REVIEWS Constraints and enablers for increasing carbon storage in the terrestrial biosphere Connor J. Nolan 1 ✉ , Christopher B. Field 1,2 and Katharine J. Mach 3,4 Abstract | Harnessing nature-based climate solutions (NbCS) to help simultaneously achieve climate and conservation goals is an attractive win-win. The contribution of NbCS to climate action relies on both biogeochemical potential and the ability to overcome environmental, economic and governance constraints for implementation. As such, estimates of additional NbCS-related terrestrial biosphere storage potential range from less than 100 GtCO2 to more than 800 GtCO2. In this Review, we assess the negative emissions contributions of NbCS — including reforestation, improved forest management and soil carbon sequestration — alongside their environmental, social and governance constraints. Given near-term implementation challenges and long-term biogeochemical constraints, a reasonable value for the expected impact of NbCS is up to 100–200 GtCO2 in negative emissions for the remainder of the twenty-first century. To sustainably reach this level, focus should be on projects with clear co-benefits, and must not come at the expense of a reduction in emissions from deforestation and forest degradation, rapid decarbonization and innovation from alternative negative emissions technologies. Rapid cuts to global carbon dioxide (CO2) emissions Whereas negative emissions technologies such as are needed to meet the Paris Agreement goal of limiting direct air capture and BECCS remain in early stages of anthropogenic warming to well below 2 °C. Stabilizing technological development and are not currently avail- climate change will eventually require net-zero emis- able at scales relevant for global climate mitigation4, sions. Slow progress towards decarbonization has CO2 removal via the terrestrial biosphere already oper- increased the emphasis on negative emissions in meeting ates on these scales. Increasing carbon storage in the long-term temperature aims1,2. terrestrial biosphere has been proposed as a climate Negative emissions describe intentional efforts to change response since at least 1990 (REF.9), but has been 3 remove CO2 emissions from the atmosphere . Examples the subject of increased interest in policy and manage- include direct air capture and bioenergy with carbon ment spheres as nature-based solutions or natural cli- capture and storage (BECCS), both of which are in the mate solutions10 (hereafter referred to as nature-based early stages of technological development and cannot climate solutions (NbCS) to circumvent defining what currently operate at scales relevant for global mitigation4. ‘natural’ is). 1Stanford Woods Institute for Other negative emissions technologies include human NbCS include land management actions, such as the Environment, Stanford University, Stanford, CA, USA. interventions that seek to increase carbon sequestration conservation, restoration and improved management in the ocean (including coastal vegetation restoration, in forests, wetlands, grasslands and agricultural fields, 2Department of Earth System 10 Science, Stanford University, enhanced ocean productivity and enhanced weather- that contribute to climate change mitigation . They Stanford, CA, USA. ing)5 and on land (including reforestation, biochar and comprise both avoided emissions and negative emis- 6 3Rosenstiel School of Marine some improved forest management actions) . Natural, sions. Some important NbCS, in particular, avoided and Atmospheric Science, unmanaged land and ocean CO2 uptake that make up deforestation and avoided forest degradation, both of University of Miami, Miami, background sinks are not negative emissions because which are valuable conservation and climate mitiga- FL, USA. they do not contribute additional CO2 removal. The CO2 tion interventions, do not contribute to negative emis- 4 Leonard and Jayne Abess uptake of these unmanaged sinks is very important in sions, even though they facilitate the perpetuation of Center for Ecosystem Science and Policy, University of both global and jurisdictional carbon budgets, and their unmanaged sinks. Miami, Coral Gables, FL, USA. future is uncertain, especially for land. But assumptions Four main interventions, avoided deforestation, ✉e-mail: [email protected] about their continued uptake are embedded in calcula- reforestation, (improved) forest plantations and soil https://doi.org/10.1038/ tions of remaining CO2 emissions budgets and negative carbon sequestration, can be used to represent the s43017-021-00166-8 emissions needs7,8. diversity of NbCS interventions and their associated 436 | JUNE 2021 | VOLUME 2 www.nature.com/natrevearthenviron 0123456789();: REVIEWS Key points persistent problem. It is