August 14, 2020 by John Sterman (1), Florian Kapmeier (2)
In 2019 Bastin et al. (2019b) published “The global tree restoration potential” in Science. The paper received intense media coverage because it claimed that planting trees is “among the most effective strategies for climate change mitigation.” The paper and media attention led to plans to plant a trillion trees around the world, accompanied by breathless claims like “…you can easily help to plant trees! No matter when or where you are, it’s never been easier to save the climate and do something good” (Plant-for-the-Planet Foundation, 2020) and “Want to stop climate change? Start by planting a trillion trees” (Parker, 2019).
However, the scientific community, including Friedlingstein et al. (2019), Grainger et al. (2019), Lewis et al. (2019), Rahmstorf (2019), Skidmore et al. (2019), and Veldman et al. (2019), and recently Fagan (2020), Martin et al. (2020), and Taylor et al. (2020) among others, criticized Bastin et al.’s assumptions, findings, and conclusions. In an initial reply, Bastin et al. (2019a) confirmed that the “original estimations are accurate”.
Our team at Climate Interactive also analyzed Bastin et al.’s paper, using the En-ROADS climate and policy simulation model (Jones et al., 2019) (see our September 2019 blog post). We concluded “that afforestation efforts could likely remove only 1-10% of anthropogenic CO2 emissions from the beginning of the industrial revolution until 2100,… in contrast with Bastin et al’s estimates of two-thirds.” Although afforestation and reforestation can have many desirable effects, the idea that planting trees will save us from climate change is incorrect.
Bastin et al. (2020) recently published an erratum acknowledging several errors and overstatements in their original paper (Bastin et al., 2019b) and in their response to early criticisms of it (Bastin et al., 2019a). The erratum acknowledge that “the authors stated in the abstract and in the main text that tree restoration is the most effective solution to climate change to date. This was incorrect.” After detailing other mis-statements and errors, they go on to state that addressing “climate change … will require a full combination of approaches.”
We here examine the study and erratum, addressing the role of forests in mitigating climate change. In our September 2019 blog post we examined what it would take to limit global warming to no more than 2 °C, using En-ROADS, the interactive climate policy simulation model developed by Climate Interactive and the MIT Sloan Sustainability Initiative. We concluded that afforestation can play a role as “part of a broad suite of policies that address climate change, but it is not a silver bullet solution.”
Why not? Limiting global warming to no more than 2 °C, and striving for 1.5 °C, requires a rapid decline in global greenhouse gas (GHG) emissions by 2030, which must reach (near) net zero levels around 2050 (see Rogelj et al. (2018) and in particular Grubler et al. (2018), Holz et al. (2018), and van Vurren et al. (2018) for studies with no or only limited technological CO2 removal (CDR) technologies). The large majority of greenhouse gas emissions today are from the combustion of fossil fuels. There is no chance of limiting warming to 2 °C without cutting fossil fuel emissions, and rapidly (Figueres et al., 2017).
Trees do remove carbon from the atmosphere. But trees grow slowly. Small trees absorb much less CO2 per year than larger ones. It takes decades or more for newly planted trees to become large enough to remove significant amounts of carbon from the atmosphere. Even if an aggressive afforestation and reforestation program were begun today, the annual rate of CO2 removal from such trees would become substantial only in the second half of the century.
Consider the following scenario from En-ROADS, showing the reference case with an aggressive afforestation and reforestation program that begins now, in 2020. The scenario assumes 900 million hectares could be devoted to the program, as suggested by Bastin et al.’s (2019b) analysis. The new trees would remove slightly more than 8 billion tons of CO2 equivalents per year (GtCO2e/yr) in 2100 (Figure 1a). That sounds like a lot, but global GHG emissions were more than 52 GtCO2e/yr in 2018, and, as shown in Figure 1b, without policies to address emissions from fossil fuels, agriculture, and industry, are expected to grow to roughly 110 GtCO2e/yr – roughly 14 times as much as the new trees remove. Figure 1c puts the impact of afforestation in perspective: humanity would have emitted about three times as much CO2 equivalent in 2100 alone than all planted trees would have removed through 2100 (348 GtCO2 by 2100). Despite the aggressive afforestation program, the impact on global warming is small, with global average surface temperature expected to only decrease by one tenth of a degree by 2100 compared to the reference case (Figure 1d).
Figure 1: Impact of aggressive afforestation and reforestation program in En-ROADS assuming “Percent available land for afforestation” = 100% and “Max available land for afforestation” = 900Mha. a) CO2 removals from afforestation; b) GHG emissions by gas, showing afforestation program causes negative emissions from forestry and land use by midcentury; c) Stock of carbon stored by the trees in the afforestation program; d) Temperature change: reference case (black); impact with afforestation (blue).
