Carbon sinks: What are they, and what is their global potential?

This article is the first in a series on carbon sequestration published as part of the Net Zero Initiative in late 2024.

Although the concept of carbon offsetting is now being criticized or rejected by a growing number of standards and leading organizations, companies are still expected to demonstrate their ability to establish a pathway for contributing to carbon sequestration, at a level of ambition consistent with the goal of global carbon neutrality.

While methodologies for setting gross emissions reduction targets—such as those of the SBTi—exist for many economic sectors, there is not yet a methodological consensus on how a company should develop a “science-based” carbon sequestration pathway.

Recent publications, such as the latest SBTi guidelines on net-zero^[1] and on the FLAG^[2] (Forestry, Land, and Agriculture), as well as the draft document from the GHG Protocol, *Land Sector and Removals Guidance*^[3] contribute to the methodological framework for implementing these carbon sequestration pathways. This series of articles aims to clarify the challenges, scope, and limitations of existing methods for carbon sequestration pathways by comparing them with the principles of the Net Zero Initiative (NZI).

A first article providing an overview of global emissions and removals from the land sector highlights issues related to the scope of accounting, on the one hand, and removal potential, on the other. The challenges of setting sequestration targets (SBTi standard) are analyzed in a second article in light of the NZI framework. The distribution of sequestration efforts among the various economic actors is addressed in a third article, based on a detailed analysis of the SBTi FLAG framework and the GHG Protocol’s draft document.

Introduction

Global carbon neutrality—a goal set by the Paris Agreement and outlined in Article 4 for the second half of the century—consists of balance greenhouse gas emissions and removals (also known as absorptions^[4]) of human origin:

With a view to achieving the long-term temperature goal set forth in Article 2, the Parties shall strive to […] achieve a balance between anthropogenic emissions by sources and anthropogenic removals by sinks of greenhouse gases during the second half of the century, on the basis of equity, and in the context of sustainable development and poverty reduction^[5].

CO2 is the only one of the major greenhouse gases (CO2, CH4, N2O) that can be removed from the atmosphere through natural or technological sequestration processes. Therefore, CO2 sequestration targets must offset the CO equivalent of all greenhouse gas emissions, not just CO2.^[6].

The Showsgreenhouse gasesare now widely adopted by governments, businesses, and civil society, which are committing to increasingly comprehensive reduction pathways.  However, carbon sequestration remains a complex issue that is difficult for all these stakeholders to fully grasp, yet on which a significant portion of our global climate goals depends. This article provides an overview of the current state of carbon sinks worldwide and their overall sequestration potential for the coming decades, in light of global emissions levels.

Inventory of Anthropogenic Emissions and Sequestration from the Land Sector

Anthropogenic sequestration falls underfrom the land sector known as “AFOLU” (Agriculture, Forestry, and Other Land-Use), which covers emissions and removals from managed lands, excluding indirect effects related to climate change^[7]. In the IPCC's Sixth Assessment Report (Chapter 7), the annual net anthropogenic emissions from the AFOLU sector estimated using global vegetation dynamics models (DGCMs)^[8] amounted to 11.9 GtCO2e/year over the 2010–2019 period, representing approximately 20% of total net anthropogenic greenhouse gas emissions worldwide. They aredistributed as follows (see Figure 1):

• About half of this—5.8 GtCO2e/year—is attributable to emissions from agriculture: enteric fermentation, rice cultivation, soil and manure management, and fertilizers (shown in shades of blue and red on the graph).
• The other half—6.1 GtCO2e/year—comes from land use, land-use change, and forestry (LULUCF)^[9], circled in green on the graph. This category includes theland-use changes, such as deforestation and afforestation, and land management, including timber harvesting and regrowth, drainage and fires in peatlands, and the management of cropland and grasslands. Global vegetation dynamics models estimate anthropogenic terrestrial CO2 flux by considering only the impact of direct effects, and only in areas that have been subject to intensive and direct management, such as clear-cutting.

UTCATF emissions (circled in green in the graph above) are “net” emissions: they represent the difference between gross emissions and removals (see Figure 2). The IPCC does not provide details on gross emissions and removals, but according to the Global Carbon Budget 2020:

• Gross emissions averaged 16.2 GtCO2/year over the 2010–2019 period and stemmed primarily from deforestation and other logging activities;
• Carbon sequestration averaged approximately -10.5 GtCO2/year over the 2010–2019 period and resulted primarily from afforestation, reforestation, and the management of cultivated lands (grasslands, hedgerows, etc.).

*Global Carbon Budget 2020: https://essd.copernicus.org/articles/12/3269/2020/. A recalibration was performed to align with the annual net UTCATF emissions from the 2022 IPCC report (+6.1 GtCO2e/year). The baseline data from the Global Carbon Budget 2020 estimate gross emissions at 16.2 GtCO2/year and removals at -10.5 GtCO2/year for the period 2010–2019, resulting in net emissions of +5.7 GtCO2/year.

What is the planet's overall carbon sequestration potential?

