Dear Dr. Hansen,

I have sent your paper for further review. Two previous referees have re-assessed your manuscript; two additional referees assessed the revised manuscript. At this place I express my gratitude to all referees for their major efforts to review this lengthy and complex manuscript.

All referees acknowledge major improvements in the manuscript compared to the ACPD version. Two referees recommend the current version of the manuscript for publication, but nevertheless provide suggestions for improvement. Referee 2 and 3 provided substantial comments and indicate a number of areas of contention, and ask for further revisions. In particular, referee 2 has undertaken not only a review of the revised manuscript, but also provided his advise on the review process; the quality of the replies to the short contributed comments and how these comments are included in the revised manuscript. Based on the referees’ comments, I decided to reconsider your manuscript after major revisions.

To assist you in addressing the referees’ comments, and given the complexity of issues raised in the reviews, please find below more detailed instructions for addressing these comments in your revised manuscript and the response letter to the editor.

I remind you that the reviews, the editors correspondence and your replies will be duly made available on ACP’s website upon acceptance for publication.

Detailed instructions:
In the following I refer to referees R1, R2, R3 and R4.

Non-adequate response to Short Contributed Comments (SCs) (R2; l. 60)

I agree with R2 that some of the replies to the SCs were not sufficiently detailed, and in some cases may have perceived to be offensive. I agree also with R2 that the diligence of referees and the contributed commenters deserves to be taken seriously and to be responded with the highest possible quality.

However, I also acknowledge that the authors have been under pressure to deliver timely replies to the vast-amount of comments. At the same time, if R2 can provide a detailed account of the SCs (and the replies), I expect the authors to reach at least a similar level as R2.

Rather than re-opening the public-discussion (as suggested by R2), I wish the authors to address in the replies and in the manuscript the analysis of R2 regarding the 10 or so ‘germane’ comments, identified by R2 in lines 529-756. In most cases, where opinions diverge, it will be sufficient to discuss these in terms of diverging opinions and uncertainties in the assumptions in this work. The updated responses to the SCs can be provided in a separate document, with reference to the specific SCs and the text and line number in the revised manuscript.

Issues raised in ‘responses to referees and letter to editors’ (R2, l 758); mischaracterization of IPCC process (l. 219)

R2 in his response to your reply in http://www.atmos-chem-phys-discuss.net/15/C8227/2015/acpd-15-C8227-2015-supplement.pdf, obviously expressed his strong disagreement and discontent with some of your statements made on the roles of referees and the IPCC process in the responses.

At this point I consider a further exchange of opinions on this topic not necessary for improving the contents of the paper. I do recognize that overall the findings of this paper are out-of-bound with the high-level messages that were communicated by IPCC AR5, and as such a reference to IPCC is appropriate and even desired, however they should be made carefully and accurately phrased- to avoid misinterpretation.

Contentious issues that need substantial revision:

Title

R3 and R4 suggest that the word dangerous needs further clarification, which in their view should be the UNFCCC definition. Several SCs have questioned the appropriateness of the title- and several others were supportive. Both R2 (l.256-264) and R3 (section 3.2) offer several alternatives that would more accurately describe the actual paper and the surrounding uncertainties. I support the suggestion:
“Ice Melt, Sea Level Rise and Superstorms: Evidence from Paleoclimate Data, Climate Modeling, and Modern Observations that 2°C Global Warming is potentially Dangerous”, as it sufficiently close to the original title. The authors should however feel free to consider the other suggestions as well, but should well motivate the choice of alternative phrasings.

Reflection on uncertainties

I support R2 (l. 266) and many commenters reviews that careful wording on uncertainties is needed in abstract and conclusion (and title see above). The need for follow up work in 10.2 is crucial to corroborate or falsify the paper’s hypothesis.

Policy discussion (section 10.3)

Both R2 and R3 raise the issue of the appropriateness of the policy discussions provided in section 10.3. R2 finds the entire policy discussion inappropriate and recommends omitting it from the manuscript (l. 169). R3 considers that the discussion after l. 1857 is inappropriate – and ineffective- for a disciplinary journal like ACP. He further suggests the journal to offer an alternative venue to the authors to talk about the possible policy implications of this work. ACP and its publisher Copernicus are currently exploring this option -upon acceptance of this manuscript. Following the recommendation of R3, in the revised manuscript I wish the authors to limit the discussion policy implications to the minimum possible.

Expansion of section 10.2

R4 expresses his consent that despite many identified weaknesses the originality and potential importance of the paper “ … will lead to much subsequent progress”. R3 states: “…conclusions can only be corroborated or falsified by work of other research groups”.
I therefore support the recommendation of R2 asking for substantial expansion of section 10.2 in terms of identifying critical modeling and observational work that can corroborate this work.


Section 9: (R2 l. 195-205)

R2 suggests that section 9 is entirely deleted. I leave it to the judgment of the authors to delete or modify it. In the latter case, a better connection to other parts of the paper needs to be made, especially section 7. In this respect, please consider the remarks of R3 requesting clarification of the discussion of ‘CO2 as the control knob’, and R1. A short implication for policy and society can be integrated in section, however avoiding too detailed policy discussion.

Boulders (R2 l 272; R1 about section 6.2; R4 comment 2).

R2 offers additional thoughts about the physical process leading to Boulder deposition. R1 finds section 6.2 not well introduced, R4 identifies this section as scholarly but original. I do not endorse R4 suggestion to move this to another paper; but moving lengthy considerations to supplementary material could be considered. Clearly this is perhaps the most disputed part of the manuscript; I invite the authors to try to reflect as good as possible the uncertainties that are associated with the ‘Boulder’ hypothesis, and consequences for the Eemian storminess hypothesis. Section 10.2 could identify further modeling and field work needed to strengthen the evidence.

Other Comments:

R1: Suggestions to be considered.
R3: Points raised in section 5 are of technical nature or requesting clarification, and should be relatively straight forward to address
R4: Detailed comments in point 3-4 are of technical nature and should be considered.
R2: l. 394-489 provides several suggestions for further streamlining the paper, and improving the readability. While many of these suggestions seem entirely sensible, and could be considered by the authors; I leave it to discretion of the authors to judge to what extent it is practical to implement such changes.
R2: l. 490-514 Color schemes in pictures. I leave it to discretion of the authors to judge to what extent it is practical to implement such changes.
R2 l. 867-1148 Mostly technical comments, to be considered from case to case. Short information needs to be provided in the replies with regard to the implementation of changes.

Dear Dr. Hansen,
following a discussion with the ACP editorial board, I will request one change in the title. In order to bring out even more clearly the element of uncertainty associated with the work to change the word would into could.

The title would thus become:
"Ice Melt, Sea Level Rise and Superstorms: Evidence from Paleoclimate Data, Climate Modeling, and Modern Observations Implies that 2°C Global Warming Above the Preindustrial Level Could Be Dangerous"

thank you for your cooperation

Dear Dr. Hansen,
following your substantial revisions, in response to my instructions and the remarks of the 4 reviewers, I am pleased to inform you that I decided to accept your
publication. At this place I would like to thank the reviewers, and the many commenters to the earlier versions of this publication, for their substantial efforts that led to a considerable improvement of the ACPD paper. I would like to encourage the scientific community to engage in the critical experiments (observations and modelling) that could corroborate or falsify the main thesis of your publication. Given the potential significance and implications of the results, I will recommend to highlight this publication to the EGU’s press officer.