CLIMATE CHANGE 01/2022 German Environment Agency Policy Paper Nature-based solutions and global climate protection Assessment of their global mitigation potential and recommendations for international climate policy by: Judith Reise, Anne Siemons, Hannes Böttcher, Anke Herold, Cristina Urrutia, Lambert Schneider Öko-Institut Berlin Ewa Iwaszuk, Hugh McDonald, Ana Frelih-Larsen, Laurens Duin, McKenna Davis Ecologic Institute publisher: German Environment Agency CLIMATE CHANGE 01/2022 Ressortforschungsplan of the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety Project No. (FKZ) 3721 42 502 0 Report No. FB000738/ENG Policy Paper Nature-based solutions and global climate protection Assessment of their global mitigation potential and recommendations for international climate policy by Judith Reise, Anne Siemons, Hannes Böttcher, Anke Herold, Cristina Urrutia, Lambert Schneider Öko-Institut Berlin Ewa Iwaszuk, Hugh McDonald, Ana Frelih-Larsen, Laurens Duin, McKenna Davis Ecologic Institute On behalf of the German Environment Agency Imprint Publisher Umweltbundesamt Wörlitzer Platz 1 06844 Dessau-Roßlau Tel: +49 340-2103-0 Fax: +49 340-2103-2285 buergerservice@uba.de Internet: www.umweltbundesamt.de /umweltbundesamt.de /umweltbundesamt Report performed by: Öko-Institut e.V. Ecologic Institut Berlin Germany Report completed in: November 2021 Edited by: Sections V 1.1 Climate Protection and V 2.6 Emissions Reduction Projects Hannah Auerochs, Friederike Erxleben (Fachbegleitung) Publication as pdf: http://www.umweltbundesamt.de/publikationen ISSN 1862-4359 Dessau-Roßlau, Janruary 2022 The responsibility for the content of this publication lies with the authors. mailto:buergerservice@uba.de http://www.umweltbundesamt.de/publikationen CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 5 Abstract: Nature-based solutions and global climate protection Nature-based Solutions (NbS) build synergies between biodiversity conservation and societal challenges such as climate change. This paper derives a working definition of NbS based on an evaluation of existing definitions, in particular the IUCN (2016) definition. It comprises the key elements of the existing definitions that we believe to be important to inform the scope of this study. It critically assesses the global mitigation potential of NbS in relevant studies for forests, croplands, grasslands, terrestrial and coastal wetlands as well as settlements. Recommendations for international climate policy are derived. The study finds that it is likely that NbS potentials provided by scientific literature overestimate the realistic potential of NbS for climate change mitigation. This is due to a lack of integrated studies, overly optimistic assumptions on land availability as well as the quality of available information. Furthermore, the influence of measures on GHG fluxes, uncertainties related to carbon fluxes and quantification methodologies as well as climate impacts are not taken into account. The majority of studies evaluating the mitigation potential of NbS focus on the technical mitigation potential. General ecological constraints such as existing threats to ecosystems, and biodiversity impacts, land use conflicts and other social, cultural and political barriers as well as the risk of non-permanence further limit mitigation potentials. The success of NbS to mitigate climate change and deliver ecological and social co-benefits will very much depend on eliminating direct and indirect pressures on ecosystems caused by current patterns of production and consumption. Nevertheless, the uncertainties related to the quantification of mitigation effects of NbS should not be used as an argument against their implementation. Neither should they be used as an excuse to delay ambitious mitigation action to reduce emissions. In the UNFCCC negotiation process, information on NbS in biennial transparency reports may serve as a basis for technical discussion to improve methodologies and indicators to assess how NbS contribute to achieving NDCs and to make further financial support available. In implementing activities under Article 6 of the Paris Agreement, the specific risks related to NbS must be taken into account. In the development of processes or support schemes to foster NbS, social and environmental safeguards need to be put in place. Coherence with work under other international policy frameworks such as the other Rio Conventions is required to foster synergies. Kurzbeschreibung: Naturbasierte Lösungen und globaler Klimaschutz Naturbasierte Lösungen (NbS) schaffen Synergien zwischen dem Schutz der Biodiversität und gesellschaftlichen Herausforderungen wie dem Klimawandel. In diesem Papier wird eine Arbeitsdefinition von NbS abgeleitet, die sich auf andere bestehende Definitionen, insbesondere auf die Definition der IUCN (2016) stützt. Sie enthält zentrale Elemente der bestehenden Definitionen, die für den Rahmen dieser Studie wichtig sind. Das globale Minderungspotenzial von NbS in relevanten Studien für Wälder, Ackerland, Grünland, terrestrische und küstennahe Feuchtgebiete sowie Siedlungen wird kritisch analysiert und es werden Empfehlungen für die internationale Klimapolitik abgeleitet. Die Studie kommt zu dem Ergebnis, dass die in der wissenschaftlichen Literatur angegebenen Potenziale das realistische Potenzial von NbS für den Klimaschutz wahrscheinlich überschätzen. Dies ist auf das Fehlen integrierter Studien, zu optimistische Annahmen zur Flächenverfügbarkeit und die Qualität der verfügbaren Informationen zurückzuführen. Außerdem werden der Einfluss von Maßnahmen auf Treibhausgasflüsse, Unsicherheiten in Bezug auf Kohlenstoffflüsse und Quantifizierungsmethoden sowie Klimawandelauswirkungen nicht berücksichtigt. Die Mehrzahl der Studien, die das Minderungspotenzial von NbS untersuchen, konzentriert sich auf das technische Minderungspotenzial. Allgemeine ökologische Einschränkungen wie bestehende Bedrohungen für Ökosysteme, Auswirkungen auf die Biodiversität, Landnutzungskonflikte und andere soziale, kulturelle und politische Hindernisse sowie das Risiko der Nicht-Permanenz von CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 6 Minderungserfolgen schränken die Minderungspotenziale weiter ein. Der Beitrag von NbS bei der Bekämpfung des Klimawandels und der Erzielung ökologischer und sozialer Co-Benefits wird in hohem Maße davon abhängen, ob die direkten und indirekten Belastungen der Ökosysteme aufgrund der vorherrschenden Produktions- und Konsummuster beseitigt werden. Dennoch sollten die Unsicherheiten in Bezug auf die Quantifizierung der Minderungseffekte von NbS nicht als Argument gegen ihre Umsetzung verwendet werden. Sie sollten auch nicht als Vorwand dienen, um ehrgeizige Minderungsmaßnahmen zur Reduzierung von Emissionen zu verzögern. Im Rahmen des UNFCCC-Verhandlungsprozesses können die Informationen über NbS in den zweijährlichen Transparenzberichten als Grundlage für technische Diskussionen dienen, um Methoden und Indikatoren von NbS im Kontext der NDCs weiterzuentwickeln, und um finanzielle Unterstützung bereitzustellen. Bei der Umsetzung von Aktivitäten unter Artikel 6 des Übereinkommens von Paris müssen die spezifischen Risiken im Zusammenhang mit NbS berücksichtigt werden. Bei der Entwicklung von Verfahren oder Unterstützungsregelungen zur Förderung von NbS müssen soziale und ökologische Schutzmaßnahmen eingeführt werden. Zur Förderung von Synergien ist eine Kohärenz mit der Arbeit im Rahmen anderer internationaler politischer Rahmenwerke wie den anderen Rio-Konventionen erforderlich. CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 7 Table of content List of tables ............................................................................................................................................ 8 List of abbreviations ................................................................................................................................ 8 Summary ............................................................................................................................................... 10 Zusammenfassung ................................................................................................................................. 14 1 Introduction ................................................................................................................................... 19 2 Definition of Nature-based Solutions ............................................................................................ 20 2.1 Core elements of the NbS definition .................................................................................... 21 2.1.1 Defining characteristics of NbS ......................................................................................... 22 2.1.2 Common qualities of NbS ................................................................................................. 25 2.2 Categorisation of NbS ........................................................................................................... 26 3 Assessment of the global potential of Nature-based Solutions .................................................... 30 3.1 Methodological approach ..................................................................................................... 30 3.2 Assessment of potentials for different ecosystems .............................................................. 32 3.2.1 Forests ............................................................................................................................... 32 3.2.2 Croplands .......................................................................................................................... 35 3.2.3 Grasslands ......................................................................................................................... 41 3.2.4 Terrestrial wetlands .......................................................................................................... 43 3.2.5 Coastal wetlands ............................................................................................................... 45 3.2.6 Settlements ....................................................................................................................... 47 3.3 Discussion .............................................................................................................................. 48 4 Nature-based solutions in international climate policy ................................................................ 54 4.1 Role of NbS under UNFCCC and the Kyoto Protocol ............................................................. 55 4.2 Role of NbS under the Paris Agreement ............................................................................... 57 4.2.1 Reporting and accounting rules for the land use sector in the Paris Agreement ............. 57 4.2.2 The role of NbS in NDCs .................................................................................................... 59 4.2.3 The role of NbS in the negotiations for Article 6 .............................................................. 60 5 Conclusions and strategic implications for international climate policy ...................................... 62 6 List of references ........................................................................................................................... 66 CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 8 List of tables Table 1: Definitions of NbS ..................................................................... 20 Table 2: Defining characteristics of NbS ................................................. 