Deep mitigation of CO2 and non-CO2 greenhouse gases toward 1.5 °C and 2 °C futures

The Paris Agreement targets limiting warming to well below 2 °C and pursuing efforts to 1.5 °C, implying stringent limits on cumulative GHG emissions and net-zero in the second half of the century. Non-CO2 GHGs contribute roughly a quarter of total CO2-eq emissions, and their residual levels influence end-of-century warming even if CO2 reaches net zero. However, assessing system-wide non-CO2 mitigation is challenging due to numerous sector- and gas-specific options, evolving techno-economics, and interactions with CO2 mitigation (e.g., fuel switching reduces both CO2 and upstream CH4). This study asks how comprehensive non-CO2 mitigation, in conjunction with CO2 abatement, affects the timing of net-zero CO2 required to achieve 1.5 °C and 2 °C outcomes. By integrating updated non-CO2 data and abatement cost curves into GCAM, the study quantifies the contributions of non-CO2 mitigation across sectors and regions and its implications for temperature targets and net-zero timing.

Prior research shows ultimate warming depends on both driving CO2 emissions to zero and limiting residual non-CO2 emissions. Studies have examined non-CO2 contributions to remaining carbon budgets and net-zero timing, finding substantial variation across models due to differing representations of non-CO2 options and economic structures. Literature highlights that fuel switching and demand reduction can cut energy-related CH4 and N2O, but dedicated measures are needed for high-GWP fluorinated gases from industry and cooling. Agricultural non-CO2 sources, particularly enteric CH4 and fertilizer-related N2O, have limited economic mitigation potential relative to technical potential, often leaving significant residuals. Differences in short-lived forcer treatments, GWP time horizons, and climate model choices further affect budget estimates and temperature outcomes.

The study integrates 2019 EPA global non-CO2 GHG emission projections and mitigation potentials into GCAM v5.3, a multi-sector integrated assessment model with 32 geopolitical regions and detailed representations of energy, industry, buildings, transport, agriculture, land, water, and climate. Historical emissions are taken from CEDS (v_2020-09-11) and harmonized with EPA emissions by region/sector/species for 1990–2015; future emissions follow EPA projections to 2050 and are extended in GCAM using derived emission factors tied to activity. EPA marginal abatement cost (MAC) curves for CH4, N2O, HFCs, PFCs, and SF6 across sectors and regions are mapped into GCAM’s non-CO2 module, with year-specific technological change (TC) parameters for 2020–2050 to replicate evolving mitigation potentials; post-2050 TC is assumed equal to the pre-2050 average in the main analysis. Maximum reductions are phased in from 2020 to 2040 to reflect stock turnover limits. Non-CO2 emissions respond to increased economy-wide carbon prices via MAC application (direct pricing of non-CO2 is not used), capturing fuel switching and targeted abatement actions (e.g., leak repair, refrigerant recovery, alternative coolants, methane control in oil/gas systems). Climate outcomes are computed by coupling GCAM to the Hector v2.5 reduced-form climate model, which converts annually interpolated emissions to concentrations, radiative forcing, and global mean temperature. GWP-100 (AR4) is used only for reporting aggregated non-CO2 CO2-eq emissions; climate results are independent of GWP choices. Scenarios: one Reference (SSP2 socioeconomic pathway, no new policies beyond 2015) and 90 mitigation scenarios pairing 30 CO2 pathways with three non-CO2 abatement levels. CO2 pathways linearly reach endpoints between 2030 and 2100: for 2 °C, 0 GtCO2/yr and remain at zero; for 1.5 °C, −8 GtCO2/yr and remain at −8, with net-zero years occurring earlier along these trajectories. Non-CO2 levels: (1) CO2 abatement only (non-CO2 as in Reference), (2) CO2-driven GHG abatement (non-CO2 reductions from fuel switching and demand changes), and (3) Comprehensive GHG abatement (adds explicit non-CO2 measures via MACs). Sensitivity analyses explore alternative technological change assumptions post-2050, alternative socioeconomic pathways (SSPs), and alternative GWP assumptions for aggregation.

