Dear authors, based on reviewer reports and your detailed responses, I would like to invite you revise your manuscript that will be then subject to another round of peer review.

Dear authors

I am now in the possession of three independent reviews of your revised manuscript. I have collated them in terms of Major and Minor comments below. Please review all three reviewer reports as posted on ESDD as I have not included all their comments in my synthesis. I invite you to respond to the comments as detailed below and revise your manuscript accordingly. Once you have done so your revised manuscript will be sent for another round of independent review. Therefore my decision is: review after major corrections.

Reviewer #1 considers the manuscripts methods are scientifically justifiable and transparent enough that others who disagree with the paper's threshold for climate change or emission-reduction scenarios can explore other approaches. The reviewer has only relatively minor comments.

Reviewer #3 considers the manuscript to be scientifically sound - given the limitations of the reviewer’s expertise. They have a number of Major and Minor comments which I ask you to address.

Reviewer #2 considers the manuscript to be scientifically sound, but that there are essentially editorial decisions that should be made. I agree with the reviewer that the manuscript has become much longer and that such length may reduce its effectiveness. Reviewer #2 has made considerable effort in outlining how the manuscript could be reduced in length. I would strongly recommend the authors critically engage with these suggestions. It is my decision that it is not necessary to limit the final manuscript to a specific number of pages. I agree with Reviewer #2 that the paper’s readability and potentially impact would be improved with their suggestions, but these suggestions in themselves are not required to ensure the scientific soundness of the manuscript.

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Reviewer #2 Major Comments

1) As a first step in paring down, it would help to distill the main message down into key points, e.g.:

(a) It is unsafe to let Earth warm beyond the point experienced during the early Holocene, because sustained warming beyond that point risks catastrophic sea level rise and other serious consequences.
(b) A less conservative limit can be framed around the maximum temperature of the Eemian.
(c) The Holocene limit corresponds to maximum warming of ~0.7K or CO2 levels of ~ 350 ppm, taking account likely changes in other GHGs. The Eemian limit corresponds to warming of ~1.0K or CO2 levels of ??.
(d) Both limits have already been surpassed.
(e) Restoring temperature and CO2 to the achieve either target is still feasible, but only by combining a rapid transition to non fossil energy sources with steps to actively remove CO2 from the atmosphere, including changes in forestry and agricultural practice.
(f) If these steps are not taken, then the burden on future generations will be very high.
Actually, the above is pretty close to the points emphasized the abstract, so the abstract probably also serves well enough as a core summary. In any case, a succinct set of points like this needed to prioritize content.

(2) Go through and cut the tangential material. Heavy cuts seem possible in Sections 3-6. Section 5 could probably almost be cut in its entirety, because all that’s needed is showing that CO2 is already beyond the threshold (point d). The new material on slow feedbacks can be heavily cut and would better be framed under heading that relates to a key point (e.g. point a). Other material could also possibly be moved around to better support key the key points. The final section labeled “Discussion” needs a clearer restatement of the main message and trimming down of tangential discussions.

(3) I urge Hansen’s coauthors to play a larger editorial role. The draft reads like a Hansen “core dump” without much additional input. I had the impression that my eyes might have been the first other than Hansen’s to actually read the current draft from start to end. More eyes are needed.

In addition to this major editorial issue, I have two other general concerns:

(1) The abstract mentions the Eemian-based limit as part of the central message, but the text is mostly framed around the Holocene-based limit, as if the draft was inconsistently revised at some point. Perhaps both limits are relevant, but the reader is not given any clear sense of how to think about them separately or collectively. Some framing is missing.

(2) The potential for large sea level rise is central to the argument, but it’s wrong to frame this as a feedback. A distinction is needed between slow feedbacks and slow impacts. The potential for slow impacts under sustained warming is key to point (a) above, and slow feedbacks are only one aspect of this.

