Carbon removals from nature restoration are no substitute for steep emission reductions

Abstract

•A “responsible development” approach to nature restoration minimizes land-use change•We assess the responsible potential for land removals at 103 GtC over the century•Land removals cannot be scaled up quickly enough to noticeably reduce peak global temperatures•Nature restoration is crucial but cannot offset fossil fuel emissions for net zero Growing commitments to net-zero emissions by 2050 to achieve the Paris Agreement goals are a welcome step forward on climate action but have also seen an increasing focus on nature restoration to remove carbon dioxide from the atmosphere. This risks over-relying on land for mitigation at the expense of phasing out fossil fuels. At the same time, a wide range of activities are being labeled “nature restoration,” some of which, such as monoculture tree plantations, degrade nature—destroying biodiversity, increasing pollution, and removing land from food production. We apply a “responsible development” framing to imagine a constrained approach to nature restoration, guided by ecological principles. Quantifying the resultant carbon uptake and temperature impacts shows that nature restoration can marginally lower peak warming, but any climate benefits are dwarfed by the scale of ongoing fossil fuel emissions. We conclude that more “zero” and less “net” is required for 2050 climate targets. The role of nature restoration in mitigating the impacts of climate change is receiving increasing attention, yet the mitigation potential is often assessed in terms of carbon removal rather than the ability to meet temperature goals, such as those outlined in the Paris Agreement. Here, we estimate the global removal potential from nature restoration constrained by a “responsible development” framework and the contribution this would make to a 1.5°C temperature limit. Our constrained restoration options result in a median of 103 GtC (5%–95% range of −91 to 196 GtC) in cumulative removals between 2020 and 2100. When combined with deep-decarbonization scenarios, our restoration scenario briefly exceeds 1.5°C before declining to between 1.25°C and 1.5°C by 2100 (median, 50% probability). We conclude that additional carbon sequestration via nature restoration is unlikely to be done quickly enough to notably reduce the global peak temperatures expected in the next few decades. Land restoration is an important option for tackling climate change but cannot compensate for delays in reducing fossil fuel emissions. The role of nature restoration in mitigating the impacts of climate change is receiving increasing attention, yet the mitigation potential is often assessed in terms of carbon removal rather than the ability to meet temperature goals, such as those outlined in the Paris Agreement. Here, we estimate the global removal potential from nature restoration constrained by a “responsible development” framework and the contribution this would make to a 1.5°C temperature limit. Our constrained restoration options result in a median of 103 GtC (5%–95% range of −91 to 196 GtC) in cumulative removals between 2020 and 2100. When combined with deep-decarbonization scenarios, our restoration scenario briefly exceeds 1.5°C before declining to between 1.25°C and 1.5°C by 2100 (median, 50% probability). We conclude that additional carbon sequestration via nature restoration is unlikely to be done quickly enough to notably reduce the global peak temperatures expected in the next few decades. Land restoration is an important option for tackling climate change but cannot compensate for delays in reducing fossil fuel emissions. The potential for atmospheric carbon-dioxide removal (CDR) is a growing area of research, and the Intergovernmental Panel on Climate Change (IPCC) Special Report on 1.5°C has confirmed that some level of CDR will be essential for limiting warming to 1.5°C, or even below 2°C, above pre-industrial levels.1Riahi K. Schaeffer R. Arango J. Calvin K. Guivarch C. Hasegawa T. Jiang K. Kriegler E. Matthews R. Peters G.P. et al.Mitigation pathways compatible with long-term goals.in: Climate change 2022: mitigation of climate change. Intergovernmental Panel on Climate Change, 2022https://www.ipcc.ch/report/ar6/wg3/Google Scholar Even the most ambitious decarbonization pathways rely on (lower) levels of CDR.2Grubler A. Wilson C. Bento N. Boza-Kiss B. Krey V. McCollum D.L. Rao N.D. Riahi K. Rogelj J. De Stercke S. et al.A low energy demand scenario for meeting the 1.5 °C target and sustainable development goals without negative emission technologies.Nat. 