the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Nov 2021
Earth System Sensitivity: a Feedback perspective
Abstract. In the field of climate-change research a lot of effort is devoted to the ‘narrowing down’ of uncertainties in the estimation of Equilibrium climate sensitivity (ECS), the global mean warming as a result of an instantaneous doubling of the CO2 concentration in the atmosphere. The present study explores possible consequences of this narrowing down of ECS for the long-term Earth system sensitivity (ESS), taking into account ‘slow’ feedbacks due to the cryosphere response (permafrost melting and ice-sheet disintegration) to a warming world. Implications for international policy making, aiming at avoiding 2 degrees Celsius of global warming, are briefly discussed.
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RC1: 'Comment on esd-2021-78', Anonymous Referee #1, 16 Dec 2021
In the paper Earth System Sensitivity: a Feedback perspective, Peter O. Passenier discusses emergenct constraints for equilibrium climate sensitivity (ECS) and argues that slow feedbacks (e.g. permafrost and icesheet dynamics) are not properly accounted for in previous work. The paper is short - which is nice in many respects, but I also have a concern whether it adds new information. Science papers need to explain the current state of science on the chosen topic to demonstrate that they are up to date (scholar.google search with '"emergent constraints" AND ECS' gave 239 hits, many of which were published since 2018 - most of the cited litereature herein are older than those). This manuscript doesn't do that. It may nevertheless, present some new ideas and insight, but I'm not able to say if it is or isn't. Analogies from the world of electronics, however, are interesting and probably quite novel within climate research.
Another question is whether some of the derivations and mathematics presented in the Metdods section sould be left in an appendix.
It is possible that slow feedbacks also affect the fast ones and that the dynamics and thermodynamics involve nonlinear interactions so that the total feedback no laonger is the sum of individual feedbacks. Hence, the paper assumes that the effect from various processes are additive, which I don't think has been convincingly demonstrated. The paper does, however, discuss combined earth-system feedbacks in the context of earrh system sensitivity. I think that this part needs to be explained more carefully.
I find it a bit har to see the 'red thred' in this paper, which presents a selection of 'facts' without sufficient context or explanation for why. It would be easier to follow the train of thoughts with a clearly stated hypothesis and explicit definitions. Explain why the mathematical derivations and why presenting e.g. Fig 1. It doesn't suffice doing so only in the introduction.
In conclusion, the paper present some interesting ideas, but I find it difficult to follow and think it needs to account for more of the recent progress concerning emergent constraints. Also, a more careful guidance through the ideas and concepts will make the paper easier to follow. It is always a bit more difficult to follow interdisciplinary work because some aspects often are a bit unfamiliar. Here, the paper relied on ideas from electronics in addition to maths.
Minor; 'IPPC' should be 'IPCC'.
Citation: https://doi.org/10.5194/esd-2021-78-RC1 -
AC1: 'Reply on RC1', Peter Passenier, 18 Jan 2022
Author Response to RC1
Thank you very much for your very useful and to-the-point comments to my manuscript.
Although you conclude that the paper presents some interesting ideas (from the world of electronics) you find it difficult to follow and you think it needs to account for more of the recent progress concerning emergent constraints (in the field of climate research).
Please find my detailed responses and corresponding (proposed) changes to the manuscript in the attached Supplement.
-
AC1: 'Reply on RC1', Peter Passenier, 18 Jan 2022
-
RC2: 'Comment on esd-2021-78', Anonymous Referee #2, 04 Jan 2022
Review of “Earth System Sensitivity: a Feedback perspective” by Peter O. Passenier.
This manuscript explores the consequences a narrowing of uncertainty in equilibrium climate sensitivity (ECS) has for the Earth system sensitivity (ESS) using a feedback analysis. The analysis compares the ECS range from IPCC AR5 (likely 1.5-4.5 K) to the narrower range taken from Cox et al. 2018 (likely 2.2-3.4 K) which was derived from an emergent constraint based on observed variations in global temperature. It then explores the implications of these two ECS ranges for ESS by assuming several possible values for Earth system feedbacks (ice sheet albedo feedback, methane, etc). It concludes by making the point that even with a narrowed (Cox et al.) ECS range, the resulting ESS values could be larger than the IPCC AR5 ECS range, with possible implications for Paris targets.
The manuscript is well written and interesting to read. The methods are based on a straightforward application of feedback analysis. However, I have several major concerns that need to be addressed before I could recommend publication.
1) Sherwood et al. 2020 and IPCC AR6 WG1 Chapter 7 are two recent community assessments of ECS that narrow the likely range to about 2.5-4 K based on multiple lines of evidence (including emergent constraints). This is the likely ECS range that should be used in the analysis instead of the single study of Cox et al. 2018 (which has been challenged on methodological grounds and may turn out to not be robust, see discussion in IPCC AR6 Chapter 7).
