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

Methane (CH₄), a potent greenhouse gas, is contributing significantly to global warming. Atmospheric CH₄ levels have increased rapidly since 2007, exceeding 1900 parts per billion (ppb), more than 2.5 times the pre-industrial average. Anthropogenic sources, primarily from fossil fuel exploitation, livestock, waste, and agriculture, dominate global CH₄ emissions. While the need for CH₄ mitigation is recognized (e.g., the Global Methane Pledge), the urgency and long-term consequences of delayed action remain unclear. Previous studies highlight the importance of CH₄ mitigation alongside CO₂ reduction, but there's limited knowledge on biogeochemical feedbacks and the long-term (multi-century) climate effects of delayed mitigation. This study addresses these gaps using an Earth system model to assess the impact of different methane mitigation timelines on meeting the Paris Agreement's temperature goals.

Existing research underscores the critical role of methane mitigation in achieving the Paris Agreement's climate goals. Studies emphasize the necessity of deep emission reductions in CH₄ alongside stringent CO₂ mitigation to limit global warming to well below 2°C. However, these studies often lack a comprehensive representation of biogeochemical feedbacks or focus on shorter-term climate effects. This paper contributes by examining the multi-century impacts of delayed methane mitigation, considering the interplay of various earth system feedbacks.

This research utilizes version 2.10 of the University of Victoria Earth System Climate Model (UVic ESCM), enhanced with a simplified global CH₄ cycle representation. This includes simulated wetland CH₄ emissions (accounting for emissions from thawing permafrost) and atmospheric CH₄ decay. The model is validated against historical CH₄ data and global CH₄ budget estimations. To assess the impact of mitigation timing, the study employs two Shared Socioeconomic Pathways (SSPs): SSP1-2.6 (immediate mitigation) and SSP3-7.0 (no mitigation). Four additional scenarios are designed, each delaying CH₄ mitigation (2020, 2030, 2040, 2050) before linearly declining to reach SSP1-2.6 levels by 2100. All scenarios assume all other anthropogenic forcings (including CO₂) follow SSP1-2.6. The model simulates changes in atmospheric CH₄ and CO₂ concentrations, surface air temperature (SAT), and global wetland CH₄ emissions under each scenario. Additional simulations with prescribed CO₂ concentrations from the early mitigation scenario are performed to quantify the carbon-climate feedback's contribution to warming differences.

The timing of CH₄ mitigation significantly affects peak warming. Each 10-year delay increases the peak CH₄ concentration by 150–180 ppb and peak warming by approximately 0.1°C. Delaying mitigation to 2040–2050 increases the peak CH₄ concentration by 450–540 ppb and warming by 0.2–0.3°C compared to immediate mitigation. The carbon-climate feedback amplifies this effect; higher CH₄ levels from delayed mitigation lead to higher SAT levels, further increasing CO₂ levels through feedback mechanisms (soil respiration and weakened ocean CO₂ uptake). The carbon-climate feedback contribution to peak warming increases with each 10-year delay, ranging from ~0.03°C (2020 mitigation start) to ~0.06°C (2050 mitigation start). In contrast, the feedback between global warming and wetland CH₄ emissions is negligible under the low CO₂ emission scenarios. Achieving the 2°C goal requires rapid and deep CH₄ emission cuts alongside stringent CO₂ mitigation. Initiating mitigation before 2030 could limit warming to well below 2°C throughout the 21st century and beyond. Delaying to 2040 results in exceeding the 2°C target for at least two decades. Even with net-zero CO₂ emissions by mid-century, failing to mitigate CH₄ this century leads to global warming exceeding 2°C for more than two centuries. While atmospheric CH₄'s lifetime is around a decade, delayed mitigation impacts global warming for over two centuries due to climate system inertia and feedback effects. Early mitigation increases the likelihood of limiting warming to 1.5°C in the long term, after an initial overshoot.

The findings confirm that deep reductions in both CO₂ and CH₄ emissions are crucial for achieving the Paris Agreement's temperature goals. The study's novel contribution lies in quantifying the amplifying effect of biogeochemical feedbacks, particularly the carbon-climate feedback, on the impact of delayed CH₄ mitigation. The model's results align with previous research emphasizing the need for immediate and substantial emission reductions. The long-term consequences of delayed action, extending beyond the 21st century, highlight the critical need for urgent policy and technological advancements to curb CH₄ emissions.

This study demonstrates that rapid and deep cuts in anthropogenic CH₄ emissions, coupled with stringent CO₂ mitigation, are essential for achieving the Paris Agreement's temperature goals. Delaying CH₄ mitigation significantly increases the risk of exceeding the 2°C warming limit and amplifies long-term climate impacts due to the carbon-climate feedback. The findings strongly support the immediate implementation of large-scale CH₄ mitigation strategies to safeguard future climate stability.

The study's limitations include uncertainties in the areal extent and dynamics of natural wetlands and the complexities of CH₄ production and oxidation processes. The model simplifies atmospheric CH₄ decay by assuming a constant lifetime, potentially underestimating the CH₄ peak in delayed mitigation scenarios. The focus on SSP1-2.6 for non-CH₄ forcings assumes effective mitigation of other greenhouse gases and aerosols, simplifying the overall climate system response.