As we start 2025, I am reflecting on my 20 year career of working in greenhouse gas estimation in the land sector, and in particular some of the questions that have remained persistent in our conversations through the years.
Questions that really never seem to get ’resolved’. But indeed that is the nature of science. We hypothesise, we observe, we increase our knowledge and we improve our understanding. If we are successful in communicating our findings in a way the wider community understands, informed decisions can be made.
One such re-emerging question we get is around the values used for methane global warming potential.
I know it is early in the year and I have opened the box on a sometimes sensitive topic in our industry, probably before we have fully recovered from our festive holidays, but this one has been sitting in my ‘to do list’ from 2024 and I was procrastinating during my first week back; so here goes.
The latest IPCC report (AR6) introduced multiple 100-year GWP values for methane, which vary based on its source:
- Fossil Fuel Methane: Adds new CO₂ to the atmosphere after oxidation.
- Biogenic Methane: Returns CO₂ previously absorbed from the atmosphere.
For GHG compilers within the land sector (encompassing agriculture, forestry and other land use) we were happy to see this distinction and many have possibly jumped to use the lower reported value for biogenic methane sources.
But as a recent publication from the GHG Management Institute communicates very well, selecting the appropriate methane GWP value is not as simple as differentiating between fossil and biogenic methane sources.
I have used this excellent example of science communication as a reference, and for those wanting more background, it is well worth the read. It not only describes the atmospheric chemistry and carbon cycle science behind the various methane GWP values provided in recent IPCC reports, but also highlights potential policy and ethical considerations in adopting various values.
This is my simple summary for Agriculture, Forestry and Other Land Use GHG compilers:
When to use the 27.9 “Solely Methane” GWP Value
- Biogenic Methane Sources:
Emissions from livestock (e.g., enteric fermentation) and biomass combustion.
These emissions involve carbon that was recently part of the atmospheric carbon cycle and do not result in a net addition of CO₂ once the methane has broken down (after approximately 12 years) to carbon dioxide.
- Fossil Fuel Combustion in Agricultural Equipment:
- Methane emissions from fuel combustion are already accounted for in CO₂ estimates, so using 27.9 prevents double counting the radiative forcing effect of oxidized CO₂.
When to use the 29.8 GWP Value
- Fugitive Methane Emissions:
- Methane released during fossil fuel production, transport, or delivery (e.g., natural gas leaks in farming operations or food processing).
- Industrial Processes in Agricultural Supply Chains:
- Processes involving fossil-origin CH4 where CO₂ from oxidation is not separately accounted for (e.g., fertilizer production).
Avoid Using the 27.0 GWP Value
- This value assumes that 25% of the CH4 at the end of its atmospheric lifetime is not oxidized to CO2 but rather converted to formaldehyde and removed from the atmosphere through deposition. The length of time this formaldehyde is stored in soils, water bodies and other surfaces has been shown to be relatively short due to its high reactivity and rapid degradation through various processes. More science will inevitably lead to improvements in Methane GWP in the short term as we strive to address the high impact gases. So watch this space. In the meantime the GHG Management Institute suggest avoiding the use of this value as it could underestimate the real impact of methane emissions.
By applying these GWP values, the accuracy of Agriculture, forestry and Other Land Use GHG inventories will be improved, and as such, will support effective decision making relating to mitigation strategies.
Well, that wasn’t so bad. I hope you feel more informed from engaging with well communicated science, as I do, and prepared for when this question inevitably comes up again.