2) The AR6 definition of ECS includes everything but the feedbacks associated with ice sheet changes and CO2. That is, it includes methane, vegetation, and many other biogeochemical/physical feedbacks whose values are assessed in AR6 Chapter 7. So, the only feedback of relevance for ESS here would be the ice sheet feedback.
Regarding the ice sheet feedback, AR6 Chapter 7 states the following:
… ice sheet mass loss leads to fresh water fluxes that can modify ocean circulation (Swingedouw et al., 2008; Goelzer et al., 2011; Bronselaer et al., 2018; Golledge et al., 2019). This leads to reduced surface warming… However, model simulations in which the Antarctic ice sheet is removed completely in a paleoclimate context indicate a positive global mean feedback on multi-millennial timescales due primarily to the surface albedo change… This net positive feedback due to ice sheets on long timescales is also supported by model simulations of the mid-Pliocene warm period… As such, overall, on multi-centennial timescales the feedback parameter associated with ice sheets is likely negative (medium confidence), but on multi-millennial timescales by the time the ice sheets reach equilibrium, the feedback parameter is very likely positive (high confidence; see Table 7.10). However, a relative lack of models carrying out simulations with and without interactive ice sheets over centennial to millennial timescales means that there is currently not enough evidence to quantify the magnitude of these feedbacks, or the timescales on which they act.
That is, on timescales of a century (of relevance for the Paris targets) the ice sheet feedback is probably negative, and only on timescales of several centuries and longer does it become positive, but with a value that is not well quantified on either timescale. In light of the AR6 assessment, a positive ice sheet feedback does not seem to be relevant for Paris targets. The ice sheet feedback values chosen in this study need to be well justified. And note that the value derived from the LGM is not suitable for a calculation of ESS relevant for future warming.
3) Given the above, I am not sure that the analysis can add much to the existing literature on ESS. While showing the impact of different hypothetical ice sheet feedback values for ESS (on top of different ECS ranges) would be a fine exercise, I don’t see what new information it would provide or how ESS relates to Paris targets which deal with warming this century. It’s also possible that I am missing something, but either way the author needs to better explain the relevance and novelty of the feedback calculation performed here.
References
IPCC AR6 WG1 Chapter 7: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_07.pdf
Sherwood et al. (2020). An assessment of Earth's climate sensitivity using multiple lines of evidence. Reviews of Geophysics, 58, e2019RG000678. https://doi.org/10.1029/2019RG000678
Citation: https://doi.org/10.5194/esd-2021-78-RC2 -
AC2: 'Reply on RC2', Peter Passenier, 18 Jan 2022
Thank you very much for your very valuable and constructive review of my manuscript.
Although you find the manuscript well written and interesting to read, you have several major concerns that need to be addressed before you could recommend publication in ESD.Please find my detailed responses and corresponding (proposed) changes to the manuscript in the attached Supplement
-
AC2: 'Reply on RC2', Peter Passenier, 18 Jan 2022
Status: closed
-
RC1: 'Comment on esd-2021-78', Anonymous Referee #1, 16 Dec 2021
In the paper Earth System Sensitivity: a Feedback perspective, Peter O. Passenier discusses emergenct constraints for equilibrium climate sensitivity (ECS) and argues that slow feedbacks (e.g. permafrost and icesheet dynamics) are not properly accounted for in previous work. The paper is short - which is nice in many respects, but I also have a concern whether it adds new information. Science papers need to explain the current state of science on the chosen topic to demonstrate that they are up to date (scholar.google search with '"emergent constraints" AND ECS' gave 239 hits, many of which were published since 2018 - most of the cited litereature herein are older than those). This manuscript doesn't do that. It may nevertheless, present some new ideas and insight, but I'm not able to say if it is or isn't. Analogies from the world of electronics, however, are interesting and probably quite novel within climate research.
Another question is whether some of the derivations and mathematics presented in the Metdods section sould be left in an appendix.
It is possible that slow feedbacks also affect the fast ones and that the dynamics and thermodynamics involve nonlinear interactions so that the total feedback no laonger is the sum of individual feedbacks. Hence, the paper assumes that the effect from various processes are additive, which I don't think has been convincingly demonstrated. The paper does, however, discuss combined earth-system feedbacks in the context of earrh system sensitivity. I think that this part needs to be explained more carefully.
I find it a bit har to see the 'red thred' in this paper, which presents a selection of 'facts' without sufficient context or explanation for why. It would be easier to follow the train of thoughts with a clearly stated hypothesis and explicit definitions. Explain why the mathematical derivations and why presenting e.g. Fig 1. It doesn't suffice doing so only in the introduction.
In conclusion, the paper present some interesting ideas, but I find it difficult to follow and think it needs to account for more of the recent progress concerning emergent constraints. Also, a more careful guidance through the ideas and concepts will make the paper easier to follow. It is always a bit more difficult to follow interdisciplinary work because some aspects often are a bit unfamiliar. Here, the paper relied on ideas from electronics in addition to maths.
Minor; 'IPPC' should be 'IPCC'.
