Summary

The authors present a perspective on an alternative pathway (called the ‘Paraglacial Transition’ view) of how Himalayan glacier systems may respond to climate change in the future, different to the common view that future climate warming will result in sustained glacier retreat and ice loss (‘Major ice loss’ view). They describe how rising rock and moraine instabilities release inscreasing amounts of debris material on glacier surfaces, with impacts on their melt, dynamics, and morphologies. A preliminary analysis of past long-term glacier retreat rates shows distinct differences across HMA. The authors suggest a potential transition into stagnant debris covered glacier tongues and eventually into rock glaciers for some glaciers, causing a prolonged life cycle for glacier ice.

General comments

The manuscript is well written and structured, and the balance of theoretical background, motivation and new insights based on a preliminary analysis is tempting and adequate. The study gives a fresh view on alternative ways how glaciers in the Himalays can develop in the future, complementary to the conventional view based on global and regional-scale simulations of sustained ice loss in the Himalayas expected by the end of the century. I like the authors’ view on future glacier response as mirroring that of the past. In my opinion the manuscript fits well into the scope of The Cryosphere and as a perspective paper. However, I suggest addressig two major points and clarify few minor points specified below, to make the study more robust.

Main points

• Debris thickness

I miss a discussion of the fact that the major part of debris supply for debris-covered tongues comes from headwalls and is transported englacially, i.e. its rate is controled by headwall erosion rates, but also the glacier’s ice dynamics and surface ablation. Therefore, I don’t see clearly how e.g. typically downwasted (concave) near-stagnant debris-covered tongues can become drastically more debris-covered and eventually transition into a rock glacier in the near or far future. Unless the authors are talking in general about very small tongues of debris-covered glaciers without lateral moraines, which might be directly connected gravitationally to nearby rock walls. But in general, I think lateral debris supply from headwall erosion, rock falls and moraines can not efficiently increase the debris thickness of debris-coverered tongues of ‘normal’ valley glaciers. I suggest addressing this point more clearly in the manuscript, since the increase in debris supply and increase in supraglacial debris covered area and debris thickness are a major part of the proposed Paraglacial Transition pathway and also play a crucial role for point 2 above (glacier-rock glacier continuum).

• Transition into rock glaciers

Many studies have disussed the presence or absence of a transition of glaciers into rock glaciers and divided the community in either the continuum- or the permafrost creep school (e.g. Berthling, 2011), a difference that comes mainly from focusing on either the genesis or morphogoly of rock glaciers. To my understanding a transition of a glacier into a rock glacier is a rather rare case, and I can not see the continuum of a typical downwasted, concave Himalayan debris-cover tongue into a rock glacier to be a common transition. Also, the presence of underlying permafrost, a requirement for the presence of rock glaciers, might also not be valid in many landscapes where currently debris-covered glacier tongues are present. Since debris-covered glaciers are rather downwasting than retreating as stated by the authors, I see the possibility of a glacier-rock glacier transition as rather small in most places in the Himalayas, unless maybe in cases where the debris-coverd tongue ends up in a small cirque located at high altitude in permafrost conditions. The authors state the uncertainty associated to this transition (e.g. 417-418) and very long time scales needed for this to happen (e.g. l. 311), but the very low probability of this transition should be stated more clearly in the text and should therefore not be menitioned in the abstract, as it might concern only a very small subsample of Himalayan glaciers, in my opinion. Instead of explicitly mentioning rock glaciers in this manuscript I strongly suggest stating e.g. ‘ice-debris landforms’ instead, when talking about stagnating dead ice bodies buried under a large amount of debris, what the PT view is essentially suggesting.

Minor points

-Thoughout the manuscript I was not sure if the authors talk about the Himalayas as a specific region of HMA or about HMA in general, targeting the entire arc. Please specify more clearly in the text.

-l. 135: “high glacier volume loss” - this contradicts partly your previous sentence (l. 132-134), in which relatively small ice volume losses were stated.

-l. 140-141: This sentence is thematically disconnected to the previous one and I suggest to add some more content here to clarify.

Fig. 1: It would be helpful for understanding and discussion to have also ice mass loss as an additional line in subfigures b, d and e, if possible.

-l. 178: “unlike the approach taken by climate models” – what do you mean here? Please clearify.

-l. 218-219: “and the slow melting of clean ice and debris-covered glaciers” – what do you mean here? This is somehow disconnected to the rest of the sentence.

-l. 230-236: Although you mention later that a combination of both responses (MIL and PT) might exist (l. 415-417), I suggest stating the likely coexistence of both views more prominently and step back partly form the impression that either one or the other is the only view (e.g. in the abstract, discussion, conclusion).

-l. 261: “ice glaciers” – do you mean “clean ice glaciers”?

-l; 269-270: supraglacial ice cliffs should be mentioned in line with supraglacial ponds here as melt hot spots, and appropriate literature cited.

-l. 367: compiled

-l. 382, 389: please clearify the meaning of “MIS 2” and “MIS 3”, as the explanation in brackets (l. 383) is not clear to me.

-l. 419: “above” – do you mean “below” instead?

-l. 471: “increase snowfall” – is this predicted?

-l. 493-494: Point 4 is unclear to me, please clearify.

References:

Berthling, I. (2011). Beyond confusion: Rock glaciers as cryo-conditioned landforms. Geomorphology, 131(3-4), 98-106.