What could be better than planting a tree? Leafy, peaceful, beautiful, carbon-sucking, climate-saving miracles, every last one of them, right? Trees have become the poster child for demonstrating commitment to environmental sustainability. There’s a constantly growing list of businesses, politicians, and non-profits, rallying around the idea that we can use forests to climb out of the climate crisis we’ve created through our use of fossil fuels. It sounds great, but are these initiatives actually removing CO2 from the atmosphere? Or is planting trees just good PR? It turns out, it depends on…can you guess? Accounting. Not what you thought, right?
On the face of it. It does make sense. Trees grow by converting CO2, into plant biomass and energy through photosynthesis. Through this process, the circa 3 trillion trees currently on earth are sequestering about a third of the human caused greenhouse gas emissions released each year. Tree planting initiatives are based on the idea of up-scaling this process, but how much can we upscale that? And how can we determine how much CO2 could be stored by those methods? In this video, we’re going to take a look at the different strategies for carbon dioxide removal or CDR that we can achieve using forests, and we’ll look at how scientists today are thinking about the total carbon dioxide storage capacity of forests on a global basis.
There are three main categories of forestry practices that can be used to decrease net greenhouse gas emissions. The first is what is called avoided emissions, or in simpler terms, don’t cut the trees down. In this case, we are avoiding conversion of forested lands to other land uses such as agriculture or pasture land. The largest drivers of deforestation are cattle ranching, soy and palm oil farming, and paper product production. Current global rates of deforestation release about 3.7 gigatons of carbon dioxide annually, about one tenth of global greenhouse gas emissions, mostly from tropical forests.
The second approach is forest management, which is defined as modifying existing land use activities in forested regions to increase their net sequestration of CO2 from the atmosphere. Examples of this type of practice include restoring forests that have been degraded through unsustainable logging practices or naturally induced biomass loss like fire or pest infestation, or increasing the age of trees that are sustainably harvested by extending harvesting rotation rates. A 2019 IPCC special report estimated that between 0.44 and 2.1 gigatons of carbon dioxide equivalent could be removed annually from the global implementation of these kinds of practices.
Finally, there is the forest CDR method that comes with immense marketing appeal: growing new trees. Technical terms for this practice are reforestation, if trees are being replanted in recently deforested land, or afforestation, which is growing trees on land that has not been forested in the last 50 years. Growing new forests has by far the largest potential impact of all forest CDR methods, but it also comes with a significant element of uncertainty.
Conservative estimates of forest carbon sequestration rates that we reported in our summary of different carbon dioxide removal techniques ranged from 0.5 to 3.6 gigatons of CO2 per year. But proponents of natural climate solutions have suggested that as much as 17 gigatons per year could be stored in new forest growth. This uncertainty exists because the amount of carbon we can store a new forest depends on just how much space we are willing to use for that effort.
Before the start of human civilization, we had nearly 6 trillion trees. Up until only 300 years ago, we had removed about 11% of those trees. But in the last 300 years, we’ve removed an additional 34%. Today, we have only 54% of the forests that were covering the planet before the start of human civilization. So the question is really simple: how many of those trees could we put back? Susan Cook-Patton is a senior forest restoration scientist at The Nature Conservancy where she works to quantify the potential of reforestation. She talked to us about her work determining where those trees might go.
Dr. Susan Cook-Patton:
So when you’re thinking about how much mitigation potential you can get from reforestation, there’s two elements you want to keep in mind. The first is how much carbon can you get? And the second is where the heck are you going to put all those trees? So when you’re developing maps of opportunity for restoring tree cover, there’s a few considerations. The first is thinking, where did those trees belong historically? You don’t want to put trees in native grasslands, that’s bad for lots of reasons, bad for biodiversity, and often the trees don’t survive. The next thing that you want to keep in mind is there are some places in the world where trees cause more warming than cooling than you would get from sucking carbon dioxide out of the atmosphere. So you want to avoid those places as well. And then of course you don’t want to put trees where people are actively using the land – places like urban landscapes, all of our agricultural landscapes, for example, we obviously need to eat. But then there’s a lot of area that’s leftover where we could restore to tree cover.
In the United States alone, Dr. Cook-Patton and her colleagues identified 133 million acres of land where we could plant new forests without appreciably decreasing the lands’ utility to humans. That is about 5% of the United States total landmass. If all of that land were to be used, about 300 million tons of carbon dioxide could be removed from the atmosphere each year. That sounds like a lot, but it’s actually only about 6% of the annual emissions in the United States.
