The Paris Agreement sets ambitious goals to limit global temperature rise this century well below 2°C above pre-industrial levels, aiming to pursue efforts to limit the increase to 1.5°C. Achieving these targets necessitates stringent limitations on cumulative greenhouse gas (GHG) emissions, requiring a global peak in emissions as soon as possible, followed by rapid reductions to achieve a balance between emissions and removals by sinks in the second half of the century. Recent net-zero emission pledges from various nations and regions underscore the urgency of this goal. Reaching net-zero CO2 emissions is crucial, but achieving stringent climate targets requires combined mitigation of both CO2 and non-CO2 GHGs. Non-CO2 GHGs, representing a significant portion of total CO2-equivalent emissions, necessitate focused mitigation efforts to lessen future climate forcing. Previous research highlights the dependence of ultimate temperature warming on achieving net-zero CO2 and managing residual non-CO2 emissions. However, inconsistencies in modeling approaches and diverse mitigation options across studies result in variability in carbon budget estimates and net-zero commitment years needed to achieve the same temperature targets. The evaluation of system-wide non-CO2 GHG mitigation benefits is complex due to numerous mitigation options, the intertwinement of CO2 and non-CO2 mitigation actions across sectors, and the evolving techno-economic potential of mitigation technologies. A robust analysis demands an integrated modeling approach combining detailed sectoral and regional mitigation data with a comprehensive representation of the economic system across various sectors (energy, industrial processes, buildings, transport, urban processes, and agriculture). This study addresses these challenges by integrating the latest non-CO2 abatement data with the Global Change Analysis Model (GCAM) to explore how CO2 and non-CO2 mitigation pathways jointly impact temperature change, specifically examining the impact of comprehensive non-CO2 mitigation on the timing of net-zero CO2 required to achieve 1.5°C and 2°C targets.

Existing literature emphasizes the critical role of both CO2 and non-CO2 greenhouse gas mitigation in achieving the Paris Agreement's ambitious temperature targets. Studies consistently demonstrate that reducing non-CO2 emissions can significantly lessen future climate forcing. However, significant variation exists in the estimated remaining carbon budgets and the years needed to reach net-zero CO2 emissions to achieve 1.5°C or 2°C temperature goals. This variation stems from differences in modeling approaches, including the representation of non-CO2 mitigation options and the economic structures of the models used. The challenge of comprehensively evaluating the climate benefits of system-wide non-CO2 mitigation arises from the difficulties in identifying and parameterizing the numerous potential mitigation options for different gases emitted from various sectors. Furthermore, the intertwined nature of CO2 and non-CO2 mitigation actions across sectors complicates analysis, along with the ongoing evolution of techno-economic mitigation potential for each source and mitigation technology. This study builds upon and contributes to the existing literature by utilizing a sophisticated integrated assessment model and up-to-date data to produce a more nuanced understanding of the interactions between CO2 and non-CO2 mitigation strategies.

This research utilizes the Global Change Analysis Model (GCAM), a state-of-the-art integrated assessment model, coupled with the latest region-, sector-, and year-specific non-CO2 abatement datasets from the U.S. Environmental Protection Agency (EPA). The study constructs 90 mitigation scenarios by combining 30 CO2 abatement pathways with three levels of non-CO2 abatement: CO2 abatement only (no additional non-CO2 measures), CO2-driven GHG abatement (including emission reductions associated with fuel switching and demand reduction driven by CO2 abatement), and Comprehensive GHG abatement (incorporating specific non-CO2 abatement measures driven by increased carbon prices). The CO2 abatement pathways are defined by the year in which net-zero CO2 emissions are reached, ranging from 2030 to 2100 for 2°C scenarios and incorporating negative CO2 emissions for 1.5°C scenarios. The non-CO2 abatement levels consider the mitigation potential of various non-CO2 GHGs (CH4, N2O, HFCs, PFCs, and SF6) across all sectors. The GWP-100 (Global Warming Potential based on a 100-year timeframe) is used to estimate CO2-equivalent emissions for non-CO2 GHGs. GCAM simulates the interactions between economic, energy, land use, water, and climate systems. The model incorporates historical emissions data from CEDS and harmonizes it with the EPA’s non-CO2 emission projections and mitigation potential data. Marginal abatement cost (MAC) curves are used to represent the cost-effectiveness of different non-CO2 mitigation options, reflecting technological advancements and regional variations. The model projects future emissions based on economic activity and technology-specific emission factors. Sensitivity analyses explore alternative technology change assumptions, socioeconomic pathways (SSPs), and GWP assumptions to assess the robustness of the findings. The study uses the Hector climate model to translate the emission projections into radiative forcing and temperature changes.

