Revisiting ice sheet mass balance: insights into changing dynamics in Greenland and Antarctica from ICESat-2

Hansen, Nicolaj; Sørensen, Louise Sandberg; Spada, Giorgio; Melini, Daniele; Forsberg, Rene; Mottram, Ruth; Simonsen, Sebastian B.

doi:https://doi.org/10.5194/tc-2023-104

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https://doi.org/10.5194/tc-2023-104

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17 Jul 2023

| 17 Jul 2023

Status: this preprint has been withdrawn by the authors.

Revisiting ice sheet mass balance: insights into changing dynamics in Greenland and Antarctica from ICESat-2

Nicolaj Hansen, Louise Sandberg Sørensen, Giorgio Spada, Daniele Melini, Rene Forsberg, Ruth Mottram, and Sebastian B. Simonsen

Abstract. The time series of observations from NASA’s latest satellite laser altimetry, the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) are now mature to revisit the methodology for estimating surface elevation change and mass balance of ice sheets as proposed by Sørensen et al. (2011). Following the original ICESat study, we combine the derived ICESat-2 surface elevation change estimates with modelled changes of both the firn and the vertical bedrock to derive the total mass balance of the ice sheets, during the northern hemisphere mass balance years of October 2018 to September 2021. The method of converting the surface elevation change to mass balance change has been refined to obtain more reliable mass balance results for both ice sheets. From 2018 to 2021, we find that the grounded ice sheet in Antarctica has lost 135.7±27.3 Gt year^-1, and the Greenland ice sheet 237.5±14.0 Gt year^-1. Compared to 2003–2008, the ICESat-2 derived mass change of the Greenland ice sheet has a similar magnitude; however, the spatial pattern is changed and we observe reduced ice loss around Jakobshavn Isbræ and in the southeast accompanied by increased loss almost everywhere else and especially in the northern sector of the ice sheet. Our results show pervasive ice sheet loss across much of Greenland in recent years and an increase in loss from Antarctica compared to earlier studies. Parallels between the two ice sheets revealed by ICESat-2 data reflect atmospheric and oceanic drivers and show the importance of understanding ice sheets as components within the Earth system.

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Received: 04 Jul 2023 – Discussion started: 17 Jul 2023

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Nicolaj Hansen, Louise Sandberg Sørensen, Giorgio Spada, Daniele Melini, Rene Forsberg, Ruth Mottram, and Sebastian B. Simonsen

Interactive discussion

Status: closed

CC1:
'Comment on tc-2023-104', Paul Summers, 20 Jul 2023

The red-blue color scales in figure 1, 2, 4 for elevation change, and figure 5 for mass balance are red-positive (increased elevation, positive mass balance). Though this is consistent with Sørensen et al. (2011), it is the exact opposite of numerous other publications (Rignot et al 2008, Mouginot et al 2019, Rignot et al 2019, Shepherd et al 2019, Berthier et al 2023, Otosaka et al 2023). The authors could consider using a blue-positive red-blue color scale if they would like to work towards a more uniform presentation of ice elevation change and mass balance change across publications.

Citation: https://doi.org/10.5194/tc-2023-104-CC1
- AC1: 'Reply on CC1', Nicolaj Hansen, 14 Aug 2023
  
  Hi Paul.
  Thanks for your comment. We see the benefit of changing the colorbar to be consistent with other studies.
  
  So, if non of the reviewers object we will change the colors as you suggest.
  
  All the best
  Nicolaj and co-author
  
  Citation: https://doi.org/10.5194/tc-2023-104-AC1
RC1: 'Comment on tc-2023-104', Anonymous Referee #1, 11 Aug 2023

The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2023-104/tc-2023-104-RC1-supplement.pdf

Citation: https://doi.org/10.5194/tc-2023-104-RC1
RC2: 'Comment on tc-2023-104', Anonymous Referee #2, 26 Nov 2023

This study aims to derive average ice sheet elevation and mass changes from ICESat-2 ATL06 data for 2018 October 18 to September 2021 for the Greenland and Antarctic ice sheets to investigate decadal scale mass change and drivers of ice sheet processes. The authors modified an algorithm initially developed for ICESat satellite laser altimetry to achieve their goal. Improvements have been introduced for estimating vertical crustal deformation and volume-to-mass change conversion. The surface elevation change detection uses the same regression algorithm as the 2011 study but with a reduced along-track resolution (5000 m instead of 500 m).

