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Threshold response to melt drives large-scale bed weakening in Greenland


Ice speeds in Greenland are largely set by basal motion1, which is modulated by meltwater delivery to the ice base2,3,4. Evidence suggests that increasing melt rates enhance the subglacial drainage network’s capacity to evacuate basal water, increasing bed friction and causing the ice to slow5,6,7,8,9,10. This limits the potential of melt forcing to increase mass loss as temperatures increase11. Here we show that melt forcing has a pronounced influence on dynamics, but factors besides melt rates primarily control its impact. Using a method to examine friction variability across the entirety of western Greenland, we show that the main impact of melt forcing is an abrupt north-to-south change in bed strength that cannot be explained by changes in melt production. The southern ablation zone is weakened by 20–40 per cent compared with regions with no melt, whereas in northern Greenland the ablation zone is strengthened. We show that the weakening is consistent with persistent basal water storage and that the threshold is linked to differences in sliding and hydropotential gradients, which exert primary control on the pressures within drainage pathways that dewater the bed. These characteristics are mainly set by whether a margin is land or marine terminating, suggesting that dynamic changes that increase mass loss are likely to occur in northern Greenland as temperatures increase. Our results point to physical representations of these findings that will improve simulated ice-sheet evolution at centennial scales.

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Fig. 1: Summary of bed-strength variations across western Greenland.
Fig. 2: Threshold change in ablation-zone bed strength.
Fig. 3: Latitudinal variations in bed strength and drainage-system variables.
Fig. 4: Controls on bed-strength threshold.

Data availability

All data used in this study are currently archived and publicly available for download. Multiyear surface velocities: Ice surface elevation: Land/Ice classification: Bed topography: Basal thermal state map: Snowline elevation: Basal melt: RACMO surface mass balance data can be requested from Brice Noël (

Code availability

Elmer/Ice files and code used to produce this paper are archived at


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This work was funded by the French National Research Agency grant ANR-17-CE01-0008. We thank B. Noël for providing the RACMO surface mass balance data. The Elmer/Ice computations presented in this paper were performed using HPC resources of both CINES under the allocation 2021- A0060106066 made by GENCI and of the GRICAD infrastructure (, which is supported by Grenoble research communities. We thank A. Gilbert and P. Christoffersen for discussions that improved this work.

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Authors and Affiliations



N.M. designed the study, analysed the data, produced the figures and is the primary author of the manuscript. F.G. contributed significantly to the data analysis and writing of the manuscript. F.G.-C. performed the friction inversions implemented in Elmer/Ice and contributed to the writing of the manuscript.

Corresponding author

Correspondence to Nathan Maier.

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Extended data figures and tables

Extended Data Fig. 1 Relationship between normalized friction and major geologic boundaries.

Geologic boundaries (dashed lines) were adapted from Dawes et al. (2009). Unit descriptions are as follows: 1: Palaeoproterozoic Precambrian Shield, 2: Meso-Neoarchean Precambrian Shield, 3: Palaeoproterozoic with reworked Archaean Precambrian Shield, 4: Basalts and intrusions, Palaeogene Volcanic Province, 5: Mesoproterozoic Red Beds, 6: Mainly Archaean Precambrian Shield, 7: Palaeoproterozoic Precambrian Shield, 8: Mesoproterozoic Porphyries and Red Beds, 9: Phanerozoic Basin Formation from the Cambrian through Silurian.

Extended Data Fig. 2 Relationship between normalized friction and radar-derived roughness proxies.

(a) Normalized friction plotted adjacent to (b) topographic roughness and (c) scattering-derived roughness proxies derived from radar from ref. 63. Topographic roughness reflects roughness at the greater than 200 m length scale, while the scattering-derived roughness proxy is estimated to reflect roughness at length scales less than 100 m.

Extended Data Fig. 3 Regression between glaciological variables and normalized friction.

Relationship between hydropotential gradients, sliding speed, JJA SMB, and ice thickness and the normalized friction for the upper and lower ablation zone. Individual markers are the mean of 100 evenly spaced latitudinal bins along the margin for the upper and lower ablation zone.

Extended Data Fig. 4 Observations contextualized with till-bed friction law.

Mean basal velocity and friction for elevation bins from each catchment are plotted against a till-bed friction law modified from ref. 71 (Methods). Circle markers are interior elevation bins that experience no summer melt (from all catchments), square markers are ablation-zone bins for north Greenland (catchments 5 - 7), stars are ablation-zone bins for south Greenland (catchments 1 - 4). Horizontal and vertical bars indicate 95% C.I. Reference scaling (solid line, r2 = 0.85) is constrained using bins with no melt forcing (circles, Methods). The ablation-zone bins for northern Greenland (squares) adhere to this relationship while the ablation-zone bins for southern Greenland (stars) are shifted downward indicating increased bed deformation compared to upgradient regions. This trend is captured by a deformation scaling (dashed line, r2 = 0.76) (Methods). A constrained, effective-pressure-based till-bed friction law71 (Methods, dotted lines) is plotted for various N. Deviation from the reference scaling (i.e. the curvature in dotted lines) results from increasing bed deformation as effective pressures decrease.

Extended Data Fig. 5 Drainage analysis.

Panel (a) shows the summer mean accumulated discharge (Qsum), while panel (b) shows the critical discharge (Qc) required for channel formation following ref. 34 (Methods). Panel (c) shows the regions where the accumulated discharge exceeds the critical discharge indicating channel formation is likely. Panel (d) shows the modelled effective pressure within active drainage pathways using a formulation from ref. 34 which incorporates cavities and conduits (Methods).

Extended Data Fig. 6 Parameterized implementation of hydrologic threshold.

Hydrologic threshold is reproduced using a parameterization which implements the reference and cavity friction scaling based on the modelled effective pressure within active drainage pathways (Methods).

Extended Data Fig. 7 Friction summary.

Relationship between basal velocity and friction for 20 evenly spaced elevation bins is plotted for each catchment. Markers with red outline are elevation bins located within ablation zone, and those with black outline are elevation bins where the mean flotation fraction is greater than zero. Relationships from all catchments are plotted together in bottom right panel. Horizontal and vertical bars indicate 95% C.I.

Extended Data Fig. 8 Data summary.

(a) Multiyear averaged surface velocities (1995–2015)24,51, (b) element nodes for the full-Stokes friction inversions, (c) grid cells used in analysis, (d) mean JJA surface mass balance (2001–2015) from RACMO 2.3p2, (e) modelled basal friction and (f) modelled basal velocity from Elmer/Ice inversion (Methods). Coloured lines show catchment boundaries. All maps are plotted using MATLAB.

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Maier, N., Gimbert, F. & Gillet-Chaulet, F. Threshold response to melt drives large-scale bed weakening in Greenland. Nature 607, 714–720 (2022).

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