[Editor’s comment: The comment below I have listed as a Major Comment as it represents a significant editorial judgement. It is my decision that discussions of a carbon fee are within the scope of the journal, but that such discussions should be appropriately framed within the rest of the manuscript and occupy commensurate space. I would recommend that the authors consider how a policy framed discussion on carbon taxes can be more integrated into the manuscript.]

1227-1237, 1246-1249: Cut. The relative merits of a carbon fee versus other approaches is tangential and also inappropriate to a science journal.

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Reviewer #3 Major Comments
General comments:

L127: In addition to the climate forcings, Earth’s measured energy imbalance and climate sensitivity, it would be interesting if the authors could put the performance of their model in the context of the transient climate response to cumulative emissions of carbon (TCRE; 0.8-2.5°C per 1000 PgC, >66% range, IPCC AR5 WGI).

L150-154: The citations of the Paris Agreement are taken from the draft agreement and not from the final agreement’s text. Please consider updating to the latest official wording which can be found on: http://unfccc.int/paris_agreement/items/9485.php

L432-440: A comparison of these values with the IPCC AR5 WGI assessment could be useful.

L508: The expression “burning of all fossil fuels” is very imprecise. Please clarify whether really all fossil fuels are meant here (that is, reserves, resources and future resources, or some smaller subset of these). Burning all known reserves and resources would result in much more than a doubling in CO2. Please clarify also whether this is a double of CO2 from today (ca. 400ppm) or from preindustrial.

L535-538: Please clarify how fast this 1°C warming was achieved and over which period the carbon losses are estimated.

L583-584: Maybe clarify here that oxidation of CH4 in the atmosphere results in a sink.

L634: Hansen and Sato (2004) does not provide the equations. It actually doesn’t provide a table either. Please update with the correct reference here for transparency of methods.

Figure 10: Indicate which airborne fraction is assumed for the right-hand axis of concentrations.

L786: The assumption that aerosol loading would increase or even stay constant is very weak. This should be put in context of the recent literature on this topic. Recent papers indeed show that future aerosol loading from air pollution is expected to decline under a wide range of socio-economic assumptions. For example, see Rao et al (2017).

Rao, S., et al. (2017). "Future air pollution in the Shared Socio-economic Pathways." Global Environmental Change 42: 346-358. http://www.sciencedirect.com/science/article/pii/S0959378016300723

L917: The burning of biomass in power plants is only one option for BECCS. Capturing the CO2 stream of ethanol production processes is an example of another.

L975: The costs cited here are “current” extraction costs. Given that a few pages further (L1030) the authors highlight the potential for rapidly falling costs of certain (renewable) technologies, one could expect that this could also be the case for negative emissions. There are clear differences to be expected in the dynamics of highly modular renewable technologies and the very bulky carbon dioxide removal (negative emissions) technologies. Nevertheless, price reductions should be considered in providing estimates over the entire century, ideally with some sensitivity assumptions.

Figure 14: Please indicate which energy accounting method is used to compare renewables and fossil fuels.

Figure 15: My understanding is that the RCPs include anthropogenic contributions only, while the observations also include additional natural sources and feedbacks. Please consider explaining this. This could be done in the caption as well as around L1078, where this data is being discussed.

L1100-1103: These statements could use a literature reference.

L1120: This statement would also require a reference.

L1136: Would it be possible to provide a range for the -1 W/m^2 value?

L1238-1240: Note that these policies did not target emissions reductions, but rather energy independence from oil. They are thus not a perfect historical analogue.

L1295: Given that we’re already in 2017 now, the authors might want to consider updating this graph with the latest data from the Global Carbon Project: Le Quéré, et al (2016). "Global Carbon Budget 2016." Earth Syst. Sci. Data 8(2): 605-649.


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Reviewer #1 Minor Comments

L490-492: Part of the text is missing. Should it read "Although there is little evidence to suggest increased C emissions from permafrost and hydrates in the past few decades, they potentially could have...."

L768-769: I suggest making the role of the Montreal Protocol more clear, e.g., ...Montreal Protocol (2016), which controls consumption and production of ozone depleting substances, to phase down.... In other words, I think it should be clear that the MP does not directly address emissions. Also, the reference is to the MP web page; can you point to the specific document used in this study?