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Contested framings of greenhouse gas removal and its feasibility: Social and political dimensions.WIREs Clim Change. 2020; 11: e649https://doi.org/10.1002/wcc.649Crossref Scopus (27) Google Scholar as well as to the potential for what has been referred to as “mitigation deterrence,” whereby expectations for future removals delay or replace near-term emission reductions.9McLaren D.P. Tyfield D.P. Willis R. Szerszynski B. Markusson N.O. Beyond “net-zero”: A case for separate targets for emissions reduction and negative emissions.Front. Clim. 2019; 1: 4https://doi.org/10.3389/fclim.2019.00004Crossref Scopus (59) Google Scholar While recent studies have sought to understand the upper bounds for removals via nature restoration,10Bastin J.-F. Finegold Y. Garcia C. Mollicone D. Rezende M. Routh D. Zohner C.M. Crowther T.W. The global tree restoration potential.Science. 2019; 365: 76-79https://doi.org/10.1126/science.aax0848Crossref PubMed Scopus (674) Google Scholar, 11Erb K.-H. Kastner T. 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Our study goes beyond existing literature that assesses the temperature impacts of nature restoration13Matthews H.D. Zickfeld K. Dickau M. MacIsaac A.J. Mathesius S. Nzotungicimpaye C.-M. Luers A. Temporary nature-based carbon removal can lower peak warming in a well-below 2 °C scenario.Commun. Earth Environ. 2022; 3: 65https://doi.org/10.1038/s43247-022-00391-zCrossref Scopus (4) Google Scholar by also interrogating the land-use removal options. We do this by differentiating between activities that restore degraded lands or forests and other AR, a distinction that is rarely made in the literature. On the basis of this distinction, we develop five land-management pathways to assess an ambitious potential for ecosystem restoration (beyond what countries have pledged) while assessing a constrained potential for reforestation (to the extent that countries have pledged). This distinction matters because reforestation requires a land-use change and therefore presents more risks and potential trade-offs than restoring degraded lands while maintaining existing land uses. We suggest that this combination presents the maximum responsible land-restoration potential that is available to contribute to climate mitigation. On the basis of this removal potential, we assess the impacts on peak warming and century-long temperature reduction. Given that healthy ecosystems are critical for combating climate change,14Lade S.J. Norberg J. Anderies J.M. Beer C. Cornell S.E. Donges J.F. Fetzer I. Gasser T. Richardson K. Rockström J. Steffen W. Potential feedbacks between loss of biosphere integrity and climate change.Glob. Sustain. 2019; 2: e21https://doi.org/10.1017/sus.2019.18Crossref Google Scholar we rely on principles of ecological restoration to guide land interventions that are inherently beneficial—to climate, biodiversity, and people—thereby building ecological resilience and human capacity.15Stefanes M. Ochoa-Quintero J.M. Roque F.D.O. Sugai L.S.M. Tambosi L.R. Lourival R. Laurance S. Incorporating resilience and cost in ecological restoration strategies at landscape scale.E&S. 2016; 21: 54https://doi.org/10.5751/es-08922-210454Crossref Google Scholar, 16Lewis S.L. Wheeler C.E. Mitchard E.T.A. Koch A. Restoring natural forests is the best way to remove atmospheric carbon.Nature. 2019; 568: 25-28https://doi.org/10.1038/d41586-019-01026-8Crossref PubMed Scopus (274) Google Scholar, 17Seddon N. Turner B. Berry P. Chausson A. Girardin C.A.J. Grounding nature-based climate solutions in sound biodiversity science.Nat. Clim. Change. 2019; 9: 84-87https://doi.org/10.1038/s41558-019-0405-0Crossref Scopus (96) Google Scholar A simplistic typology of these key characteristics is depicted in Table 1, which is used to guide the selection of five land-management pathways that represent different approaches to ecosystem restoration. These are represented by primary land-use objective (restoration or production), management intervention (land-use change, land-cover change, or change in production intensity), and impact on ecosystem integrity.Table 1Land-use and management characteristics of land-management pathwaysPrimary land useManagement interventionPathwayImproved ecosystem integrityrestorationland-use changeforest restoration (from productive use to restoration, allowing secondary forests to reach their biological potential)land-cover changereforestation (from deforested to forested land via natural regeneration)productionreduced productionreduced harvest (reduced logging intensity) and silvopasture (reduced grazing intensity)increased productivityagroforestry (increased crop productivity) and silvopasture (alternative feed sources) Open table in a new tab The carbon-sequestration potential of the five land-management pathways that aim to restore ecosystem integrity (which we refer to as ECORES) is quantified between 2020 and 2100 by an area-based approach and estimates of land carbon flux (details are provided in the experimental procedures). While peatland, coastal, and marine ecosystems are among the most carbon dense in the world, at a global scale, the potential for carbon sequestration in coastal and marine ecosystems is orders of magnitude lower than that in terrestrial ecosystems,18Hoegh-Guldberg O. Northrop E. Lubchenco J. The ocean is key to achieving climate and societal goals.Science. 