Based on Dr. Cook-Patton’s United States study, planting one tree can sequester about 4.9 kilograms of carbon dioxide per year. And on average, one acre of land can hold about 511 trees. The United States greenhouse gas emissions rate in 2019 was 16.06 tons of carbon dioxide per person per year. That means that each United States citizen would need to plant 3, 277 trees or 6.4 acres of new forest to balance their carbon footprint. And for the United States to compensate entirely for their emissions by planting new forest, they would need to create 2.1 billion acres of new forest, which comes to about 86% of the total US landmass area. So we can’t offset all of our emissions with forests, but they can play a role. The good news is that the relationship between sequestration needs and real estate looks better globally than it does just in the United States, because different forests have different carbon densities
Dr. Susan Cook-Patton:
When picking locations where you want to prioritize reforestation. The other thing to keep in mind is the amount of carbon you can get really varies across the globe. We find over a hundred fold variation across the board with our own research. For example, trees in the tropics tend to grow a lot faster than trees in the boreal.
Using almost 11,000 measurements of carbon uptake from regrowing forests in 250 studies around the world, Cook-Patton and her colleagues calculated that the biophysical maximum estimate of global reforestation potential would sequester about 8.9 gigatons of carbon dioxide per year, or about a quarter of the current global emissions rate. It would require converting about 5% of global land into forests. But as some of that land is currently used to support livestock, making full use of forestation’s potential requires global dietary shifts toward more plant-based diets. Cook-Patton and her colleagues propose a more realistic maximum estimate of forest carbon sequestration of about 5.9 gigatons of CO2 per year, which is based in part on existing national commitments from countries to regrow forests.
So if we can find the space to at least partially mitigate our greenhouse gas emissions with forestry growth and improved management, then what is wrong with politicians and companies saying they are doing it? Let’s be clear. There is nothing wrong with planting trees and regrowing forests. We need to do this and we need to do a lot of it. The problem comes when we try to measure how much of our carbon footprint we are offsetting when we plant those new trees. And there are some specific challenges when it comes to this point. These are the issues of permanence, leakage, and additionality.
Permanence is the idea that when we remove carbon dioxide from the atmosphere, it needs to stay removed. The likelihood of achieving permanence can be increased by making forests less vulnerable. Some examples of this include increasing diversity in forests to make them less susceptible to disease outbreak and insect infestation, as well as widening space between trees and removing dead wood to mitigate forest fire spread, and planting thicker tree species to foster resilience to forest fires. To properly account for the carbon captured by trees requires a commitment to trace the growth and preservation of new forests now, and over ensuing decades. As a reasonable approximation for measuring the amount of carbon stored by each tree, we can use remote sensing technologies to measure forest growth and health on a per acre basis.
Additionally, it is very difficult to assess whether halting deforestation activities, or replacing pasture land in one place won’t lead to the transfer of logging to a different forest or conversion somewhere else to make new pasture land. This is the concept of leakage, and like the concept of permanence, it makes it extremely difficult to assess whether the good work of managing or growing forests is actually resulting in a measurable decrease in atmospheric CO2.
Finally, there’s the concept of additionality. Additionality addresses whether specific emissions reductions, or CO2 removals that are claimed by a given entity would have happened anyway without the intervention by that entity. This particularly applies to limiting deforestation or reducing harvesting rates in existing forests. In order to prove that a company or an individual’s investment in forest management prevented the loss of forest stock, we would have to know what the intentions of the landowner were prior to receiving any financial incentive not to harvest. Essentially this requires being able to predict the future. We simply can’t do it. While we cannot predict what a landowner might do, we can give landowners the tools and resources to better manage their land or seek to protect lands that are more likely to be converted.
The results of all of this is that while protecting, managing, and growing forests can significantly reduce concentrations of carbon dioxide in the atmosphere and should absolutely be done, the act of planting a tree to compensate for your emissions isn’t enough. Companies should absolutely support forest growth, but instead of focusing on offsetting their emissions, they should focus on reducing their emissions to zero.
Ep 2.4 Forest CDR citations
00:52, 3:48 Crowther et al. (2015) Mapping tree density at a global scale. Nature 525:201-205
00:57 Ciais et al. (2013) Chapter 6: Carbon and Other Biogeochemical Cycles. IPCC 5th AR, Working Group 1
1:42, 2:15, 2:34 NAP: Negative Emissions Technology and Reliable Sequestration: A Research Agenda (2019). Chapter 3: Terrestrial Carbon Removal and Sequestration.
1:50, 1:59 Ritchie, H. and M. Roser. Deforestation and Forest Loss. Our World In Data. https://ourworldindata.org/deforestation
Curtis, P. et al. (2018) Classifying drivers of global forest loss. Science
1:59 Baccini et al. (2012) Estimated carbon dioxide emissions from tropical deforestation improved by carbon-density maps, Nature Climate Change 2:182-185.