24 Table 3: Categorisation of NbS ............................................................... 27 List of abbreviations AFOLU Agriculture, Forestry and Other Land Use BECCS Bioenergy with Carbon Capture & Storage C Carbon CBD Convention on Biological Diversity CDM Clean Development Mechanism CH4 Methane CO2 Carbon dioxide CO2e Carbon dioxide equivalents COP Conference of the Parties FAO Food and Agriculture Organisation GHG Greenhouse gas Ha hectar IPCC Intergovernmental Panel on Climate Change ITMO Internationally transferred mitigation outcomes IUCN International Union for Conservation of Nature JI Joint Implementation K Potassium KJWA Koronivia Joint Work on Agriculture LULUCF Land Use, Land Use Change and Forestry MEA DaRT Data Reporting Tool for Multilateral Environmental Agreements Mha Million hectar MPGs Modalities, procedures and guidelines for the transparency framework for action and support Mt Mega tonnes N Nitrogen N2O Nitrous oxide NbS Nature-based Solutions NDC Nationally Determined Contributions under the Paris Agreement P Phosphor pH Potential of hydrogen CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 9 AFOLU Agriculture, Forestry and Other Land Use RCP Representative concentration pathway REDD+ Reducing Emissions from Deforestation and Forest Degradation SBI Subsidiary Body for Implementation SBSTA Subsidiary Body for Scientific and Technological Advice SOC Soil organic carbon t Tonne UNFCCC United Nations Framework Convention on Climate Change yr year CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 10 Summary The recognition that nature can contribute to addressing and solving societal challenges, including climate crisis, is referred to as “Nature-based Solutions” (NbS). NbS build synergies between biodiversity conservation and societal challenges and deliver environmental and social benefits. Although the ‘NbS’ term is widely used, there is still no common understanding of NbS in the scientific and political debate. This paper derives a working definition of NbS based on an evaluation of existing definitions, in particular the IUCN (2016) definition: Nature-based Solutions are locally appropriate, adaptive actions to protect, sustainably manage or restore natural or modified ecosystems in order to address targeted societal challenge(s) - such as climate change mitigation -, while simultaneously enhancing human well-being and providing biodiversity benefits. It comprises the key elements of the existing definitions, that we believe to be important to inform the scope of this study. On the basis of this definition, the study critically assesses the global mitigation potential of NbS. Furthermore, recommendations for international climate policy are derived. It is crucial to critically assess the mitigation potential associated with NbS in order not to overestimate their contribution to climate protection. The paper reviews a number of prominent studies on measures associated with NbS towards their mitigation potential, and their methodologies and assumptions in order to develop a better understanding of the potential and limits of NbS measures as a mitigation strategy. The studies were assessed and compared with regard to their scope, the range of mitigation potential provided, approaches towards the quantification of this potential, assumptions regarding safeguards and co-benefits as well as costs, constraints and uncertainties included in the studies. Potentials provided for forests through reforestation, afforestation, forest protection as well as forest management vary greatly depending on assumptions regarding land availability and constraints on co-benefits and trade-offs (afforestation/reforestation), assumed future baselines of drivers (avoided emissions from deforestation) as well as forest growth and assumed harvest intensity (forest management). Differences in assumptions but also definition of activities makes a comparison of different estimates difficult if not impossible. Given the wide concept of NbS adopted by the reviewed studies, there is a risk that potentials are largely overestimated. The risk of overestimation is larger for afforestation/reforestation (up to five times higher) and lower but still significant for forest management (about two times lower). Deviation of estimates for avoided deforestation were found to be between the two. NbS in croplands mainly contribute to climate change mitigation by increasing CO2 sequestration in mineral soils and on farmland and by reducing CH4 emissions from rice cultivation. Estimates for global sequestration potentials in croplands range from 0.2 GtCO2e/yr to 11 GtCO2e/yr and have high uncertainties. Global estimates derived from global soil models do not reflect the high natural variability of carbon stocks and there is currently a lack of systematic and reliable measurement of soil carbon in mineral soils in countries. However, constant sustainable soil management in croplands (e.g. planting cover crops during fallow periods, increasing the returns of organic input to soils) is needed to maintain the capacity for soil carbon sequestration and to protect soil carbon stocks. Estimates of the CO2 sequestration potential of agroforestry range from 0.3 GtCO2e/yr to 5.7 GtCO2e/yr. Estimates often reflect enhancements of SOC, which is also constrained by the issues mentioned above, and increases in biomass, which is currently not systematically assessed in most countries. The mitigation potentials of agroforestry systems are strongly influenced by soil and climate variables, as well as by the system under consideration. Estimates for reducing CH4 emissions from rice cultivation range from 0.08 to 0.87 GtCO2e/yr. The diversity of rice cultivation systems poses a CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 11 strong limit to assessing generalised potentials and trade-offs between the reduction of CH4 emissions. Increasing N2O emissions need to be considered, if alternative water management is not accompanied by improved fertiliser management. Compared to other ecosystems, NbS in grasslands show a very wide range of climate mitigation potentials assumingly because of differing assumptions for soil carbon sequestration rates, e.g. for improved grazing (0.15 and 1 tCO2/ha/yr, Griscom et al. 2017; Conant et al. 2017) and for the potentially suitable area extent of this NbS. Hence, total mitigation potentials from improved grazing range from 0.15 (Griscom et al. 2017) to 1.5 GtCO2e/yr (Smith et al. 2008). The highest mitigation potentials can be expected from the avoidance of grassland conversion to cropland, although the total estimate of avoided emissions varies according to the underlying soil carbon assumptions. Also, the active restoration of abandoned cropland substantially increases the soil carbon sequestration (1.9 tCO2/ha/yr and 3.3 tCO2/ha/yr; Yang et al. 2019; Conant et al. 2017). Yet, there are no estimates on the potential restoration area. Although the overall climate mitigation potential of grasslands due to NbS is very uncertain, the co-benefits of NbS protecting grasslands from conversion and restoring them can be very high for biodiversity and ecosystem services like flood control and improved soil structure (Griscom et al. 2017). Protection and restoration of terrestrial wetlands can avoid and reduce further carbon loss primarily from soils. Maximum global mitigation estimates for peatland restoration is estimated at 0.8 GtCO2e/yr (Griscom et al. 2017) and 0.9 GtCO2e/yr (Leifeld and Menichetti 2018). Additionally, the avoidance of further loss of peatlands could mitigate about 0.7 GtCO2e/yr (Griscom et al. 2017). Main uncertainties related to these mitigation potentials result from different estimates for degraded peatland areas as well as different estimates regarding a full implementation of the global restoration potential. Also, there is a lack of emission factors that better reflect the different phases of peat degradation in order to make more accurate assumptions. Another uncertainty are future GHG fluxes of peatlands under climate change that could lead to increased emissions from intact peatlands (Leng et al. 2019). Finally, global mitigation potentials for terrestrial wetlands are predominantly limited to peatlands but do not consider impacts on the emission fluxes from lake and river sediments as well as alluvial (floodplain) forest soils and biomass (Ramsar Convention Secretariat 2018). The restoration of coastal wetlands (mangroves, seagrass meadows and saltmarshes) can mitigate up to 0.8 GtCO2e/yr but this mitigation potential could be lower especially because of potentially lower emission factors for seagrass meadows. However, there are high uncertainties in the number of sequestration rates, area extent as well as the impact of disturbances on the emission fluxes of coastal wetlands (Jia et al. 2019; Pendleton et al. 2012; Howard et al. 2017; IUCN 2021b), which makes mitigation potential estimates very challenging. The effect of climate change on coastal ecosystems and their carbon stocks is still highly debated and probably has a high geographic variation but is not considered in the studies. Sea level rise could be beneficial for coastal ecosystems, while marine heatwaves, storms and altered availability of fresh water could have a negative impact (Macreadie et al. 2019). Currently, impacts of disturbances to seafloor sediments of the open sea mainly due to bottom trawling have not been considered in global assessments so far (Jia et al. 2019; Griscom et al. 2017). Enhancing urban green infrastructure in settlements can contribute to mitigating emissions as well as to cities’ adaptation to climate change. At the same time, they involve co-benefits for food security, improve air quality and can have positive impacts on soil and water. Overall, the potential to abate pollution is evaluated as more substantial than the potential to mitigate GHG emissions (Baro et al. 2017). Additionally, the circumstances for urban greening are very different across the globe. Local data remains fragmentary (Nowak et al. 2013). CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 12 More research is required, as it is likely that NbS potentials provided by the scientific literature overestimate the realistic potential of such activities for climate change mitigation. This is partly due to the lack of integrated studies that achieve a consistent and comprehensive assessment of activities competing for land and financial resources, affecting production levels and causing displacement of production to provide the net mitigation potential. Moreover, many studies make overly optimistic assumptions on land availability and do not consider negative impacts on ecosystems, human well-being or non-GHG effects (e.g. albedo) of measures. Additional constraints relate to the quality of available information on the current state of ecosystems and the influence of measures on their GHG fluxes and other ecosystem components like biodiversity. Furthermore, underlying assumptions towards ecosystem carbon fluxes as well as quantification methodologies bear significant uncertainties. Climate impacts are not taken into account in any of the studies assessed. Also, the majority of studies focus on the technical mitigation potential which can differ significantly from economic potentials and related assumptions are not always clear. A lot of studies do not consider opportunity, transaction or transition costs. Also, land use conflicts and other social, cultural and political barriers to the implementation of NbS are barely taken into account. General ecological constraints such as existing threats to ecosystems, consumption