X more CO2 than others, pragmatically, it should be favored as it would substantially increase the efficiency of the C02 absorption project. Therefore, a 40/60 division seems more practical. Planting 200 million Paulownia trees and 92.5 billion mixed variety of trees, as per the suitability of the plantation region, would be a more realistic approach.
Such an endeavor would cost $212 million for planting Paulownias and $93 billion for planting a mixed variety of trees.
Depending on the region and the bureaucracy involved, there may be other operational and idiosyncratic costs involved as well.
Such an endeavor would also require 1.2 million acres for the Kiri trees, and 231 million acres for conventional trees, 231.2 million acres in total.
Broadly, we can claim that planting enough trees globally to achieve carbon neutrality, in an intelligent way, would cost approximately $100 billion. At a global level, this cost cannot be considered exceptionally high, considering the insidious impact of carbon emissions and the damage being caused to fragile ecosystems.
The Kiri or Empress tree would achieve the objective much more efficiently in terms of land required and the costs; however, it may require a higher degree of management and may not be suitable for all regions of the world.
Finally, there would be other benefits of such a project beyond the obvious objective of carbon capture; deforestation and related activities have harmed many ecosystems and species of animals, driving many to extinction; increasing the global forests by 231.2 million acres would mean an increase in global forests comparable to about 14 percent of the size of Amazon Forest, which is 1.7 billion acres in area. Such an increase in forests, if undisturbed, would gradually increase naturally and help endangered species over time, providing them a safe habitat. Trees, through the process of evapo-transpiration, also aid in increasing rainfall, which can help prevent droughts as well. Thus, such a project would have 'positive externalities.'
Chasan, E. (2019). We Already Have the World's Most Efficient Carbon Capture Technology. Bloomberg.
Hannah Ritchie and Max Roser (2020) - "CO₂ and Greenhouse Gas Emissions." Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions' [Online Resource]
Hannah Ritchie and Max Roser (2020) - "Energy." Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/energy' [Online Resource]
Hardie, I., J. Kundt, and E. Miyasaka. 1989. Economic feasibility of U.S. Paulownia plantations. Journal of Forestry 87:19-24.
Hemmerly, T.E. 1989. New commercial tree for Tennessee: Princess tree, Paulownia tomentosa Steud. (Scrophulariaceae). Journal of the Tennessee Academy of Science 64(1):5-8
Graves, D. H. 1989. Paulownia plantation management: a guide to density control and financial alternatives. University of Kentucky Cooperative Extension Service Forestry Extension Series No. 1. Lexington, KY. 32 pp.
Stancil, J. M. (2019). The Power of One Tree - The Very Air We Breathe. U.S. DEPARTMENT OF AGRICULTURE. Retrieved from https://www.usda.gov/media/blog/2015/03/17/power-one-tree-very-air-we-breathe