Evaluating the near- and long-term role of carbon dioxide removal in meeting global climate objectives

The Intergovernmental Panel on Climate Change (IPCC) uses Integrated Assessment Models (IAMs) to explore pathways for meeting global climate targets. Mitigation involves deep cuts in greenhouse gas (GHG) emissions and various CDR methods. However, the IPCC's Sixth Assessment Report (AR6) lacked a complete assessment of total CDR deployment due to inconsistencies in reporting methodologies for land-based CDR (afforestation and reforestation) across different modeling frameworks. Existing analyses of AR6 scenarios either omitted scenarios without land removal data or used net-negative CO2 emissions from the agriculture, forestry, and other land use (AFOLU) sector as a proxy for land-based removals, neglecting current removals and near-term land sector dynamics. This creates a critical data gap, hindering a comprehensive understanding of the mitigation solution space. A complete picture requires information on both gross emissions reductions and total removals to evaluate the contributions and trade-offs of different mitigation options. This study aims to address this gap by providing a comprehensive global and regional assessment of total CDR in mitigation scenarios, using a novel dataset of land-based carbon fluxes derived from the AR6 scenario database. The analysis focuses on three IPCC pathway categories (C1: limiting warming to 1.5°C with limited overshoot; C2: returning warming to 1.5°C after a high overshoot; C3: limiting warming to 2°C) and two CDR categories: conventional land-based CDR (afforestation, reforestation) and novel CDR (BECCS, DACCS, enhanced weathering). Understanding the contributions of these different mitigation strategies across various scenarios and regions is crucial for informing effective climate policy.

Previous research has highlighted the importance of carbon dioxide removal (CDR) in achieving ambitious climate targets. Studies such as those by Strefler et al. (2018) and Prütz et al. (2023) have explored the role of CDR in different mitigation pathways, emphasizing the trade-offs between the scale of CDR deployment and its associated costs. Other research has focused on the fairness and feasibility of deep mitigation pathways incorporating novel CDR technologies, considering institutional capacity (Gidden et al., 2023). The IPCC's AR6 report provided valuable insights into mitigation pathways, but lacked a comprehensive assessment of total CDR deployment, particularly concerning land-based removals. This absence of complete data on gross emissions reductions and total removals created a gap in the understanding of mitigation options. The current work builds on this existing literature by providing a more comprehensive and detailed analysis of CDR across a wider range of scenarios and regions, utilizing a novel dataset to address the methodological limitations of previous studies. This improved data analysis allows for a more robust assessment of the role of CDR in achieving various climate objectives, including consideration of both near- and long-term contributions. Analysis of the AR6 scenarios has revealed inconsistencies in how land-based CDR was accounted for across the models, highlighting the need for standardization in reporting methodologies.

This study utilizes a novel dataset of land-based carbon fluxes derived from the IPCC AR6 scenario database. This dataset, created using the compact Earth system model OSCAR v3.2, allows for the separation of gross carbon dioxide removal in the land sector into its components. The input variables include reported CO2 emissions from the AFOLU sector, land cover data (cropland, forest, pasture), and climate assessment data from the MAGICC v7.5.3 model. The analysis covers three IPCC pathway categories (C1, C2, C3) defined by their climate outcomes (limiting warming to 1.5°C with limited overshoot, returning warming to 1.5°C after high overshoot, and limiting warming to 2°C respectively). Two CDR categories are considered: conventional land-based CDR and novel CDR. The study assesses the difference in GHG emission reduction rates across different timeframes and pathway categories. It evaluates the contributions of gross emission cuts, residual emissions, and total CDR to overall net GHG reductions. The regional assessment is conducted at the R5 regional level defined by the IPCC. The analysis explores the regional distributions of gross emissions and total CDR in cost-effective mitigation pathways, considering the varying contributions of CO2 and non-CO2 emissions. The impact of delaying mitigation action on the volume and composition of CDR is also evaluated, considering the time to halve net CO2 emissions and the time to reach global net-zero CO2. The relationship between global bioenergy demand at net-zero CO2 and the deployment of conventional and novel CDR is also investigated to explore broader sustainability concerns. This research uses median values and interquartile ranges to report results and better understand the range of uncertainty associated with the scenarios.

