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Decadal slowdown of a land-terminating sector of the Greenland Ice Sheet despite warming

Abstract

Ice flow along land-terminating margins of the Greenland Ice Sheet (GIS) varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet. Such inputs lubricate the ice–bed interface, transiently speeding up the flow of ice1,2. Greater melting results in faster ice motion during summer, but slower motion over the subsequent winter, owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice-sheet bed that have a high basal water pressure2,3. However, the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained. Here we show that annual ice motion across an 8,000-km2 land-terminating region of the west GIS margin, extending to 1,100 m above sea level, was 12% slower in 2007–14 compared with 1985–94, despite a 50% increase in surface meltwater production. Our findings suggest that, over these three decades, hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion (not a speed-up, as previously postulated1). Increases in meltwater production from projected climate warming may therefore further reduce the motion of land-terminating margins of the GIS. Our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought.

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Figure 1: Study area in the ablation zone of the western GIS.
Figure 2: Surface melting and ice motion averaged over the study area.
Figure 3: Ice velocities along three transects in the study area.

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Acknowledgements

A.J.T. acknowledges UK Natural Environment Research Council (NERC) studentships NE/152830X/1 and NE/J500021/1, a Scottish Alliance for Geoscience, Environment and Society (SAGES) Postdoctoral/Early Career Researcher Exchange (PECRE) award, and a University of Edinburgh GeoSciences Moss scholarship. N.G. acknowledges European Space Agency Dragon 3 grant 10302, the Centre National d’Etudes Spatiales Tosca CESTENG project, and a fellowship from the Centre National d’Etudes Spatiales to A.D. This work made use of the resources provided by the Edinburgh Compute and Data Facility (ECDF) (http://www.ecdf.ed.ac.uk/). We thank P. Huybrechts for his work on the runoff/retention model used in this study. The Landsat imagery was provided by the United States Geological Survey and the European Space Agency third party missions program.

Author information

Authors and Affiliations

Authors

Contributions

A.J.T., P.W.N. and N.G. designed this study. A.D., N.G. and A.J.T. developed the processing chain used for feature tracking of Landsat imagery. A.J.T., A.D. and N.G. processed the Landsat imagery. A.J.T. and D.G. calculated the impact of changing ice geometry upon ice motion. E.H. processed the melt data. A.J.T., N.G. and P.W.N. analysed the results. A.J.T., P.W.N. and N.G. wrote the manuscript. All authors discussed the results and edited the manuscript.

Corresponding author

Correspondence to Andrew J. Tedstone.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Sensitivity of extracted ice motion to variations in baseline duration.

For each period are shown: a, the average start day-of-year (DOY) of all pairs used in the period; b, the average baseline duration of all pairs used in the period; c, the proportion of the baseline duration that is attributable to summer, which is defined as 1 May to 31 August; and d, the annual velocity that would be expected in the ablation zone of the Leverett glacier catchment, based on the average proportion of summer versus winter and the average baseline duration for each year.

Extended Data Figure 2 Statistical significance of three different periods of surface meltwater production.

Hypothesis test of the Wilcoxon rank sum test at 95% confidence (see Methods), showing that three periods of surface melt separated by the specified dates have statistically different medians (outlined white region). The colouring shows the residuals of a three-trend linear segmented regression model fitted to melting at each possible combination of two break dates, expressed as the root-mean-squared error. The grey area is shaded as such for simplicity; it would otherwise be a mirror image of the coloured area.

Extended Data Figure 3 Statistical significance of two different periods of ice motion.

a, Hypothesis test of the Wilcoxon rank sum test for equal medians, testing the probability that the two populations (separated by the specified date) are similar, with 95% confidence. 0 signifies that the hypothesis of equal medians cannot be rejected, and 1 signifies that the hypothesis of equal medians can be rejected. b, Residuals shown as the sum-of-squares (m yr−1)2 of a two-trend model fitted to velocities at each possible break date.

Extended Data Figure 4 Ice velocities during each period.

The velocities have uncertainties < 60 m yr−1 and were observed across at least 30% of the study area in each period (see Methods).

Extended Data Figure 5 Impact of changing ice geometry on ice motion.

Ice thinning of 10 m (purple) and 20 m (blue) at the ice margin, through to 0 m at 100 km inland, was applied to transect A. a, Prescribed change in ice thickness over transect length. b, The ratio of velocity change, calculated from equation (4). c, Left axis, observed ice velocity during 1985–94 (dotted green) and 2007–14 (dotted red). Modelled velocities in 2014 (solid lines) for the prescribed ice thicknesses. Right axis, ice thickness (dashed grey)24.

Extended Data Table 1 Statistical relationship between melting and ice motion

Supplementary information

Supplementary Data 1

This file contains a list of all the pairs of Landsat images processed to derive velocity fields. (XLSX 19 kb)

Supplementary Data 2

This file contains lists of the processed Landsat pairs which contribute to each velocity period. (XLSX 21 kb)

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Tedstone, A., Nienow, P., Gourmelen, N. et al. Decadal slowdown of a land-terminating sector of the Greenland Ice Sheet despite warming. Nature 526, 692–695 (2015). https://doi.org/10.1038/nature15722

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