2019; 365: 1372-1374https://doi.org/10.1126/science.aaz4390Crossref PubMed Scopus (37) Google Scholar and peatland restoration results in (significant) avoided emissions rather than additional sequestration.19Leifeld J. Menichetti L. The underappreciated potential of peatlands in global climate change mitigation strategies.Nat. Commun. 2018; 9: 1071https://doi.org/10.1038/s41467-018-03406-6Crossref PubMed Scopus (235) Google Scholar For these reasons, terrestrial carbon removals are the focus of this study, although the impact of avoided emissions on temperature is included in the baseline options. The pathway characteristics are summarized in Table 2 (details in Tables S2–S7). We then present a 1.5°C compatible scenario combining ecosystem restoration with deep decarbonization pathways (the RESTORE scenario). The ecosystem restoration pathways (ECORES) included here build on previous work in Teske et al.20Teske S. Achieving the Paris Climate Agreement Goals: Global and Regional 100% Renewable Energy Scenarios with Non-energy GHG Pathways for +1.5°C and +2°C. Springer, 2019Crossref Scopus (91) Google Scholar and Littleton et al.21Littleton E.W. Dooley K. Webb G. Harper A.B. Powell T. Nicholls Z. Meinshausen M. Lenton T.M. Dynamic modelling shows substantial contribution of ecosystem restoration to climate change mitigation.Environ. Res. Lett. 2021; 16: 124061https://doi.org/10.1088/1748-9326/ac3c6cCrossref Scopus (2) Google Scholar by using updated datasets and extending the analysis from these previous studies.Table 2Five ecosystem restoration pathways (ECORES)PathwayDescriptionForested landsforest restorationset aside degraded natural (secondary) forests for conservation purposes; all biomes (1,893 Mha or 25% of secondary forests)reduced harvestreduction of harvest intensity in temperate and boreal managed forests (221 Mha or 19%) and ceasing industrial harvest in tropical forests (532 Mha or the remaining 75%)reforestationreforestation of mixed-native species in tropical and temperate biomes; maintained for conservation purposes (the 211 Mha pledged for reforestation under the Bonn Challenge as of 2021 included here)Agricultural landsagroforestryintegrating trees in existing croplands over 20% of temperate and tropical croplands (278 Mha)silvopastureincreased trees and shrubs over 10% of temperate and tropical pastureland (308 Mha) via reduced grazing intensity Open table in a new tab The results show the median gross cumulative potential of additional CDR with five ECORES to be 103 Gt carbon (GtC) (5%–95% range of −91 to 196 GtC) between 2020 and 2100. The peak annual sequestration rate from all ECORES (forest restoration, reforestation, reduced harvest, agroforestry, and silvopasture) is a median of 2.6 GtC per year (5%–95% range of 1–5 GtC per year), although this rate is only maintained for 1 to 2 decades (Figure 1). The average annual sequestration rate from 2020 to 2100 is 1.2 GtC per year. These removals will be canceled out to some extent by ongoing net land-use emissions, discussed in section “temperature pathways.” These results are approximately 10% higher than the carbon removals found when the same land-management pathways were modeled in a dynamic global vegetation model (DGVM); the difference is largely due to the inclusion of soil carbon response to land-use change in the DGVM.21Littleton E.W. Dooley K. Webb G. Harper A.B. Powell T. Nicholls Z. Meinshausen M. Lenton T.M. Dynamic modelling shows substantial contribution of ecosystem restoration to climate change mitigation.Environ. Res. Lett. 2021; 16: 124061https://doi.org/10.1088/1748-9326/ac3c6cCrossref Scopus (2) Google Scholar However, given the very different methodological approaches to quantifying carbon removal, the similarity increases confidence in the results. Figure 1 shows that while the sequestration potential of restoration pathways scales up over time, pathways reliant on removing disturbances see a jump in annual sequestration potential after a 20-year implementation period. During the 20-year implementation period, sequestration from regrowth is considered non-additional. We include the sequestration only after 20 years, when removal factors for old forests apply.22Harris N.L. Gibbs D.A. Baccini A. Birdsey R.A. de Bruin S. Farina M. Fatoyinbo L. Hansen M.C. Herold M. Houghton R.A. et al.Global maps of twenty-first century forest carbon fluxes.Nat. Clim. Change. 