The study's key findings demonstrate the significant contribution of non-CO2 emission reductions to achieving 1.5°C and 2°C climate targets. The analysis reveals a crucial interplay between CO2 and non-CO2 mitigation strategies. Achieving the same temperature targets without implementing specific non-CO2 abatement measures would require reaching net-zero CO2 emissions approximately two decades earlier than scenarios that incorporate comprehensive non-CO2 mitigation. Specifically, for the 1.5°C target, the model shows that comprehensive GHG abatement, incorporating non-CO2 mitigation, allows for net-zero CO2 emissions by 2053, whereas CO2 abatement alone or CO2-driven GHG abatement (only considering reductions from fuel switching) requires net-zero CO2 to be reached much earlier. Similar differences in the timing of net-zero CO2 are observed for the 2°C target. Decarbonization-driven fuel switching effectively reduces non-CO2 emissions from fuel extraction and end use. However, specific non-CO2 abatement measures are essential for significantly reducing fluorinated gas emissions from industrial processes and cooling sectors. The analysis identifies HFCs (used in refrigeration and air conditioning) and SF6 (used in electrical systems) as having significant mitigation potential. The model shows a substantial reduction in these emissions with comprehensive GHG abatement. Agriculture remains a major source of non-CO2 emissions, with limited cost-effective mitigation options currently available; therefore, further technological innovations are necessary to substantially reduce emissions from this sector. Sensitivity analyses, exploring alternative technology change assumptions, socioeconomic pathways, and GWP assumptions, largely support the study’s main findings, although the socioeconomic pathways significantly impact the results. The spatial analysis shows that comprehensive GHG abatement achieves greater non-CO2 reductions across all regions in 2050, particularly in China and the US, and also in rapidly developing economies where industrial and cooling demands are increasing.

The results of this study underscore the importance of a comprehensive approach to greenhouse gas mitigation that fully integrates both CO2 and non-CO2 reductions. Underestimating the role of non-CO2 mitigation can lead to a biased assessment of carbon budgets and premature net-zero CO2 commitments. The findings highlight the significant mitigation potential of targeted non-CO2 measures, especially concerning HFCs and SF6 emissions from industrial processes and cooling sectors. These actions can significantly reduce the burden of achieving net-zero CO2. However, the analysis also reveals challenges associated with reducing agricultural non-CO2 emissions. The relatively limited cost-effective mitigation options currently available in this sector necessitate further technological advancements and/or economic mechanisms to incentivize changes in agricultural practices. The sensitivity analyses confirm the robustness of the main conclusions, although the socioeconomic pathway significantly impacts the results. These findings reinforce the need for policy frameworks that effectively incentivize and support both CO2 and non-CO2 mitigation measures to achieve ambitious climate targets. Future research could focus on developing more precise methods for estimating non-CO2 emissions, specifically in the agricultural sector and fossil fuel production, which would allow for an even more informed assessment of mitigation potential and inform effective policymaking strategies.

This study demonstrates the critical need for comprehensive GHG abatement strategies that fully integrate non-CO2 mitigation measures to achieve the Paris Agreement's ambitious climate goals. Underestimating non-CO2 mitigation leads to inaccurate carbon budget estimates and accelerates the need for net-zero CO2 emissions. The research highlights the significant potential for reducing HFC and SF6 emissions, urging targeted measures in industrial processes and cooling sectors. While decarbonization reduces non-CO2 emissions from fuel sources, additional actions are required for substantial reductions in agricultural emissions. The study highlights the importance of continued research into the technological and economic potential for all GHGs to inform policies and drive further innovation in reducing residual non-CO2 emissions. Furthermore, the integration of economic mechanisms, such as direct pricing of CH4, to incentivize behavioral changes should be investigated in future studies.

The study's analysis is performed within a single modeling framework (GCAM), employing a consistent socioeconomic pathway, economic structure, and climate modeling assumptions. Although sensitivity analyses address uncertainties around socioeconomic pathways and technological changes, the analysis might not fully capture the range of potential outcomes that could arise from using different integrated assessment models or incorporating more varied assumptions. Uncertainty remains in the estimation of emission factors, especially for methane emissions from the oil and gas supply chain and coal mining. Further improvements in measurement and quantification techniques are crucial for a more accurate representation of the mitigation potential in these sectors. Finally, the study focuses primarily on GHGs, neglecting other climate forcers like aerosols and black carbon, whose emissions are largely influenced by CO2 mitigation actions. Future research could expand the scope of analysis to encompass additional climate forcers and potentially explore different climate feedback mechanisms.