Brief communication: Solar radiation management not as effective as CO<sub>2</sub> mitigation for Arctic sea ice loss in hitting the 1.5 and 2&thinsp;°C COP climate targets

Ridley, Jeff K.; Blockley, Edward W.

doi:https://doi.org/10.5194/tc-12-3355-2018

Articles | Volume 12, issue 10

https://doi.org/10.5194/tc-12-3355-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-12-3355-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 10

Brief communication

|

23 Oct 2018

Brief communication |

| 23 Oct 2018

Brief communication: Solar radiation management not as effective as CO₂ mitigation for Arctic sea ice loss in hitting the 1.5 and 2 °C COP climate targets

Jeff K. Ridley and Edward W. Blockley

Download

Final revised paper (published on 23 Oct 2018)
Preprint (discussion started on 03 Jan 2018)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

- Printer-friendly version

- Supplement

RC1: 'Review of « The significance for the IPCC targets of 1.5°C and 2.0°C temperature rise for an ice-free Arctic » by Ridley and Blockley', Anonymous Referee #1, 10 Jan 2018
- AC1: 'Response to reviewer 1', Jeff Ridley, 16 Mar 2018
RC2: 'Review', Anonymous Referee #2, 31 Jan 2018
- AC2: 'Response to reviewer 2', Jeff Ridley, 16 Mar 2018
- AC1: 'Response to reviewer 1', Jeff Ridley, 16 Mar 2018
RC3: 'Review of Ridley and Blockley', Anonymous Referee #3, 28 Mar 2018
- AC3: 'Respnse to referee 3', Jeff Ridley, 04 Apr 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Jeff Ridley on behalf of the Authors (04 Apr 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 Apr 2018) by Xavier Fettweis

RR by Anonymous Referee #2 (23 Apr 2018)

Suggestions for revision or reasons for rejection

2nd Review of Ridley and Blockley

This is my second review of the brief communication, “The significance for the IPCC targets of 1.5°C and 2.0°C temperature rise for an ice-free Arctic.” This version of the manuscript is much improved, but unfortunately, I still don’t think it is ready for publication, as some of the newly added text is confusing, some of it is incorrect, and the figures are still all out of focus and not acceptable (despite a reply to review that stated these figures were updated in response to review). Figures should be vector graphics (eps or pdf) to avoid them being this pixelated at a normal print and screen resolution (100%). Furthermore, since the first review, three new papers on this exact same subject have now appeared (see below for the new references), with another in open review in earth system discussions (Iversen et al., see below). Those need to discussed in a further review cycle (at least the published ones, the under review one maybe not, not sure on the policy for those articles in open review). I don’t think these new studies preclude the publication of this paper, as they use different models and methodologies, and the solar radiation management approach is unique to this study. And it is very interesting that the results agree quite well. But these studies need to be discussed, and the introduction needs to be rephrased in order to reflect that there are previous answers to this question, and that the main contribution here is to look at the impact of solar radiation management, as alternative to reduced emissions, as mentioned in the reply to review. I think this will actually help the article to be more interesting and significant.