Citation: https://doi.org/10.5194/esd-2021-78-RC1 -
AC1: 'Reply on RC1', Peter Passenier, 18 Jan 2022
Author Response to RC1
Thank you very much for your very useful and to-the-point comments to my manuscript.
Although you conclude that the paper presents some interesting ideas (from the world of electronics) you find it difficult to follow and you think it needs to account for more of the recent progress concerning emergent constraints (in the field of climate research).
Please find my detailed responses and corresponding (proposed) changes to the manuscript in the attached Supplement.
-
AC1: 'Reply on RC1', Peter Passenier, 18 Jan 2022
-
RC2: 'Comment on esd-2021-78', Anonymous Referee #2, 04 Jan 2022
Review of “Earth System Sensitivity: a Feedback perspective” by Peter O. Passenier.
This manuscript explores the consequences a narrowing of uncertainty in equilibrium climate sensitivity (ECS) has for the Earth system sensitivity (ESS) using a feedback analysis. The analysis compares the ECS range from IPCC AR5 (likely 1.5-4.5 K) to the narrower range taken from Cox et al. 2018 (likely 2.2-3.4 K) which was derived from an emergent constraint based on observed variations in global temperature. It then explores the implications of these two ECS ranges for ESS by assuming several possible values for Earth system feedbacks (ice sheet albedo feedback, methane, etc). It concludes by making the point that even with a narrowed (Cox et al.) ECS range, the resulting ESS values could be larger than the IPCC AR5 ECS range, with possible implications for Paris targets.
The manuscript is well written and interesting to read. The methods are based on a straightforward application of feedback analysis. However, I have several major concerns that need to be addressed before I could recommend publication.
1) Sherwood et al. 2020 and IPCC AR6 WG1 Chapter 7 are two recent community assessments of ECS that narrow the likely range to about 2.5-4 K based on multiple lines of evidence (including emergent constraints). This is the likely ECS range that should be used in the analysis instead of the single study of Cox et al. 2018 (which has been challenged on methodological grounds and may turn out to not be robust, see discussion in IPCC AR6 Chapter 7).
2) The AR6 definition of ECS includes everything but the feedbacks associated with ice sheet changes and CO2. That is, it includes methane, vegetation, and many other biogeochemical/physical feedbacks whose values are assessed in AR6 Chapter 7. So, the only feedback of relevance for ESS here would be the ice sheet feedback.
Regarding the ice sheet feedback, AR6 Chapter 7 states the following:
… ice sheet mass loss leads to fresh water fluxes that can modify ocean circulation (Swingedouw et al., 2008; Goelzer et al., 2011; Bronselaer et al., 2018; Golledge et al., 2019). This leads to reduced surface warming… However, model simulations in which the Antarctic ice sheet is removed completely in a paleoclimate context indicate a positive global mean feedback on multi-millennial timescales due primarily to the surface albedo change… This net positive feedback due to ice sheets on long timescales is also supported by model simulations of the mid-Pliocene warm period… As such, overall, on multi-centennial timescales the feedback parameter associated with ice sheets is likely negative (medium confidence), but on multi-millennial timescales by the time the ice sheets reach equilibrium, the feedback parameter is very likely positive (high confidence; see Table 7.10). However, a relative lack of models carrying out simulations with and without interactive ice sheets over centennial to millennial timescales means that there is currently not enough evidence to quantify the magnitude of these feedbacks, or the timescales on which they act.
That is, on timescales of a century (of relevance for the Paris targets) the ice sheet feedback is probably negative, and only on timescales of several centuries and longer does it become positive, but with a value that is not well quantified on either timescale. In light of the AR6 assessment, a positive ice sheet feedback does not seem to be relevant for Paris targets. The ice sheet feedback values chosen in this study need to be well justified. And note that the value derived from the LGM is not suitable for a calculation of ESS relevant for future warming.
3) Given the above, I am not sure that the analysis can add much to the existing literature on ESS. While showing the impact of different hypothetical ice sheet feedback values for ESS (on top of different ECS ranges) would be a fine exercise, I don’t see what new information it would provide or how ESS relates to Paris targets which deal with warming this century. It’s also possible that I am missing something, but either way the author needs to better explain the relevance and novelty of the feedback calculation performed here.
References
IPCC AR6 WG1 Chapter 7: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_07.pdf
Sherwood et al. (2020). An assessment of Earth's climate sensitivity using multiple lines of evidence. Reviews of Geophysics, 58, e2019RG000678. https://doi.org/10.1029/2019RG000678
Citation: https://doi.org/10.5194/esd-2021-78-RC2 -
AC2: 'Reply on RC2', Peter Passenier, 18 Jan 2022
Thank you very much for your very valuable and constructive review of my manuscript.
Although you find the manuscript well written and interesting to read, you have several major concerns that need to be addressed before you could recommend publication in ESD.Please find my detailed responses and corresponding (proposed) changes to the manuscript in the attached Supplement
-
AC2: 'Reply on RC2', Peter Passenier, 18 Jan 2022
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