the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Apr 2025
Normalizing the permafrost carbon feedback contribution to TCRE and ZEC
Abstract. As permafrost thaws, the permafrost carbon feedback (PCF) can amplify the Transient Climate Response to Cumulative Carbon Emissions (TCRE) and the Zero Emissions Commitment (ZEC) by introducing additional greenhouse gases into the atmosphere. Using a basic permafrost carbon response model coupled to the simple climate model FaIR, we estimate this feedback's contribution to TCRE and ZEC100 and find that it can substantially increase estimates of these climate metrics. The results also show that this contribution is robust in scenario with various emission rates for TCRE and also for ZEC100 when time-integrated warming is considered. Relating these climate metrics to permafrost carbon emissions allows the normalization of the PCF contribution to TCRE and ZEC by discounting its uncertainties.
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RC1: 'Comment on egusphere-2025-1714', Chris Jones, 02 May 2025
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
- 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)
- 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?)
- 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
- 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?
Citation: https://doi.org/10.5194/egusphere-2025-1714-RC1 -
AC1: 'Reply on RC1', Norman Julius Steinert, 27 Jun 2025
Dear Chris Jones,
Thank you for your thorough and constructive review. Please find attached our detailed responses to each of your points including our strategies to address your criticisms in our revised manuscript. Note that we provide the responses to both reviewer comments in one document for your reference.
-
RC2: 'Comment on egusphere-2025-1714', Andrew MacDougall, 02 Jun 2025
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/I75BZ0While 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.Citation: https://doi.org/10.5194/egusphere-2025-1714-RC2 -
AC2: 'Reply on RC2', Norman Julius Steinert, 27 Jun 2025
Dear Andrew MacDougall,
Thank you for your thorough and constructive review. Please find attached our detailed responses to each of your points including our strategies to address your criticisms in our revised manuscript. Note that we provide the responses to both reviewer comments in one document for your reference.
-
AC2: 'Reply on RC2', Norman Julius Steinert, 27 Jun 2025
Status: closed
-
RC1: 'Comment on egusphere-2025-1714', Chris Jones, 02 May 2025
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
- 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)
- 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?)
- 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
- 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?
Citation: https://doi.org/10.5194/egusphere-2025-1714-RC1 -
AC1: 'Reply on RC1', Norman Julius Steinert, 27 Jun 2025
Dear Chris Jones,
Thank you for your thorough and constructive review. Please find attached our detailed responses to each of your points including our strategies to address your criticisms in our revised manuscript. Note that we provide the responses to both reviewer comments in one document for your reference.
-
RC2: 'Comment on egusphere-2025-1714', Andrew MacDougall, 02 Jun 2025
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/I75BZ0While 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.Citation: https://doi.org/10.5194/egusphere-2025-1714-RC2 -
AC2: 'Reply on RC2', Norman Julius Steinert, 27 Jun 2025
Dear Andrew MacDougall,
Thank you for your thorough and constructive review. Please find attached our detailed responses to each of your points including our strategies to address your criticisms in our revised manuscript. Note that we provide the responses to both reviewer comments in one document for your reference.
-
AC2: 'Reply on RC2', Norman Julius Steinert, 27 Jun 2025
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