Specific comments:
Page 1,Line 1: I would recommend reconsidering the title to better reflect the unique aspects of this study. Maybe “Brief Communication: Probabilities of an ice-free Arctic for limiting warming to 1.5C and 2.0C though solar radiation management”
Page 1, Line 8-9: It is unclear to me what “is used to improve the signal to noise associated with the internal variability” means, so that is not good in an Abstract, which should convey the main message clearly and concisly.
Page 1, Line 9-10: I don’t think this statement is correct, based on the analysis presented: “It is found that the continuing loss of Arctic sea ice can be halted if the Paris Agreement temperature goal of 1.5oC is achieved”. The paper only assessed September ice extent, whereas this statement implies any ice loss, which can include winter month and/or ice volume. Also, arguably also the 2C stabilization stops the ice-loss (in September), but at a lower level. So this statement is just not correct and can’t be in the Abstract. Instead there should be a statement on the actual finding, as stated in the conclusions (incorrectly, as increased rather than decreased, see comment later) and supported by the analysis presented: “The risk for a seasonally ice-free Arctic is reduced for a target temperature for global warming of 1.5°C (0.1%) compared to 2.0°C (42%).”
Page 1, line 18-19: Given that there are now three more 2018 published studies on this subject, and a News and Views text by Screen (2018), in addition to the cited two 2017 studies (Screen and Williamson and Sanderson et al), “Here we investigate if Arctic sea ice cover is one such system.” Really doesn’t seem appropriate anymore. All of these studies have shown that the Arctic sea ice cover is such a system. So while this may have been the initial motivation, I would suggest re-casting it in terms of whether global temperature control through solar radiation management also shows such large impacts between 1.5C and 2.0C for Arctic sea ice, or whether solar radiation management would lead to different results. That requires re-writing large parts of the initial paragraph, but I think it will make the paper much stronger.
Page 2, line 11-12: This is incorrect “Sanderson et al., 2017) used a climate model emulator, calibrated against CESM1, to produce simulations at constant 1.5°C and 2.0°C”. They used a climate model emulator to design emission scenarios that lead to CESM1 simulations at constant 1.5°C and 2.0°C. But the results shown are from the CESM1, not from an emulator. This needs to be correctly stated.
Page 2, line 12-14: This is also incorrect “Another study (Screen & Williamson, 2017) used all the CMIP5 model simulations and regressed the September sea ice extent for 1.5°C and 2.0°C against the 2007-2016 mean sea ice extent. Bayesian statistics were then used to estimate the probability of an ice-free Arctic for 1.5°C or 2.0°C.”. Screen & Williamson used CMIP5 simulations until the year before they first reached a global warming of 1.5°C and 2.0°C and regressed those against the 2007-2016 mean sea ice extent.
Page 2, line 14: A discussion of the new three studies need to go here.
Page 2, line 15: Now of six different approaches
Page 3, line 6-7: I know this was added in response to reviewer 3 (The use of an ensemble mean will not be available in reality to plan implementation of SRM.), but without context, this is not very useful and should be expanded upon, so the reader can understand why this is here, without having to read the reviews.
Page 3, line 12-16: This is an important and very interesting point, but I needed to read it three times to get it. Please re-write it in shorter, more precise sentences, so it is easier to follow.
Page 4, line 17-19: In order to really understand Figure 3b, one need to know how many years contribute to each of the pdfs. I would recommend adding the temperature bands in Figure 1, so it is clear how many years of ensemble 1, 2, and 3 end up in those bands and are used for the assessment of these probabilities. Because Sigmond et al showed that this probability increases the longer the sample period is, as also discussed by Screen (2018). In addition, (or alternately, if those boxes make Figure 1 too busy), the authors should mention how many years are in each of the bands, and from which ensembles they are (RCP2.6, RCP4.5, ensemble-1, ensemble-2, ensemble-3). This is important to understand the methodology.
Page 4: Line 21: This is the central finding, but there is a typo: The risk is significantly DECREASED, not increased as stated, for a target temperature of 1.5C.
Page 4, Line 22: It is unclear to me how this statement fits here, as the first sentence isn’t about timing, but general probabilities. So “Another climate model, with a thicker Arctic sea ice in its mean state, would be expected to produce a later date for an ice-free Arctic (Bitz, 2008) “ doesn’t fit here. Please either remove it or add a sentence that actually refers to the date of ice-free conditions. But that wasn’t analyzed here, so really I think this needs to go.
Page 4, line 23-24: Screen and Williamson assessed the probability of any ice-free conditions before reaching 1.5C and 2.0C, not the probability in a given year, as is done here, and was done in Sanderson et al. (2017). That needs to be clarified. See Screen 2018 (see below for ref) for an explanation of the differences of those probabilities.
Page 4, Line 25: Sanderson et al did not use a climate model emulator for these results, as stated here incorrectly, but used a climate model emulator to develop the emission scenarios that would lead to stabilized warming at 1.5C and 2.0C in the CESM1. That’s clearly explained in Sanderson et al.
Figures: All figures are still low quality and out of focus. They need to be vector graphics.

