This is undoubtedly a major paper, which poses challenges at various levels.

It should be published but not in its current form. I propose:

- to change the title

- to leave out ethical, juridical, policy considerations from the conclusions,

- to offer the authors a guest editorial or other appropriate forum to discuss the above issues

- to do further efforts to make the paper shorter and more readable

- to address some major issues/questions that, in my opinion, still remain





1. Challenge for the reviewer



When I started to review the first version of the paper I eventually withdrew because I felt unable to judge the quality/correctness/coherence of the assertions made. I could basically not digest the variety of material, coming from oceanography, glaciology, atmospheric physics, paleoclimatology, geomorphology, …



When the editor asked me to look at the revised version, he wanted me to reflect on language and conclusions that seemed to go beyond what is normally expected in a scientific paper. I started again reading the full paper and found it more digestible. The first revision and reorganization of the paper might have been of help. However it is still a difficult paper to read. A lot of the discussion is about explaining why the coarse model is still appropriate to deal with often sub-grid processes. Sometimes there is text that is not to the point, sometimes explanations are scattered throughout the paper or even are missing. There are here and there are signs of sloppiness. These major and minor issues are listed in section 5, and should be addressed.



In general, I still find it a challenge to judge the validity of the conclusions made, especially regarding the severity and timing of the events described (non-linear ice-melt and multi-meter sea-level rise). I think those conclusions can only be corroborated / falsified by work of other research groups that must address the same issues. That work should appear better sooner than later.





2.Challence for ACP



The editors of ACP have decided to review the paper. They now need to make sure that the open discussion has delivered a comprehensive review process, in which specific aspects of each the various disciplines where critically looked at, as well as their integration, in order to accept the conclusions of the paper.



But there is a greater challenge



The authors ostensibly cross boarders when it comes to describing the possible consequences of the papers results. In the discussion, many have argued that the use of words like “highly dangerous” in the title, and the adding of a (shallow) discussion regarding ethics, justice and prescribing policy action might be out of the scope for a disciplinary natural sciences journal like ACP.



I want to be clear from the beginning. I strongly believe that a collective issue such as climate change can no longer be discussed in scientific terms only, but that the scientific discourse must inform, or even go along ethical and political discourses. There is indeed a role for scientists to engage in the latter discourses, based on the former. However, my point is that there are many other ways to engage in these discourses and to have the voice of science heard. (The first author has made use of these other means frequently and successfully in the past). Putting the full scientific, ethical, juridical, and policy prescriptive discussion in an ACP paper is, in my opinion, not only totally ineffective, but it may put also a good disciplinary journal into problems. It is something for the ACP editors to consider, but gain, there are other means. I will make a suggestion later for ACP for how deal with this with respect to the current paper.



3. My comments and suggestions to ACP are as follows.



3.1 Title



The title presently use by the authors is rather journalistic. That would be OK if the paper would also be written in a journalistic (but correct) way. It isn’t. It is a complex 50 pages long technical paper that even scientists are struggling with. It is too much to hope for that journalists would read it, including all the caveats it contains.



If the ACP editors would like to keep the title to the standards of a disciplinary paper, and stick to what the paper is really about, the title should, in my opinion, be more like:

Non-linear Ice Melt, Multi-meter Sea Level Rise and Superstorms at 2°C Global Warming: clues from Paleoclimate Data, Climate Modeling, and Modern Observations.

That is already a strong enough title, covering the content of a strong piece of science.



However, in the climate change debate, the term “dangerous” has been given a rather precise definition in Art 2 of he UNFCCC, namely a change that doesn’t allow ecosystems to adapt, that threatens food production and that prevents economic development in a sustainable manner. It is obvious that a multi-meter sea-level rise within a century would be “dangerous” in that sense, at least in low-lying islands and coastal areas.

I could therefore also live with a title that reads.

Ice Melt, Sea Level Rise and Superstorms: Evidence from Paleoclimate Data, Climate Modeling, and Modern Observations that 2°C Global Warming is “Dangerous”.

Where the quotes indicate that a special meaning is given to “Dangerous”. A reference could be made to UNFCCC in the abstract, and a fuller explanation in the body of the text.



Given the many uncertainties and need for further analyses mentioned by the authors themselves and by reviewers in the Discussion, one should however argue for the following title:

Ice Melt, Sea Level Rise and Superstorms: Evidence from Paleoclimate Data, Climate Modeling, and Modern Observations that 2°C Global Warming could be “Dangerous”.