patterns, as well as biodiversity impacts further limit the mitigation potentials provided in the literature. Also, the risk of non-permanence inherent to mitigation activities in the land use sector needs to be accounted for when quantifying mitigation potentials from NbS. A tonne of CO2 removals achieved through NbS can thus not be considered equivalent to one tonne of CO2 of fossil fuel avoided that has a much lower risk of non-permanence. Available global mitigation potentials therefore need to be considered as rough estimates with considerable constraints. Nevertheless, the uncertainties related to the quantification of mitigation effects of NbS should not be used as an argument against their implementation. Advancing NbS is often described as a ‘no-regret’ option, as they entail benefits to people in a range of scenarios. To realise these benefits, NbS need to be carefully designed, be based on metrics that take into account their various benefits to human beings and the environment and have robust social and biodiversity safeguards in place. At the same time, the mitigation benefits implied by NbS will be an important contribution to reaching the goals of the Paris Agreement, but they should always be seen as a complement to ambitious mitigation action to reduce emissions. The success of NbS to mitigate climate change and deliver ecological and social co-benefits will also very much depend on a successful implementation of the goals and targets of the Rio Conventions, in particular the CBD and its global biodiversity framework. This will mean to eliminate direct and indirect pressures on ecosystems related to recent drivers of global change, including land- and sea-use change, ecosystem and species exploitation and pollution, caused by current patterns of consumption and production. For the UNFCCC negotiation process, the following recommendations can be derived: ► If Parties also report on the implementation of NbS in their biennial transparency reports, this may serve as a basis for technical discussion to improve methodologies and indicators to assess how NbS contribute to achieving NDCs and to direct capacity building resources to support the development of better policies to enhance and promote their implementation. ► Including NbS in market-based mechanisms involves risks related to the uncertainty in setting baselines, monitoring carbon stock changes, non-permanence of achieved mitigation and social and environmental safeguards. In implementing cooperative approaches under Article 6 of the Paris Agreement, these risks must be taken into account. Particularly, eligible activities need to be designed in a careful manner in order to manage reversal risks. A CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 13 prudent policy approach would be to use crediting mechanisms only for those activities for which the likelihood of additionality is high and for which baselines can be estimated with reasonable certainty. ► Under the UNFCCC negotiations, more attention should be paid to those types of NbS that are less prevalent but bear significant benefit to people and the preservation of ecosystems such as coastal and marine habitats. In the development of processes or support schemes to foster NbS under the UNFCCC process, special attention needs to be paid to ensuring that social and environmental safeguards are put in place. While the Warsaw framework for REDD+ explicitly requires the conservation of biodiversity and the respect of indigenous peoples’ and local communities’ rights, guidance on such safeguards for other types of NbS measures is too vague under the UNFCCC. Progress on NbS safeguard frameworks has been made for example under the CBD (ecosystem approach; principles and safeguards for ecosystem- based approaches to climate change adaptation and disaster risk reduction) or the IUCN Global Standard for NbS on which the UNFCCC process can further build on. ► An integrated view on NbS is necessary, as they need to be understood as measures to enhance mitigation as well as adaptation and biodiversity conservation. Coherence with other ongoing work under the UNFCCC (e.g. KJWA, Nairobi Work Programme, Standing Committee on Finance) and close collaboration with the work under the other Rio Conventions, in particular the CBD global biodiversity framework, is required to foster synergies. For this purpose it would be beneficial to work on common or at least aligned concepts as well as indicators for reporting and tracking NbS activities under these different processes, e.g. under initiatives such as MEA DaRT1. Making use of NbS for long-term carbon storage can only be successful if the sink potential of forests, wetlands and soils is maintained through sustainable land use and existing carbon stocks are protected. Synergies between conservation and use objectives can be realised if climate protection, biodiversity conservation and climate adaptation are thought together. 1 See https://dart.informea.org/. https://dart.informea.org/ CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 14 Zusammenfassung Die Erkenntnis, dass die Natur zur Bewältigung und Lösung gesellschaftlicher Herausforderungen, einschließlich der Klimakrise, beitragen kann, wird unter dem Begriff "naturbasierte Lösungen" (NbS) subsumiert. NbS schaffen Synergien zwischen dem Schutz der Biodiversität und gesellschaftlichen Herausforderungen und bringen somit ökologische und soziale Vorteile mit sich. Obwohl der Begriff "NbS" weit verbreitet ist, gibt es in der wissenschaftlichen und politischen Debatte noch kein gemeinsames Verständnis von NbS. In diesem Papier wird eine Arbeitsdefinition von NbS abgeleitet, die auf einer Bewertung der bestehenden Definitionen, insbesondere der Definition der IUCN (2016) basiert: Naturbasierte Lösungen sind lokal angemessene, anpassungsfähige Maßnahmen zum Schutz, zur nachhaltigen Bewirtschaftung oder zur Wiederherstellung natürlicher oder veränderter Ökosysteme, um gezielte gesellschaftliche Herausforderungen - wie die Abschwächung des Klimawandels - anzugehen und gleichzeitig das menschliche Wohlergehen zu verbessern und die biologische Vielfalt zu fördern. Die Arbeitsdefinition enthält die zentralen Elemente der bestehenden Definitionen, die für den Rahmen dieser Studie wichtig sind. Sie bildet somit die Grundlage, um das globale Minderungspotenzial von NbS kritisch zu bewerten. Außerdem werden Empfehlungen für die internationale Klimapolitik abgeleitet. Es ist von entscheidender Bedeutung, das mit NbS verbundene Minderungspotenzial kritisch zu bewerten, um ihren Beitrag zum Klimaschutz nicht zu überschätzen. In dem Papier wird eine Reihe prominenter Studien über Maßnahmen im Zusammenhang mit NbS im Hinblick auf ihre Minderungspotenziale sowie ihre Methoden und Annahmen untersucht, um ein besseres Verständnis für das Potenzial und die Grenzen von NbS-Maßnahmen als Minderungsstrategie zu entwickeln. Die Studien wurden hinsichtlich ihres Umfangs, des Umfangs des Minderungspotenzials, der Ansätze zur Quantifizierung dieses Potenzials, der Annahmen zu Schutzmaßnahmen und Zusatznutzen sowie der in den Studien enthaltenen Kosten, Einschränkungen und Unsicherheiten bewertet und verglichen. Die Potenziale, die für Wälder durch Wiederaufforstung, Aufforstung, Waldschutz und Waldbewirtschaftung angegeben werden, variieren stark, je nach den Annahmen über die Flächenverfügbarkeit und die Einschränkungen bei den Zusatznutzen und Kompromissen (Aufforstung/Wiederaufforstung), dem angenommenen künftigen Referenzszenario der Einflussfaktoren (vermiedene Emissionen aus Entwaldung) sowie dem Waldwachstum und der angenommenen Nutzungsintensität (Waldbewirtschaftung). Unterschiede in den Annahmen, aber auch in der Definition der Aktivitäten machen einen Vergleich der verschiedenen Schätzungen schwierig, wenn nicht gar unmöglich. Angesichts des weit gefassten Konzepts von NbS, das in den untersuchten Studien verwendet wird, besteht die Gefahr, dass die Potenziale weitgehend überschätzt werden. Das Risiko einer Überschätzung ist bei Aufforstung/Wiederaufforstung größer (bis zum Fünffachen) und bei der Waldbewirtschaftung geringer, aber immer noch signifikant (etwa das Zweifache). Bei der vermiedenen Entwaldung liegen die Abweichungen zwischen den beiden Schätzungen. NbS in Anbauflächen tragen hauptsächlich zur Abschwächung des Klimawandels bei, indem sie die CO2-Sequestrierung in Mineralböden und auf Ackerland erhöhen und die CH4-Emissionen aus dem Reisanbau verringern. Die Schätzungen für das globale Sequestrierungspotenzial von Ackerland reichen von 0,2 GtCO2e/Jahr bis 11,0 GtCO2e/Jahr und sind mit großen Unsicherheiten behaftet. Globale Schätzungen, die aus globalen Bodenmodellen abgeleitet werden, spiegeln die hohe natürliche Variabilität der Kohlenstoffvorräte nicht wider, und es fehlt derzeit an einer systematischen und zuverlässigen Messung des Bodenkohlenstoffs in Mineralböden in den einzelnen Ländern. Allerdings ist es notwendig, eine nachhaltige CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 15 Bodenbewirtschaftung im Ackerbau (z. B. Anbau von Zwischenfrüchten während der Brachezeiten, Erhöhung des Rückflusses von organischen Stoffen in die Böden) dauerhaft beizubehalten, um die Bindung von Kohlenstoff im Boden sicherzustellen und die Kohlenstoffvorräte im Boden zu zu erhalten. Die Schätzungen des CO2-Bindungspotenzials der Agroforstwirtschaft reichen von 0,3 GtCO2e/Jahr bis 5,7 GtCO2e/Jahr. Die Schätzungen beziehen sich häufig auf die Erhöhung des Anteil an organischem Bodenkohlenstoff (SOC), die ebenfalls durch die oben genannten Probleme eingeschränkt wird, und auf die Erhöhung der Biomasse, die in den meisten Ländern derzeit nicht systematisch bewertet wird. Das Minderungspotenzial agroforstwirtschaftlicher Systeme hängt stark von den Boden- und Klimavariablen sowie von dem jeweiligen System ab. Die Schätzungen für die Reduzierung der CH4-Emissionen aus dem Reisanbau reichen von 0,08 bis 0,87 GtCO2e/Jahr. Die Vielfalt der Reisanbausysteme stellt eine starke Einschränkung für die Bewertung der verallgemeinerten Potenziale und der Kompromisse bei der Verringerung der CH4-Emissionen dar. Ein Anstieg der N2O-Emissionen muss in Betracht gezogen werden, wenn ein alternatives Wassermanagement nicht mit einem verbesserten Düngemittelmanagement einhergeht. Im Vergleich zu anderen Ökosystemen weisen NbS im Grünland eine sehr große Bandbreite an Klimaschutzpotenzialen auf, vermutlich aufgrund unterschiedlicher Annahmen für die Kohlenstoffbindung im Boden, z. B. bei verbesserter Beweidung (0,15 und 1 tCO2/ha/Jahr, Griscom et al. 2017; Conant et al. 2017) und für die potenziell geeignete Flächengröße dieser NbS. Die gesamten Minderungspotenziale durch verbesserte Beweidung reichen daher von 0,15 (Griscom et al. 2017) bis 1,5 GtCO2e/Jahr (Smith et al. 2008). Die höchsten Minderungspotenziale können von der Vermeidung der Umwandlung von Grünland in Ackerland erwartet werden, obwohl die Gesamtschätzung der vermiedenen Emissionen je nach den zugrunde liegenden Annahmen zum Bodenkohlenstoff variiert. Auch die aktive Wiederherstellung von aufgegebenem Ackerland erhöht die Kohlenstoffbindung im Boden erheblich (1,9 tCO2/ha/Jahr und 3,3 tCO2/ha/Jahr; Yang et al. 2019; Conant et al. 2017). Es gibt jedoch keine Schätzungen über die potenzielle Wiederherstellungsfläche. Obwohl das gesamte Klimaschutzpotenzial von Grünland durch NbS sehr ungewiss ist, können die Zusatznutzen von NbS, die Grünland vor der Umwandlung schützen und wiederherstellen, für die biologische Vielfalt und Ökosystemleistungen wie Hochwasserschutz und verbesserte Bodenstruktur sehr hoch sein (Griscom et al. 2017). Der Schutz und die