Technical Comments

Response to Comment on “The global tree restoration potential”

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Science  29 Nov 2019:
Vol. 366, Issue 6469, eaaz0493
DOI: 10.1126/science.aaz0493


Our study quantified the global tree restoration potential and its associated carbon storage potential under existing climate conditions. Skidmore et al. dispute our findings, using as reference a yearly estimation of carbon storage that could be reached by 2050. We provide a detailed answer highlighting misunderstandings in their interpretation, notably that we did not consider any time limit for the restoration process.

Skidmore et al. (1) calculated the yearly carbon storage associated to restoration from numbers provided in the IPCC special report Global Warming of 1.5°C (2) and projected them for 30 years. They use this to suggest that our estimates of total potential carbon storage in restoration areas are too high (3). Based on their analysis, they claim that we overestimated the amount of carbon that can be captured in restored trees by a factor of 3. Although we understand their concerns, we highlight several misunderstandings in their interpretation of the analysis that invalidate their comment.

First, in our analysis, we never considered any time limit for the restoration process. Indeed, our study estimates the total carbon stock that can be associated to the optimal state of each ecosystem. As mentioned in the main text of our paper, the accumulation of carbon in forests would take a very long time to reach maximum potential carbon storage—considerably greater than the 30-year analysis (3). Consequently, the comparison proposed by Skidmore et al. is not valid.

We stress that our estimations of carbon stock were not calculated from simulated values of yearly accumulation of carbon. Instead, our values correspond to ground-based estimates of the extent of carbon stored in mature forests. Yearly accumulation could eventually be compared to our numbers if the related simulation includes the appropriate nonlinear carbon accumulation and a stationary phase associated to the mature state of the ecosystem. The number of years required to reach this stationary phase should then be calculated for each pixel and then converted in a total increase of carbon stocks. This was not done in the analysis proposed by Skidmore et al.

In the second part of the comment by Skidmore et al., there is a critical misunderstanding. Contrary to what they suggest, our estimation of the 205 gigatonnes of carbon (GtC) that could be captured in restored ecosystems certainly does not reflect the “remaining budget” that we should overcome in order to limit global warming to 1.5°C. Instead, that is the amount of carbon that could potentially be stored in the vegetation and soil of terrestrial ecosystems if forests were restored across all available land. This is a misunderstanding of both our analysis and the IPCC special report (2).

We would like to reexplain the main principles of our analysis. Simply put, we provide a quantitative estimation of the tree cover that could be naturally reached in any place of the world if the human activity were close to null [figure 2A of (3)]. As such, we assessed the natural forest potential of the planet under existing climate conditions. These natural conditions were approximated in building our model only considering protected areas. Here, it is important to note that in contrast to the suggestion of Skidmore et al., the representativeness of each biome in protected areas was very good—a point illustrated by the extremely low level of extrapolation versus interpolation in our methods section [see figure S12 of (3)]. To realistically quantify areas that can potentially be restored, we then removed currently existing forest, urban, and cropland areas from the total forest potential. This gives the “global tree restoration potential” illustrated in figure 2, B and C, of (3). The area of canopy cover (and not forest cover) available for restoration—i.e., 900 million hectares—was then used to estimate the total amount of carbon stored. As previously mentioned, our estimation of 205 GtC does not correspond to the sum of the yearly uptake of carbon in vegetation. It corresponds to the quantity of carbon associated to the mature state of each ecosystem considered. This calculation was done using numbers published in the literature based on field plot observation in each ecosystem (4, 5). We hope that this explanation helps to clarify our approach.

Finally, our colleagues caution against interpreting these potential restoration estimates as a guide for climate action. Here we must again disagree. Although we would never want to discourage investment in all other climate solutions, we believe that continuing to underappreciate the value of restoration for climate mitigation, as well as for biodiversity and human well-being, is a greater danger. Until now, most of our understanding of restoration potential stemmed from Earth system models with high uncertainties (6) or from “expert opinion” pieces (7) that cannot reflect the full global potential for carbon capture. Quantitative global approaches based on observations, as presented in our study, are needed to understand and promote restoration as one of the most promising solutions at our disposal in the fight against the global threats of climate change and biodiversity loss.


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