the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Oct 2025
Estimating the Thermodynamic Contribution to Recent Greenland Ice Sheet Surface Mass Loss
Abstract. The Greenland Ice Sheet has become the largest single frozen source of global sea level rise following a pronounced increase in meltwater runoff in recent decades. The pivotal role of anomalous anticyclonic circulation patterns in facilitating this increase has been widely documented; however, this change in atmospheric circulation has coincided with a rapidly warming Arctic. While amplified warming at high latitudes has undoubtedly contributed to trends in Greenland's mass loss, the contribution of this shift in background conditions relative to changes in regional circulation patterns has yet to be quantified. Here, we apply the pseudo-global warming method of dynamical downscaling to estimate the contribution of the change in the thermodynamic background state under global warming to observed Greenland Ice Sheet surface mass loss since the turn of the century. Our analysis demonstrates that, had the recent atmospheric dynamical forcing of the Greenland Ice Sheet occurred under a preindustrial setting, anomalous surface mass loss would have been reduced by over 62 % relative to observations. We show that the change in the thermodynamic environment under amplified Arctic warming has augmented melt of the ice sheet via longwave radiative effects accompanying an increase in atmospheric water vapor content. Furthermore, the thermodynamic contribution to surface mass loss over the exceptional melt years of 2012 and 2019 was less than half that of the long-term average, demonstrating a reduced influence during periods of strong synoptic-scale atmospheric forcing.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4140', Anonymous Referee #1, 03 Nov 2025
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RC2: 'Comment on egusphere-2025-4140', Jason Box, 30 Nov 2025
SummaryThe submitted article is very clearly written and applies a clever pseudo-global warming (PGW) method to estimate of the relative contributions of dynamical versus thermodynamic change to recent Greenland ice sheet surface climatic mass loss.It's an interesting angle that "the change in the background thermodynamic environment, and its resulting impact on GrIS SMB, represents a more robust signal of climate change than the potential dynamical response**high level critique** in no particular order of importance...I think this is an attribution study, and so think the a-word belongs in article s titleThe article would benefit from stating and focusing some discussion on assumptions, to what extent they may or may not be violated, e.g. coupling between ocean and atmosphere is assumed to be well represented by the approach... the relatively minor SMB response to changing sea ice, line 428 has implicit assumption "is evident that sea-surface conditions alone exert minimal influence on the ice sheet" .. Further discussion lines 434 to 440 rests on the assumption the modeling accurately captures ocean/atmosphere energy exchange.A sub point, assuming the modeling accurately captures ocean/atmosphere energy exchange, where around Greenland has the greatest sea ice change response? And can that regional signal be credibly interpreted?A recurring question for me:a) 503-504 "suggests no appreciable contribution by the observed change in local sea-surface conditions to runoff production during these exceptional melt years" and/or the coupling of runoff to sea ice in the modeling is off/muted/incompleteb) 614 "Even so, this analysis reveals a minimal influence." or the coupling is incomplete/ineffective?c) 626-627 "It should also be noted that" or the coupling here is incomplete/ineffective?...alternative hypothesis, the not-explicitly-treated interplay between dynamics and thermodynamics is to blame for the counterintuitive result that is to reject the idea "that higher SST and lower SIC may promote earlier melt onset in the spring"?KEY: "io" means "instead of", NUMEBRS: line numbers87-88 the idea of warmer atmosphere, more water vapor appears as a statistically robust feature of observational data, specifically N Atlantic SST and Northern Hemisphere near-surface air temperatures correlation with Greenland snow accumulation, matching theory articulated by Treberth (doi: 10.3354/cr00953), see https://doi.org/10.1175/JCLI-D-12-00373.1 figure 9a, Table 3 and related discussion, e.g. "Paeth et al. (2002) link the increasing precipitation to increasing SST and atmospheric CO2."libe 603 "remotely sourced heat" is a point made by https://doi.org/10.1175/JCLI-D-12-00373.1 and that study also examines N Atlantic SST as being less remote than the N Hemisphere526 "2019 underwent more frequent melt at lower elevations", 2019 had a low accumulation anomaly across the west, preconditioning the albedo feedback to do its thing alongside the anomalous synoptic forcingTo make the text easier to follow, the abbreviations PGW1 PGW2 should be PGW_T_q PGW_SST_SIC, stacked super sub script, e.g. LaTeXPGW_{\text{SST}}^{\text{SIC}}% or shorterPGW_\text{SST}^\text{SIC}% or evenPGW_{\mathrm{SST}}^{\mathrm{SIC}}or HTML: PGW SST<sup>SIC</sup> or ...idea around Figure 7.: The delta LWD correspond to a delta T, if assuming same emissivity for control vs the two experiments (or emissivity as a function of water vapor), is the delta in effective radiative temperature consistent with the observed delta TThe reader would appreciate capturing the main points with a smaller word count, otherwise I would have rated the article in 4) Presentation quality, as excellent because 4) Presentation quality includes "concise"** lower level critique **Main Text50 statistically "significant"? In any case, the s-word should be reserved for the result of a statistical test.55,396,515 downward io incoming (find and replace); downward is the PMOD World Radiation Center standard59-60 (Gallagher et al., 2018; Lenaerts et al., 2019; Noël et al., 2019; Wang et al., 2019). read and cite also http://dx.doi.org/10.1002/2016GL07221262-63 cite also http://dx.doi.org/10.1002/2016GL067720 and http://dx.doi.org/10.3389/feart.2016.00082195 delete un-needed "quite", and in general to avoid ambiguity, avoid adjectives and adverbs252 delete un-needed "precipitous"288 "bouts", have a read and consider incorporating ideas/results from https://doi.org/10.1029/2024GL110121306 supporting info in https://doi.org/10.1002/met.2134 figs 5 and 6 and related discussion312 periphery io terminusFig. 