Wiederherstellung von terrestrischen Feuchtgebieten kann weitere Kohlenstoffverluste vor allem aus Böden vermeiden und verringern. Die maximalen globalen Minderungsschätzungen für die Wiederherstellung von Moorgebieten werden auf 0,8 GtCO2e/Jahr (Griscom et al. 2017) und 0,9 GtCO2e/Jahr (Leifeld und Menichetti 2018) geschätzt. Zusätzlich könnte die Vermeidung eines weiteren Verlusts von Torfgebieten etwa 0,7 GtCO2e/Jahr einsparen (Griscom et al. 2017). Die größten Unsicherheiten in Bezug auf diese Minderungspotenziale resultieren aus unterschiedlichen Schätzungen für degradierte Moorflächen sowie aus unterschiedlichen Schätzungen hinsichtlich einer vollständigen Umsetzung des globalen Wiederherstellungspotenzials. Außerdem fehlt es an Emissionsfaktoren, die die verschiedenen Phasen der Torfdegradation besser widerspiegeln, um genauere Annahmen treffen zu können. Eine weitere Ungewissheit sind die zukünftigen THG- Flüsse von Torfgebieten unter dem Klimawandel, die zu erhöhten Emissionen aus intakten Torfgebieten führen könnten (Leng et al. 2019). Schließlich beschränken sich die globalen Minderungspotenziale für terrestrische Feuchtgebiete in erster Linie auf Torfgebiete, berücksichtigen aber nicht die Auswirkungen auf die Emissionsflüsse von See- und Flusssedimenten sowie von Auwaldböden und Biomasse (Ramsar Convention Secretariat 2018). CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 16 Die Wiederherstellung von küstennahen Feuchtgebieten (Mangroven, Seegraswiesen und Salzwiesen) kann bis zu 0,8 GtCO2e/Jahr eindämmen, aber dieses Minderungspotenzial könnte niedriger sein, insbesondere aufgrund potenziell niedrigerer Emissionsfaktoren für Seegraswiesen. Es bestehen jedoch große Unsicherheiten in Bezug auf die Sequestrationsraten, die Flächenausdehnung sowie die Auswirkungen von Störungen auf die Emissionsflüsse von Küstenfeuchtgebieten (Jia et al. 2019; Pendleton et al. 2012; Howard et al. 2017; IUCN 2021b), was Schätzungen des Minderungspotenzials sehr schwierig macht. Die Auswirkungen des Klimawandels auf Küstenökosysteme und ihre Kohlenstoffvorräte werden immer noch heftig diskutiert und weisen wahrscheinlich eine große geografische Variation auf, werden aber in den Studien nicht berücksichtigt. Der Anstieg des Meeresspiegels könnte sich positiv auf die Küstenökosysteme auswirken, während marine Hitzewellen, Stürme und die veränderte Verfügbarkeit von Süßwasser negative Folgen haben könnten (Macreadie et al. 2019). Die Auswirkungen von Störungen der Meeresbodensedimente auf offener See, die vor allem auf die Grundschleppnetzfischerei zurückzuführen sind, wurden bisher in globalen Bewertungen nicht berücksichtigt (Jia et al. 2019; Griscom et al. 2017). Die Verbesserung der städtischen grünen Infrastruktur in Siedlungen kann sowohl zur Emissionsminderung als auch zur Anpassung der Städte an den Klimawandel beitragen. Gleichzeitig bringen sie einen Zusatznutzen für die Ernährungssicherheit mit sich, verbessern die Luftqualität und können positive Auswirkungen auf Boden und Wasser haben. Insgesamt wird das Potenzial zur Verringerung der Umweltverschmutzung als größer eingeschätzt als das Potenzial zur Minderung von Treibhausgasemissionen (Baro et al. 2017). Hinzu kommt, dass die Bedingungen für die Stadtbegrünung weltweit sehr unterschiedlich sind. Lokale Daten sind nach wie vor lückenhaft (Nowak et al. 2013). Es besteht weiterer Forschungsbedarf, da die in der wissenschaftlichen Literatur angegebenen NbS-Potenziale das realistische Potenzial solcher Aktivitäten für den Klimaschutz wahrscheinlich überschätzen. Dies ist zum Teil auf das Fehlen integrierter Studien zurückzuführen, die eine kohärente und umfassende Bewertung von Aktivitäten vornehmen, die um Land und finanzielle Ressourcen konkurrieren, das Produktionsniveau beeinflussen und Produktionsverlagerungen verursachen, um das Nettominderungspotenzial zu ermitteln. Darüber hinaus gehen viele Studien von zu optimistischen Annahmen hinsichtlich der Flächenverfügbarkeit aus und berücksichtigen nicht die negativen Auswirkungen der Maßnahmen auf die Ökosysteme, das menschliche Wohlergehen oder die Nicht-THG-Effekte (z. B. Albedo). Weitere Einschränkungen betreffen die Qualität der verfügbaren Informationen über den aktuellen Zustand des Ökosystems und den Einfluss der Maßnahmen auf die Treibhausgasflüsse und andere Ökosystemkomponenten wie die biologische Vielfalt. Darüber hinaus sind die zugrundeliegenden Annahmen zu den Kohlenstoffflüssen in Ökosystemen sowie die Quantifizierungsmethoden mit erheblichen Unsicherheiten behaftet. Klimaauswirkungen werden nicht in allen untersuchten Studien berücksichtigt. Außerdem konzentrieren sich die meisten Studien auf das technische Minderungspotenzial, das sich erheblich von den wirtschaftlichen Potenzialen unterscheiden kann, und die damit verbundenen Annahmen sind nicht immer klar. In vielen Studien werden Opportunitäts-, Transaktions- oder Übergangskosten nicht berücksichtigt. Auch Landnutzungskonflikte und andere soziale, kulturelle und politische Hindernisse für die Umsetzung von NbS werden kaum berücksichtigt. Allgemeine ökologische Zwänge wie bestehende Bedrohungen für Ökosysteme, Konsummuster sowie Auswirkungen auf die biologische Vielfalt schränken die in der Literatur genannten Minderungspotenziale weiter ein. Außerdem muss bei der Quantifizierung des Minderungspotenzials von NbS das Risiko der Nicht-Permanenz von Minderungsmaßnahmen im Landnutzungssektor berücksichtigt werden. Eine Tonne CO2- Entfernung durch NbS kann daher nicht als gleichwertig mit einer Tonne CO2 aus vermiedenen CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 17 fossilen Brennstoffen angesehen werden, bei denen das Risiko der Nichtdauerhaftigkeit viel geringer ist. Die verfügbaren globalen Minderungspotenziale müssen daher als grobe Schätzungen mit erheblichen Einschränkungen betrachtet werden. Dennoch sollten die Unsicherheiten im Zusammenhang mit der Quantifizierung der Minderungseffekte von NbS nicht als Argument gegen ihre Umsetzung dienen. Die Förderung von NbS wird oft als "No-regret"-Option bezeichnet, da sie für die Menschen in einer Reihe von Szenarien Vorteile mit sich bringt. Um diese Vorteile zu realisieren, müssen NbS sorgfältig konzipiert werden, auf Metriken beruhen, die ihren verschiedenen Vorteilen für Mensch und Umwelt Rechnung tragen, und über solide soziale und biodiversitätsbezogene Schutzmechanismen verfügen. Gleichzeitig werden die mit den NbS verbundenen Minderungsvorteile einen wichtigen Beitrag zur Erreichung der Ziele des Pariser Abkommens leisten, doch sollten sie stets als Ergänzung zu ehrgeizigen Minderungsmaßnahmen zur Reduzierung der Emissionen gesehen werden. Der Erfolg der NbS bei der Abschwächung des Klimawandels und der Erzielung ökologischer und sozialer Zusatznutzen wird auch in hohem Maße von der erfolgreichen Umsetzung der Ziele und Vorgaben der Übereinkommen von Rio abhängen, insbesondere des Übereinkommens über die biologische Vielfalt (CBD) und ihres globalen Rahmens für die biologische Vielfalt. Dies bedeutet, dass die direkten und indirekten Belastungen der Ökosysteme im Zusammenhang mit der veränderten Land- und Meeresnutzung, der Ausbeutung von Ökosystemen und Arten sowie der Umweltverschmutzung aufgrund der vorherrschenden Produktions- und Konsummuster beseitigt werden müssen. Für den UNFCCC-Verhandlungsprozess lassen sich die folgenden Empfehlungen ableiten: ► Wenn die Vertragsparteien in ihren zweijährlichen Transparenzberichten auch über die Umsetzung von NbS berichten, kann dies als Grundlage für technische Diskussionen zur Verbesserung der Methoden und Indikatoren dienen, um zu bewerten, wie NbS zur Erreichung der NDCs beitragen, und Ressourcen zu mobilisieren, um die Entwicklung besserer Strategien zur Verbesserung und Förderung ihrer Umsetzung zu unterstützen. ► Die Einbeziehung von NbS in marktbasierten Mechanismen birgt Risiken im Zusammenhang mit der Unsicherheit bei der Festlegung von Baselines, der Überwachung von Kohlenstoffbestandsveränderungen, der fehlenden Dauerhaftigkeit der erzielten Minderungsmaßnahmen und den sozialen und ökologischen Garantien. Bei der Umsetzung von kooperativen Ansätzen unter Artikel 6 des Übereinkommens von Paris müssen diese Risiken berücksichtigt werden. Insbesondere müssen förderfähige Aktivitäten sorgfältig gestaltet werden, um das Risiko zu verringern, dass erzielte Minderungsergebnisse wieder aufgehoben werden. Ein umsichtiger politischer Ansatz wäre es, den internationalen Handel mit Zertifikaten nur für solche Tätigkeiten zu verwenden, bei denen die Wahrscheinlichkeit der Zusätzlichkeit hoch ist und für die die Referenzszenarien mit angemessener Sicherheit geschätzt werden können. ► Im Rahmen der UNFCCC-Verhandlungen sollte jenen Arten von NbS mehr Aufmerksamkeit geschenkt werden, die zwar weniger verbreitet sind, aber einen erheblichen Nutzen für die Menschen und die Erhaltung von Ökosystemen wie Küsten- und Meereslebensräumen haben. Bei der Entwicklung von Verfahren oder Unterstützungsregelungen zur Förderung von NbS im Rahmen des UNFCCC-Prozesses muss besonders darauf geachtet werden, dass soziale und ökologische Schutzmaßnahmen eingeführt werden. Während der Warschauer Rahmen für REDD+ ausdrücklich die Erhaltung der biologischen Vielfalt und die Achtung der Rechte indigener Völker und lokaler Gemeinschaften vorschreibt, sind die Leitlinien für solche Schutzmaßnahmen für andere Arten von NbS-Maßnahmen unter der UNFCCC zu vage. Fortschritte im Bereich der Schutzmaßnahmen für NbS wurden CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 18 beispielsweise im Rahmen des CBD-Übereinkommens (Ökosystemansatz; Grundsätze und Schutzmaßnahmen für ökosystembasierte Ansätze zur Anpassung an den Klimawandel und zur Verringerung des Katastrophenrisikos) oder des Globalen Standards der IUCN für NbS erzielt, auf denen der UNFCCC-Prozess weiter aufbauen kann. ► Eine integrierte Sichtweise auf NbS ist notwendig, da sie als Maßnahmen zur Verbesserung des Klimaschutzes und der Klimaanpassung sowie der Erhaltung der biologischen Vielfalt verstanden werden müssen. Kohärenz mit anderen laufenden Arbeiten im Rahmen des UNFCCC (z. B. KJWA, Nairobi Arbeitsprogramm, Ständiger Finanzausschuss) und eine enge Zusammenarbeit mit den Arbeiten im Rahmen der anderen Rio-Konventionen, insbesondere der CBD, sind erforderlich, um Synergien zu fördern. Zu diesem Zweck wäre es von Vorteil, gemeinsame oder zumindest angeglichene Konzepte und Indikatoren für die Berichterstattung und Verfolgung von NbS-Aktivitäten für diese verschiedenen Prozesse zu erarbeiten, z.B. im Rahmen von Initiativen wie der MEA DaRT2. Die Nutzung von NbS für die langfristige Kohlenstoffspeicherung kann nur dann erfolgreich sein, wenn das Senkenpotenzial von Wäldern, Feuchtgebieten und Böden durch eine nachhaltige Landnutzung erhalten bleibt und bestehende Kohlenstoffvorräte geschützt werden. Synergien zwischen Schutz- und Nutzungszielen können realisiert werden, wenn Klimaschutz, Biodiversitätserhalt und Klimaanpassung zusammen gedacht werden. 