|It is clear that a lot of thoughtful work went into the revisions in response to reviewer comments. By themselves, the revisions seem to address the main concerns. In the mean time, however, another major issue has emerged, which is that the paper has grown from being very long to being unwieldy. Reading from start to end is now very tedious. The main text of the paper is now 35 pages, up from 24 on the last draft. Is there not a limit on length for ESD articles? Some significant paring down and focusing is essential. The article is not publishable until this central issue is addressed.|
Fortunately, there is a lot of room for shortening because much of the text involves digressions that are not central. The authors will need to come up with their own solutions, but here’s my advice:
(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. The author list might be described as “Hansen and the Eleven Dwarfs”. Sorry for the pejorative analogy, but I’m not happy as a reviewer to be forced to point out limitations that might easily have been caught by coauthors.
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.
On my rereading the paper, I also noted some more specific issues:
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
1227-1237, 1246-1249: Cut. The relative merits of a carbon fee versus other approaches is tangential and also inappropriate to a science journal.