- Comprehensive non-CO2 abatement substantially relaxes the required timing of net-zero CO2 for a given temperature goal. For identical temperature outcomes, omitting specific non-CO2 measures advances the net-zero CO2 date by about two decades compared with comprehensive GHG abatement. - Net-zero timing by pathway: 1.5 °C requires net-zero CO2 by ~2032 under CO2-driven GHG abatement but by ~2053 with comprehensive GHG abatement; CO2 abatement only cannot achieve 1.5 °C in any modeled pathway. For 2 °C, net-zero CO2 years are 2030 (CO2 abatement only), 2045 (CO2-driven), and ~2075 (Comprehensive). Net-negative CO2 is not required for 2 °C. - Emission trajectories: In Reference, CO2 and non-CO2 reach ~69 and ~25 Gt CO2-eq/yr in 2100 with ~3.8 °C warming. Under 1.5 °C pathways, CO2-driven GHG abatement reduces non-CO2 to ~18 Gt CO2-eq/yr in 2100 (~28% below Reference) but still on an increasing trend; Comprehensive GHG abatement reverses this trend, lowering non-CO2 to ~9.8 Gt CO2-eq/yr in 2100. - Sectoral/species contributions: Decarbonization-driven actions mainly reduce non-CO2 from fuel extraction and end use; explicit measures in Comprehensive GHG abatement strongly cut F-gases. HFCs fall ~68% by 2050 and ~92% by 2100 versus Reference, yielding ~85% reduction in HFC forcing by 2100. SF6 increases under CO2-driven abatement due to electrification but falls by ~68% by 2100 with targeted SF6 abatement; PFCs also decline substantially under comprehensive measures. - Agriculture is the dominant residual non-CO2 source across the century, with limited cost-effective mitigation options; CH4 from enteric fermentation and N2O from fertilizer application remain challenging to abate economically. - Spatial patterns: Largest additional non-CO2 reductions in 2050 under Comprehensive abatement occur in China and the U.S., and in rapidly growing economies (e.g., Brazil, South Asia, Western Africa), driven by large potential in cooling and industrial process emissions; HFCs reduce by >80% in most regions by 2050. - Sensitivity analyses: Post-2050 technology change assumptions have limited influence on end-of-century forcing; some SSPs fail to achieve 1.5 °C even with comprehensive abatement, though all SSPs achieve well-below 2 °C; socioeconomic and technology assumptions can shift non-CO2 emissions by roughly −34% (SSP1) to +44% (no technological change) in the main 1.5 °C comprehensive scenario.

Findings demonstrate that fully integrating non-CO2 mitigation can materially ease the CO2 mitigation burden by delaying the required net-zero CO2 date by about two decades for the same temperature objective. This implies that policy strategies focusing solely on CO2 risk underestimating the remaining carbon budget and prematurely accelerating net-zero commitments. The most cost-effective non-CO2 opportunities are in F-gases from cooling and industrial processes, where leak repair, end-of-life refrigerant recovery, alternative coolants, and industrial process controls produce large reductions across all regions. Conversely, agriculture remains the primary source of residual non-CO2 emissions, indicating the need for technological innovation, potential structural or behavioral changes, and possibly direct pricing of CH4 and N2O to unlock further reductions. The results support comprehensive GHG abatement policies that combine economy-wide CO2 pricing with targeted non-CO2 measures, leveraging lower-cost abatement options to reduce overall mitigation costs and to stabilize temperatures consistent with 1.5 °C and 2 °C. Sensitivity analyses underscore the importance of socioeconomic development pathways in determining feasibility of 1.5 °C, while showing that technology evolution post-2050 and alternative aggregation metrics (GWPs) do not qualitatively alter the climate conclusions.

By combining updated, detailed non-CO2 mitigation data with an integrated assessment framework, the study quantifies how comprehensive non-CO2 abatement can delay required net-zero CO2 dates by about two decades for equivalent temperature targets and make 1.5 °C stabilization achievable when paired with net-zero CO2. Major, cost-effective reductions in F-gases complement decarbonization-driven cuts in energy-related CH4 and N2O, while agriculture persists as the dominant residual source. Policy-relevant insights include the need to integrate targeted non-CO2 measures alongside CO2 reductions to avoid underestimating carbon budgets and overstraining CO2 mitigation timelines. Future work should prioritize: advancing technological and institutional innovations for agricultural CH4 and N2O; evaluating realistic policy instruments beyond uniform carbon pricing; refining emissions inventories (especially CH4 from fossil systems and coal mining); and further exploring socioeconomic pathways and sectoral transitions that enable comprehensive, system-wide GHG mitigation.

- Model framework and data: All 90 mitigation scenarios share a single modeling framework (GCAM-Hector) and SSP2 socioeconomic pathway to isolate non-CO2 effects; results may differ under alternative model structures or socioeconomic assumptions. - Technology assumptions: Non-CO2 MACs include technological change to 2050 based on EPA; post-2050 extensions are assumed (average TC), introducing uncertainty in late-century abatement potentials. - Climate forcers and modeling choices: Treatment of other forcers (e.g., aerosols, ozone, black carbon) and climate model parameters can affect temperature outcomes; these forcers are primarily influenced by decarbonization level rather than specific non-CO2 measures. - Aggregation metrics: While climate results are independent of GWP choices, different GWP time horizons change the reported CO2-eq shares of gases, potentially affecting interpretation of relative contributions. - Emission factor uncertainties: Historical inventories for CH4 from oil/gas systems and coal mining (e.g., U.S., China) show discrepancies with measurement-based studies, affecting estimated mitigation potentials and regional contributions. - Policy realism: The analysis uses an economy-wide carbon price to induce CO2 mitigation and applies MAC-based responses for non-CO2; real-world policy mixes and adoption barriers may differ from modeled idealized pricing and implementation.