2 Siehe https://dart.informea.org/. https://dart.informea.org/ CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 19 1 Introduction Improved sustainable management, protection and restoration of the world’s diverse ecosystems are increasingly recognised as powerful tools for climate change mitigation (IUCN 2016a). The recognition that nature can contribute to addressing and solving societal challenges, including the climate crisis, is referred to as “Nature-based Solutions” (NbS). NbS imply reliance on natural ecosystems and delivery of environmental and social benefits. Although the term ‘NbS’ is widely used, there is still no common understanding of NbS in the scientific and political debate. Given the prolific use of the term, a definition with clear standards and criteria is needed in order to avoid potential trade-offs and greenwashing of measures which might not entail NbS in a strict sense. Moreover, various studies provide estimates of mitigation potentials of NbS or land use measures in a broader sense on a global, regional or ecosystem-specific scale. To evaluate these potentials the underlying assumptions of the studies have to be critically assessed. Also, there are methodological differences in the assessment of mitigation potentials which lead to considerably different estimates in these studies. It is crucial to critically assess the mitigation potential associated with NbS in order not to rely on an overly optimistic assessment of their role as a climate mitigation option. Firstly, the measures covered in given potentials might not adhere to strict criteria for NbS, therefore entailing social or environmental drawbacks. Secondly, methodologies for quantifying the potential of NbS imply significant uncertainties and rely on a number of assumptions, so that conservative estimates might be the more realistic ones. Additionally, relying on the notion that NbS bear large mitigation potentials which could be used to counterbalance GHG emissions might divert attention from putting all possible effort into decarbonising economies (Seddon et al. 2021). Against this background, this paper critically assesses the global mitigation potential of NbS as provided in existing literature and to derive recommendations for international climate policy. To do so, it first compares and evaluates different understandings of NbS and summarises a definition based on IUCN (2016) to inform the scope of this study (Chapter 2). Chapter 3 compares and critically reviews relevant studies that provide estimates of the mitigation potential of NbS in order to get a sound understanding of a plausible contribution of NbS towards long-term climate targets. Additionally, Chapter 4 analyses the role of NbS in international climate policy, focusing on the UNFCCC context. On the basis of these analyses, Chapter 5 provides conclusions and recommendations for future treatment of NbS in the UNFCCC process. CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 20 2 Definition of Nature-based Solutions ‘Nature-based Solutions’ (NbS) is an umbrella concept encompassing a variety of established approaches stemming from different sectoral and geographic backgrounds across policy, practice and academia and established in their respective sectors, such as e.g. ecosystem-based adaptation and mitigation, green and blue infrastructure or ecological restoration (Seddon et al. 2020; Pauleit et al. 2017; Nesshöver et al. 2017; IUCN 2016a; EEA 2021). The term itself indicates that through a provision of ecosystem services nature can provide solutions to societal challenges such as climate mitigation and adaptation, air quality, public health and well-being, water management or disaster risk reduction. Most established definitions highlight the aspect of multifunctionality, specifying that NbS should address specific social and environmental challenges, while also producing wider co-benefits. The first publication on NbS from the World Bank (2008) did not define the term but made the case that the sustainable use of natural ecosystems was critical to fulfilling the World Bank’s mission of alleviating poverty and supporting sustainable development. The publication also underlined that the sound management of ecosystems provides society with multiple benefits and opportunities (Sobrevila et al. 2008). Two widely used definitions of NbS stem from the International Union for Conservation of Nature (IUCN 2016a) and the European Commission (EC 2015; 2020) (see Table 1). The EC’s publication introducing NbS emphasised their relevance in urban areas and framed NbS in the context of green growth (EC 2015). The IUCN subsequently released a Global Standard for NbS, providing clear parameters for defining NbS and a common framework to help benchmark progress (IUCN 2020). More recently, the Nature-based Solutions Initiative, a consortium of conservation and development organisations and research institutions led by the University of Oxford created a set of NbS guidelines for successful, sustainable nature-based solutions in the context of climate change mitigation that were submitted to the UK COP presidency (Seddon et al. 2021). NbS principles were also proposed by WWF (WWF 2020). Other sources consulted for this study (e.g. Albert et al. 2017; Balian et al. 2014; Chausson et al. 2020; EC 2017; IIED 2018; IPCC 2019b; Maes and Jacobs 2017; Nature-based Solutions Initiative 2021; WWF 2020; UNEP 2021) either directly use one of these definitions or a variant thereof, building on similar terms and elements. Although the other definitions are not listed here, specific elements of relevance are discussed in Sections 2.1.1 and 2.1.2. Table 1: Definitions of NbS Source Definition IUCN (2016b) NbS are defined by IUCN as actions to protect, sustainably manage and restore natural or modified ecosystems, which address societal challenges (e.g. climate change, food and water security or natural disasters) effectively and adaptively, while simultaneously providing human well-being and biodiversity benefits. European Commission (2015) NbS aim to help societies address a variety of environmental, social and economic challenges in sustainable ways. They are actions inspired by, supported by or copied from nature; both using and enhancing existing solutions to challenges, as well as exploring more novel solutions, e.g. mimicking how non-human organisms and communities cope with environmental extremes. NbS use the features and complex system processes of nature, such as its ability to store carbon and regulate water flow, in order to achieve desired outcomes, like reduced disaster risk, improved human well-being and socially inclusive green growth. Maintaining and enhancing natural capital, therefore, is of crucial importance, as it forms the basis for CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 21 Source Definition implementing solutions. These NbS ideally are energy- and resource-efficient, and resilient to change, but to be successful they must be adapted to local conditions. European Commission (2020) NbS to societal challenges are solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social and economic benefits and help build resilience. Such solutions bring more, and more diverse, nature and natural features and processes into cities, landscapes and seascapes, through locally adapted, resource-efficient and systemic interventions. NbS must benefit biodiversity and support the delivery of a range of ecosystem services. Source: International Union for Conservation of Nature (2016b); European Commission (2015; 2020) Climate change is among the societal challenges that can be addressed with NbS which has increasingly been recognised by major international scientific bodies and governments. NbS have been highlighted in e.g. the Global Commission on Adaptation Report and the three IPCC Special Reports published since 2018. Also in the context of the UNFCCC process, NbS are playing an increasing role (see Chapter 4). This increased interest in NbS for climate mitigation highlights the importance of clearly defining NbS and ensuring individual interventions fulfil shared basic standards and criteria. For example, the 2019 “Nature-based Solutions for Climate Manifesto” supported by 70 governments, made calls to scale-up NbS for mitigation and mainstream NbS within climate policy-related instruments, and collected nearly 200 initiatives and best practices of NbS (Nature-based Solutions (NBS) Facilitation Team 2019; UNEP 2020; NbS for Climate Coalition 2020) – yet without ever defining what NbS actually are, or providing any qualifying criteria for NbS. Under carbon market approaches, the term NbS is often used rather specifically, e.g. as a synonym for mitigation activities in the land use sector or to achieve a price premium, when selling certificates for mitigation outcomes. Business-driven initiatives often convey the notion that there exists a solution to climate change and there is no need to decarbonise economies and curtail the use of fossil fuels (see e.g. the Natural Climate Solutions initiative3 by IETA or Shell’s communication on NbS).4 While ‘NbS for climate change mitigation’ are increasingly cited as such, there is a number of related, more narrow terms used in climate discussions. Some of these are defined based on the intended outcome (e.g. ecosystem-based mitigation, natural climate solutions, sustainable climate action), while others highlight the specific actions involved (ecological restoration, green and blue infrastructure). These terms and approaches share a common focus on enhancing the provisioning of ecosystem services as a means to address societal challenges and build on the understanding of the key roles that ecosystems play in supporting human well-being and safety (EEA 2021). 2.1 Core elements of the NbS definition This section discusses key defining characteristics of NbS as considered in existing definitions in order to arrive at a working definition of NbS. The definition in Section 2.1.1 clearly distinguishes between measures that do and do not constitute NbS, despite their use of nature or 3 https://ncs.ieta.org/ 4 https://www.shell.com/energy-and-innovation/new-energies/nature-based- solutions.html#iframe=L3dlYmFwcHMvMjAxOV9uYXR1cmVfYmFzZWRfc29sdXRpb25zL3VwZGF0ZS8 https://ncs.ieta.org/ https://www.shell.com/energy-and-innovation/new-energies/nature-based-solutions.html#iframe=L3dlYmFwcHMvMjAxOV9uYXR1cmVfYmFzZWRfc29sdXRpb25zL3VwZGF0ZS8 https://www.shell.com/energy-and-innovation/new-energies/nature-based-solutions.html#iframe=L3dlYmFwcHMvMjAxOV9uYXR1cmVfYmFzZWRfc29sdXRpb25zL3VwZGF0ZS8 CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 22 working with natural processes. Section 2.1.2 discusses characteristics of NbS that are included in some of the existing NbS definitions, but were excluded in the working definition created for this study. 