While using the same method might provide an advantage for estimating Greenland Ice Sheet changes (section 4.1), there are several issues with applying it to ICESat-2 data because of their higher spatiotemporal resolution. The 5000-meter along-track segment size is very large considering the intricate details of elevation change captured by ICESat-2's 20 m along-track sampling and the complexity of ice sheet elevation changes. It is especially true in the coastal regions, as demonstrated by the significant noise of the elevation change estimates, especially in Greenland (Fig. 1A). A high-resolution elevation change product, ATL15 (quarterly at 1 km resolution), is already distributed. This study doesn't demonstrate comparable or better performance to ATL15.

Moreover, the density model used for the volume-to-mass conversion includes some decisions that appear to be arbitrary, for example, assuming the regions moving faster than 30 m/yr exhibit dynamic thinning (negative SMB could also be a reason, e.g., snowfall minimum) or that a priori knowledge about dynamic thickening is assumed.

To summarize, the authors should improve the elevation change method and the volume-to-mass change conversion to take advantage of the high-resolution ICESat-2 observations and recent advances in firn densification modeling to derive robust estimates.

The manuscript also falls short in interpreting the elevation and mass changes and fails to provide new insights into ice sheet processes. The discussion is limited to comparing the ice sheet-wide mass balance estimates to other studies with limited analysis of the drainage basin scale results. The qualitative comparison of the SMB anomalies in Antarctica (lines 297-311) is interesting but could be further enhanced by quantitative comparisons. The interpretation of decadal mass changes in Greenland is also disappointing. It mostly repeats statements from the introduction confirming changes detected by other studies without providing more details from the ICESat & ICESat-2 comparison.

Detailed comments:
Line 20: Forsberg et al., 2017 only use data after 2000, so they could not detect changes in the 1980s

Line 21: Please change floating glaciers to floating ice shelves

Lines 33-35: several of these studies are model-based only, e.g., Verjans et al., 2021; Pelle et al., 2020; Stokes et al., 2021. Observational studies of recent changes and/or altimetry applications would better serve the statements here.

Line 63: “These studies” refer to studies (in the previous sentence) that include a much shorter period than 1992-2018. I suggest keeping only the relevant references in this sentence.

Line 67: the manuscript refers to several papers discussing numerical ice sheet models. Mentioning vertical crustal deformation and firn models here would clarify the improvement due to numerical models.

Line 81: ICESat had a near-infrared laser, not a red laser.

Line 86: the posting of ATL06 is 20 meters, not 40 meters (40 meters is the segment length). ATL06 height is defined by straight-line fitting and not by averaging.

Line 88: Sørensen et al., 2011 used 500 meter segments. Why was the length increased? Are elevation changes computed from single tracks or pair tracks?

Line 94: The SEC from ICESat-2 was interpolated not the ICESat-2 data.

Line 144: what topographic/bedrock data set was used for Greenland?

Figs 1-2: Are the elevation changes indeed equivalent in ice? The kriging interpolation was described before the correction for density, firn-compaction, etc.

Line 176: I assume the Pritchard reference is included because of the significant dynamic thickening signals over some surging glaciers in NE Greenland. Dynamic thickening with smaller amplitude as a response to millennial-scale changes is also likely in other GrIS regions and in Antarctica (e.g., Schlegel et al., 2012).

Line 183: the dynamic thickening of the Siple Coast ice streams are well documented and attributed to changes in subglacial hydrology. This description here is misleading and should be improved.

Results: Please label the regions mentioned in the results and discussion sections, eg., lines 214, 222.

Line 235: why are ice rises and rumples thickening? Is it a new result or detected by previous studies?

Figure 5: Consider adding the numbers for the drainage basins. Referring to their relative location, especially in Antarctica, is unusual and confusing (e.g., Siple coast basin as Midwest in line 244).

Line 256: showing results for the grounded area of the ice sheets would require the separation of grounded glaciers and the grounded portion of the ice sheet – therefore, the statement here is misleading. It is sufficient and important to indicate which ice sheet masks are used and what they cover, i.e., ice sheet, ice caps, etc.

Line 276: Smith et al., 2020 used densities from a firn densification model, not ice densities for the entire ice sheets.