2:20 U.S. Forest Service Images of Forest Restoration from NPR.org (2012) “Our changing forests: an 88-year time lapse” https://www.npr.org/sections/pictureshow/2012/08/23/159614784/our-changing-forests-an-88-year-time-lapse
2:27 Hancock, L. WWF. What is forest degradation and why is it bad for people and wildlife? https://www.worldwildlife.org/stories/what-is-forest-degradation-and-why-is-it-bad-for-people-and-wildlife
Family Owned Forests: How to unlock the carbon potential in America’s backyard. Benefits of improved forest management. https://www.forestfoundation.org/stuff/contentmgr/files/1/2c45ad7945b0a0647378ab0d49acd2ab/miscdocs/aff_2020carbonreport_final.pdf
2:34 Fargione et al. (2018) Natural climate solutions for the United States, Science Advances 4(11):eaat1869
2:41 IPCC (2019) Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems. [Shukla et al., eds.]
3:09, 3:18 Fuss et al. (2018) Negative Emissions – Part 2: Costs, potentials and side effects. Environ. Res. Lett. 13:063002
3:30 Griscom et al. (2017) Natural climate solutions. PNAS 114:11645-11650
3:40, 8:02 Cook-Patton, S.C. et al. (2020) Mapping carbon accumulation potential from global natural forest regrowth, Nature 585:545-550
3:58 Mackey et al. (2015) Policy options for the worlds primary forests in multilateral environmental agreements, Conservation Letters 8:139-147
4:10 Note: Currently, forests store about 372 gigatons of carbon (Spawn et al., 2020, Nature Scientific Data 7:112). They report 409 Pg total vegetation biomass (woody + herbaceous), and 37.4 Pg total herbaceous biomass. 409 – 37.4 = 371.6 Pg = 371.6 GtC in woody biomass (above and below ground). So in order to sequester the approximately 270 gigatons of anthropogenic carbon currently in the atmosphere (Ciais et al., 2013, IPCC AR5), we would have to regrow about 85% of the forests that we destroyed (372 Gt C = 0.54*(original trees) → original trees = 689 Gt C. → trees lost = 689-372 = 317 Gt C. → anthropogenic carbon/trees lost = 270/317 = 0.61).
4:48 Betts, R. (2000) Offset of the potential carbon sink from boreal forestation by decreases in surface albedo. Nature 408: 187–190. https://www.nature.com/articles/35041545
5:22 Cook-Patton, S.C., et al. (2021) Lower cost and more feasible options to restore forest cover in the contiguous United States for climate mitigation, One Earth 3:739-752
5:27 Google.com: US land area = 3.797 million mi2 = 2.43 billion acres
6:09 300 million tons of carbon dioxide, equivalent to: 3% of global annual total CO2 needing to be removed; or taking 72 million cars off the road (289.5 million total cars in the US in 2021 – https://financesonline.com/number-of-cars-in-the-us/)
5:35, 5:50, 5:56 Cook-Patton et al. https://www.reforestationhub.org/
5:40, 6:06, 7:28 https://ourworldindata.org/co2-emissions
7:14 Pearce, F. Is it better to plant trees or let forests regrow naturally? Wired.com, published Oct. 30, 2020
7:25 The conversion for Cook-Patton et al. (2020)’s reported 2.43 Pg C/yr = 8.91 Gt CO2/yr
7:44 Maximum estimate uses 678 million hectares (C-P20, p. 547); global land area = 13,003 million hectares (https://ourworldindata.org/land-use)
7:54 The conversion for Cook-Patton et al. (2020)’s reported 1.60 Pg C/yr = 5.87 Gt CO2/yr
8:41, 10:15 Chagas et al. (2020) A close look at the quality of REDD+ carbon credits. Published by: Climate Focus. https://www.climatefocus.com/publications/close-look-quality-redd-carbon-credits
8:55 Frankel, S. et al. (2012) Forest tree diseases and climate change. Published by: The USDA Climate Change Research Center. https://www.fs.usda.gov/ccrc/topics/forest-disease
9:05 Malcolm, N. et al. (2019) Tamm Review: reforestation for resilience in dry western U.S. forests. Forest Ecology and Management, 432: 209-224. https://www.fs.fed.us/psw/publications/north/psw_2019_north002.pdf
9:45 USDA: Office of Sustainability and Climate. Forest Carbon FAQs. https://www.fs.usda.gov/sites/default/files/Forest-Carbon-FAQs.pdf