Definition: Emissions|Kyoto Gases - which GWP version?

[jkikstra]

Currently, the template is as follows:

variable	description	unit
Emissions|Kyoto Gases	Emissions of Kyoto GHG emissions, including fossil CO2, CH4, N2O and F-gases	Mt CO2-equiv/yr

Unfortunately, "Mt CO2-equiv/yr" is poorly defined.
To get to this, you need to make define how you translate other gases into CO2 equivalent values.
Modelling teams may very well use different versions. Generally, most in the IAM community use GWP100, but there are multiple versions.

In previous templates, often the IPCC 4th assessment report GWP100 values were suggested to use as a default, I think to align with certain reporting templates.

It needs to be decided what is to be used.

Would it again be AR4GWP100? Or is it better to go with the newer AR6GWP100 value set? And how can we ensure this to be forward-compatible? Should there be info in the variable, or unit column about what GWP set is used?

[danielhuppmann]

Good question - any thoughts @IAMconsortium/common-definitions-emissions?

[volker-krey]

UNFCCC national GHG emissions inventory reporting makes it mandatory as of 31 December 2024 to report GHG emissions using IPCC AR5 GWP100 UNFCCC common metrics. From my perspective we can either decide to go with this UNFCCC convention or with the latest science, i.e. IPCC AR6 GWP100.

[gidden]

Because our pathways are regularly compared against country climate inventories, I would suggest to use AR5 GWP. This is also in line with past practice (I believe).

In any case, I think we should also include a variable with AR6 GWPs for completeness.

This also has interactions with the accounting differences between NGHGIs and models - where to get a "proper" like-for-like comparison, we would want Kyoto gases with AR5 GWPs and Kyoto gases with AR5 GWPs + NGHGI alignment.

Do we envision having a readme page somewhere that explains different FAQs with respect to our variables? I would happy write this section.

[jkikstra]

UNFCCC national GHG emissions inventory reporting makes it mandatory as of 31 December 2024 to report GHG emissions using IPCC AR5 GWP100 UNFCCC common metrics. From my perspective we can either decide to go with this UNFCCC convention or with the latest science, i.e. IPCC AR6 GWP100.

It does have a small caveat, "excluding fossil methane" but - while I'm downloading AR5 report (366MB on my slow internet) to check - I assume this is nog a big issue. We don't normally report the split between fossil and biogenic anyway.

In any case; then let's include both options separately in the next variable template update.
Emissions|Kyoto Gases [AR5GWP100] and Emissions|Kyoto Gases [AR6GWP100]

I also note that when the climate-assessment is run, we automatically add AR5 and AR6 Kyoto Gases as well - but that sum may be different than the sum the IAM teams do internally, because of reporting availability. The post-processing sum code could easily be updated to have more different options depending of reporting of e.g. F-Gases, if that is deemed necessary.

[jkikstra]

Okay, yes the table is actually very clear, all good.

For reference, with climate-assessment we're going through many openscm unit conversion functions, to eventually end up at this file: https://github.com/openclimatedata/globalwarmingpotentials/blob/main/globalwarmingpotentials.csv

[danielhuppmann]

For cross-reference of tool-development and use case: IAMconsortium/nomenclature#427