The overall technical potential for additional carbon sequestration^[10] The land sector is subject to very high levels of uncertainty and is heavily dependent on the assumptions made: the pace of adoption of agricultural and forestry practices, competition for land use related to dietary patterns and energy demand, the costs and financing of these efforts, climate-related risks, and cultural and institutional barriers, among other factors. The IPCC distinguishes between the following:

• The overall technical potential for additional carbon sequestrationestimated at 22.2 GtCO2/year, which corresponds to the biophysical potential of these carbon sinks. This theoretical potential amounts to the assumption that the development of carbon sinks takes precedence over all other objectives (food, energy, economic, etc.) and that all barriers (financial, human, political, climate-related, etc.) do not are not limiting.
• The Economic Potential of Additional Carbon Sequestration estimated at between 1.1 GtCO2e/year (cost less than 20 USD/tCO2e) and 8.3 GTCO2e/year over the 2020–2050 period, which is considered likely to be achievable by a cost of less than 100 USD/tCO2e. Although cost is by no means the only factor in realizing carbon sequestration potential, the economic potential can be considered both an ambitious and realistic estimate.

In the paper by Roe et al.^[11] (2019), which serves as the basis for the SBTi’s FLAG framework, the potential for additional carbon sequestration in the land sector (compared to sequestration trends) is estimated at 7.6 GtCO2/year, which is in line with the IPCC’s economic potential at a cost of less than 100 USD/tCO2e (8.3 GtCO2/year). These sequestration measures fall into four distinct categories and are shown in the graph below according to their varying levels of technical and financial feasibility:

• The restoration of forests, coastal wetlands, and peatlands
• Improved Forest Management and Agroforestry
• Carbon Sequestration in Agricultural Soils and the Use of Biochar
• The Deployment of BECCS Technologies^[12]

The IPCC report rightly emphasizes that the AFOLU sector thus offers significant short-term mitigation potential at a moderate cost (compared to other sectors), particularly through carbon sequestration measures, but it cannot compensate for the lag in emissions reductions in other sectors. In fact, the “economic potential” of additional carbon sequestration—estimated at approximately 8 GtCO2e/year (at a cost of less than 100 USD/tCO2e), is far from sufficient to offset total GHG emissions, estimated at an average of 56 GtCO2e/year between 2010 and 2019, as illustrated in the figure below. Furthermore, effectively harnessing this potential faces major challenges in practice and is far from guaranteed: technical and organizational challenges related to changes in practices; competition for land use in the context of population and/or economic growth; access to appropriate and sufficient financing; the impact of climate change on ecosystems.

In accordance with the principles of the Net Zero Initiative, achieving global carbon neutrality therefore requires a massive and simultaneous effort to reduce emissions (Pillar A) and sequester carbon (Pillar C), with no possibility of offsetting between these two types of action at the organizational level.

The issue of how to mobilize the technical and financial resources needed to harness this limited sequestration potential among the various stakeholders is therefore as central as it is challenging. The GHG Protocol Land Sector and Removals (draft) and SBTi FLAG guidelines, released in 2022, provide new methodological frameworks on this subject, which are analyzed in the following articles in light of NZI’s principles and methods.

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Appendix - Scope of Accounting for Anthropogenic Sequestration

The concept of global carbon neutrality requires a precise definition of the scope of these anthropogenic removals. However, defining this scope is complex and varies depending on the methods used, particularly among the scientific models of climate dynamics and vegetation used by the IPCC^[13] or the Global Carbon Budget^[14] and national greenhouse gas inventories.

In global vegetation dynamics models, other net carbon fluxes from the atmosphere to vegetation are estimated but are considered non-anthropogenic and are not included in the scope of their land-use inventory. These net carbon fluxes are estimated at 12.5 GtCO2/year for the period 2010–2019 and are broken down as follows:

• Approximately 5.5 GtCO2e/year on managed lands, corresponding to indirect effects environmental changes affecting non-intact “managed” forests (climate change, CO2 concentrations, or nitrogen deposition).
• Approximately 7 GtCO2e/year on unmanaged lands, corresponding to the same indirect effects on unmanaged intact forests (boreal forests and certain parts of tropical forests)

In other words, this means that for “managed lands,” global vegetation models distinguish and separate, in terms of vegetation change, (i) factors resulting from direct management effects, such as logging or afforestation; and (ii) effects considered indirect, linked to global environmental changes such as increased biological growth due to rising CO2 concentrations.

However, it is virtually impossible to distinguish between direct and indirect fluxes on managed lands in the monitoring exercises conducted by national greenhouse gas inventories. As a result, a difference in the definition and scope of the anthropogenic fluxes considered across climate models that exclude indirect effects, and national inventories which include these effects on managed lands (see Table 1 below).

In AR6, the IPCC proposes reconciling the two approaches by reincorporating the indirect effects on managed lands into the scope of anthropogenic carbon sequestration in global models, so that the results are comparable to national inventories. This recalibration of the scope of national inventories results in average annual net emissions for UTCATF estimated at 0.6 GtCO2/year over the 2010–2019 period, as shown in Figure 3. The annual net balance of carbon fluxes between land and the atmosphere corresponds to sequestration of 6.4 GtCO2/year.

These differences in scope highlight the various ways of considering and accounting for carbon fluxes from the atmosphere to vegetation:

• 10.4 GtCO2/year of gross carbon sequestration is accounted for within the UTCATF anthropogenic sequestration category (see Figure 2) as a result of afforestation, reforestation, regrowth after harvesting, and other carbon-sequestering cultivated land management practices;
• An additional 5.5 GtCO2/year of carbon sequestration on managed lands resulting from indirect effects of climate and environmental changes. These indirect effects are considered anthropogenic and are included in national GHG inventories, but are considered non-anthropogenic and are not included in the IPCC model inventories;

An additional 7 GtCO2/year of carbon sequestration resulting from indirect effects on unmanaged lands is considered non-anthropogenic and is not included in any of these inventory scopes (IPCC models or national inventories).