|Aspects of this work have been greatly improved since the initial draft, notably a much simplified Methods section, and the idea behind the work remains interesting. However, other aspects of the manuscript still leave much to be desired, including the organization of the paper (which is muddled in places), the extensive comparison to Tedstone (which was unconvincing), and the majority of the Discussion text (which almost wholly lacks synthesis). Unfortunately I do not think that the manuscript meets the standard of the journal at this time.|
The work that the authors put into this section shows well. The methods are much more condensed and clear. However, I do still find some issues.
- I remain convinced that the outflow elevation of marine glaciers would be correctly defined as sea level, as AR1 also pointed out. The authors’ treatment of this (P4 L10-13, P8 L27, and responses to reviewers) states otherwise. In my understanding, a parcel of water discharged at the base of a glacier may remain at that elevation, but it displaces a parcel of water upward, which itself displaces the next parcel upward. When these gains in potential energy are integrated, the net result is sea-level potential. However, this is only a small worry at this point.
- I am confused that the authors study only basal potential energy (rho*g*b) while discarding potential energy from the surface (rho*g*s), which is much greater in magnitude. The paragraph on supraglacial rivers suggests that the authors recognize this (P3 L16-17 especially). Perhaps my memory of the first manuscript colors this, though.
- The treatment of PTT in the calculation seems to be inconsistent (P5 L11-14). The calculation ignores PTT changes when ice thins along the basal hydraulic gradient, but includes it when ice thickens along the water path.
The manuscript suffers from poor organization. For instance:
In Results, a methods paragraph (P4 L26-31) appears.
In Discussion, a results sentence appears for the first and only time in the paper (P7 L8).
The Discussion is repetitive of the Introduction (P7 L21-25 especially, with 4 of 5 references repeated for the same facts).
There may be others. These basic offenses contributed to the difficulties I had in following the logic of this work.
In the revision, the authors added a substantial component to their analysis by attempting to parallel the Tedstone et al. (2015) study. To their original Historical / Present comparison (which I thought was good), the authors added a Reference / Baseline pair as well (which I found hard to keep track of, and which I think added little), based on Tedstone. I could not find that any of the reviewers asked for this addition.
The authors bluntly state that one of the Tedstone conclusions is “likely” not true (P8 L9-10) and provide an alternative hypothesis. I found the support for this new idea (section 5.2.1) entirely unconvincing and difficult to even understand the authors’ path of reasoning. The authors say that if the increase in VHD in the Tedstone basin caused the observed slowdown there (P8 L12), then this is incompatible with the ice-sheet-wide decrease in VHD observed over that period having caused the generally observed ice-sheet-wide speedup (P8 L14) — although it seems to me that these ideas do not contradict one another at all. (I am putting aside obvious issues with comparing velocity changes observed over slightly different time periods, and extrapolating land-terminating ice in western Greenland to the whole ice sheet.) But a few paragraphs later, the authors contradict themselves by extrapolating the Tedstone observations over the whole ice sheet after all (P8 L23-24). Such inconsistencies greatly undermined my confidence in this work.
The immaturity of the Discussion and the failure of this study to integrate its results with past work are major issues. While the authors include relevant citations, they do so largely in the form of a literature review. They do not demonstrate how their results build on this literature, except in the broadest strokes (i.e., VHD enhances conduit formation, which slows ice near the margin but not far from the margin) which were already made clear in the introduction. In short, the authors leave unclear what their work adds to the community’s understanding.
Upon a closer read, I did find a few sentences where the authors introduce a nice (and perhaps novel) line of reasoning / speculation about the competing effects of increasing runoff and increasing VHD (P7 L14-17). However, that idea is buried deep within the text and not revisited. The rest of the subsection (3 paragraphs), which would normally be used to build on the idea, consists only of a rambling literature review. While I remain interested in this topic and would enjoy continuation along this line of thought, I am at a loss for specific suggestions at this point.
Alternately, it may still be possible to frame the paper differently from runoff / speedup — for example, around GHF vs VHD, which is presently an interesting section, and one that perhaps could be expanded toward greater ideas. (In fact, the abstract suggests that this is the main point of the paper, but this is not borne out.) But as it is framed now around ice sheet mass balance, the paper makes only a vague contribution. At this point, I agree with AR1 who said that the study merely converts future runoff rates into future VHD rates, which was not particularly illuminating.
Some of the section headings (4.1 Volume of subglacial hydrology; 4.3 Flow-routed spatial distribution of VHD) are inscrutable.
Figure 2 caption — how could water flow both uphill AND toward ice that thickens along flow?
Figures 4/5 — Tedstone basin should be indicated
Figure 7 — it is hard to compare the small blue dots with the large black bars. Lightening the background relief map might help, as well as plotting the dots on the top layer and using colors that provide better contrast than blue/black.