3.2 Conclusions of the paper



The text in lines 1767 – 1770 could be rewritten to define “Dangerous” according to UNFCCC.



For completeness, these lines should also include a discussion about the fact that accelerating ice melt would also lead to a reduced global warming. What positive effects would that have on human health (heat waves) or agriculture (droughts)? In fact the latest IPCC WG II report put a large emphasis on the impact on future food security.



Section 10.3 Practical consequences, should really stop with line 1856. It could however contain a section on the shift from a focus from global mean temperature as an indicator to SLR of radiative forcing. That could have practical consequences for future policy discussions.



What follows after line 1856 is in fact not correct. The Copenhagen accord (downloadable from the UNFCCC website) never uses the term “guardrail”, it in fact never mentions that 2 degrees would be safe, it even mentions to look into 1.5 degrees as a possible target. I am sorry to say, but in lines 1857 – 1861 the authors are going over the top.



Although I can personally relate to what is written in lines 1864-1890, I do not believe it fits within a paper in a disciplinary journal like ACP, for the reasons mentioned above.



4. Final suggestion to the ACP editors.



The final section of the paper, as it is submitted, raises the question about the place of science and scientists within the climate change debate, and, in fact, within a debate on any collective issue. These questions and ensuing discussions are of utmost importance to find scientifically and socially robust solutions to those problems.



I would suggest to the ACP editors that , while keeping the paper a (multi-) disciplinary scientific paper, following the suggestions above, they offer the first author a forum (for instance a guest editorial if that is possible), in which he can expresses his opinion about the role of science and scientists in the debate. Some of the material used in section 10.3 could be used there. I am sure that such a guest editorial would help reflect ACP and any other disciplinary journal about how to deal with facts and figures on the one hand and concerns and opinions on the other.



5. Major and minor issues after having read the full paper



Circular reasoning?



The study begins with a hypothesis that disintegration of an ice sheet in contact with the ocean is nonlinear and better characterized by a doubling time for mass loss rate, … Modeling then revealed amplifying feedbacks that lead to such nonlinear response. The question arises whether the feedback is not a result from the nonlinear increase? Have the authors performed experiments with a linear increase in ice melt, and to what extent did the feedbacks still show up?



How is the ocean water column stabilized by melt water?



A short explanations of the working of the THC would be very useful background information. An attempt is made in Fig. 18, but it still doesn’t explain the vertical stratification of the cean column. Maybe it is only a matter of adding a few words. I stress this because it is, after all the core issue of the study. The paper never clearly explains why stratification happens: the fact that it is fresh water (lower salinity, lower density) helps stratification, but it would be helpful to explicitly mention that. It is also never said whether the temperature (-15 degrC) of the meltwater is still warmer than the upwelling cold water? Does slowing down the ocean circulation helps stratification? Is it all three? Summarizing all this effects in a few lines, rather than giving partial hints in various places throughout in the paper would increase the understanding.



Is the Eemian a good surrogate for the Holocene?



The temperature during the Eemian was about 1 degrC higher that today, while the CO2 concentration was 275 ppm and not 400 ppm as today. That would indicate that warming processes, not linked to CO2 were active during the Eemian. Although orbital is forcing is mentioned at several places driving climate change in the Eemian, it could find a more prominent place in the discussion or conclusion. How is orbital forcing related to the point made by the authors that CO2 is the “control knob” for global mean temperature.



Ice melt reduces global surface warming



While the effect of multi-meter sea level rise is discussed, the reduction of global surface warming and its potential effects is not sufficiently elaborated. Clearly one effect will not offset the other, but as a reader you expect that something would be written about it.



Some examples of things that make the text difficult to read, and that could be addressed



- The abstract mentions ice-melt doubling times of 10, 20, 40 years, whereas model experiments seem to be performed with doubling times of 5,10 20.

- Unclear use of the terms “radiative forcing”, “climate forcing” “Energy imbalance”. “Net forcing”. In line 1356 they are treated the same. In line 539 and Fig 15, “net forcing” seems to be different from “energy imbalance”: the text mentions 5 W/m2 at 2100, Fig 15 shows around 1,5 W/m2.

- The authors frequently use the words “We suggest that”, which is sometimes a mere suggestion, or which is sometimes a hypothesis that is corroborated later on in the text.

- Certain processes are described in unnecessary detail, like e.g. the various carbon pumps (lines 1412 – 1417). A short explanation of how the THC works would me more useful, instead!

- The issue of ocean column stratification upon ice-melt is explained at various places in the paper. A single comprehensive explanation would suffice (see comment above)