L1055: In the past decade or so, growth in forcing of N2O and the sum of other trace halogenated gases (HFCs, HCFCs, minor-CFCs, halons, and SF6) has been equal to or greater than that for CH4. My point is that many non-CO2 GHGs are contributing, although increasing CO2 is dominating.

L1117: add "stratospheric" before ozone.

L114: Williamson, 2016

L252: ..Shakun, 2015)

L256: ..Hansen et al. (2016)...

L265: add degree symbol

L322: Uncertainties ... are....

L410: Is "unknown unknowns" redundant? Would "unknowns" be sufficient?

L595: Clark, 2014

L778: Liu and Zhang, 2011

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Reviewer #2 Minor Comments

133: Suggest “magnitude and time scales is not possible”

193: “The series of monthly records has terminated;…” Confusing wording. Better to say that the series peaked in October 2016…

270-277: This material should better fit in the section on slow feedbacks/climate responses.

282-284: The factor of 1.3 hinges on equilibrium land warming being twice the ocean warming, but Figure A2a establishes this only for transient warming. The correct factor must be closer to 1.0 for equilibrium warming.

303: Missing here connection between the 2 or 1.5C limits and this new limit of 350 ppm. What temperature would 350 ppm correspond to?

346: While this sentence is not incorrect, it may confuse the non expert, because it appears to equate net forcing with energy imbalance, a common point of confusion. Perhaps another way to start this would be say: “A positive radiative forcing causes Earth to warm, and given enough time the Earth will be hot enough…. “ A relevant point to follow is that, during the transient warming phase, Earth is out of energy balance..

476-478: Cut. Foster and Rohling neglected direct orbital forcing of ice volume, e.g. via seasonal changes in insolation, and they assume CO2 is the sole forcing agent of past climate changes, when in fact it almost certainly is partly responding to warming rather than driving it. In any case, this purported link between CO2 and sea level is not central and can be cut on that basis, whether or not the authors agree with my concern (I note that Rohling is a coauthor).

491: “to suggest and potentially” missing word here?

634: There is no Table 1 in Hansen and Sato (2004). Probably should Hansen et al 2000.

643-658: An example of tangential material.

698-700: Need reference for BP data.

Figure 9: The terms Delta Fe, MPTG and OTG are not defined.

Equation 1: This Equation still looks wrong. I’m pretty sure that R(t’) needs to be replaced by R(t-t’) and R(t) needs to be replaced by R(t=infinity), where R(t=infinity) is the equilibrium climate response.

761: Just put this footnote content in the main text. Might fit best at line 755, where some editing could reduce repetition.

765-782: Cut or move this text elsewhere. It gets in the way of understanding the method.

881-888: An example of an unnecessary digression.

959-961: Confusing. Is the scenario discussed here is among those shown in Figure 11 and 13? If not, better to cut to avoid the confusion. I urge cutting all discussion here and elsewhere of the alternate scenario, because it is tangential.

962-965: This text repeats methods already discussed. Cut.

962-970: These details belonged earlier, when the extraction scenarios were first described. They don’t belong here under Costs.

971: Confusing. It would have helped me if these points had been made here: (1) The total extraction requirements for different scenarios were previously summarized in Figure 11. (2) Costs estimates here use totals that are 100Pg less than in Figure 11, under the assumption that 100 PgC can be stored via improved agricultural and forestry practices at little net cost.

1048: a priori

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Reviewer #3 Minor Comments

L56: “of” instead of “if”

L85: China is now the largest source of “annual” fossil fuel emissions. At least that is what figure 1 shows. If the authors want to indicate which country is the largest cumulative contributor, please clarify this explicitly.

