| 04 Feb 2026
Comparing calving laws at Greenland’s three largest ice shelves
Abstract. The retreat of Greenlandic glaciers through calving has major implications for the ice sheet's mass balance and future sea-level rise contributions. Despite its importance, the implementation of calving in ice sheet models remains contested, with several calving laws suggested to parametrise this process. While the performance of some of these calving laws has been tested for Antarctic ice shelves and Greenland's grounded outlet glaciers, it is unclear which calving law would best capture the observed behaviour of Greenland's ice shelves. Petermann, Ryder, and Nioghalvfjerdsbræ (79N) glaciers are fronted by Greenland's three largest ice shelves, accounting for 90 % of the remaining floating ice and buttressing ~15 % of the ice sheet's mass. Here we build on other systematic calving studies by comparing five calving laws at Greenland's three largest ice shelves using the Ice-sheet and Sea-level System Model (ISSM). We begin by constraining the performance of each law against observed terminus fluctuations between 2008 and 2024, and continue with projections to 2300 under various climate forcings. When evaluated against observed terminus changes, we recommend the use of a von Mises or Crevasse Depth calving law owing to their consistent performance and similar tuning parameters across the three ice shelves. However, in our extended projection runs, we find that calving parametrisations have little influence on grounding line discharge rates, which are instead driven by the choice of climate forcings. Large ice shelf calving or collapse events are scarce, and only in these rare cases do we find any pronounced grounding line response. Our results indicate either continued buttressing potential from Greenland's ice shelves into the coming centuries or fundamental flaws in the current set of calving laws that involve calibrating to contemporary ice-shelf behaviour.
Overall, I find the manuscript to be a very well written and clearly presented investigation into the impact of using one of five different calving on the future behaviour of the calving front of three Greenlandic ice shelves. This is a very topical and impactful area of research, as our ability to simulate ice shelf calving is a known area of uncertainty when making predictions for how ice sheets will respond to a warming climate. I am happy to recommend publication, provided my comments can be addressed.
My first comment is that the calibration exercise does not currently show the impact of calving front shape on ice discharge (although this can be inferred from time 0 on Fig7). It might be good to have a figure showing this.
Currently you are tuning to minimise the misfit in total shelf area. Potentially, this could result in a shelf front being greatly advanced on one side of the domain whilst retreated by the same amount on the other. This is likely to have a quite different amount of buttressing when compared to a shelf position that is everywhere on the observed position, despite similar misfits. It can also potentially ignore an ice shelf that has detached from a lateral boundary. For example, the observed position of Petermann in Fig 2(i) compared to the MT law on the left hand boundary. Perhaps incorporating a comparison to observed discharge rates in combination with the area match might be a better metric for calibration?
My second comment is in regard to the melt profiles used for forcing. My understanding is that all simulations have a minimum melt layer that never experiences any melting near the surface. Whilst I gather this is a good match to present day observations, can you comment on how likely this is to continue in a warming climate? Do you see any evidence of ice thickness at the calving front being strongly linked to your choice of this minimum melt layer (is it ~110m at Ryder to match the minimum melt depth, for example)? Perhaps a figure showing flow line profiles of ice thickness could help here. Do you results appear unduly influenced by the choice of the size of this melt layer?
Line comments/typos below.
L103: Can you comment on the different melt profiles used for Petermann and Ryder, despite them being so geographically close to each other? Are the different profiles assumed to be a result of differences in ocean forcing or due to the shape of each ice shelves cavity?
L111: How often is this limiting maximum migration rate used in the simulations? For an example, is there an approximate percentage of time for the run where this maximum is reached?
L115: For clarity, define whether a positive calving rate advances or retreat the front position.
L115: In which direction is this calving rate applied? Perpendicular to ice geometry at the calving front, or antiparallel to ice velocity at the calving front?
L130: For the Crevasse Depth and Minimum thickness laws. Are these applied at every model time step or some longer interval? (Monthly, Yearly, etc)
L189: "kept fixed in our"
L200: To clarify, does this mean that all simulations still maintain their minimum melt sections of 0 melting during these warming scenarios? If so, the term oceanFull may be a little misleading.
L209: Are there any cases where you chose a parameter that gave you a qualitatively better match to ice front position even if the quantitative misfit value was worse?
L295: In the static case for Petermann, VM has a greater front retreat rate than CD during the 2100s, and yet CD has a greater retreat of the grounding line. Can you comment on this?
L328: Should velocities be ice discharge here?
L331: VM and EC appear the best on average, but do noticeable worse on Petermann. Can you comment on this?
L395: Is it unreasonable to calibrate for individual Glaciers in large scale ice sheet simulations? It will certainly be more work, and will require some technical challenges around the boundaries between different glaciers, but I don't foresee any absolute barriers to doing so. Your results from just three glaciers already imply differences in how each law performs at different glaciers.
L448 able to parameterise