Methane (CH4) is a powerful greenhouse gas, second only to carbon dioxide (CO2) in the contribution to global mean temperature increase over the last two centuries. For the last four decades at least, more than 60% of global CH4 emissions have originated from anthropogenic activities such as fossil fuel exploitation, livestock production, agriculture and waste. Recent trends of observed atmospheric CH4 levels are tracking future scenarios of unmitigated emissions, making it a serious challenge for climate change mitigation.
A key outcome of the 2015 Paris Agreement is the international commitment to hold the increase in global mean temperature to well below 2°C above pre-industrial levels, and pursue efforts to limit global warming to 1.5°C above pre-industrial levels. Limiting global warming to 1.5 or 2°C above pre-industrial levels will require reaching net-zero CO2 emissions as well as deep reductions in CH4 and other non-CO2 emissions by or around the year 2050.
For many years, researchers and policymakers have focused on CO2 mitigation. It is only recently – following the 2021 UN Climate Change Conference held in Glasgow, Scotland – that more than 100 countries (representing 70% of the global economy) jointly agreed to urgently act on reducing CH4 emissions to comply with the Paris Agreement climate goal.
While policymakers now accept the need for urgent CH4 mitigation, it is necessary to assess the importance of immediate versus delayed CH4 mitigation to comply with the temperature goal in the Paris Agreement – particularly taking into account potential Earth system feedbacks.
In our latest article published in Communications Earth & Environment, we use an Earth system model with a coupled CH4 cycle to assess the importance of immediate versus delayed CH4 mitigation to comply with the 2°C warming limit above pre-industrial levels. In particular, we aim at investigating the role of feedbacks in the carbon and methane cycles in the context of the timing of CH4 mitigation to achieve the Paris Agreement climate goal, as well as the long-term climate impacts of delaying or failing to mitigate CH4 in the current century, which have not been explored previously.
We drive the model with anthropogenic CH4 emissions according to two scenarios: a scenario featuring immediate CH4 mitigation, and a scenario without CH4 mitigation throughout the 21st century. We design four additional scenarios of anthropogenic CH4 emissions with initiation of global-scale CH4 mitigation ranging from the year 2020 to year 2050 (Figure 1). We prescribe the same future anthropogenic forcings – including CO2 emissions reaching net zero around mid-century in all scenarios – implying that the only difference in outcomes between scenarios results from the difference in anthropogenic CH4 emissions.
Our results suggest that global warming relative to the pre-industrial period could be limited to well below 2°C if global-scale CH4 mitigation is initiated before the year 2030 (Figure 2). However, delaying CH4 mitigation to the year 2040 or beyond will increase the risk of breaching the 2°C warming limit above pre-industrial levels, even if net-zero CO2 emissions are achieved.
According to our model simulations featuring low CO2 emission scenarios, every 10-year delay in CH4 mitigation will result in additional peak warming of about 0.1°C. Our results show that the peak warming is amplified by the carbon-climate feedback whose strength increases with delayed CH4 mitigation (Figure 2). The simulated contribution from the carbon-climate feedback to the peak warming ranges from about 0.03°C to 0.06°C for CH4 mitigation initiated in 2020 and 2050, respectively. In contrast, our model simulations suggest a negligible contribution from wetland CH4 emissions to temperature change for every 10-year delay in CH4 mitigation under the scenarios explored in the study.
Despite that CH4 stays in the atmosphere for only about 10 years, we demonstrate that delaying CH4 mitigation by 2-3 decades will have an impact on global warming over many centuries (Figure 2). Most importantly, we show that a failure to mitigate CH4 in the current century implies a high risk for global warming to exceed the 2°C warming limit for several centuries in the future, even under net-zero CO2 emissions (Figure 2).
Our study supports recent concerns about a sustained atmospheric CH4 growth and the associated potential challenge for achieving the Paris Agreement climate goal even under stringent CO2 mitigation. We demonstrate that the carbon-climate feedback amplifies the warming response for delayed versus early CH4 mitigation – with the feedback strength increasing for every 10-year delay in CH4 mitigation. In addition, we show that aggressive reductions of CO2 emissions without CH4 mitigation could still push the Earth system beyond the 2°C warming limit for several centuries in the future.
Therefore, we highlight the need for immediate cuts in anthropogenic CH4 emissions globally, along with stringent CO2 mitigation, in order to increase the likelihood of holding the rise in global mean temperature to well below 2°C above pre-industrial levels. We emphasize that actions associated with the Global Methane Pledge should not be delayed, because every year of delayed CH4 mitigation implies additional global warming – making it harder and harder to achieve the Paris Agreement climate goal.