Limiting warming by carbon dioxide and methane mitigation in an expanded scenario space

How much do methane emissions need to fall for different climate targets?

A seemingly simple question, but one that has lacked a clear answer so far.

Background

The scientific consensus as summarized by the Intergovernmental Panel on Climate Change (IPCC) is that "[...] limiting human-induced global warming to a specific level requires limiting cumulative CO₂ emissions, reaching at least net zero CO₂ emissions, along with strong reductions in other greenhouse gas emissions. [...]".

What it does not say is how strong the reductions in different greenhouse gases (GHGs) need to be.

Methane (CH₄) is the second most important GHG after CO₂ and contributed as much as 0.6 °C to global warming as of 2025. CH₄ is primarily emitted in agriculture, fossil fuel production and distribution, and waste management. In contrast to CO₂, it stays in the atmosphere for only around a decade. This means that a sustained reduction in CH₄ emissions (i.e., sustained CH₄ mitigation) can reduce warming within decades, whereas CO₂-induced warming is practically irreversible for millennia. 

Discussions of climate targets usually focus on CO₂, or on all GHGs combined and converted into CO₂-equivalents. In both cases the differences between CO₂ and CH₄ stay hidden, as does the necessary CH₄ mitigation. There is a whole zoo of conversion methods (e.g., GWP100, GWP20, GWP*) and depending on the conversion method, CH₄ mitigation can either appear as an immediate priority or be framed as almost unnecessary.

CO₂ and CH₄ are coupled in scenarios that guide policy

Integrated assessment models (IAMs) used by scientists to generate future, often cost-optimal, emission scenarios couple CO₂ and CH₄ in two ways:

1) Technologically, because both gases are sometimes released by the same activities, such as in the fossil fuel supply chain, and 2) economically, because IAMs commonly use GWP100 for conversion to CO₂-equivalents before optimising mitigation costs of all greenhouse gases.

Across the emission scenarios assessed by the last IPCC Report, CO₂ and CH₄ emissions are therefore closely correlated, while many combinations of emission pathways are not assessed.

This coupling makes it difficult to isolate the effect of CH₄ reductions independent of CO₂ reductions. And as emission reductions in IAM-generated scenarios are driven by costs, economically suboptimal combinations of CO₂ and CH₄ reductions are not represented, even though reality is messy and thus emission pathways are never purely cost-effective.

Separation of mitigation targets for CO₂ and CH₄ 

To make the required CH₄ mitigation explicit while staying largely independent of assumptions made in IAMs, we separated mitigation targets for CO₂ and CH₄. We systematically combined CH₄ emission pathways with net zero CO₂ (or GHG) emission pathways and calculated peak warming with the simple climate model FaIR.

This allows us to determine the minimum compatible CH₄ mitigation target for a given global peak temperature and net zero CO₂ (or GHG) emission target. This is particularly relevant, as many countries and companies already have net zero CO₂ or GHG targets, and climate policy is typically framed around a global temperature outcome. 

Future emission pathways are of course not unambiguously defined by two single emission targets, so we restricted ourselves to linear pathways for net CO₂ and CH₄ emissions and built a simple model to determine compatible emissions of other GHGs and aerosols.

Figure 1 shows how peak warming until 2100 depends on the year of net zero CO₂ emissions and the simultaneous change in CH₄ emissions.

This allows you to pick a peak global warming level (a line of equal warming) and read off the minimum compatible CH₄ mitigation target from the y-axis for different net zero CO₂ emission years. This already gives an answer to our initial question.

For example, a world reaching net zero CO₂ in 2050 with the aim of limiting peak warming to 1.7 °C needs CH₄ emissions to fall by at least two thirds by 2050 relative to 2020.

The table below gives the minimum compatible CH₄ mitigation targets for several peak warming levels and net zero CO₂ or GHG emission targets.

However, current legislation implies emissions around 20% above 2020 levels (purple bar in Figure 1), which gives peak warming above 2.1 °C even if CO₂ emissions were to reach global net zero by 2050.

No country seems to target net zero CO₂ emissions before 2040, so we judge part of the displayed emission space in Figure 1 implausible (Finland formally has a 2035 carbon neutrality target, however, this includes natural uptake in the land-use sector, which recently turned into a net CO₂ source). Following the horizontal line where CH₄ emissions stay at 2020 levels shows that lack of CH₄ mitigation leads to peak warming exceeding 1.85 °C. This is already above what has been argued to be consistent with the well-below 2 °C limit of the Paris Agreement.

Reducing CH₄ emissions by around a third by 2030 (orange bar in Figure 1) could reduce peak warming by 0.15 °C, of which 0.05 °C could be achieved at no net cost in the fossil fuel sector (red bar in Figure 1).

The global remaining carbon budget

The global remaining carbon budget refers to the amount of cumulative CO₂ emissions that can still be emitted until crossing a particular global warming threshold. Current estimates of the 2 °C remaining carbon budget (50% likelihood) after 1 January 2025 lie around 1000-1150 GtCO₂ (e.g., from the latest IPCC Report, or a later update).

We find that these estimates imply linear CH₄ emission reductions of 27-35% by 2050 relative to 2020, which is consistent with another independent analysis. The usually communicated values of the global remaining carbon budget are therefore only valid under substantial CH₄ mitigation. If CH₄ emissions are not reduced at all in the future, our results show that the remaining carbon budget for 1.7 °C has already been exhausted.

Implications

How much do CH₄ emissions need to fall? This depends on the peak warming level and the additional target for CO₂ or GHG emissions.

However, strong CH₄ mitigation is essential to keep warming well-below 2 °C, the upper limit of the Paris Agreement.

Aggregating GHG emissions to CO₂-equivalents, both in climate targets and in models that generate emission scenarios, hides the necessity and opportunity of CH₄ mitigation. Decoupling CO₂ and CH₄ emissions opens up a perspective that is thus far missing, and helps explore the consequences of different (including economically suboptimal) mitigation decisions. 

So far, only a few countries have emission targets specifically for CH₄. We provide the data to set them and help refine existing net zero emission targets.