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https://doi.org/10.5194/egusphere-2026-624
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2026-624
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
20 Feb 2026
| 20 Feb 2026
Status: this preprint is open for discussion and under review for The Cryosphere (TC).
Diverging runoff drives uncertainty in Antarctic surface mass balance projections under a high emission scenario
Abstract. Three recent downscalings of CESM with MAR, RACMO, and HIRHAM under SSP5-8.5 produce consistent contemporary Antarctic surface mass balance (SMB), but diverge strongly by 2100, especially over ice shelves. HIRHAM simulates a large SMB decline driven by strong runoff increases, MAR a moderate decrease, while RACMO maintains near balance. These differences mainly reflect contrasting melt–albedo feedbacks, present-day melt and refreezing levels, and a persistent 1–2 °C temperature offset between MAR and RACMO. CESM shows a decline similar in magnitude to MAR, with high melt partly compensated by extensive refreezing. Over grounded ice, all models project increased SMB from higher snowfall, though runoff still drives their spread. Despite shared boundary conditions and similar contemporary SMB, model behavior diverges, and CESM’s integrated results resemble MAR's despite its coarse resolution. Performance differences in present-day melt suggest that uncertainty estimates should account for model skill, motivating Bayesian treatment of multi-model ensembles.
How to cite. Heurgue, B., Amory, C., Kittel, C., Boberg, F., Durand, G., Favier, V., Fettweis, X., Glaude, Q., Goelzer, H., Hansen, N., Jourdain, N. C., Mottram, R., Olesen, M., Van de Berg, W. J., Van den Broeke, M. R., and Wijngaard, R. R.: Diverging runoff drives uncertainty in Antarctic surface mass balance projections under a high emission scenario, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-624, 2026.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
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Benjamin Heurgue
CORRESPONDING AUTHOR
Institut des Géosciences de l’Environnement (IGE), Univ. Grenoble Alpes/CNRS/IRD/G-INP, Grenoble, France
Charles Amory
CORRESPONDING AUTHOR
Institut des Géosciences de l’Environnement (IGE), Univ. Grenoble Alpes/CNRS/IRD/G-INP, Grenoble, France
Christoph Kittel
Physical Geography Research Group, Department of Geography, Vrije Universiteit Brussel, Brussels, Belgium
Fredrik Boberg
Department of National Centre for Climate Research (NCKF), Danish Meteorological Institute, Copenhagen, Denmark
Gaël Durand
Institut des Géosciences de l’Environnement (IGE), Univ. Grenoble Alpes/CNRS/IRD/G-INP, Grenoble, France
Vincent Favier
Institut des Géosciences de l’Environnement (IGE), Univ. Grenoble Alpes/CNRS/IRD/G-INP, Grenoble, France
Xavier Fettweis
Physical Geography Research Group, Department of Geography, Vrije Universiteit Brussel, Brussels, Belgium
Quentin Glaude
Physical Geography Research Group, Department of Geography, Vrije Universiteit Brussel, Brussels, Belgium
Applied Computer Electronics Laboratory, University of Liège, Liège, Belgium
Heiko Goelzer
Norwegian Research Centre (NORCE), Bjerknes Centre for Climate Research, Bergen, Norway
Nicolaj Hansen
Department of National Centre for Climate Research (NCKF), Danish Meteorological Institute, Copenhagen, Denmark
Nicolas C. Jourdain
Institut des Géosciences de l’Environnement (IGE), Univ. Grenoble Alpes/CNRS/IRD/G-INP, Grenoble, France
Ruth Mottram
Department of National Centre for Climate Research (NCKF), Danish Meteorological Institute, Copenhagen, Denmark
Martin Olesen
Department of National Centre for Climate Research (NCKF), Danish Meteorological Institute, Copenhagen, Denmark
Willem Jan Van de Berg
Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands
Michiel R. Van den Broeke
Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands
René R. Wijngaard
Department of Physical Geography, Utrecht University, Utrecht, the Netherlands
Short summary
We studied how the Antarctic ice sheet surface mass balance may change by 2100 using three high-resolution climate models forced by the same future climate scenario. While the models agree for present-day conditions, they project very different futures, especially over floating ice shelves. These differences mainly come from how melting, refreezing, and temperature are represented. Our results show that future sea level projections strongly depend on how well models simulate today’s climate.
We studied how the Antarctic ice sheet surface mass balance may change by 2100 using three...
This is a well written paper, focused on downscaling projections from a single global climate model (CESM) under high-emission scenario using three regional climate models, in order to look at the differences between models (model uncertainty). The methodology is largely sound. The focus is on SMB and shows that the downscaled projections of this diverge strongly by 2100, which is due to differences in how the models represent melt-albedo feedbacks, present-day melt and refreezing levels, and temperature differences.