6, aren't the units Gt per day?throughout instead of [] around units, suggest ", unit"345 "greatest over the northern ice sheet." consistent with Orsi study?448 with io experiencing. The latter is for sentient beings.451 incur (or exhibit) io experience491-494 the following main insight deserves highlight in abstract and/or conclusions "This suggests that the relative importance of a changing background state under global climate change may be minimized during periods of strong synoptic-scale atmospheric forcing. In other words, the record melt observed during those two summers is more a consequence of exceptional atmospheric circulation patterns than it is a direct consequence of the long-term warming trend"500 "turbulent heat fluxes (Fausto et al., 2016a, b)." also Box et al 2022 GRL524 remove un-needed "quite"534 "anomalously high atmospheric" io "anomalous atmospheric"546 "southwestern ice sheet" io "western slope of the southern ice sheet", elsewhere PLEASE use "ice sheet" instead of "GrIS" once it's obvious the geographic focus is Greenland537 "it relatively[?] susceptible" io "it more susceptible"566,elsewhere "downward" io "downwelling" (find and replace)609 un-needed "It is important to note that"618 nu-needed "It should also be noted that" but seems a new paragraphCitation: https://doi.org/
10.5194/egusphere-2025-4140-RC2 -
RC3: 'Comment on egusphere-2025-4140', Anonymous Referee #3, 30 Dec 2025
This manuscript presents an interesting analysis using the so-called ‘pseudo global warming’ approach to quantify the thermodynamic contribution to GrIS SMB changes. The manuscript is very well written, and the analysis is easy to follow. I especially appreciate the excellent Introduction and the Discussion sections. Below, I list some critics of the manuscript, which can be addressed in a revised version. I think these all fall on the minor side of the 'major revisions' and I am supportive of this paper’s eventual publication.
I am raising a potential issue with interpreting the ~62% reduction in anomalous surface mass loss under “preindustrial” conditions. As MAR is forced by ERA5 boundary conditions – and the PGW approach is designed to hold the synoptic evolution as similar as possible between experiments – the “62% statement” is inherently conditional on the imposed large-scale circulation. As such, the results quantify thermodynamic amplification given the observed circulation anomalies, but they do not quantify (and should not be worded as if they quantified) the forced versus internal origin of the circulation anomalies themselves, hence are not to be used as full attribution. This distinction should be made explicit in the abstract and conclusions wherever the “62%” number (and related statements about the “substantial contribution of external forcing”) are presented, to avoid the impression of a full causal partitioning of the observed SMB anomaly into ‘dynamic vs anthropogenic’. I’d suggest adding the phrasing for example ‘conditional on the ERA5 2000–2019 circulation’. In addition, consider revising the terminology around ‘anthropogenic’ versus ‘externally forced’ changes unless the perturbation is explicitly anthropogenic-only.
Second, I have a methodological question related to that the PGW perturbations applied at the MAR boundaries are constructed from long-term monthly-mean CESM-LE differences and then linearly interpolated to 6-hourly forcing. To me, this approach yields a regime-invariant mean-state shift and may therefore miss regime-dependent thermodynamic changes that are particularly relevant for Greenland melt and runoff (often a small number of high-impact events related to blocking or moisture intrusions). A single monthly-mean Δq applied to all synoptic situations can misrepresent how moisture and cloud emissivity change specifically during blocking/moisture intrusion events. In addition, extremes are not expected to scale with the mean: any change in the variance/skewness of specific humidity, temperature or coastal inversion will be missed by a monthly-mean shift. Also, a monthly mean perturbation does not preserve event-specific vertical gradients, which may influence cloud phase, boundary-layer stability and downwelling longwave. If the added/removed moisture in PGW1 is not representative during the high-impact synoptic regimes, then the inferred thermodynamic contribution might be biased high or low to some extent. As the manuscript emphasizes a moisture/downwelling-longwave pathway as a key mechanism, I would suggest demonstrating that the imposed monthly Δq (and associated radiative effects) remains representative during the circulation regimes that dominate extreme SMB anomalies, including the extreme years in 2012 and 2019. The authors could condition key diagnostics (near-surface q, integrated water vapor, downwelling longwave or melt onset) on circulation regimes (blocking days vs moisture-intrusion days) to show that the thermodynamic signal is robust across regimes; or provide sensitivity tests using alternative ways of the perturbation (seasonal mean deltas or smoothed daily climatological deltas) to assess how sensitive the ~62% percent reduction is to the monthly-mean assumption. In any case, some discussion on that the current method does not capture changes in the variance could help.
