Cutting one third of methane emissions by 2030 will reduce peak warming by 0.15°C

Reduction of global methane emissions by about one-third between now and 2030 may limit the 21^st century peak global warming by some 0.15°C, and without such reduction warming will peak at nearly 2°C over pre-industrial levels. And that will happen even if the net zero CO₂ emissions target is achieved as early as 2050. Those findings were published by a research team at the Swiss Federal Institute of Technology Zurich.

Carbon dioxide and methane do not affect climate in the same way: CO₂ can stay in the climatic system for a very long time accumulating gradually, so in terms of temperature change the humanity’s contribution to the global carbon dioxide emissions over an extended period is what really matters. Methane’s behavior is different: during an average period of nine years its concentration in the atmosphere diminishes notably due to certain chemical processes. Therefore its effect on temperature is especially sensitive to its current level of emissions. If emissions are reduced promptly its concentration in the atmosphere will start to go down relatively fast, and so will its contribution to further warming. The Swiss researches estimated that anthropogenic methane has already contributed more than 0.6°C of contemporary global warming. It means that emissions reduction can have a relatively fast effect of slowing down future temperature growth.

In order to estimate this effect the researchers simulated multiple scenarios of how emissions of both gases may change. Climate scenarios usually assume reductions in CO₂ and methane emissions as part of a common strategy: models select the most cost efficient mix of measures, so for both gases, emissions are normally reduced simultaneously. In the new research project, the authors decided to treat both parameters independently. For instance, they estimated the impact of quick CO₂ emission reductions with methane emissions hardly changed; and, vice versa, the impact of significant cuts in methane with slower reduction of CO₂ emissions.

To this end the team set various time frames for net zero CO₂ emissions, extending all the way to 2100. In parallel, they altered methane emission levels independently, reducing them by up to 90% or otherwise increasing by 50% from 2020 levels. For each of those combinations, additional estimates were made for emissions of other climate-impacting substances, and then global temperature change was calculated with the FaIR model, which was applied in preparation of the IPCC Sixth Assessment Report on climate change. As many as 2,000 variants were simulated for each combination to account for uncertainties of future emissions and climatic system responses.

Calculations clearly point to the key role of methane. With no emissions reduction, peak warming will exceed 1.7°C even with lower CO₂ emissions. And if the net zero CO₂ target is achieved by 2050, without additional methane abatement incremental global temperature will approach 2°C against pre-industrial levels.

The more the global net zero CO₂ target is delayed, the more of methane emissions will have to be cut in order to keep warming within 2°C. For instance, if it is achieved by 2050 the model even allows for a moderate increase in methane emissions – up to до 8% above the 2020 level. However methane emissions will have to be reduced by 25% if the net zero CO₂ target is not met until 2070, and by as much as 83% if the target is delayed until 2100. All those scenarios are based on a 50 percent probability of maintaining warming within 2°C.

The research employs two different scenarios to demonstrate the extent to which methane can affect the outcome. In the first scenario, the net zero CO₂ target is achieved as early as 2040 while methane emissions grow by 20%. In the second scenario, net zero CO₂ emissions do not materialize until 2090 but methane emissions are cut by half. In both cases peak warming is found to be approximately the same, slightly below 2°C. Notably, the second scenario implies that the humanity additionally releases more than 1 trillion tons of CO₂. Accelerated decline in methane emissions can therefore partially compensate for slower decline in CO₂ emissions in terms of maximum temperature impact. It cannot, however, be a substitute for CO₂ reduction.

The impact of methane is obvious also when looking at the carbon budget – the amount of CO₂ that the humanity can release without exceeding the pre-determined warming limit. The faster methane emissions go down the larger the buffer. The authors estimate that cutting methane emissions another 10% by 2050 will add about 165 bn tons to the affordable CO₂ emissions given the 2°C target. This is comparable to global carbon dioxide emissions over a four-year period.

Without cutting methane emissions the carbon budget shrinks much more. For example, given that methane emissions are fixed at the 2020 level, the capacity to limit warming to 1.7°C was exhausted before 2025. We are now less than 600 bn tons of CO₂ short of the 2°C threshold – at the current level of global emissions, that volume will be taken up in less than 1.5 years.

In fact, a significant part of methane emissions could be avoided even with existing methods. The authors provide estimates suggesting that currently available technology allows for anthropogenic methane reduction by approximately one-third by 2030. That includes, inter alia, leakage detection and elimination in the process of fossil fuel extraction and transportation. Cutting methane emissions in this way may reduce peak global warming by some 0.15°C.

However methane abatement will not take CO₂ reduction off the agenda. It may slow down temperature increase and reduce peak warming, but long-term climate stabilization is conditional on achieving net zero CO₂ emissions. Based on the researchers’ estimate, there is hardly any time left to limit warming to 1.5°C. For this target to be achieved with at least a 50 percent probability, anthropogenic CO₂ and methane emissions would have to almost entirely stop approximately by 2028. The authors consider this scenario unrealistic.