2.1.1 Defining characteristics of NbS The core element of the NbS definition is a specification of the term “nature-based”. This refers to measures which are aligned with natural ecosystems. NbS are different from other measures that may use natural processes but which lack alignment with natural ecosystem processes and functions, require an ongoing and significant human intervention using engineered structures, ongoing provision of energy or water, or lead to soil sealing, ecosystem destruction, exploitation or harmful effects to biodiversity. NbS are differentiated from solutions that are nature-derived, i.e. which come from the natural world, but which are not directly based on functioning ecosystems (e.g. wind and solar energy) and solutions that are inspired by nature or modelled on biological processes (e.g. biomimicry), but which are not based on functioning ecosystems’ ability to provide natural services either (IUCN 2021a). In the context of climate change mitigation, “nature-based” refers to measures that use natural ecosystem processes such as CO2 uptake by photosynthesis and biomass build-up which can be used in a diverse cascade by different organisms of the specific ecosystem. The focus of NbS is on the protection, restoration, sustainable management or creation of ecosystems to build on their capacity for self-regulation, renewal, nutrient cycling and provision of various services (IUCN 2016b). At the same time, it is important to bear in mind that nearly all NbS measures include some degree of design or alteration of existing ecosystems, such as selecting certain species or prioritising a given ecosystem service over another, depending on the primary societal challenge to be addressed by the intervention (Nesshöver et al. 2017). Moreover, NbS can be used together with other measures to form so-called “hybrid solutions” that combine elements of grey infrastructure with natural elements, e.g. Sustainable Urban Drainage Systems. NbS include working with a variety of ecosystems, including modified or novel ecosystems in urban areas. Second, an important element distinguishing NbS from other solutions that use nature or natural processes is the explicit expectation that NbS provide benefits to biodiversity through enhancement of diverse ecosystem functions, ecosystem resilience and ecosystem health or protection or enhancement of species richness in a given ecosystem, or ecosystem richness in a given area. While not all climate NbS may realise the full potential of biodiversity benefits, positive contributions distinguish NbS from “actions that exploit nature to address societal challenges, but which create trade-offs and can damage biodiversity in doing so”, e.g. BECCS or commercial monoculture plantations that can disrupt natural ecosystem processes, remove or fragment habitats or directly harm habitats and species (Seddon et al. 2021). This stance is in line with both the IUCN (2021a) and the EC (2020) definitions of NbS. Long-term planning, preparing for change and maintaining natural adaptability in the context of NbS are further critical considerations (Nesshöver et al. 2017). This criterion is elaborated in the IUCN’s Global Standard: NbS implementation plans should consider the uncertainty inherent to ecosystem management, given their complex, dynamic and self- organising nature and consequently enable adaptive management “to effectively harness ecosystem resilience” to be able to “respond to unanticipated social, economic or climate events” with a wider range of options. Such adaptive management should result in a greater resilience and adaptive capacity of ecosystems (IUCN 2021a). The synergies of climate adaptation and NbS are also specifically included in the definition of “NbS for climate change” proposed by the WWF. They indicate that NbS for climate change “have human development and biodiversity co- benefits managing anticipated climate risks to nature that can undermine their long-term CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 23 effectiveness” (WWF 2020). The EC definition indicates that NbS should “help build resilience” (EC 2020). Another element is the need for NbS to be locally adapted (EC 2020) or locally appropriate (UNEP 2021), suggesting e.g. the use of native species and consideration of economic and social local conditions as well as tradition and culture. This is an important consideration in the context of NbS for climate mitigation, given the emphasis on tree planting as a way to increase carbon sinks. Not every afforestation or tree-planting project qualifies as NbS. For example, evidence shows that plantations involving fast-growing non-native species can introduce new pests and diseases or themselves become invasive and monoculture plantations harm biodiversity (Seddon et al. 2020) (see also Section 3.2.1). Moreover, the NbS concept represents a paradigm shift in ecosystem management, shifting away from single-objective management (e.g. separating conservation from water issues) and focusing on solutions that are multifunctional (i.e. providing numerous (co-)benefits in parallel for human beings and the environment) (Nesshöver et al. 2017). Furthermore, this delineates NbS from interventions such as BECCS, which do not generate additional ecosystem services (IUCN; Oxford University 2019). The multifunctionality aspect also differentiates NbS from Natural Climate Solutions. The two terms are sometimes used interchangeably but have a different meaning: Natural Climate Solutions is a narrower concept, focusing on only one objective (climate mitigation), although pointing to the associated co-benefits (Osaka et al. 2021). Finally, the explicit focus of NbS to address societal challenges distinguishes NbS from traditional conservation activities focused on e.g. the protection of individual species, without considering how they address societal challenges. According to the EC (2020) definition, NbS should simultaneously provide environmental, social and economic benefits. IUCN uses a more open framing, indicating that NbS should provide human well-being and biodiversity benefits while addressing societal challenges. We propose to use the IUCN framing of ‘human well-being’ due to the fact that traditional concepts of economic benefits and their measurement in monetary terms do not appropriately take into account natural capital or costs of biodiversity loss. The inclusion of economic benefits as a prerequisite could also put undue emphasis on generating short-term economic benefits at the cost of a long-term delivery of a full range of ecosystem services or an emphasis on functions quantifiable in monetary terms compared to functions for which this is difficult (see also IUCN 2016a). The reference to economic benefits would also require a more standardised methodological framework related to the measurement of economic benefits of ecosystems which is not yet widely implemented. The concept of ‘human well-being’ includes economic aspects, but in a more holistic and qualitative way, avoiding such potential bias arising from quantification methodologies. Table 2 summarises the arguments presented in this section, comparing key characteristics of NbS versus measures that may use nature or natural processes but do not meet the criteria outlined above and are therefore not qualified as NbS in light of scientific discourse. Based on the existing definitions, in particular the IUCN (2016) definition and following the previous arguments as well as the resulting delineation between NbS and non-NbS, we derive the following working definition of NbS. It comprises the key elements of the existing definitions, that we believe to be important to inform the scope of this study, as outlined above: Nature-based Solutions are locally appropriate, adaptive actions to protect, sustainably manage or restore natural or modified ecosystems in order to address targeted societal challenge(s) - such as climate change mitigation -, while simultaneously enhancing human well-being and providing biodiversity benefits. CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 24 Table 2: Defining characteristics of NbS NbS Non-NbS Alignment with natural ecosystem processes Use natural ecosystem processes (e.g. CO2 uptake by photosynthesis and biomass build-up which can be used in a diverse cascade by different organisms of the specific ecosystem); build on ecosystem capacity for self-regulation, renewal, nutrient cycling and the provisioning of various services. Lacks alignment with natural ecosystem processes; require an ongoing and significant human intervention using engineered structures, ongoing provision of energy or water; lead to soil sealing and ecosystem destruction. May come from natural world or be modelled on biological processes but is not directly based on functioning ecosystems. Benefit biodiversity Benefits for biodiversity are achieved by protecting and restoring natural ecosystem processes. They support the adaptive capacity and quality of ecosystems and habitats. Can damage biodiversity by disrupting natural ecosystem processes, removing or fragmenting habitats or directly harming habitats and species. Adaptability Is planned in a manner that supports the natural adaptability of ecosystems. Does not include considerations of adaptability and ecosystem resilience. Locally appropriate actions Consider local economic and social conditions and use native species. Uses non-native species; does not consider the characteristics, importance and ecological resilience of local ecosystems; is not designed with the local social and economic conditions in mind. Multi-functional Provides numerous (co-)benefits in parallel for people and the environment. Focuses specifically on one objective and/or does not generate additional ecosystem services. Address societal challenges and enhance human well-being Provides benefits to human well-being and helps to address societal challenges. Focuses on conservation, biodiversity or other objectives, without considering how they address societal challenges. Source: Own compilation, Ecologic Institute. CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 25 Figure 1: Graphic illustration of the core elements of Nature-based Solutions Design: Erik Tuckow, sichtagitation.de 2.1.2 Common qualities of NbS This section discusses two elements that are common to many NbS and are included in some of the existing NbS definitions and explains why these elements were excluded from the working definition created for the purpose of this study. CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 26 First of all, the EC definition states that NbS are solutions that are “cost-effective” (EC 2020). Cost-effectiveness is seen as a by-product of multifunctionality, with significant long-term (co-) benefits being produced alongside the primary targeted impact. The IUCN Global Standard for NbS requires that they are “economically viable”, i.e. that long-term gains are balanced against short-term costs, with short-term actions developed with a long-term perspective in mind. Sufficient consideration should be given to returns on investment, efficiency, and effectiveness and equity in the distribution of benefits and costs (IUCN 2020). However, a number of challenges are related to assessing cost-effectiveness. While literature on Natural Climate Solutions provides quantified evidence of their cost-effectiveness as a mitigation option (Osaka et al. 2021), assessing cost-effectiveness of the multifunctional NbS is more complex as: (1) many co-benefits are often not accounted for; (2) calculations need to factor in trade-offs between different measures or between stakeholder groups who may have different preferences and therefore perceive the benefits and costs differently; (3) it may be difficult to quantify changes in ecosystem service provisioning over time, not least due to uncertainty regarding future conditions; and (4) it may be challenging to attribute a positive change or benefit to a specific activity. Finally, while NbS might be a more effective solution in the long term, the full scale of the benefits might not be evident while the costs are incurred (IUCN; Oxford University 2019). As a consequence, we argue against including this qualification in the NbS definition – while agreeing with the IUCN’s indication that NbS projects should be appraised to the extent possible to ensure economic viability. Good governance including the wide variety of stakeholders impacted by or able to impact the delivery and maintenance of NbS is key to their effective implementation and long-term viability (Seddon et al. 2021; Nesshöver et al. 2017). The aspect of governance is considered in a definition proposed by the NbS Initiative of the University of Oxford, stating that NbS should be “designed and implemented with the full engagement and consent of local communities and Indigenous Peoples” (Nature-based Solutions Initiative 2021). Considerations of good governance are also included in the IUCN Global Standard for NbS, which requires that NbS are based on an “inclusive, transparent and empowering governance process” (IUCN 2020). However, while inclusive and transparent governance should be a standard for the design and implementation of NbS, it does not need to form part for the definition itself – as good governance is not something that specifically delineates NbS from other solutions. 