Metrics for methane emissions

Methane is the most important anthropogenic greenhouse gas after CO₂. To meet ambitious climate change mitigation targets, it is crucial to effectively reduce the emissions of methane along with other greenhouse gases. The selection of mitigation measures depends on factors such as feasibility, costs and the amount of emission savings. When measures that target different greenhouse gases are compared, the climate impact of these different gases must be made comparable. For this purpose, metrics have been introduced.

The most common metric is the global warming potential over 100 years (GWP₁₀₀), which compares the warming effect of a gas over 100 years with the effect of CO₂. This metric is also used for greenhouse gas inventories under the United Nations Framework Convention on Climate Change and under the Paris Agreement.

In the current discussion, the global warming potential of methane over a period of 20 years (GWP₂₀) is sometimes used. In addition, other metrics for comparing the impact of different greenhouse gases have been proposed in the relevant literature. The contribution of various greenhouse gases to total emissions, and thus the need to reduce emissions, can change substantially depending on the choice of metric.

A comparison of various metrics shows that GWP₁₀₀ is well suited to prioritising measures as it is designed for the long term and has a robust scientific foundation. If metrics with a shorter time horizon – such as GWP₂₀ – are used, the long-term effect of greenhouse gases beyond the 20-year period is not taken into account. This would limit the informative value of total emissions for the periods under consideration, such as the middle of the century.

Another metric, the global temperature change potential (GTP), addresses the effect of a greenhouse gas on temperature at the end of a defined time period. It is associated with higher uncertainties than GWP; when using this metric, it is important to align the chosen time horizon with that of the temperature target.

A variation of the global warming potential, which is referred to as the GWP*, focuses on the change in emissions of short-lived substances compared to historical emissions. Negative GWP* values represent a reduction in emissions compared to the past. However, even with negative GWP*, there may be significant greenhouse gas emissions. This makes it difficult to use this metric in planning climate change mitigation.

Overall, GWP₁₀₀ constitutes a good choice for prioritising climate change mitigation measures. The use of other metrics can be useful for various issues, but it should be made clear when quantifications of greenhouse gas emissions differ from those set out in the Paris Agreement, and what sectors or parties involved would benefit from what metrics.