the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jan 2025
Limited global effect of climate-Greenland ice sheet coupling in NorESM2 under a high-emission scenario
Abstract. The Greenland ice sheet is melting at an accelerating rate due to the warming climate. In order to understand the potentially important ice-climate feedback processes, evolving ice sheets need to be included in global climate models. Here, we present results from the first bi-directional coupling of the Earth System model NorESM2 with the ice sheet model CISM2 for the Greenland ice sheet under an extended high emission SSP5-8.5 forcing from 1850 to 2300. Comparing simulation results to an otherwise identical simulation with a fixed Greenland ice sheet, we see the same global trends in air, ocean and sea ice changes. The main signals are a 10 °C global air temperature increase from 2000 to 2300, a reduced maximum AMOC at 26.5° N from average 23 to 9 Sv and an all-year free Arctic by 2200. Similar to other coupled CMIP models, the warming trend dominates the changes of the climate components and the relatively minor changes of the Greenland ice sheet do not enhance or shift the simulated patterns. At the regional scale, elevation changes become an important part of the Greenland surface mass balance after 2100. By the end of the simulation, the ablation area covers 93 % of the ice area and the cumulative ice mass loss since 1850 is equivalent to 1.4 m of sea-level rise. With a low climate sensitivity and relatively low weak amplification in NorESM2, these results are on the lower end of the spectrum of expected ice mass loss under CMIP6 model forcing.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-3785', Anonymous Referee #1, 20 Feb 2025
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RC2: 'Comment on egusphere-2024-3785', Anonymous Referee #2, 18 Mar 2025
In this paper, the authors analyze the first NorESM simulation including an interactive Greenland ice sheet model. The authors indicate a minor impact of interactive model on the global climate dynamics simulation. They do find differences in Arctic climate by 2100 and a large impact of the elevation-melt feedback on the overall ice sheet mass loss
General comments
The results are very interesting since there are very few examples of this type of coupled ice sheet-climate simulations. However, the analysis of results is difficult to follow as there are too many loose numbers in the text – these could be presented in tables instead – and with inconsistent metrics – sometimes ranges or extremes are presented, sometimes mean values with standard deviations -. The paper would benefit of some assessment of statistical significance of the differences between simulations (e.g, Figure 2), as well of (attempt of) explanations of these differences. In the conclusions, the authors highlight how their results differ as those from previous work from EMICs. It would be interesting that the authors provide more detail on that.
The design of the simulations needs some more clarity, taking some parts of the model description (submitted) here where they are relevant to explain the results. For instance, Figure 2 does not provide information about initial NorESM2-NorESM2fixed (temperature, surface elevation) differences (for a pre-industrial or 1850 climate). Also, the treatment of meltwater fluxes in the “fixed” simulation is not clear; perhaps these fluxes could be compared in the manuscript.
Title and main conclusion
The title can be misleading, as it refers to “Limited global effect of climate-Greenland ice sheet coupling (…)”. The conclusions rather refer to the limited effects of Greenland ice sheet change on the global climate. In the title the word climate is first, and the coupling could be uni-directional (“one-way”). Therefore, the title can read as if climate change does not affect Greenland or global sea levels when both climate and ice sheet are modelled together within NorESM … In general, I would highlight more the added value of the coupling in title and conclusions, e.g., along the lines of mapping climate and (land, sea) ice change with a single model that permits to establish direct connections within the Earth System.
Abstract
Lines 1-2: it seems from the title that the results do not justify the coupling? I would disagree, as this paper presents a consistent, comprehensive yet fully detailed account of (Artic, global) climate and Greenland ice sheet co-evolution within the same model. Perhaps worth to highlight more the relationships between the timing of NAMOC decline, sea-ice decline, and melt acceleration?
Line 9: “an important part” -> quantify; e.g., how much is the mass loss enhanced by 2100, how much by 2300
Line 11: “1.4 m sea-level rise” -> consider to give this earlier, before line 8 “relatively minor changes”
Line 11: “low weak amplification” -> remove one adjective, amplification of what?
