Add diagnostic to calculate Transient Climate Response to Emissions (TCRE)

[schlunma]

Description

This PR adds a diagnostic and recipes to calculate the Transient Climate Response to Emissions (TCRE). For CMIP7, this can be simply calculated from the esm-flat10 experiment as the change in global mean near-surface air temperature averaged between years 90 and 110 of that simulation (see https://doi.org/10.5194/egusphere-2024-3356). However, since CMIP7 data does not exist yet, this PR adds an example using CMIP6 data and the esm-1pctCO2 experiment. This second approach needs the variable fco2antt (anthropogenic CO2 emissions) in addition to tas (near-surface air temperature). The implementation here is 100% compatible with the desired approach using the CMIP7 esm-flat10 experiment outlined in the paper mentioned above.

Example plot:

• Closes Add Transient climate response to cumulative emissions of carbon dioxide (TCRE) for REF #3879
• Link to documentation:
  • https://esmvaltool--3904.org.readthedocs.build/en/3904/recipes/recipe_tcre.html
  • https://esmvaltool--3904.org.readthedocs.build/en/3904/recipes/recipe_ref.html

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[alistairsellar]

Hi @schlunma, this is great work!

Not a complete science review yet, but a couple of early suggestions for you to consider.

Also, do you have examples of TCRE metrics calculated for a couple of CMIP6 models that could be compared against values in the literature as assurance of the science output?

[schlunma]

Thanks for reviewing, @alistairsellar!

Also, do you have examples of TCRE metrics calculated for a couple of CMIP6 models that could be compared against values in the literature as assurance of the science output?

Good point! For the MPI-ESM1-2-LR model, we get TCRE=1.64 K/EgG here, while the literature gives 1.65 K/EgC:

Unfortunately, this is the only CMIP6 model where we have all the available data (tas and fco2antt from the esm-1pctCO2 and esm-piControl). You could also diagnose the anthropogenic CO2 emissions from the atmospheric, land, and ocean CO2 fluxes, but this is not supported here to keep the diagnostic as simple as possible. For the REF, fco2antt is requested, so we shouldn't have any problems for the CMIP7 models.

[alistairsellar]

I've reviewed the rest now @schlunma, and it's great work! Just some minor suggestions / questions remaining.

Since I'm not an expert in TCRE, I had a chat with @ChrisJones-MOHC, who is. He took issue with the statement in the first PR comment that TCRE could be calculated for any run with increasing CO2 emissions. Technically it could, but there are some experiments for which it shouldn't (for example it must be an idealised experiment with only CO2 forcing, not e.g. historical). To reduce risk of the diagnostic being used in unrecommended ways, I updated that comment to say "TCRE can be calculated for the esm-1pctCO2, as tested in this PR.". Apologies for direct edit of your comment - I thought it was simpler than asking you to change.

[alistairsellar]

For the MPI-ESM1-2-LR model, we get TCRE=1.64 K/EgG here, while the literature gives 1.65 K/EgC:

Unfortunately, this is the only CMIP6 model where we have all the available data (tas and fco2antt from the esm-1pctCO2 and esm-piControl). You could also diagnose the anthropogenic CO2 emissions from the atmospheric, land, and ocean CO2 fluxes, but this is not supported here to keep the diagnostic as simple as possible. For the REF, fco2antt is requested, so we shouldn't have any problems for the CMIP7 models.

Fantastic - thanks! That's a solid assurance.

[schlunma]

Great, thanks for reviewing @alistairsellar and @ChrisJones-MOHC! I think I addressed all your comments, let me know that you think!

[alistairsellar]

Great work @schlunma ! I'm happy to sign off science review. Thanks @ChrisJones-MOHC for your help!

[bouweandela]

Why are there two almost identical copies of the recipe? I would recommend adding just one copy to avoid future synchronization issues.

[schlunma]

My idea was to have a collection of all recipes used in the REF in one place, but I agree that this will lead to duplication. I can remove it.

[schlunma]

Done in fbf6311, but I added the recipe to the doc in 3a0bcff