Major comment:
+ I realise that the aim is to focus on the differences between the RCMs, and therefore one global climate model (CESM) is enough for this aim. However, I feel the manuscript needs to better explain the limitations of this in understanding the future climate, i.e. it is only one realisation of the future climate. Maybe all that is necessary is an additional line or two in the final paragraph of the Introduction. But it would be good to have some information on e.g. how CESM compares with other CMIP6 models on key aspects such as future warming, sea ice loss, climate sensitivity, even SMB. Also, I’m not convinced that referring to the model as CESM is correct, as CESM1 and CESM2 are very distinct models, with CESM2 shown to have a reasonable representation of Antarctic climate and SMB (Dunmier et al., 2022)
+ The mention of satellite observations AMSR2 and SSM/I in the results section was rather out of the blue. These datasets should be mentioned in Section 2 (including limitations) and discussed as part of the methodology – for example, why are two datasets being employed. This is explained in the paragraph on page 12, but this should be in the methods section.
+ The map plots are quite small, making it difficult to identify small-scale features such as ice-shelves (e.g. Fig. 3). This makes it difficult to follow certain statements such as HIRHAM shows that SMB decreases over ‘all’ ice shelves in the future (Fig. 3a).
+ There are occasions in the results where the stand alone paragraphs seemed inappropriate, especially explaining Fig. 4.
Minor Comments
+ Line 31: This sentence on warmer ice shelves requires a citation.
+ Line 33: I think also need to mention GCMs, as GCMs and ESMs do differ in their complexity and representation of a range of processes. For example, GCMs do not include biological and chemical processes, while ESMs do.
+ Line 36: They face more than two major limitations, with representation of cloud microphysics being another one, for example. But this sentence rather suggests that there are only two major limitations.
+ Line 53: I would suggest citing Hawkins and Sutton (2009) here, which clarifies the role of model uncertainty in projections, along with scenario uncertainty and natural variability. Hawkins and Sutton (2009) The potential to narrow uncertainty in regional climate predictions. Bulletin of the American Meteorological Society, 90 (8). pp. 1095-1107. ISSN 1520-0477 doi: 10.1175/2009BAMS2607.1
+ Line 57: This sentence is a little repetitive, and the previous sentence also mentioned high-emission scenario.
+ Line 81: Maybe ‘capturing’ is better than ‘resolving’ in this sentence, as some of these processes are maybe parameterised.
+ Line 84: Define CMIP6.
+ Line 98: CESM2 used here but not defined.
+ Line 144: Its not clear what the ‘few widely spaced layers’ is referring to here. Surface? Atmosphere?
+ Section 2.1: The model descriptions jump straight into snow schemes and firn schemes, but without really explicitly saying that these schemes are responsible for computing melt, percolation, retention, refreezing, and runoff. Maybe an additional sentence could be added to the Introduction to clarify this, which currently only really says ‘… representation of the snowpack … remains rudimentary’.
+ Section 2.2 heading: I don’t think use of the word ‘analysis’ is correct here.
+ Table 1: Maybe should be ‘Surface albedo’ and also ‘Maximum water capacity of firn’. Also the title says ‘main characteristics and parameterisations’ which is inappropriate, as these are the ones for SMB.
+ Section 2.3: I think a comment is necessary to explain that the 10 km common grid is finer than the actual native model grids, and in the case of CESM which is >100 km this represents quite a jump.
+ Table 2: Need to specify in the heading that the results are integrated over Antarctica.
+ Line 234: Need to include a citation for this statement.
+ Figure 3 caption: Is the term ‘anomaly’ correct here? Is the top row not just the differences between the means for the two periods? This is not the same as ‘anomalies’. See also use of ‘anomalies’ in text.
+ Line 275: Its not possible to identify many of the ice shelves from these maps, as they are not shown. I also don’t think that the statement that they mostly show a decline in SMB (negative anomalies) is true – as this is only readily identifiable for HIRHAM over Ross and Ronne-Filchner ice shelves.
+ Line 276: These place names (and others mentioned) need to be shown on a map.
+ Lines 295-296: This does not constitute a standalone paragraph. Please revise. Its also not clear what ‘analyse them separately’ refers to.
+ Figure 4 caption: It says ‘parts of the ice sheet’, which is rather vague. Better to say ‘Antarctic ice sheet’ or even ‘ice sheets’. Does this include only grounded ice, or also ice shelves … Please check that all captions have full information.
+ Line 332: I would not use the term ‘observed differences’ as these are not observations.
+ Summary: Mention of ‘Bayesian’ treatment of multi-model ensembles is used in the abstract, yet no where else, including the summary.