2.2 Categorisation of NbS For the purposes of an assessment and discussion of NbS mitigation potentials in this study, we categorise NbS along three criteria: the ecosystem they are applied in, the type of greenhouse gas (GHG) emission mitigation as well as the type of management change, as shown in Table 3. These NbS are described and qualified further in Chapter 3. Related to the type of mitigation, we refer to a reduction of emissions when the activity that generates emissions is already occurring and when the amount of emissions not released into the atmosphere as a result of an NbS is compared against an existing level of emissions. For example, natural forest management involves decreased harvest intensity which leads to a decrease in emissions. We use the term avoided emissions when the emission-generating activity has not yet occurred and when the amount of emissions not released into the atmosphere is calculated against a hypothetical level of future emissions that would have occurred without the intervention (e.g. protecting an existing forest from being degraded or deforested). The removal of emissions is an active process where CO2 is removed from the atmosphere by e.g. photosynthesis and carbon is stored in biomass for long periods. CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 27 The third criterion for categorisation distinguishes NbS that require a change in management practices of an existing land use (practice shift), including ecosystem protection, from those that include land use changes (e.g. conversion of agricultural land to forest) or those that prevent a land use change (e.g. avoided grassland conversion). Table 3: Categorisation of NbS Type of GHG emission mitigation Management change Natural or modified ecosystem NbS Reduction Removal Avoided Practice shift Land use change Forests Reforestation & Afforestation X X X Natural forest management X X X Avoided forest conversion X X Forest protection X X Improved plantations X X X Croplands Nutrient management X X Agroforestry/ Trees in croplands/ Alley cropping X X X Improved manure management X X Conservation agriculture X X Cover crops X X X Improved rice cultivation X X Grasslands Grazing optimisation X X X Legumes in pastures X X X Grassland restoration X X X Avoided grassland conversion X X Terrestrial wetlands Peatland restoration X (X)+ X X X Peatland protection (X) + X X CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 28 Type of GHG emission mitigation Management change Avoided degradation/conversion of peatlands X X Coastal wetlands Coastal wetland restoration X X X X Coastal wetland protection X X X Avoided degradation/conversion of coastal wetlands X X Settlements Urban greening X X X Source: Own compilation, Öko-Institut, with selection of NbS pathways based on Griscom et al. (2017) and Roe et al. (2019). Notes: NbS are categorised according to ecosystems and the type of mitigation effect they have on greenhouse gas emissions as well as the change in management they imply. + Removals by peatlands are very slow and long-term compared to other ecosystems like forests and are therefore not the main benefit of this measure. Biochar as NbS? In simple terms, biochar is charcoal that is incorporated into soils. To produce biochar, biomass is heated in the absence of oxygen (pyrolysis) or under controlled low-oxygen conditions (gasification). Sources of the biomass can be wood, organic waste or other natural feedstocks. Biochar is traditionally used in some regions, e.g. in Thai traditional kiln biochar from Eucalyptus (Ding et al. 2016). The key assumption is that biochar persists for hundreds or thousands of years (under right conditions), thus storing carbon that would otherwise decompose. Various benefits of biochar are discussed in the literature: Many studies evaluate nutrient (N, P and K) availability from biochar and related higher crop yields. However, the findings of several studies have not been tested in field experiments (Ding et al. 2016). Jones et al. (2012) demonstrated that biochar had no effect on the growth of maize but increases growth of a subsequent grass crop. Effects on crop yields were related to the biomass source of the biochar, pyrolysis temperature and soil type. Some research indicates that biochar reduced N2O emissions from different soils to a large extent, however some studies found no such effect or even increase of N2O emissions after biochar application (Ding et al. 2016). Ding et al. (2016) conclude that the effect on N2O emissions could be dependent on biochar pyrolysis temperature, soil types, fertiliser doses and types, and soil water contents. Additionally, biochar has been found to increase microbial abundance in soils, change microbial composition and activity. But it can also have negative effects on microbial community due to harmful components (e.g. phenolic and polyphenolic substances, see below) (Ding et al. 2016). Moreover, biochar has been found to improve physical soil qualities and water holding capacities, but it is unclear whether these effects can be maintained over longer periods or whether they only occur immediately after biochar application. Overall, most laboratory and field studies were focused on the short-term effects of biochar on soil properties and few studies have conducted long-term experiments (Ding et al 2016). Large-scale industrial pyrolysis plants have not been built so far (Schmidt and Hagemann 2021) and costs for large-scale production infrastructure are not accounted for in the literature. The IPCC indicates a global mitigation potential of biochar of 0.03-4.9 GtCO2e/yr by 2050 and up to 6.6 GtCO2e/yr if energy substitution is included (Jia et al. 2019). According to Lal et al. (2018), CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 29 biochar bears sequestration potentials of 1.6-3.5 GtCO2e/yr. Griscom et al. (2017) estimate that biochar could deliver 1.1 GtCO2e/yr of carbon removals by 2030, if approx. 80% of biochar carbon remains stored for more than 100 years and assuming that there is no impact on methane or nitrous oxide emissions. Studies giving higher sequestration potentials of biochar assume that all crop residues globally are used for the production and subsequent burial of biochar (e.g. Lenton 2014). However, such assumptions on the availability of biomass are not realistic due to a high competition regarding the use of biomass for different purposes. The availability of excess feedstock biomass is limited for the production of biochar, leading to a lower “sustainable” global potential of 0.3-2.0 GtCO2e/yr in 2050 (Fuss et al. 2018; a similar order of magnitude is provided in Minx et al. 2018 and Hepburn et al. 2019). If biomass is removed from cropland areas for the production of charcoal/biochar, biomass inputs to soils will be missing for the formation of soil organic carbon which will reduce soil fertility. Biochar will also compete with biomass needs for bio-based products or for biomass as an energy source. Additionally, biochar needs large land areas for the production of biomass to produce charcoal. This adds to the competition for land. A broader lifecycle assessment is therefore necessary in order to determine the overall mitigation effect of biochar as an exogenous carbon input. It will depend on where and how the offsite biomass is removed, how it is transported and processed, what their alternative end use would be (burning, adding to landfill or left in place as residues), how it interacts with other soil GHG- producing processes and the condition of the soil to which the inputs are added (Paustian et al. 2016; Minasny et al. 2017; National Academies of Sciences, Engineering, and Medicine 2018). Additionally, the precise interactions of biochar with soils are uncertain (Smith 2016; Tammeorg et al. 2016). For example, in a study conducted by Budai et al. (2016), high temperature-produced biochar with a half-life 60 times higher than the parent material, enhanced the positive priming (increased mineralisation rate) of soil organic carbon (SOC), causing changes in the composition of bacterial and fungal communities due to increase in pH levels (see also Paustian 2016). Also, pollutants can be introduced into the soil by the pyrolysis of waste products (UBA 2016) and biochar application can also release black carbon aerosols which diminish air quality (Ravi et al. 2016). Biochar might change the albedo of soils when applied to large areas, which can lead to an increase in soil temperature and therefore loss of SOC. Hence, according to Bozzi et al. (2015) the biochar mitigation potential might be reduced by up to ∼30%. Also, experience with large-scale production and use of biochar is still missing and “feasibility, long-term mitigation potentials, side-effects and trade-offs therefore remain largely unknown” (Fuss et al. 2018, p. 26). The uncertain effects of biochar on biodiversity and ecosystem functions (UBA 2016) raise the question whether it can be considered an NbS according to the definition set in this paper. Especially the differing effects of the amount and duration of biochar application to soil microbial diversity mainly due to changes in altered soil pH (Jiang et al. 2016; Budai et al. 2016; Hardy et al. 2019) raise doubts about positive effects on biodiversity. Additionally, the production of biochar requires the provision of external energy input. For these reasons, and even though it is often listed as an example of NbS (e.g. Griscom et al. 2017; Bossio et al. 2020; Fargione et al. 2018), biochar should not be considered an NbS in the view of the authors (see NbS requirements in Table 2). CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 30 3 Assessment of the global potential of Nature-based Solutions Few studies have estimated the global mitigation potentials which could be achieved through NbS measures and measures in the land sector. Two studies stand out in terms of comprehensiveness and visibility. Griscom et al. (2017) have estimated that NbS (or "natural climate solutions") could deliver 37% of the necessary cost-effective CO2 mitigation potential 2030 (23.8 GtCO2 e/yr) and 20% by 2050. The highest potentials in the study are associated with forest-related measures such as reforestation and avoiding conversion of forest to other land uses. Moreover, establishing agroforestry systems and avoiding conversion of wetlands have a high climate change mitigation effect. Other global studies by Roe et al. (2019, 2021) use mainly results by Griscom et al. (2017, 2020) for measures in agriculture, forestry andwetlands as well as some additional studies on specific measures (e.g. Pendleton et al. 2012; Humpenöder et al. 2020; Paustian et al. 2016). Beside measures in the land sector, Roe et al. (2019) take into account the effects of bioenergy, BECCS and consumption behaviour (e.g. reducing food waste). They conclude that all of these measures could contribute up to 30% (15 GtCO2 e/yr) of global GHG mitigation required until 2050 to reach the 1.5 °C target. While these studies suggest that there is significant mitigation potential associated with NbS, there is an ongoing debate around how much NbS can realistically contribute to climate change mitigation because the potential estimates very much depend on time frames, considered land availability and other assumptions (Girardin et al. 