Planting trees for climate mitigation?

18 Oct 2019

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Planting trees for climate mitigation ? (200 billion tons of carbon stored using a trillion trees?)

Original Science Paper (sci-hub pdf)

Main claims of the paper:

  1. 205 gigatonnes can be captured by creating an extra 0.9 billion hectares of canopy cover [claim 1]
  2. global tree restoration as our most effective climate change solution to date [claim 2]
  3. climate change will drive the loss of 450 million hectares of existing tropical forest by 2050 [claim 3]
  4. live stem, leaf, root, and necromass are all also zero prior to restoration [claim 4]
  5. future environmental change will have no impact on forest carbon stocks [claim 5]
  6. reaching this maximum restoration potential [205 GtC] would reduce a considerable proportion of the global anthropogenic carbon burden (~300 GtC) to date [claim 6]

Rebuttals:

Blogpost Stefan Rahmstorf (16 July 2019)

Blogpost Stefan Rahmstorf

-> “paper was immediately welcomed by those who still dream of climate mitigation that doesn’t hurt.”

-> “Unfortunately, it is [claim 2] also too good to be true.”

Introduction : anthropogenic CO2 emissions and sequestration by natural systems : correction of claim 6

Effect of reducing 200 GtC from the atmosphere : necessity to take into account natural buffers (forests, oceans)

Time-dimension of reforestation

Climate mitigation lies in reducing emissions of energy, heating and transport systems

Responses to original paper in Science (18 October 2019)

Simon Lewis et al.

Science Commentary

  1. Capturing 205 GtC on 0.9 billion hectares [claim 1] is a too high estimate, because:
    • “Anthropogenic land-use change, since 1750, has emitted 200 GtC” –> unlikely that restoring only a fraction of previously deforested land will itself sequester 205 GtC
    • “Using a fully coupled climate model to restore 100% of agricultural land predicts 240 GtC uptake on 2020 Mha of land” –> only 107 GtC on 0.9 billion hectares
    • “Using mapped restoration areas regenerating to climatically supportable carbon stock levels results in 108 GtC sequestered over 0.9 billion hectares”
    • “Mapping exercise that identified restoration areas, with no economic constraints and published sequestration rates, gave a median estimate of 89 GtC when scaled to 0.9 billion hectares”
  2. Assuming that ive stem, leaf, root, and necromass are all zero prior to restoration [claim 4] is not correct, because:
    • “substantial proportion will have some trees already”
    • “Recent restoration potential analysis showed 42.1 GtC uptake from restoring 350 Mha of forest, in addition to the 10.6 GtC already in biomass on the land prior to restoration. Using the Bastin et al. method, sequestration would thus have been overreported by ~25%”
  3. Future environmental change [claim 5] needs to be accounted for to estimate forest carbon stocks
    • “Higher atmospheric CO2 concentrations increase carbon stocks more than higher temperatures and altered rainfall reduce it. Given the intrinsic uncertainty of future emissions, this impact could be as low as ~7% under a low-emission scenario, or as high as a ~20% increase under a mid-range scenario for the tropical forest biome.” Taking the latter:
    • ~92 GtC is expected to be sequestered in 0.9 billion hectares of new forest cover = less than half the Bastin et al. estimate.
  4. Reaching a maximum restoration potential [205 GtC] would reduce a considerable proportion (67%) of the global anthropogenic carbon burden (~300 GtC) to date [claim 6] is not correct:
    • Human actions have added 640 GtC since 1750; thus, 205/640 is 32%.
    • “~55% of anthropogenic CO2 emissions were removed from the atmosphere into land and ocean sinks”
  5. global tree restoration as our most effective climate change solution to date [claim 2] is false, because:
    • in physical terms, keeping fossil carbon in its original geological storage is self-evidently a more effective solution
    • 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
    • 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

Pierre Friedlingstein et al.

Science Commentary

  1. [claim 6] is false, as this is “only about half of the historical anthropogenic emissions of about 600 GtC”
  2. [claim 1] is false, because they “ignore the carbon that is currently stored in those regions”
  3. “forests affect climate through biophysical feedbacks, such as changes in albedo or evapotranspiration, which can counteract the cooling effect from CO2 uptake”. “biophysical feedbacks were not discussed in the article and could substantially reduce the potential of forest reforestation in some of the considered regions”
  4. [claim 2] is false, as “The only long-term and sustainable way to stabilize the climate at any temperature target is to reduce anthropogenic CO2 emissions to zero (over the coming 30 to 50 years to meet the temperature targets of the Paris Climate Agreement)”

Joseph Veldman et al.