Some advocates for afforestation and reforestation favor faster-growing species such as Loblolly pine, which are commonly grown in plantations, for example, in the southern United States, where they supply pulpwood for the paper industry and pellets for bioenergy. However, these fast growing species do not take up as much CO2 as the slower growing ones (Sterman et al., 2018a) and have a lower equilibrium carbon density than natural forests (Schulze et al., 2012). Furthermore, plantations generally consist of single species, planted in rows, and are harvested with rotation periods of 20-30 years (Sterman et al., 2018b), which puts most of the carbon they absorbed back into the atmosphere, particularly if used for bioenergy. And plantations do not support the habitat for other species and biodiversity found in natural forests.
Additionally, trees do not live forever. When they are cut and used for fuel, the carbon they slowly removed from the atmosphere is once again released into the air. And the carbon in wood products also returns to the atmosphere; relatively quickly for pulp and paper, somewhat slower for lumber. Even if left standing, trees eventually die and decay, releasing their carbon back into the atmosphere. Fire, disease, insect pests, drought, and storms speed this process (Morehouse et al., 2008). Climate change is increasing the risk of all these sources of tree mortality in many places around the world.
Existing forests absorb about 30% of annual CO2 emissions. Preserving them is crucial. But they, like newly planted forests, face risks from legal and illegal logging (Ferrante et al., 2018; Ferrante et al., 2020), and harvest for bioenergy (Thrän et al., 2017). Deforestation is particularly damaging because the annual CO2 uptake from older, larger trees is higher than younger, smaller trees (Moomaw et al., 2019; Sterman et al., 2018a).
Many regions in the world, including the European Union (European Commission, 2003), still incorrectly claim that burning wood is carbon neutral and allow power plants that burn wood to ignore the CO2 released at the point of combustion in official carbon accounting. Doing so falsely assumes that the wood harvested for bioenergy immediately grows back, offsetting the emissions when that wood is burned. Yes, eventually, a forest harvested for bioenergy might grow back, but regrowth takes time and is not certain. In the meantime, burning wood for bioenergy increases CO2 emissions above what they would have been, even if the wood displaces coal, the most carbon-intensive fuel. The period required for regrowth to offset those excess emissions—the “carbon debt repayment time”—varies from many decades to more than a century, depending on the species and local climatic conditions, even if the wood displaces coal, and is far longer if the wood displaces oil or natural gas (Sterman et al., 2018a, 2018b), as also described here.
The scale of the reforested area that Bastin et al. suggest is immense: 900 million hectares, roughly 2.8 times the size of India (Figure 2). Where will all that land come from? What is that land used for now? Large-scale afforestation would require securing huge areas of land, both public and private. The process would need to be fair, transparent, and address equity. Some clever possibilities exist, such as shade coffee, where the coffee plants grow under a canopy of trees (Takahashi et al., 2013). However, in many places, converting farmland or pasture into forests would compete with crop cultivation or livestock, which might in turn increase food prices (Kreidenweis et al., 2016) and worsen food insecurity, especially for the poor. What about ecologically valuable habitat including grassland and heathland (Martin et al., 2020)? What about the great steppes and tundra areas in the northern hemisphere? Besides issues of soil fertility, water supply, and other resources needed for forests to thrive, forests in many regions lower the albedo of the land compared to unforested land, which could offset to some degree the benefits of carbon uptake by trees (Betts, 2000; Lewis et al., 2019).
Figure 2: 900 Mha of land for planting trees approximates and area three times the area of India (dotted line) as shown in En-ROADS with the same scenario settings as above.
Afforestation and reforestation are often highly desirable. Planting trees can restore land that has been deforested, especially if it is restored as a diverse, multi-species, un-managed forest similar to natural ones, and not as a monoculture of managed plantations. Restored “natural” forests promote biodiversity and limit extinction risk for many species, reduce soil erosion, improve water supplies, and can generate tourism, recreation, and other services to support local economies. In cities, trees provide shade and beauty, and moderate the urban heat island effect.
But even planting a trillion trees, and quickly, would not solve the climate crisis. Worse, it may actually distract our attention, undermine policies, and consume capital needed to address the most important source of global warming: emissions from fossil fuels. Policies and investment should instead promote energy efficiency and renewables like wind and solar—the fastest, safest, and cheapest way to cut fossil fuel emissions and reduce future warming (IRENA, 2020; Lazard, 2019).
You don’t need to take our word for it. Try your own scenarios and assumptions using the interactive En-ROADS climate simulator. You can try different assumptions for afforestation, including how much land to devote to new trees, and how long it takes to acquire the land and plant the trees. Using the Assumptions menu, you can change how much land is potentially available for afforestation and reforestation, how long on average it takes trees to grow, how much carbon can be stored by trees per hectare, and what fraction of the trees die and release their carbon back to atmosphere each year. You can also try a wide range of other policies, actions, and assumptions. See what it would take to limit warming to no more than 2°C. Then share your scenario with your friends and colleagues using the Share Scenario button.
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