Please see the attached PDF

Review of

Changes in Antarctic surface conditions and potential for ice shelf hydrofracturing from 1850 to 2200

By Jourdain and others

General

This is an interesting paper that addresses an important problem: when can we expect runoff from Antarctic ice shelves, leading to mass loss and/or meltwater ponding which is commonly regarded as a precursor for ice shelf hydrofracture. The authors use multiple MAR simulations of future Antarctic climates, with which they calibrate a simple statistical model to emulate melt, runoff so as to approximate SMB and (instantaneous) firn saturation. Not considered are sublimation, rain and the time it takes to saturate the firn. When compared directly to MAR output, the emulator gives mixed results, yet provides valuable insights in the uncertainties that arise from the driving global models. The writing can be clarified in places (see below), the figures are generally clear.

Major comments

The abstract is at places confusing. It goes in one step from describing runoff emulation to AIS SLR contribution. Although it is stated that the mass loss numbers are based on SMB alone, the link that is made to hydrofracturing in the title and the presented numbers in terms of sea level rise could trick the reader into thinking that dynamical effects are also considered. It should be made clear from the outset is that this study treats runoff in two ways: as a source of mass loss and as a threshold to induce ponding and hydrofracturing (after which the dynamical effect on the ice sheet is not considered).

Also in the abstract: "Based on a more limited and uncorrected ensemble, we find a considerable uncertainty in the contribution to sea level from 2000 to 2200". It is unclear whether this enhanced uncertainty derives from the fact that the ensemble is smaller and uncorrected, or from the method, or all of these. Please only include major results in the abstract, the details can be elaborated upon in the text.

l.59: The statistical model is based on MAR data, which is a fair choice given that this model has been used for multiple future runs. But for the statistical model to be useful, MAR must provide reliable runoff. Here it is stated that  the atmosphere in MAR is coupled to a 30 layer snow model "...representing the first 20 m of snow/firn with refined resolution at the surface". But in many places the firn layer in Antarctica is considerably deeper than 20 m? What does this imply for runoff simulated by MAR? See also my comments on treatment of percolation and runoff evaluation below.

l.65: " If liquid water is not able to percolate further down, then it fills the entire porosity space of surface layers, and the excess is considered as runoff and removed from the snow/firn model (there is no representation of ponds or horizontal routing)". From this description it is not clear to me how water percolation interacts with ice lenses in the firn layer and how the process of filling up works before runoff starts. 

l.67: "The surface mass balance and melting conditions produced by MAR have been evaluated in comparison to observational." Has there also been an attempt to evaluate the modelled meltwater ponding/runoff? I seem to remember that Van Wessem et al. (2023) compared their predicted ponding potential to satellite observations of melt ponds.

Figure 1 shows results for the integrated grounded ice/ice shelves only. How do the spatial patterns look?

Minor comments

l. 12: "Based on a runoff criteria.." Please note that 'criteria' is plural, so either use "Based on runoff criteria.." or "Based on a runoff criterion.."

l. 12: "we identify the emergence of surface conditions prone to hydrofracturing" Suggest: "we identify the timing of surface conditions that make ice shelves prone to hydrofracturing" or something similar.

l. 13: " A majority of ice shelves could remain safe" Suggest to reformulate: "Our results suggest that the majority..."

l.16: precipitation -> snowfall

l. 18: However, if air temperatures exceed -> However, model simulations suggest that, if atmospheric warming exceeds

l. 22: can be conducive of -> can be conducive to

l. 31: sea level -> sea level change

l. 36: in particular with regard to firn saturation by meltwater and subsequent runoff -> in particular with regard to firn saturation by meltwater and subsequent ponding

l. 87: Eq. 1: At what level is the warming specified? For this expression to be accurate, the warming should be weighted over the atmospheric column in which the water vapour resides. This is different for Eq. 2 (l. 92), where near-surface warming can be used.

l. 103: surface -> near-surface (I assume)

l. 108: " The m parameter is introduced to avoid unrealistically high melt rates" Can you provide a value, and how is it determined? Ah, it is provided in l. 120, but is given in kg m-1 s-1. Can you provide a more intuitive value, i.e. what this means per year?

l. 109-112: This is unclear, please clarify.

l. 180: typo "ooverestimated"

l.231: " Our projections over the grounded ice until 2100 agree quite well with previous estimates of sea level contribution reported by the IPCC for the three scenarios". Can these numbers be compared, i.e. did these IPCC assessments also report estimates based on SMB-only?

Section 3.2: It would be informative to provide the modelled atmospheric warming of the various models in addition to the SMB and runoff. Main motivation is that some models project negative SMB over the grounded ice sheet in the 22nd century, which must require a very strong warming, as well as significant rainfall. For these extremes, how well does the initial assumption hold that rainfall remains small?

l. 293: lead -> led

l. 294: Georges -> George 

l. 323: "...while it can take more than a decade to saturate it if melt rates are just above the threshold..." Theoretically, it can take an almost infinite amount of time if the threshold is just passed.

l. 363: criteria -> criterion