The study's key findings demonstrate that reducing gross CO2 and non-CO2 emissions is crucial for meeting climate targets. Specifically: 1. **Dominance of Gross Emission Reductions:** Over 80% of net GHG reductions between 2020 and global net-zero CO2 are achieved through cuts in gross emissions, irrespective of climate objective stringency. This highlights the paramount importance of reducing current emissions sources. 2. **Critical Near-Term Role of Conventional Land-Based CDR:** In 1.5°C pathways with limited overshoot (C1), conventional CDR (afforestation, reforestation) accounts for approximately 10% of net GHG reductions between 2020 and 2030, nearly doubling CDR volume in this decade. This underscores the immediate importance of sustainable land-use strategies. 3. **Long-Term Role of Novel CDR:** Novel CDR technologies such as DACCS scale to multi-gigatonne levels by 2050 and beyond to balance residual emissions and draw down warming. They become a more significant component of mitigation strategies post-net-zero CO2. 4. **Regional Variations:** The composition of CDR and gross emission reductions varies across world regions. Asia consistently shows the highest cumulative gross GHG emissions and CDR, followed by the OECD and EU region. The Middle East and Africa region exhibits a higher proportion of non-CO2 emissions, primarily CH4 and N2O, reflecting diverse energy systems and land-use practices. Latin America, conversely, frequently displays net negative GHG emissions owing to substantial CDR. 5. **Impact of Delayed Mitigation:** Delaying mitigation efforts increases the reliance on novel CDR technologies, particularly after 2050, to achieve net-zero CO2 emissions. The amount of total CDR deployed until net-zero CO2 increases with delays in emissions reductions. Delaying net-zero CO2 increases the total CDR needed by a substantial margin. 6. **Sustainability Concerns and Bioenergy:** While increasing crop yields for second-generation biofuels can partially mitigate land-use competition concerns linked to BECCS, the level of global bioenergy demand at net-zero CO2 only weakly correlates with total CDR. This highlights the need to balance CDR deployment with broader sustainability considerations to reduce impacts on land, water, and biodiversity.

This study's findings emphasize that achieving ambitious climate goals requires a multifaceted approach combining deep cuts in current emissions with significant CDR deployment. While gross emission reductions dominate in the near term, novel CDR technologies become increasingly crucial in the long term to address residual emissions and potentially reverse warming. The substantial regional variations highlight the need for equitable distribution of mitigation efforts and CDR deployment, acknowledging differences in historical emissions, current capacities, and future development paths. The observed correlation between delayed mitigation and increased reliance on yet-to-be fully developed novel CDR technologies underscores the importance of immediate and ambitious emission reduction targets to avoid dependence on unproven and potentially problematic large-scale technologies. Moreover, the findings stress the necessity to integrate broader sustainability considerations into climate policy decisions, balancing the need for CDR with potential environmental and social impacts, particularly concerning land use, biodiversity, and energy demand. Further research should focus on resolving uncertainties around climate-related feedbacks on land-based CDR potentials and fully understanding the lifecycle emissions associated with novel technologies.

This paper presents the first comprehensive assessment of gross emissions reductions and CDR across AR6 mitigation scenarios, utilizing a novel dataset separating land-use sector emissions into their components. The study strongly emphasizes the necessity of deep cuts in current emission sources, with conventional land-based CDR playing a vital near-term role, particularly in Latin America and Asia. However, novel CDR technologies, while currently expensive and under-developed, will become increasingly important for long-term mitigation. The significant regional variations in CDR deployment highlight the importance of incorporating fairness and sustainability considerations into future scenario assessments. Limitations include the absence of comprehensive climate-related feedbacks in the scenarios and variations in accounting conventions between models and UNFCCC reporting. Future research should address these gaps and focus on fair distribution of mitigation investments, consistent with the principles of the Paris Agreement.

The study acknowledges some limitations. The scenario literature lacks comprehensive representation of climate-related feedbacks (e.g., sink strength changes due to drought or wildfires) that might substantially reduce land-based CDR potentials. The regional-level analysis, while informative, might obscure variations within regions. Moreover, the study uses model-based land-use accounting conventions that may differ from those employed by parties to the UNFCCC, highlighting the importance of translating between these conventions. The lack of full accounting for potential co-benefits and side-effects of CDR technologies is another area for future research. Finally, this work considers cost-effectiveness in the models as the main driver of technology deployment which may not reflect realities of equitable distribution of responsibility for climate action.