Section 4.1: this section repeats several references to other studies from the introductions without providing more details, e.g., Jakobshavn Isbrae: lines 41-43 and lines 326-330. The repetitions should be avoided; rather, a discussion of the new findings should be included here. Also, some of the changes discussed, such as the decrease in thinning in SE Greenland (lines 330-332) happened during the ICESat period and are well documented. A more interesting line of inquiry would be to investigate the decadal change of outlet glacier behavior as detected by the comparison of ICESat and ICESat-2. Finally, the discussion about the potential drivers of northern Greenland mass loss is essentially a review of the studies mentioned in the introduction instead of explaining new findings. Perhaps the spatial pattern of the mass loss can help attribute the patterns to certain processes.

References:
Schlegel, N.-J. et al. Application of GRACE to the assessment of model-based estimates of monthly Greenland Ice Sheet mass balance (2003–2012). Cryosphere 10, 1965–1989 (2016).

Citation: https://doi.org/10.5194/tc-2023-104-RC2

Interactive discussion

Status: closed

CC1:
'Comment on tc-2023-104', Paul Summers, 20 Jul 2023

The red-blue color scales in figure 1, 2, 4 for elevation change, and figure 5 for mass balance are red-positive (increased elevation, positive mass balance). Though this is consistent with Sørensen et al. (2011), it is the exact opposite of numerous other publications (Rignot et al 2008, Mouginot et al 2019, Rignot et al 2019, Shepherd et al 2019, Berthier et al 2023, Otosaka et al 2023). The authors could consider using a blue-positive red-blue color scale if they would like to work towards a more uniform presentation of ice elevation change and mass balance change across publications.

Citation: https://doi.org/10.5194/tc-2023-104-CC1
- AC1: 'Reply on CC1', Nicolaj Hansen, 14 Aug 2023
  
  Hi Paul.
  Thanks for your comment. We see the benefit of changing the colorbar to be consistent with other studies.
  
  So, if non of the reviewers object we will change the colors as you suggest.
  
  All the best
  Nicolaj and co-author
  
  Citation: https://doi.org/10.5194/tc-2023-104-AC1
RC1: 'Comment on tc-2023-104', Anonymous Referee #1, 11 Aug 2023

The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2023-104/tc-2023-104-RC1-supplement.pdf

Citation: https://doi.org/10.5194/tc-2023-104-RC1
RC2: 'Comment on tc-2023-104', Anonymous Referee #2, 26 Nov 2023

This study aims to derive average ice sheet elevation and mass changes from ICESat-2 ATL06 data for 2018 October 18 to September 2021 for the Greenland and Antarctic ice sheets to investigate decadal scale mass change and drivers of ice sheet processes. The authors modified an algorithm initially developed for ICESat satellite laser altimetry to achieve their goal. Improvements have been introduced for estimating vertical crustal deformation and volume-to-mass change conversion. The surface elevation change detection uses the same regression algorithm as the 2011 study but with a reduced along-track resolution (5000 m instead of 500 m).

While using the same method might provide an advantage for estimating Greenland Ice Sheet changes (section 4.1), there are several issues with applying it to ICESat-2 data because of their higher spatiotemporal resolution. The 5000-meter along-track segment size is very large considering the intricate details of elevation change captured by ICESat-2's 20 m along-track sampling and the complexity of ice sheet elevation changes. It is especially true in the coastal regions, as demonstrated by the significant noise of the elevation change estimates, especially in Greenland (Fig. 1A). A high-resolution elevation change product, ATL15 (quarterly at 1 km resolution), is already distributed. This study doesn't demonstrate comparable or better performance to ATL15.

Moreover, the density model used for the volume-to-mass conversion includes some decisions that appear to be arbitrary, for example, assuming the regions moving faster than 30 m/yr exhibit dynamic thinning (negative SMB could also be a reason, e.g., snowfall minimum) or that a priori knowledge about dynamic thickening is assumed.

To summarize, the authors should improve the elevation change method and the volume-to-mass change conversion to take advantage of the high-resolution ICESat-2 observations and recent advances in firn densification modeling to derive robust estimates.

The manuscript also falls short in interpreting the elevation and mass changes and fails to provide new insights into ice sheet processes. The discussion is limited to comparing the ice sheet-wide mass balance estimates to other studies with limited analysis of the drainage basin scale results. The qualitative comparison of the SMB anomalies in Antarctica (lines 297-311) is interesting but could be further enhanced by quantitative comparisons. The interpretation of decadal mass changes in Greenland is also disappointing. It mostly repeats statements from the introduction confirming changes detected by other studies without providing more details from the ICESat & ICESat-2 comparison.