Plain Language Summary
It is not very clear what is happening already and what is expected to happen.
Line 20: “is”
Intro
Line 48: not all citations are CMIP6 models, please check
Figures
Please add statistical significance. I suggest increasing the size, e.g., until at least page width. For Figure 2, it would be interesting to zoom in on the Greenland area/high latitude differences, and to relate with elevation differences.
Results
Please introduce the structure of this long section. Also, consider adding numbered sub-sections. The differences in Arctic climate in the *fixed versus coupled simulations are a very interesting result, it would be great if the authors could go into more depth there.
Table 1 – please add standard deviations. Can you indicate which of the coupled versus fixed differences are statistically significant?
Line 132: “PI” is a bit misleading, perhaps “1870s”/”1870” for consistency ?
Line 134: usually 2000s would refer to the decade 2000-2009
Global average changes:
Consider adding rates of contribution to sea level rise (mm/yr) as this is an important metric of mass loss (see e.g., AR6).
Line 135: “Global average changes” -> “Time evolution of climate and ice sheet change”
Structure of this part is not straightforward, e.g., global temperature and ice sheet SMB are together in the same paragraph
Line 139: “has a strong impact”-> but temperature change is not per se the cause of SMB change – more melt energy and changes in snowfall are … -
Line 141 – “fluctuating around 600 Gt yr-1” -> give mean and standard deviation, e.g., in table
Line 142 – “SMB with climate forcing” -> ?
Line 142 “2000” this time of emergence is an interesting result, perhaps highlight more?
Line 146 is ice discharge increasing? Please explain
Line 146 “200 years” -> shouldn’t it be more?
Line 146 Figure 1d seems to show a trend for the pre-industrial simulation? Can you quantify this and comment on it?
Line 147: “show a similar pattern” -> do you mean a spatial pattern? Or similar time evolution?
Line 148 I don’t see a graph or table in support of the global SST analysis
Line 148: “discernible” -> what is the criteria you used?
Line 149 refer to Fig. 1e in this paragraph
Line 155-156: range is compared with mean plus standard deviation, please consolidate.
Line 173: here and in other places “up to” is not very representative, please use tables comparing e.g., averages and standard deviations
Lines 200-212: precipitation, is there a figure/table in support of “staying the same”? Also, numbers in the text can be replaced by a table.
Oceanic changes: figures are visually described, please add statistical significance, compare areal means, and look for some potential explanations for the results.
Greenland ice sheet changes:
There is not much analysis of surface mass balance or energy components here, is there any reason not to do this?
Please increase size and add elevation contour lines to Figure 7, to illustrate changes in equilibrium line altitude. Perhaps interesting to add a figure/table with evolution of the mean equilibrium line altitude? (can be calculated from the ablation area % and the function of cumulative area below a certain elevation, or “hypsometric curve”)
Line 246: here and in other places “visible” -> replace by scientific criteria, such as “statistically significant”, quantification of differences, explanation of mechanisms, etc.
Line 280: “up to “ , not representative of overall change -> use representative metrics, e.g., table with e.g., mean snowfall over ablation/accumulation area
Figure 7h: there seems to be a strange “corner” along the equilibrium line altitude, do you know what causes this?
Discussion
Please check (and introduce) structure, e.g., line 325 “as mentioned earlier” marks an unexpected break.
Line 310 “the lack of calving might underestimate” -> “and ocean forcing to the calving front likely”
Line 353 SAT of 5 C for which greenhouse forcing? Do you mean SSP5?
Line 355 move this before the previous paragraph?
Conclusions
Line 368 see -> analyze, assess, etc
Lines 369-371 This is an interesting result, please quantify, explain (at least tentatively)
Line 372 “major contributor” -> quantify
Line 373 “longer” with respect to?
Line 379 interesting result, please specify/quantify
Line 381 advantage -> added value
Citation: https://doi.org/10.5194/egusphere-2024-3785-RC2
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