Review of “normalising the permafrost carbon feedback contribution to TCRE and ZEC” by Steinert and Sanderson.

This is a very clearly written and presented study of how permafrost response – especially release of frozen carbon – will affect TCRE and ZEC. TCRE is the transient response to cumulative CO₂ emissions and if CO₂ (or CH₄) is released as permafrost thaws then greater warming will result from given CO₂ emissions. Similarly, ZEC is the Zero Emissions Commitment and quantifies how climate will continue to change after human emissions stop – again released CO₂ from permafrost will tend to increase long term warming. Hence both TCRE and ZEC may be increased by considering permafrost. To date very few ESMs have included permafrost and so quantitative information on how TCRE and ZEC are affected is lacking.

The study is simple and clearly designed to answer this specific question. It uses a development to the FaIR climate model explicitly to add permafrost. FaIR has been widely used to study this issue and is an appropriate tool to use as a baseline for considering the effects of permafrost.

I make a few minor comments below which may help, but the paper is in good shape and can be published with only minor revisions.

Chris Jones

1. Can you comment on the uncertainty due to the assumed CO₂:CH₄ ratio? Burke et al (http://www.the-cryosphere.net/6/1063/2012/tc-6-1063-2012.html) considered this and the spread in that study was quite wide. Why do you choose a 6:1 ratio? (line 62)
2. Related – in the same way that anthropogenic CO₂ and CH₄ have different relative contributions over time, the same is presumably true for permafrost-released CO₂ and CH₄. So can you at least comment on how the split into these components affects TCRE and ZEC? (i.e. if all the carbon is released as CO₂ it has a longer lived impact, but if it is released as CH₄ it has a more immediate, but shorter lived, impact?)
3. Probably worth making clear the definition here of “permafrost” as it means different things to different people. You are (quite reasonably) referring to the response of currently-frozen carbon in the soil. A wider definition for example might include changes in vegetation dynamics on frozen ground (warming/thawing allows forest expansion) – e.g. Pugh et al find this can be as large, and is a long term sink: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018EF000935. But this is not what you cover. Somewhere inbetween these things, there is the impact of thawed nitrogen on enhancing vegetation growth – this is an interaction term between siol BGC and vegetation (see eg Burke et al: https://www.mdpi.com/2504-3129/3/2/23), but again I think this is explicitly not what you cover?

Minor comment

1. You say the results that can be applied as scaling to existing results (line 122). This is just for TCRE right? Whereas for ZEC it would be an addition rather than a scaling?

Review of: "Normalizing the permafrost carbon feedback contribution to TCRE and ZEC"

Overall Evaluation:

The study adds a module to the FaIR climate emulator to simulate the effect of permafrost carbon feedback to climate change. The modified version of FaIR is then used to estimate how the permafrost carbon feedback's effect on the magnitude of ZEC and TCRE. The study suggests that the permafrost carbon feedback increases TCRE values by about 6\% and ZEC value by 0.08K, consistent with past results. While the paper is well written, scientifically useful and succinct I would like to see a more thorough evaluation of PerCX before publication.

General Comments:

While the functional form of PerCX appears like it should roughly capture the permafrost carbon feedback, PerCX  is never tested against existing models. Unfortunately none of the existing permafrost MIPs provide the parameters needed to test PerCX against a suite of models. However the CO₂ component of PerCX could be compared to the results from MacDougall 2021. These results are publicly available at: https://doi.org/10.5683/SP2/I75BZ0

While it is sub-ideal to test PerCX against only a single other model it is better than testing it against no models.    

Specific Comments:

Line 11: Delete 'carbon'. While we usually give ZEC values from CO₂ only ZEC, all forcing ZEC is a well established concept. 

Line 48: Cp(0) is a constant, so probably should not be shown in functional form.

Line 90: The TCRE values from MacDougall and Friedlingstein (2015) is an effective-TCRE value, not a CO₂ only TCRE value, as it is derived from all forcing RCP simulations. The difference between effective-TCRE and TCRE was not well established when the paper was published. eTCRE values tend to be higher than TCRE values.

Technical:

(1) Quotes in Latex are `'  Using '' will give two end quotes.