Minor comments:

Figure 2: As noted last time, figure 2 does not show the “spatial pattern of the Arctic sea ice extent”, but the “spatial pattern of the sea ice edge”. Other changes were made, but this one wasn’t.
Figure 3: The caption now better explains the figure. But the newly added line for ice-free is a purple line, not a dark blue line.
Page 3, line 18: As for the next two lines, this should read: Ensemble-1 : starts at 1.5C on RCP2.6 and levels out at 1.5C above pre-industrial  
Page 3, line 29: “with ice retreating further in the Atlantic sector” In which ensemble, ensemble-1 or ensemble-2?
Page 1, line 21: This is not a “salinity freezing point”, but “salinity-dependent freeze point” or “freezing point of the ocean”.
Page 1, line 23: Missing “a” before “warmer atmosphere”

New references that need to be included:
Sigmond, M., Fyfe, J. C. & Swart, N. C., Ice-free Arctic projections under the Paris Agreement, Nat. Clim. Change https://doi.org/10.1038/s41558-018-0124-y (2018).
Jahn, A., Reduced probability of ice-free summers for 1.5 °C compared to 2 °C warming, Nat. Clim. Change https://doi.org/10.1038/s41558-018-0127-8 (2018).
Niederdrenk, A.L. & Notz, D., Arctic Sea Ice in a 1.5°C Warmer World, Geophys. Res. Lett. 45, 1963–1971 (2018).

Maybe to include, based on journal policies on non peer reviewed and under review articles:
Screen, J. A., Arctic sea ice at 1.5 and 2 °C, Nature Climate Change (2018), doi:10.1038/s41558-018-0137-6
Iversen, T. et al., The NorESM1-Happi used for evaluating differences between a global warming of 1.5 °C and 2 °C, and the role of Arctic Amplification, Earth Syst. Dynam. Discuss. https://doi.org/10.5194/esd-2017-115 (2017).

Hide

RR by Anonymous Referee #3 (09 May 2018)

Suggestions for revision or reasons for rejection

The revised version of "The significance for the IPCC targets of 1.5°C and 2.0°C temperature rise for an ice-free Arctic." addresses some, but not all of the concerns I had with the original paper.

My major concern was that an SRM experiment was being used as a proxy for climate impacts at 1.5 and 2 degrees, with particular reference to those temperatures as described in the Paris Agreement. This remains the case with this version, and it is troubling for 2 reasons:

The high latitude warming at a given stabilization level will be greater in a simulation where some greenhouse gas forcing is offset with SRM. This is supported by the paper's new Figure 3a, which clearly shows that the mean sea ice extent at 1.5 degrees in the geoengineered simulations is less than that seen in the mitigation-only experiments (i.e. the mean of the distribution of black points is less than the mean of the distribution of red points at the 1.5 degree warming level). However, the authors argue the opposite - that their analysis demonstrates that the SRM simulation is a good proxy for mitigation only experiments.

This is potentially misleading to the community. The Paris Agreement targets are usually interpreted as a mitigation goal - not as targets for SRM geo-engineering. This paper is too quick to dismiss the difference between the climates which would be achieved in the two cases, despite the fact that their results show clear differences.

As I said in my initial report, this paper would be quite acceptable if it acknowledged that there are differences in SRM simulations and mitigation only experiments (and categorized them in detail for Arctic sea-ice) - but the paper still reads as if SRM and mitigation are interchangeable. This is a strong statement, with huge policy implications - and it is not even supported by the author's own results.

Minor Issues:
- Remove apostrophes in "RCP's"
- Section 3, last but 1 para " SRM itself is not having an effect on the Arctic sea ice " - this statement is not supported by the plot, the two distributions are clearly distinguishable at the 1.5 degree threshold.
- "Sandersen et al. (2017)" -> "Sanderson et al. (2017)" throughout
- Section 4 - to say Sanderson et al used a climate model emulator is misleading - the emulator was only used to produce the emission pathways. The relationship between global mean temperature and arctic sea ice found in that paper was derived from a full GCM.
- Section 4 - 2nd para - "independent" is a strong word with statistical implications which do not apply here. Consider using simply "different"

Hide

ED: Reconsider after major revisions (14 May 2018) by Xavier Fettweis

AR by Jeff Ridley on behalf of the Authors (13 Jul 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 Jul 2018) by Xavier Fettweis

RR by Anonymous Referee #2 (27 Jul 2018)

Suggestions for revision or reasons for rejection

The third version of this manuscript is finally in good shape (beside the figures). By refocusing the manuscript on the SRM difference to CO2 mitigation, the paper makes a clear and novel contribution to the discussion, while citing most of the recent relevant literature (should add Tilmes et al 2014 (GRL), which assessed the effect of global versus regional shortwave radiation management on Arctic sea ice.) I can now almost recommend publication, provided final edits are done as outlined below, including finally replacing the figures with higher quality figures, as requested twice already to no anvil.
As the wrong manuscript file was submitted, I can only comment on the version included in the reply to review, assuming this is the version the authors wanted to submit. Unfortunately, that has no line numbers, so I can’t refer to the exact place in the paper in my comments below.