2021). This means single estimates cannot be easily compared with each other, but rather need to be interpreted in the light of their differing assumptions and methods. This chapter critically reviews a number of prominent studies on measures associated with NbS in terrestrial (forests, croplands, grasslands, wetlands) and marine (coastal wetlands) ecosystems as well as settlements regarding their estimated mitigation potentials, applied methodologies and assumptions in order to develop a better understanding of the potential and limits of NbS measures as a mitigation strategy. Potentials provided in the literature comprise measures which do not necessarily meet the definition of NbS developed in Chapter 2 because they do not consider or specify ecological and social constraints. Therefore, important ecological and social requirements were formulated for each ecosystem considered to assess whether the considered measures qualify for NbS. 3.1 Methodological approach In a first step, the ecosystems were briefly introduced with respect to their role for the climate system, biodiversity protection and other ecological services. Also, the current drivers of destruction and degradation from land or marine resource use were highlighted. Finally, measures were defined which qualify as NbS and specifically address climate mitigation. In a second step, a summary of global mitigation potentials was compiled for each ecosystem based on a literature review, drawing on studies published after 2010 to include most recent methodological approaches. Another selection criterion was that measures are comparable regarding target and approach, especially in view of the study by Griscom et al. (2017) as the most comprehensive and most frequently cited study on NbS mitigation potential. In some cases, regional studies were included in the assessment of global potentials in order to better evaluate the specific potential (mitigation potential per area unit). These studies were assessed and compared with regard to the following aspects: CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 31 ► Scope: The scope of the study determines which GHGs have been covered, which carbon pools have been addressed, and which time frame has been set. The potential and conditions for the implementation of measures can differ with biogeographical regions, which can be divided into e.g. boreal, temperate, subtropical, tropical regions. The scope also includes the different type of mitigation measures, i.e. GHG emission reductions, CO2 removal and/or avoided GHG emissions (see Section 2.2 above). Moreover, the scope includes the type of management change that can be either a practice shift within one land use or a land use change. Finally, the alignment of the conceptualisation of mitigation measures in the studies with the definition of NbS developed in this paper is assessed. ► Range of total as well as specific mitigation potential: The mitigation potential is expressed in absolute terms, e.g. as MtCO2e per year but can also be related to the area on which it is implemented (specific potential), e.g. as tCO2e per ha and year. ► Approaches towards quantification of potential: Which approach is used to estimate the mitigation potential? The assessment can be top-down, e.g. using global simulation models or bottom-up, e.g. based on empirical data, project information and statistics. Is the input data geographically specific or unspecific? Are ecological processes explicitly modelled (e.g. tree growth) or does the study apply default values and generic assumptions? Quantifying the mitigation potential of NbS also requires assumptions on the baseline development. For measures related to afforestation and reforestation, the baseline typically assumes the original land use and can therefore be set easily and transparently, e.g. using historic data. Baseline setting for measures of avoided land conversion is more complex, as it needs to assume an expected future rate of carbon stock depletion. Baseline setting can considerably affect the mitigation potential, especially for the latter type of measures. Baseline setting is also used for assessing whether activities are additional to a country’s emissions development path. In this context, baselines determine whether mitigation measures can be considered as ambitious. Additionality also plays a role in the funding of mitigation measures, e.g. through results-based finance. ► Assumptions regarding safeguards and co-benefits: The implementation of NbS to achieve mitigation outcomes might imply other negative environmental or social effects, e.g. regarding biodiversity, food security, land tenure. The assessment evaluates to what extent safeguards are taken into account as constraints in the calculation of potentials. A focus will be put on biodiversity implications. At the same time, mitigation measures can also have co- benefits for other environmental and social goals. Co-benefits form an essential element of the definition of NbS (see Section 2.1.1). ► Assumptions on costs: Do the studies assess technical or economic potentials? Which types of costs of measures are assumed to achieve the given potential? Which assumptions are made with regard to the development of CO2 prices and opportunity costs? ► Constraints and uncertainties: Studies need to make assumptions and simplifications that limit their results, e.g. in terms of generalisability. Moreover, the approach taken, and the underlying data used determine the level of uncertainty associated with estimates. The question is whether and how uncertainties have been estimated and communicated regarding the mitigation potential, including the type of uncertainties that are associated with the estimation of potentials for different measures. Constraints can also include assumptions on how interactions with the climate system have been considered. The question is whether the studies take effects of climate change on ecosystems or other CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 32 climatic effects of e. g. vegetation changes into account. Such impacts can have positive or negative effects on the mitigation potential. 3.2 Assessment of potentials for different ecosystems 3.2.1 Forests State of the ecosystem and measures for NbS Globally, forests cover about 4,000 Mha (Harris et al. 2021) comprising important reservoirs that store carbon in living and dead biomass, forest soil and harvested wood products but also regulating water cycles, filtering the air, providing habitat to a large diversity of species and being an essential source for human well-being. Between 2001 and 2019 global forests removed carbon of about -15.6 GtCO2e/yr from the atmosphere but deforestation and forest disturbances resulted in global gross GHG emissions of about 8 GtCO2e/yr, mainly occurring in rainforests of South America and Southeast Asia due to commodity-driven deforestation (~3 GtCO2e/yr, Harris et al. 2021). Through different measures, NbS targeting forests can contribute to avoiding GHG emissions and increasing removals of CO2. Reforestation and afforestation activities introduce trees on areas without or only sparse tree cover in order to increase CO2 removals compared to the previous land use (e.g. agricultural land). Tree planting is a very popular activity which also gains a lot of public attention but also raises a lot of concerns (Seddon et al. 2021), which are further discussed below. Forest protection measures instead aim at avoiding potential emissions from forest conversion or forest degradation. Forest management for climate mitigation mainly addresses an increase in living and dead biomass as well as soil carbon through better management. Hence, harvesting cycles can be adjusted. Also, selective harvesting and a minimum diameter per tree species could be introduced to ensure forest reproduction. Forest soil protection can be addressed by applying a minimally invasive harvesting strategy (WBGU 2020). Furthermore, to comply with biodiversity needs, the diversity of site native tree species and natural tree age class distribution should be secured. Also, natural successional states of the forests should be represented in managed forests to maintain habitat structures. Harvested wood products cannot remove CO2 from the atmosphere but retain carbon from being emitted after biomass harvest. Therefore, the extension of lifetime of harvested wood products is also considered a measure to reduce emissions, although these measures extend to activities required beyond ecosystem boundaries. Hence, prolonging the use of harvested wood products is not considered as an NbS because it has no direct benefits for biodiversity. Also, mitigation options involving harvested wood products are independent from ecosystem processes once they have been extracted from the ecosystem. But they are a very important co-benefit for people managing forests. Forests deliver significant co-benefits, including ecosystem and biodiversity preservation, reduction of flooding, erosion, eutrophication as well as enhanced water quality and quantity (Nabuurs et al. 2007). Therefore, measures to preserve forests or extend their coverage are typically associated with these positive trade-offs. However, similarly to other land use changes, afforestation and reforestation can significantly affect provision of goods from the land to be afforested, including biodiversity services (e.g. in the case of afforestation of grassland), food and feed supply (e.g. in the case of afforestation of agricultural land) and thus increase competition for land. Previous land use is therefore an important factor for assessing the risk of leakage that occurs if the supply of goods is negatively affected and production is displaced to other areas with potential negative effects. NbS involving afforestation can reduce negative CLIMATE CHANGE Nature-based solutions and global climate protection - Assessment of their global mitigation potential and recommendations for international climate policy 33 impacts by constraining activities to unused land, bearing in mind that the definition of such lands can be challenging. Changes in forest management do not constitute a land use change and thus bear lower risks of leakage and negative trade-offs. However, measures leading to a reduction of timber and biomass supply can indirectly affect other areas through trade if not accompanied with demand-side measures, e.g. for improved efficiency and reduced consumption of wood. NbS related to forests may include changes of tree species composition. If such activities lead to the dominance of one species (e.g. monocultures) and reduction of ecosystem structure (e.g. even-aged forests) they could negatively affect biodiversity and would thus not comply with the definition of NbS. This applies to forest restoration and afforestation as well as to forest management activities. For example, evidence shows that plantations involving fast-growing non-native species can introduce new pests and diseases or themselves become invasive. Monoculture plantations harm biodiversity and negatively affect ecosystem resilience (Seddon et al., 2019). Another example is afforestation of fire-adapted savannah and dryland grassland ecosystems in which increased levels of biomass can lead to changes in the fire regime towards hotter fires and associated higher carbon losses (Bennett and Kruger 2015). Establishing resilient and healthy tree stands that secure the delivery of co-benefits of forests to establish a complex system of interactions among ecosystem elements, requires careful management over decades (Seddon et al. 2021, p. 1529). This holds especiall