Delaying methane mitigation increases the risk of breaching the 2 °C warming limit

Methane (CH₄) is the second most important greenhouse gas after CO₂ in terms of contribution to observed warming since pre-industrial times. Atmospheric CH₄ has risen rapidly since 2007, now exceeding 1900 ppb (>2.5× pre-industrial), and recent trends track unmitigated future emission scenarios. Anthropogenic sources—fossil fuels, livestock, waste, and agriculture—dominate global CH₄ emissions. The Paris Agreement commits to holding warming well below 2 °C and pursuing 1.5 °C, necessitating net-zero CO₂ and deep non-CO₂ reductions around mid-century. Despite recognition of CH₄ mitigation urgency (e.g., Global Methane Pledge), uncertainties remain about the roles of biogeochemical feedbacks and long-term consequences of delaying CH₄ mitigation. This study uses an Earth system model with an interactive CH₄ cycle to assess how the timing of CH₄ mitigation—immediate versus delayed—affects the feasibility of staying below 2 °C under low CO₂ emissions and to quantify the influence of carbon–climate and climate–CH₄ feedbacks over multi-century horizons.

Prior work underscores the essential role of CH₄ reductions alongside stringent CO₂ mitigation to meet temperature targets and highlights rising anthropogenic CH₄ emissions from both agricultural and fossil fuel sources. Studies have examined the benefits of reducing short-lived climate pollutants, the importance of non-CO₂ gases in deep mitigation pathways, and air quality co-benefits from CH₄ control. However, earlier ESM-based analyses often prescribed atmospheric CH₄ concentrations without explicit emissions pathways, interactive sinks, or multi-century perspectives. There is limited understanding of biogeochemical feedbacks (carbon–climate, climate–CH₄ via wetlands, fires, oxidation) in the context of meeting Paris goals and of the long-term climate impacts of delayed or absent CH₄ mitigation. This work addresses these gaps by using an ESM with interactive CH₄ sources/sinks and by exploring the timing of CH₄ mitigation under low-CO₂ trajectories out to 2300.

The University of Victoria Earth System Climate Model (UVic ESCM) v2.10 is used, comprising an energy–moisture balance atmosphere, 3-D ocean GCM with biogeochemistry, sea ice, and a land model with dynamic vegetation and terrestrial carbon. The model includes multi-layer ground to 250 m depth to represent permafrost processes and a dynamic wetland module (TOPMODEL-based) that simulates wetland areal extent changes. A simplified global CH₄ cycle is implemented: wetland CH₄ emissions are computed from microbial production and oxidation across soil layers as functions of moisture (anoxia), soil carbon, temperature, and depth, with oxidation governed by an oxic zone; production is zero in dry or frozen layers and includes emissions from thawed permafrost carbon. Atmospheric CH₄ burden evolves via a one-box model: dB/dt = E − S, with E the sum of prescribed anthropogenic emissions, simulated wetland emissions, and fixed non-wetland natural sources (assumed constant at 60 Tg CH₄ yr⁻¹). Sinks are parameterized with a constant atmospheric CH₄ lifetime τCH4 = 9.3 years. CH₄ concentrations are derived from burden using −2.8 Tg CH₄ per ppb; radiative forcing for CH₄ follows Etminan et al. Non-CH₄ forcing: natural (volcanic, solar) and anthropogenic forcings from CMIP6 are applied; CO₂ emissions, other non-CH₄ GHGs, aerosols, and land-use change follow SSP1-2.6 (with negative CO₂ emissions later in the century). Aerosol and LUC forcings are held fixed at 2100 values beyond 2100; other forcings extend to 2300. Scenarios: anthropogenic CH₄ emissions from SSP1-2.6 (early mitigation) and SSP3-7.0 (no mitigation), plus four delayed-mitigation scenarios that follow SSP3-7.0 until 2020, 2030, 2040, or 2050, then decline linearly to meet SSP1-2.6 CH₄ emissions by 2100 and follow its extension thereafter. All scenarios assume the same non-CH₄ forcings (SSP1-2.6). Model validation was performed against historical CH₄ concentrations and budget estimates. Additional simulations prescribing CO₂ concentration from the early-mitigation case isolate the carbon–climate feedback contribution to warming.

