The study
Thomas W. Crowther et al., The global tree restoration potential, Science 05 Jul 2019: Vol. 365, Issue 6448, pp. 76-79
Abstract
The restoration of trees remains among the most effective strategies for climate change mitigation. We mapped the global potential tree coverage to show that 4.4 billion hectares of canopy cover could exist under the current climate. Excluding existing trees and agricultural and urban areas, we found that there 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. However, climate change will alter this potential tree coverage. We estimate that if we cannot deviate from the current trajectory, the global potential canopy cover may shrink by ~223 million hectares by 2050, with the vast majority of losses occurring in the tropics. Our results highlight the opportunity of climate change mitigation through global tree restoration but also the urgent need for action.
The hype
Unfortunately, the hype begins with the very same issue of Science, in the introduction to the study:
The potential for global forest cover
The restoration of forested land at a global scale could help capture atmospheric carbon and mitigate climate change. Bastin et al. used direct measurements of forest cover to generate a model of forest restoration potential across the globe (see the Perspective by Chazdon and Brancalion). Their spatially explicit maps show how much additional tree cover could exist outside of existing forests and agricultural and urban land. Ecosystems could support an additional 0.9 billion hectares of continuous forest. This would represent a greater than 25% increase in forested area, including more than 500 billion trees and more than 200 gigatonnes of additional carbon at maturity. Such a change has the potential to cut the atmospheric carbon pool by about 25%.
Science, this issue p. 76; see also p. 24
[emphasis mine]
Even more unfortunately, the lead author of the study himself is contributing to the hype in interviews:
Tree planting 'has mind-blowing potential' to tackle climate crisis
Research shows a trillion trees could be planted to capture huge amount of carbon dioxide
Damian Carrington, Environment editor; The Guardian; July 4, 2019
As trees grow, they absorb and store the carbon dioxide emissions that are driving global heating. New research estimates that a worldwide planting programme could remove two-thirds of all the emissions that have been pumped into the atmosphere by human activities, a figure the scientists describe as “mind-blowing”.
[emphasis mine]
“This new quantitative evaluation shows [forest] restoration isn’t just one of our climate change solutions, it is overwhelmingly the top one,” said Prof Tom Crowther at the Swiss university ETH Zürich, who led the research. “What blows my mind is the scale. I thought restoration would be in the top 10, but it is overwhelmingly more powerful than all of the other climate change solutions proposed.”
[emphasis mine]
And of course the hype is picked up and amplified by the popular press, with the following example and the above mentioned story from The Guardian being fairly typical:
Trees could reduce carbon in the atmosphere to levels not seen in nearly 100 years
Stephanie DeMarco, Los Angeles Times, 4 Jul 2019
Those trees would eliminate about two-thirds of the carbon that's in the atmosphere today as a result of human activities, according to a study in Friday's edition of the journal Science.
[emphasis mine]
Unfortunately, all these grandiose claims fail to pass sanity check, as the following two very 'inconvenient truths' (hat tip—or rather deep genuflection--to Al Gore) show.
First inconvenient truth
On the high end (i.e., young trees in the tropics), one tree sequesters about 50 pounds of carbon dioxide per year:
How to calculate the amount of CO2 sequestered in a tree per year
We at Trees for the Future estimate that our agroforestry trees, planted in tropical climates, will sequester atmospheric carbon dioxide at an average of 50 pounds of carbon dioxide per tree per year.
The rate of carbon sequestration depends on the growth characteristics of the tree species, the conditions for growth where the tree is planted, and the density of the tree's wood. It is greatest in the younger stages of tree growth, between 20 to 50 years.
[emphasis mine]
Humans, on the other hand, are adding about 40 billion metric tons of excess carbon dioxide to the atmosphere every year:
Global Carbon Emissions, CO2 Earth
Global carbon (C) emissions from fossil fuel use were 9.795 gigatonnes (Gt) in 2014 (or 35.9 GtCO2 of carbon dioxide).
Based on the above two figures, a simple calculation shows that if we could magically overnight plant one trillion trees, they would sequester in toto about 20 billion metric tons of carbon dioxide per year, or only about half of present human emissions. Even if we assume that the worlds oceans would continue to absorb about a quarter of human emissions, the pool of carbon dioxide in the atmosphere would continue to go up—not down—at a rate of about 10 billion metric tons per year. (Note: It is true that historically the land biosphere has also been absorbing about a quarter of emissions, but I am making the assumption that without human intervention, this rate of uptake is unlikely to continue.)
Second inconvenient truth
The study calls for a large portion of the 500+ billion trees to be planted in the form of boreal forest, especially in Russia. Unfortunately, other research shows that planting boreal forests on ground presently covered with snow or ice, increases rather than decreases warming, by reducing Earth’s albedo, due to the fact that dark tree leaves reflect less incoming sunlight back out to space, than do snow or ice:
A 150-year simulation of worldwide deforestation finds that tropical forests are carbon sinks and boreal forests contribute to warming
Nikhil Swaminathan, Scientific American, 10 April 2007
… these natural carbon sinks [forests] only do their job effectively in tropical regions; in other areas, they have either no impact or actually contribute to warming the planet.
Not everyone is adding to the hype
One good example:
Can Planting Trees Solve Climate Change?
Unfortunately, a new scientific paper overstates forests’ potential
JESSE REYNOLDS, Legal Planet, 5 Jul 2019
Reforestation and “other natural climate solutions” should be part of the diverse toolbox to reduce climate change and manage its risks. But statements and media coverage like this feed the false belief that we could stop climate change– through “natural” [means] to boot — if there were only sufficient public awareness and political will. However, this is not true.
Conclusion
As the old saying goes, "If it seems too good to be true, it probably is," and all the hype around this study does not provide a counterexample.
[Addendum: In the title I referred to tree planting as climate mitigation, but it just occurred to me that that was not really correct, the reason being that, in standard climate-change parlance, mitigation means emissions reduction, whereas direct carbon dioxide removal is generally regarded as a form of geoengineering.]