especially acute for soil carbon sequestration15,16 (Table 1). • Land management interventions can contribute to climate change mitigation Estimates for NbCS potential cover a wide range6,17–19, through avoided emissions from deforestation and forest degradation, and through spanning ~100 to ~1,200 GtCO . Given the poten- negative emissions from increasing carbon dioxide removal via reforestation, soil 2 tial value of NbCS, policy and management inter- carbon sequestration and more. est in them has also increased. For instance, several • The largest existing estimates of negative emissions potential from nature-based non-governmental organizations focused on climate and climate solutions implicitly rely on a potentially risky strategy of increasing carbon storage beyond historical bounds. conservation have initiated or expanded ‘Trillion Tree’ and other NbCS programmes. Moreover, the ‘Trillion • More conservative estimates that focus on refilling past carbon losses from the terrestrial biosphere are likely to be more feasible and have more co-benefits. Trees Act’ was introduced in the United States House of Representatives (H.R. 5859) in February 2020. Of • Successful implementation of nature-based climate solutions requires rapid increases in financing, increased on-the-ground capacity, and robust policy and governance course, tree planting is just one example of the kinds mechanisms. of NbCS interventions that have seen large increases • In the absence of broader climate action, climate change impacts on the biosphere in attention and ambition. NbCS have featured prom- will limit the potential for nature-based climate solutions to contribute negative inently in many nationally determined contributions to emissions. the Paris Agreement and are expected to have a large platform at the 2021 United Nations Climate Change Conference, also known as COP26, in Glasgow20. challenges (Table 1). For example, avoided deforestation Yet, despite increasing emphasis on NbCS, land use and reforestation have higher potential environmen- and land cover change remain a source of 4–7 GtCO2 per tal co-benefits, such as biodiversity protection, than year, mainly from tropical deforestation8,21. Current efforts forest plantations11. Intermediate strategies include to prevent or offset these losses through NbCS, while some improved forest management actions, which increasing, remain limited. A broad suite of NbCS and can simultaneously increase carbon storage in forests related interventions have resulted in cumulative emis- (REF.22) and still provide economic benefits from occasional sions reductions of 1.1 GtCO2 from 2006 to 2018 . lumber harvest12, and proforestation, which can max- More specifically, forest-based net removals (including imize carbon storage in intact forests and encourage afforestation, reforestation, agroforestry, improved for- the growth of large, old trees13. Soil carbon sequestra- est management and avoided deforestation) resulted tion can be done on the same land as agricultural pro- in a cumulative total of 0.365 GtCO2 over 2010–2016 duction and, thus, has relatively low opportunity costs. (REF.23). The 67 afforestation or reforestation projects that The situation is similar for forest plantations, which can were part of the Kyoto Protocol’s Clean Development provide their own economic benefits via the sale of tim- Mechanism represent emission reductions of 0.002 GtCO2 ber products. By contrast, durable climate mitigation per year (REF.24). With cumulative negative emissions of from avoided deforestation and reforestation requires 550–1,017 GtCO2 likely needed through 2100 to limit the land be allocated permanently (at least timescales anthropogenic warming to 1.5 °C (REF.1), it is important to ~100+ years) to the NbCS intervention14. These time- ask how much can come from NbCS, which can provide scales and opportunity costs lead to differing amounts a relatively small but important contribution to overall of coordination required for successful implementation. carbon removal25–29. If NbCS do not meet the total need, Ability to measure the additional carbon storage is a a portfolio approach will be required30. Table 1 | Characteristics of different mechanisms and measures through which carbon is stored in the terrestrial biosphere Characteristic Unmanaged Unmanaged Avoided Reforestation Forest Soil carbon fertilization regrowth deforestation plantations Carbon storage Terrestrial carbon Yes Yes Yes Yes Yes Yes category storage? Nature-based No No Yes Yes Yes Yes climate solution? Negative emissions? No No No Yes Yes Yes Intervention required? No No Yes Yes Yes Yes Total potential contribution to negative Low Low None Medium Medium Medium/high emissions needs Stability Medium High Medium Medium Low Medium Level