End of Abstract: Should be “produces a higher amplification”.

Introduction 3rd paragraph: I think this whole paragraph should go, as it isn’t directly related to anything that is later discussed, and it makes the Introduction longer than the results section, which is odd for a paper.

Introduction 3rd paragraph (if kept): Missing comma and please check the grammar in this sentence after “The thinner the ice the less that survives the summer melt and consequently the area of perennial ice declines”
Introduction 3rd paragraph the Introduction is longer than the results section.: Should be “ice-albedo feedback”

Introduction 4th paragraph: Please take out “The current minimum…”, as this statement is unnecessary (never referred to again), is based on daily sea ice extent (when the rest is based on monthly means), and there is no pre-industrial or 1980s reference value given, so just one number does not support the following statement of “Such a sharp drop off in sea ice..”. This fits much better if it follows the 11% decline per decade in the sentence before the one I would recommend taking out.

Introduction 5th paragraph: The coastal erosion is not just a problem if the ice loss contributes to increase extreme weather in mid-latitudes, but in general as there is a longer open-water season in the Arctic. So this should be “Furthermore,”, not “Thus”, which implies a causal link to the mid-latitude weather changes.

Introduction 6th paragraph: Should be “It has been suggested”, not “It is suggested” at start of red text. In this discussion, or later, the Tilmes et al 2014 (GRL) paper should also be cited, which assessed the effect of global versus regional shortwave radiation management on Arctic sea ice.
Conclusions: two .. after “SMR and RCP..
Conclusions: Last sentence should be clarified, and include the cause of this: “Here we show that SRM is not as effective as conventional mitigation in reducing Arctic sea-ice loss, due to a higher polar amplification for SRM for the same amount of global warming.” As this is the last sentence, having it be easy to follow and strong is important. The suggested rewrite also sounds less like an endorsement of SRM, which the authors made clear in another place is not what they are doing.
References: Several references appear out of order (Overland in between McLaren and Ming; Rayner between Sanderson and Shepherd), one is cut off (ure, N, Saches, T., Helm, V., and Fritz, M.). So extra care should be taken to ensure all references cited are included, in the correct order, and without mistakes.
Figures: These figures are still of very bad quality and can not be published like this. The authors replied to my previous comment on that saying “I have no problem with the figures when I downloaded the paper from the Cryosphere. In any case the final graphics will be high resolution”. I have a problem with the figures, they are pixelated in the versions I have, making the text in them hard to read (like legend text) and the lines blur together. If high quality versions exist, why not include them now, after two requests for that?

Hide

RR by Anonymous Referee #3 (05 Sep 2018)

ED: Publish subject to minor revisions (review by editor) (05 Sep 2018) by Xavier Fettweis

AR by Jeff Ridley on behalf of the Authors (07 Sep 2018) Author's response Manuscript

ED: Publish subject to technical corrections (14 Sep 2018) by Xavier Fettweis

AR by Jeff Ridley on behalf of the Authors (24 Sep 2018) Manuscript

Short summary

The climate change conference held in Paris in 2016 made a commitment to limiting global-mean warming since the pre-industrial era to well below 2 °C and to pursue efforts to limit the warming to 1.5 °C. Since global warming is already at 1 °C, the 1.5 °C can only be achieved at considerable cost. It is thus important to assess the risks associated with the higher target. This paper shows that the decline of Arctic sea ice, and associated impacts, can only be halted with the 1.5 °C target.

Brief communication: Solar radiation management not as effective as CO2 mitigation for Arctic sea ice loss in hitting the 1.5 and 2 °C COP climate targets

Download

Interactive discussion

Peer-review completion

Brief communication: Solar radiation management not as effective as CO₂ mitigation for Arctic sea ice loss in hitting the 1.5 and 2 °C COP climate targets