2021; 11: 234-240https://doi.org/10.1038/s41558-020-00976-6Crossref Scopus (121) Google Scholar All five ECORES are implemented in temperate and tropical regions, while only two pathways (forest restoration and reduced harvest) are also implemented in the boreal region. The highest uncertainty levels are confined to the temperate region as a result of uncertainties in removal factors in temperate managed forests.22Harris N.L. Gibbs D.A. Baccini A. Birdsey R.A. de Bruin S. Farina M. Fatoyinbo L. Hansen M.C. Herold M. Houghton R.A. et al.Global maps of twenty-first century forest carbon fluxes.Nat. Clim. Change. 2021; 11: 234-240https://doi.org/10.1038/s41558-020-00976-6Crossref Scopus (121) Google Scholar,23Gibbs D. Harris N. 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Our land area for the reforestation pathway is based on current Bonn Challenge pledges,26The Bonn Challengehttps://www.bonnchallenge.orgGoogle Scholar of which only around 5% are in temperate regions (and none in boreal), meaning that the majority of reforestation included here occurs in the tropics. However, unlike existing pledges, we assume that all areas will be reforested with a diversity of native species that are thereafter maintained as standing carbon stocks. Analysis shows this is not the case in reality given that around half of these pledges are for commercial plantations.16Lewis S.L. Wheeler C.E. Mitchard E.T.A. Koch A. Restoring natural forests is the best way to remove atmospheric carbon.Nature. 2019; 568: 25-28https://doi.org/10.1038/d41586-019-01026-8Crossref PubMed Scopus (274) Google Scholar The difference between natural forest restoration and commercial timber plantations can be as much as a 90% reduction in long-term carbon sequestration and storage,16Lewis S.L. 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The key difference between the two pathways is in the intensity of changed land management, such that forest restoration allows full recovery of carbon carrying capacity in one-quarter of secondary forests (removing these forests from production and increasing the proportion of conservation areas). Reduced harvest, on the other hand, represents a reduction in harvest intensity, which research suggests would allow an increase in forest carbon stocks over time while commercial harvest continues.27Pingoud K. Ekholm T. Sievänen R. Huuskonen S. Hynynen J. Trade-offs between forest carbon stocks and harvests in a steady state – A multi-criteria analysis.J. Environ. Manag. 2018; 210: 96-103https://doi.org/10.1016/j.jenvman.2017.12.076Crossref PubMed Scopus (25) Google Scholar, 28Law B.E. Hudiburg T.W. Berner L.T. Kent J.J. Buotte P.C. Harmon M.E. Land use strategies to mitigate climate change in carbon dense temperate forests.Proc. Natl. Acad. Sci. 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Syst. 2019; 93: 1-9https://doi.org/10.1007/s10457-019-00366-8Crossref Scopus (63) Google Scholar The total carbon removal from all five ECORES—a median of 103 GtC cumulative sequestration by 2100 in addition to ongoing land-management activities—is shown in Figure 2. This represents the lower end to the middle of estimated ranges for land-based removals (approximately 30–217 GtC),38Nolan C.J. Field C.B. Mach K.J. Constraints and enablers for increasing carbon storage in the terrestrial biosphere.Nat. Rev. Earth Environ. 2021; 2: 436-446https://doi.org/10.1038/s43017-021-00166-8Crossref Scopus (10) Google Scholar reflecting the conservativeness of our approach in avoiding double counting and minimizing land-use change. The regional differences shown in Figure 2 relate primarily to the climatic biome differences already discussed above. Higher rates of sequestration are seen in Asia, Latin America, and Africa, where tropical biomes see higher net primary productivity. Greater land area is also included in the tropics because the Bonn Challenge pledges are predominantly located in tropical forested countries. The difference in the reduced harvest pathway between temperate and tropical biomes, where commercial harvest of native forests was entirely halted in the tropics, is also a key contributor to higher sequestration rates in these regions. For temperature projections, we use a reduced-complexity probabilistic climate emulator that reflects the updated climate science understanding in line with the IPCC