Detailed comments:
Line 20: Forsberg et al., 2017 only use data after 2000, so they could not detect changes in the 1980s

Line 21: Please change floating glaciers to floating ice shelves

Lines 33-35: several of these studies are model-based only, e.g., Verjans et al., 2021; Pelle et al., 2020; Stokes et al., 2021. Observational studies of recent changes and/or altimetry applications would better serve the statements here.

Line 63: “These studies” refer to studies (in the previous sentence) that include a much shorter period than 1992-2018. I suggest keeping only the relevant references in this sentence.

Line 67: the manuscript refers to several papers discussing numerical ice sheet models. Mentioning vertical crustal deformation and firn models here would clarify the improvement due to numerical models.

Line 81: ICESat had a near-infrared laser, not a red laser.

Line 86: the posting of ATL06 is 20 meters, not 40 meters (40 meters is the segment length). ATL06 height is defined by straight-line fitting and not by averaging.

Line 88: Sørensen et al., 2011 used 500 meter segments. Why was the length increased? Are elevation changes computed from single tracks or pair tracks?

Line 94: The SEC from ICESat-2 was interpolated not the ICESat-2 data.

Line 144: what topographic/bedrock data set was used for Greenland?

Figs 1-2: Are the elevation changes indeed equivalent in ice? The kriging interpolation was described before the correction for density, firn-compaction, etc.

Line 176: I assume the Pritchard reference is included because of the significant dynamic thickening signals over some surging glaciers in NE Greenland. Dynamic thickening with smaller amplitude as a response to millennial-scale changes is also likely in other GrIS regions and in Antarctica (e.g., Schlegel et al., 2012).

Line 183: the dynamic thickening of the Siple Coast ice streams are well documented and attributed to changes in subglacial hydrology. This description here is misleading and should be improved.

Results: Please label the regions mentioned in the results and discussion sections, eg., lines 214, 222.

Line 235: why are ice rises and rumples thickening? Is it a new result or detected by previous studies?

Figure 5: Consider adding the numbers for the drainage basins. Referring to their relative location, especially in Antarctica, is unusual and confusing (e.g., Siple coast basin as Midwest in line 244).

Line 256: showing results for the grounded area of the ice sheets would require the separation of grounded glaciers and the grounded portion of the ice sheet – therefore, the statement here is misleading. It is sufficient and important to indicate which ice sheet masks are used and what they cover, i.e., ice sheet, ice caps, etc.

Line 276: Smith et al., 2020 used densities from a firn densification model, not ice densities for the entire ice sheets.

Section 4.1: this section repeats several references to other studies from the introductions without providing more details, e.g., Jakobshavn Isbrae: lines 41-43 and lines 326-330. The repetitions should be avoided; rather, a discussion of the new findings should be included here. Also, some of the changes discussed, such as the decrease in thinning in SE Greenland (lines 330-332) happened during the ICESat period and are well documented. A more interesting line of inquiry would be to investigate the decadal change of outlet glacier behavior as detected by the comparison of ICESat and ICESat-2. Finally, the discussion about the potential drivers of northern Greenland mass loss is essentially a review of the studies mentioned in the introduction instead of explaining new findings. Perhaps the spatial pattern of the mass loss can help attribute the patterns to certain processes.

References:
Schlegel, N.-J. et al. Application of GRACE to the assessment of model-based estimates of monthly Greenland Ice Sheet mass balance (2003–2012). Cryosphere 10, 1965–1989 (2016).

Citation: https://doi.org/10.5194/tc-2023-104-RC2

Nicolaj Hansen, Louise Sandberg Sørensen, Giorgio Spada, Daniele Melini, Rene Forsberg, Ruth Mottram, and Sebastian B. Simonsen

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Short summary

We use ICESat-2 to estimate the surface elevation change over Greenland and Antarctica in the period of 2018 to 2021. Numerical models have been used the compute the firn compaction and the vertical bedrock movement so non-mass-related elevation changes can be taken into account. We have made a parameterization of the surface density so we can convert the volume change to mass change. We find that Antarctica has lost 135.7±27.3 Gt per year, and the Greenland ice sheet 237.5±14.0 Gt per year.


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