Third, I would be cautious about interpreting the results from the current experimental design as sea-surface conditions exert only a minimal influence on SMB. I think so, because of a lack of atmosphere-only runs (retain CTRL sea-surface conditions (ERA5 SST/SIC) rather than swapping to Hadley-OI 1880-1899 SST/SIC). The manuscript interprets PGW1 as the total thermodynamic contribution and PGW2 as the SST/SIC-only contribution. However, without an atmosphere-only PGW experiment (T/q perturbed with modern SST/SIC), it is not possible to say whether marine boundary effects depend on the atmospheric background state (or to quantify the interaction term between marine boundary changes and the atmospheric background state).
Once PGW_atm exists, you can do the separation: CTRL (modern atm + modern SST/SIC), PGW2 (modern atm + PI SST/SIC), PGW_atm (PI atm + modern SST/SIC), and PGW1 (PI atm + PI-like SST/SIC), one can estimate
- an atmosphere-only main effect as PGW_atm minus CTRL
- a marine-only main effect as PGW2 minus CTRL
- a nonlinear interaction term as PGW1 minus PGW_atm minus PGW2 plus CTRL
The interaction term is especially important given the authors current discussion of katabatic wind adjustments and marine-gradient effects (PGW2) versus water-vapor/longwave effects (PGW1): those pathways might not add linearly. Should the full 2000–2019 reruns be too expensive, a compromise is to run PGW_atm for (a) May–Sep only, or (b) a targeted set of years (e.g., 2012, 2019 plus a few 'typical' years) and show whether the inferred “minimal marine influence” and melt-timing conclusions hold once the interaction term is quantifiable.
Additionally, a comparison between 500hPa geopotential heights or the GBI between the CTRL and all PGW runs would be nice. Just to rule out that by adding a vertically varying T/q perturbation does not influence u/v (by changing pressure gradients) too much (which is assumed in the current manuscript).
My last concern is that CTRL uses ERA5 sea-ice concentration as the modern forcing baseline, and any systematic bias in ERA5 SIC relative to satellite-based products (NSIDC) would directly affect the magnitude of PGW2-minus-CTRL (potentially biasing the conclusion toward a small marine effect if CTRL SIC is already too ice-covered, or inflating it if too low). While Fig. 3 usefully motivates why applying CESM SIC deltas directly can create ice-edge artifacts, it does not validate the modern SIC forcing against independent satellite products. I would suggest including a brief evaluation of ERA5 SIC against a satellite benchmark in the key seas and seasons that matter for Greenland moisture intrusions.
Citation: https://doi.org/10.5194/egusphere-2025-4140-RC3
Data sets
Modèle Atmosphérique Régional (MAR) version 3.12 regional climate model pseudo-global warming experiment output, 2000-2019, Greenland domain, 20 kilometer (km) horizontal resolution Jonathon Preece et al. https://arcticdata.io/catalog/view/doi:10.18739/A2TT4FV6W
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General remarks
This is an excellent and novel quantitative analysis of the contribution of the local change in background thermodynamic contribution to the Greenland Ice Sheet surface mass balance loss. It effectively separates out the role of sea-surface temperature and sea-ice concentration forcing, which is found (corroborating some but not all previous studies) to be relatively minor. A valuable analysis of the extreme melt case studies of 2012 and 2019, and the difference in terms of climatic forcing between these events, is provided. The analysis is thorough and the paper is clearly written. It will be of wide interest to Greenland climate scientists and ice-sheet specialists. I recommend publication following a minor revision addressing the points below.
Specific comments
Line 26: please add the following recent relevant references:
Otosaka et al. (2023) https://essd.copernicus.org/articles/15/1597/2023/
Hanna et al. (2024) https://www.nature.com/articles/s43017-023-00509-7#publish-with-us
Figure 1: define PGW-1, PGW-2 etc. I the figure caption.
Lines 167-179: please add that while GCMs may capture the periodicity of internal climate variability they may not capture the magnitude of such variability. Will taking the CESM-LE ensemble mean be affected by signal-to-noise issues with the GCMs in capturing North Atlantic circulation change?
l.201: Are there issues with the accuracy of prescribed SIC and SST for 1880-1899 that may affected the method used?
ll.252 & 480: please add the following highly relevant reference:
Hanna et al. (2021) https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/joc.6771
The green and blue lines on some plots look a bit similar; could they be distinguished more clearly?
l.496: please add the following highly relevant reference to “more persistent circulation regimes under global warming”:
Overland et al. (2012) https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL053268