- Each 10-year delay in CH₄ mitigation increases the atmospheric CH₄ peak by roughly 150–180 ppb; delaying to the 2040–2050 decade raises the CH₄ peak by 450–540 ppb relative to mitigation starting around 2020. - Each 10-year delay yields an additional peak global mean surface air temperature (SAT) warming of about 0.1 °C; delaying to mid-century increases peak warming by 0.2–0.3 °C versus early mitigation. - Atmospheric CO₂ peaks are also higher with delay due to the carbon–climate feedback: +2–3 ppm per decade of delay; a 2040–2050 delay adds about 6–9 ppm to the CO₂ peak. - Even though total CH₄ emissions converge by 2100 across mitigation scenarios, delayed mitigation leaves a larger atmospheric CH₄ burden near 2100 because of lagging sinks; CH₄ sinks in 2100 are ~65 Tg CH₄ yr⁻¹ higher in delayed (2040–2050) versus early mitigation. - The carbon–climate feedback amplifies warming differences between early and delayed CH₄ mitigation. Its contribution to peak warming increases with delay: ~0.03 °C (mitigation starting 2020) up to ~0.06 °C (starting 2050). - Wetland CH₄ emission feedback is small under low-CO₂ scenarios; differences between early and delayed cases remain <1 Tg CH₄ yr⁻¹ for over two centuries, contributing negligibly to SAT differences. - Meeting 2 °C: With CH₄ mitigation initiated before 2030, along with SSP1-2.6 non-CH₄ forcings, warming remains well below 2 °C throughout the 21st century. Delaying to 2040 causes an overshoot of 2 °C for at least two decades; longer delays imply longer overshoot. - Long-term impacts: Although CH₄’s atmospheric lifetime is ~a decade, delaying mitigation by 10–30 years affects SAT for centuries due to system inertia and carbon–climate feedback; SAT differences persist for more than two centuries even after CH₄ concentrations converge in the early 22nd century. - Early CH₄ mitigation before 2050 increases the likelihood of limiting warming to 1.5 °C in the long run (second half of the 22nd century onward) after earlier overshoot. Failure to mitigate CH₄ this century leads to >2 °C warming throughout the 21st century and beyond, even with net-zero CO₂ by mid-century.

The study demonstrates that the timing of CH₄ mitigation critically influences whether stringent Paris temperature limits can be achieved under low-CO₂ pathways. Delays raise the CH₄ concentration peak and, via increased warming, elevate atmospheric CO₂ through reduced natural sinks and enhanced terrestrial respiration and weakened ocean solubility, thereby amplifying peak SAT beyond the direct CH₄ effect. The quantified carbon–climate feedback contribution to peak warming grows with delay (~0.03–0.06 °C), highlighting its policy relevance in timing decisions. Under the scenarios examined, climate–CH₄ feedbacks from wetlands are minor, but the authors acknowledge potential additional feedbacks (e.g., OH chemistry, wildfires) that could further increase warming under delays. Practically, early CH₄ mitigation coupled with stringent CO₂ reductions keeps warming well below 2 °C this century and reduces multi-century warming, whereas delayed or absent CH₄ mitigation risks overshooting 2 °C for prolonged periods, with cascading risks for extremes and tipping elements.

Urgent, large-scale CH₄ mitigation alongside stringent CO₂ reductions substantially improves the likelihood of meeting the Paris 2 °C goal and reduces multi-century warming. Every decade of delay adds about 0.1 °C to peak warming and increases the CH₄ and CO₂ concentration peaks, with carbon–climate feedbacks amplifying impacts. Early action (before 2030) can keep warming well below 2 °C this century; delays to 2040 or beyond increase overshoot risk and duration. Persistent SAT differences for centuries underscore that near-term CH₄ mitigation has long-term climate benefits. Future research should refine representations of methane sinks and lifetimes (e.g., OH chemistry), improve constraints on natural CH₄ sources (wetlands and non-wetland), and explore interactions with aerosols and other non-CO₂ forcings under varied socioeconomic pathways.

Key limitations include: (i) uncertainties in wetland extent/dynamics and microbial/biogeochemical controls on CH₄ production and oxidation; (ii) exclusion of time-varying non-wetland natural CH₄ sources (e.g., lakes, termites, wildfires, geologic seeps, hydrates), which are held constant; (iii) a constant atmospheric CH₄ lifetime (no interactive OH chemistry), potentially underestimating CH₄ peaks in delayed scenarios; (iv) prescribed non-CH₄ forcings following SSP1-2.6 (including CO₂, other GHGs, aerosols), with aerosol and land-use forcings fixed after 2100; and (v) model-specific feedback strengths (UVic ESCM has relatively high carbon–climate feedback) that may place estimates toward the upper half of CMIP6 outcomes. Despite these, sensitivity tests suggest the main conclusion—delays in CH₄ mitigation increase the risk of breaching 2 °C—remains robust across plausible CH₄ lifetimes (7–11 years).