L111-112: Rogelj et al 2016b is not the appropriate reference here. A more appropriate alternative would be: Rogelj, J., G. Luderer, R. C. Pietzcker, E. Kriegler, M. Schaeffer, V. Krey and K. Riahi (2015). "Energy system transformations for limiting end-of-century warming to below 1.5°C." Nature Clim. Change 5(6): 519-527.

L135: Explain CMIP5 in full.

L135: I always thought that integrated assessment models were abbreviated by “IAM” instead of the here introduced acronym “IMAC”. Moreover, this acronym seems not to be used further in the manuscript.

L189: It could be helpful to mark these super-El Niños in the figure itself.

L200-201: This point has been made also by Huber and Knutti: Huber, M. and R. Knutti (2014). "Natural variability, radiative forcing and climate response in the recent hiatus reconciled." Nature Geosci 7(9): 651-656.

L490-492: This sentence reads very odd.

L654: the “global fossil fuel CO2 emissions *increase* accelerated”

L675: “negative growth” sounds a bit strange. Maybe consider using “contraction” or something similar instead?

L769: insert “annually” before “added”. Given the long life-times of HFCs, it is unclear to me if “added forcing” of these gasses in general will soon become zero or negative.

L852: Maybe also include the term used in the scenario literature: carbon-dioxide removal (CDR).

Footnote 14: program*s*

L932: Why “but”? I don’t understand the juxtaposition. One can also just start a new sentence.

L1048: “a” priori

L1065: This sentence does not make much sense to me. RCP6.0 is not a “chemistry model”, but a multi-gas concentration scenario. I thus am not really sure what the authors are trying to say here.

L1096: Helps “to” assure

L1100: CO2 emissions only would not be sufficient to reduce methane emissions strongly, as illustrated in Figure S3 in Rogelj, J., et al (2014). "Disentangling the effects of CO2 and short-lived climate forcer mitigation." Proc Natl Acad Sci U S A 111(46): 16325-16330.

L1142: It would be useful to also include the more specific term “solar radiation management” (SRM) here.

L1201: This sentence reads as if half a meter to a meter of sea level rise would not have any humanitarian consequences. Maybe consider rephrasing.

L1252-1253: I’m not sure I understand what the authors are trying to say here in this sentence about the role of developing countries. Please clarify.

L1277: At which rate are emissions assumed to be reduced in this context? Please specify.

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Dear authors

We are now in receipt of two reviews of your revised manuscript. Both reviews are positive. While they identify a number of issues, these are relatively minor. I invite you to respond to these reviewer reports and to revise your manuscript accordingly. I will review your resubmitted manuscript.

The required corrections are either minor error correction & clarifications, or further recommendations for improvement. It is my opinion that both are readily addressed and so I am hopeful that you will be able to quickly revise your manuscript. It is clear that there is agreement on the scientific soundness on your manuscript.

Best wishes
James Dyke

Dear Authors

I have completed my review of your corrected manuscript. You have addressed all the points required in the previous round of minor corrections. Consequently, I am very pleased to publish your manuscript, subject to the technical correction(s) as detailed below.

TC 1
As per discussion, please revise Appendix A13, line 1376:

"RCP2.6 makes a more optimistic assumption: that CH4 will decline monotonically to 1250 ppb in 2100, yielding a forcing of −0.3 W/m2 (relative to today’s 1800 ppb CH4), but again the IPCC projections of RCP2.6 reduce this to −0.2 W/m2."

to:

RCP2.6 makes a more optimistic assumption: that CH4 will decline monotonically to 1250 ppb in 2100, yielding a forcing of −0.3 W/m2 (relative to today’s 1800 ppb CH4), but the IPCC projections of RCP2.6 reduce this to −0.2 W/m2 (Kirtman et al., 2013).

TC 2
You may wish to revise/update L60:
“The 21st Conference of the Parties (Paris Agreement, 2015), currently ratified by 120 nations”

In the light of 148 of the 197 Parties to the Convention have now ratified (http://unfccc.int/paris_agreement/items/9444.php).

Thank you for choosing to submit your manuscript to Earth System Dynamics.

Best wishes
James G Dyke