By Chris Lang
In July 2019, Science magazine published a paper titled, “The global tree restoration potential”. The paper was written predominantly by scientists from the Crowther Lab at ETH–Zürich, and from the UN Food and Agriculture Organisation. The paper resulted in a very large amount of media coverage, and some criticism from climate scientists.
The paper claimed that,
“[T]here is room for an extra 0.9 billion hectares of canopy cover, which could store 205 gigatonnes of carbon in areas that would naturally support woodlands and forests. This highlights global tree restoration as our most effective climate change solution to date.“
Today, Science magazine published four technical comments and three letters in response to the Crowther Lab’s paper. The lead author of the paper was Jean-Francois Bastin, of the Crowther Lab.
This post looks at each of the technical comments – and at Bastin et al.’s response.
We believe this conclusion is wrong because of the authors’ misunderstanding of both carbon storage potential and the global carbon cycle response to anthropogenic emissions.
They note that “regardless of the exact amount of carbon that could be stored via forest restoration, this solution can only temporarily delay future warming”. They write that,
Without radical reductions in fossil carbon emissions, forest restoration can only offset a share of future emissions and has limited potential. The only long-term and sustainable way to stabilize the climate at any temperature target is to reduce anthropogenic CO 2 emissions to zero.
Friedlingstein et al. conclude that,
Bastin et al. strongly overestimate the potential of forest restoration to mitigate climate change. The claim that global tree restoration is our most effective climate change solution is simply incorrect scientifically and dangerously misleading.
False assumptions and overestimates
We show that Bastin et al. (i) overestimated soil carbon gains from increased tree cover by a factor of 2; (ii) modeled new tree cover in regions where trees reduce albedo and increase climate warming; and (iii) relied heavily on afforestation of grasslands and savannas – biodiverse ecosystems where fires and large herbivores have maintained low tree cover for millions of years.
The proposed afforestation of grasslands and savannas is particularly problematic. Veldman et al. write that,
The carbon sequestration estimate of Bastin et al. is also dependent on the false assumption that natural grasslands and savannas with fewer trees than predicted by their statistical model are “degraded” and in need of restoration. Ecological restoration of savannas and grasslands rarely involves planting trees, and more often requires tree-cutting and prescribed fire to promote biodiversity and ecosystem services.
Veldman et al. conclude that,
Although ecological restoration, if carefully implemented, can have a role in mitigating climate change, it is no substitute for the fact that most fossil fuel emissions will need to stop to meet the targets of the Paris Agreement. Such action should be accompanied by policies that prioritize the conservation of intact, biodiverse ecosystems, irrespective of whether they contain a lot of trees.
They point out that Bastin et al.’s assumption for the amount of carbon that can be stored per hectare by tree planting is “double previously published estimates”. Bastin et al. do not explain this anomaly.
Lewis et al. write that,
Bastin et al. state in their abstract that “[our study] highlights global tree restoration as our most effective climate change solution to date.” This statement is not supported by the evidence provided. Furthermore, such a statement can never be supported because (i) in physical terms, keeping fossil carbon in its original geological storage is self-evidently a more effective solution to climate change than releasing it and capturing it later in trees; (ii) allowing trees to grow where they once grew is largely merely replacing carbon that was previously lost through land-use change, and so does not address fossil fuel emissions; and (iii) sequestering ~100 GtC into new forests is equivalent to just 10 years of current emissions, which clearly shows that forest restoration is of lower importance than rapidly reducing fossil fuel emissions.
Grainger et al. point out that Basin et al. “neglect considerable research into forest-based climate change mitigation during the 1980s and 1990s”. Grainger et al. note the need to address “social and environmental issues” in tree restoration, but are in favour of planting large areas of the planet with trees and do not challenge the findings of Bastin et al.
If Bastin et al.’s paper gives new impetus to using forests to mitigate climate change, then the results of this early research can finally be used for the purpose for which they were originally intended.
“Original estimations are accurate”
Bastin et al.’s response argues that “our original estimations are accurate”.
They have however “corrected” the abstract of their paper:
Before: “This highlights global tree restoration as our most effective climate change solution to date.”
After: “This highlights global tree restoration as one of the most effective carbon drawdown solutions to date.”
In their response, Bastin et al. write,
By revealing the potential of restoration as a carbon drawdown solution, our study does not preclude the urgent need to reduce greenhouse gas emissions from the combustion of fossil fuels, from deforestation and forest degradation. There is no compromise or trade-off between the two.
That may be true, but it completely ignores the recent announcements from oil companies Shell and Eni about their plans to plant trees in order to offset a small part of their emissions, while continuing to expand their polluting oil and gas operations.
But Bastin et al.’s original paper “The global tree restoration potential” makes no mention whatsoever of fossil fuels.
If Bastin et al.’s response is to be taken seriously, it exposes that the scientific community cannot agree on how to measure the carbon stored in trees. For example, Bastin et al. deny overestimating how much carbon could be stored in trees. The studies cited by Lewis et al. are “based on a different definition of forest”, they write.
Bastin et al. “completely agree” that planting trees in some regions could increase climate warming by changing albedo. But “calculating the changes in albedo and evapotranspiration associated with restoration is beyond the scope of the present study”.
So, er, tree planting in some places would make the climate crisis worse. Yet the authors, who claimed that “trees could save the climate” didn’t consider this important enough to include in their study.
Regarding Veldman et al.‘s point about tree planting in drylands and savannas, Bastin et al. write that,
Generally, we much highlight that our analysis does not ever address whether any actions “should” or “should not” take place. Our analysis simply estimated the biophysical limits of global forest growth by highlighting where trees “can” exist.
In the conclusion to their response, Bastin et al. write
As a scientific contribution, we do not state what “should” be done at any location around the world, but instead highlight what is possible.
Bastin: You can plant trees everywhere
Recently, The Economist interviewed Tom Crowther of ETH-Zürich’s Crowther Lab. Crowther admitted that capturing 205 GtC “would be very slow, it would be over 200 years”.
Crowther added that, “It’s not like 200 gigatons are really going to come out of the atmosphere immediately, it would be amazing if we even reached 10% of that full potential.”
Crowther also told The Economist that, “We would never, ever propose that tree planting works everywhere.”
It presumably slipped Crowther’s mind that lead author Jean-Francois Bastin proposed exactly that in a Crowther Lab publicity video released the same day as the study:
“And this is a beautiful thing, just to think that in order to fight climate change what you have to do is to plant trees, and you can do that everywhere.”