Science Commentary

  1. [claim 1] is false because of a “~98 GtC overestimation of potential carbon sequestration”:
    • “They mistakenly assumed that treeless areas have no soil organic carbon (SOC) and that SOC increases in direct (1:1) proportion to tree cover. The contribution of SOC to total carbon stocks is substantial in most terrestrial ecosystems.”
    • Humid tropical savannas: 86% of all carbon is in soils
    • Boreal forests: 64% of carbon occurs in soils
    • North American grasslands store as much carbon in soils as tropical forests store as biomass
  2. Bastin et al. did not account for the warming effect of trees due to decreased albedo
    • “trees planted in low-latitude, semi-arid regions can produce net warming for decades before carbon sequestration benefits are realized”
  3. Bastin et al. did not account for fire : “any statistical approach to predict tree cover at intermediate precipitation (500 to 2500 mm annually) must include the effects of fire and, where they still exist, large grazing and browsing animals”
  4. Bastin et al.’s model suggesting grasslands and savannas as potential sites for restoration using trees is inaccurate and misguided:
    • “Earth’s savannas and grasslands predate humans by millions of years; their formation is a result of complex ecological and evolutionary interactions among herbaceous plants (grasses and forbs with extensive roots and underground storage organs), environmental change (climatic cooling, drying, changes in atmospheric CO2), fires (first ignited by lightning, then by people), and large herbivores””
  5. [claim 2] is false :
    • “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”

Alan Grainger et al.

Science Commentary

The authors mainly point out the lack of linkages with important historical research and data (1980s and 1990s: REDD+, IPCC reports, …) on the subject, and refer to another commentary on the paper of Chazdon and Brancalion who point out that “an assessment of the biophysical capacity for restoring global tree cover provides a necessary but insufficient foundation for evaluating where tree cover can be feasibly increased”

Response of Bastin et al. (18 October 2019)

Science Technical Comments

  1. Response to criticism on [claim 2] : “we intended to highlight that we are aware of no other viable climate change solution that is quantitatively as large in terms of carbon drawdown. We did not suggest that tree restoration should be considered as the unique solution to climate change. To avoid this confusion, we have corrected the abstract accordingly.”
  2. Response to criticism on claim 6 that restored trees cannot capture two-thirds of anthropogenic carbon emissions:
    • “Absolutely recognize the constant airborne fraction of 45%” (after accounting for ocean and forest capture), but state that their estimate of ~300 GtC “certainly does not contradict their point” because “if we could store an extra 205 GtC in newly formed ecosystems, this process would indeed reduce a considerable amount of the excess carbon that resides in the atmosphere following human activity”
  3. Response to criticism on claim 1 (estimate of 205 Gt):
    • Discrepancies because:
    • “misinterpretations or confusion between the definitions of forest cover and associated carbon pools”
      • “Three of the four examples provided are based on a different definition of forest: namely forest area, rather than tree canopy cover. Global forest area (land containing at least 10% tree cover) across the globe is considerably larger than global tree canopy cover (cumulative tree cover)”
      • “numbers provided by Lewis et al. in their restoration study (7) concern only two of the five carbon pools for vegetation ecosystems (aboveground and belowground plant biomass)” –> need to “all five pools of carbon, including soil, litter, and dead wood”
    • “lack of sufficient detail in the original manuscript on how existing carbon in potential restoration areas was removed for estimating the global restoration potential” - Response to Lewis et al. argument of storage potential being unrealistic because “anthropogenic land-use change since 1750 has emitted only 200 GtC in total”:
    • “In many regions, substantial deforestation occurred before 1750”
    • Admit uncertainty : “uncertainty only highlights the need for more quantitative analyses, such as the present study, that are needed to refine these early IPCC estimates”
  4. Response to effects of albedo (Veldman et al.):
    • “agree that changes in forest cover resulting from restoration would also affect the climate through a range of mechanisms including changes in surface albedo and evapotranspiration”
    • “calculating the changes in albedo and evapotranspiration associated with restoration is beyond the scope of the present study”
  5. Response to criticism of considering drylands for forest restoration (Veldman et al.), disagree because:
    • “Veldman et al. neglect that large areas of dryland that are classed as savannas have, since the 1970s, been designated by UNEP as suffering from various degrees of vegetation and soil degradation, called desertification”
    • “Research since 1990 has suggested that restoring tree cover that would naturally exist on these lands would help in soil restoration”
    • “Indeed, the current climate differs from the climate of past decades, leading to a natural increase in land available for tree cover in some regions” - “Veldman et al. stress that our model had low predictive power across many of the open-canopy biomes, suggesting that it fails to account for natural fire and the presence of large mammals”
    • “natural fires and large mammals exist in protected areas. They are therefore indirectly accounted for in our model”
    • “as natural fire cannot be distinguished from human-made fire, it cannot be accounted for as a variable of the model to extrapolate the natural tree cover outside protected areas”
  6. Response to criticism of not accounting for uncertain future (Lewis et al.):
    • “agree that the uncertainty in our estimates is important, but this is already fully recognized in our paper”
    • “Our risk assessment is an extrapolation, not an interpolation, and there are considerable uncertainties in our model and in future climate projections”
    • “when considering future changes in vegetation, it is important to recognize the importance of feedbacks. We stress in the paper that “it is possible that elevated CO2 concentrations under future climate scenarios might enhance the growth of those existing trees.””
    • “Such feedbacks must be considered using process-based biogeochemical models to fully represent the mechanisms underpinning the future changes in vegetation”