|2nd review of Bathiany et al. 2016.|
I previously reviewed this paper, and have positively noted that this version is much improved compared to the earlier version. However, I do still have strong objections to some aspects of the manuscript, which do not allow me to recommend publishing it in this form in the peer reviewed literature. My main objections are related to the motivation of this study, which in my opinion overstates the implications of this study, as the discussion implies there might be bifurcations on the way to an ice-free Arctic, even though it also mentions that comprehensive models do not show that (or only for the opposite state, snowball earth, for paleo simulations, which is a completely different state, and not a state the authors investigate). Similarly, the last sentence of the conclusions again overstates the implications of the study, by suggesting that it can help to provide “early warning for potential extreme events”, which isn’t supported by the results. Nevertheless, I can see how this study can become publishable after further revisions (and a change of title), limiting its stated implications to what it actually does do. For example, the Abstract reads well now and is limited to what the study does deliver, so I hope the authors can change the title, introduction, and conclusions to match the scope of the Abstract.
• I would like to strongly re-iterate that the title should be changed to better reflect the focus of the manuscript on the statistics of sea ice variability on the way to an ice-free Arctic. A changed title was suggested by all reviewers and in the comment by T. Wagner, but it was not changed and no strong reasons were given for why not. I strongly feel that the title is is misleading, and the nature of the paper, focusing on statistical indicators, needs to be reflected in the title, rather than only in the Abstract.
• Line 22: I would suggest rephrasing “To this extent, the prospects to find statistical early warning signals before an abrupt sea-ice loss at a “tipping point” seem very limited.” To “Based on these results, the prospects to find statistical early warning signals before an abrupt sea-ice loss at a “tipping point” seem very limited. “, as “to this extent” makes no sense in English.
• Line 35-37: Isn’t one of the main finding that statistical early warning signs are not able to warn about tipping points (as stated in the Abstract)? So this sentence “Moreover, natural climate variability can be an indicator of climate stability and provide “early warning signals” of an approaching tipping point (Scheffer et al., 2009). “ needs to be rephrased, maybe to “Furthermore, previous studies (Scheffer et al., 2009) have suggested that natural climate variability can be an indicator of climate stability and provide “early warning signals” of an approaching tipping point.
• Page 2, Line 5-9: Tietsche et al. (2012) also clearly showed that sea ice loss is completely reversible, and should be cited. And Wagner and Eisenman (2015) showed why simple models show this bifurcation, and that it was an artifact of the simple models that did not include seasonal cycle or latitudinal differences. So this question has been solved, and should not be presented as a current topic of debate just to make the current study seem more relevant. Comprehensive models don’t show bifurcations in sea ice loss, physical understanding does not support it, and the reason simple models nevertheless simulated it has been found. The examples for bifurcations for paleo simulations (which I am not familiar with, but which tend to use models with much reduced resolution compared to present day study) for snowball earth is the opposite of the earlier discussed bifurcation of a irreversible sea ice loss, so it is misleading to cite these here and to imply that therefore there might be bifurcations in the future climate as well. It seems that this only serves to motivate the study presented in this manuscript, but it does not do it in a well reasoned way and this motivation needs to be revised again to be publishable, as it severely currently overstates the implications. The current study has merit as an analysis of statistical indicators of sea ice variability, with some limited implications for the real world, and should be presented as such rather than trying to imply it does more than it can.
• Page 13, Line 507: The last sentence of the conclusions is, in my opinion, not supported by the results, and overstates the implications of this study. If the authors are happy to publish this manuscript under a title that clearly reflects that they study “Statistical indicators of sea-ice variability on the way to an ice-free Arctic” (or similar), and remove this final sentence of the conclusions as well as rephrase the introduction, this manuscript should be publishable. But trying to overstate the impactions of the study through the title or the conclusions make it so I can not sign off on for publication in the peer reviewed literature. The Abstract reads fine and does not make this last statement, so it does not seem to be central to the paper.
Tietsche, S.,D. Notz, J.H. Jungclaus, and J. Marotzke: Recovery mechanisms of Arctic summer sea ice,Geophys. Res. Lett.
, 38, L02707, doi:10.1029/2010GL045698, 2011.