|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.
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.
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).