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Ice-sheet acceleration driven by melt supply variability

Abstract

Increased ice velocities in Greenland1 are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice–ocean interactions in water-terminating outlet glaciers2 and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration2,3,4,5,6 have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another7, suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt4 but an increase in water input variability8 that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure9,10,11, with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure12,13,14,15. Using a model that captures the dynamic switching12 between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning16, indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles4,14 and an increase in rain and surface lake drainage events8,17,18 rather than an increase in mean melt supply3,4.

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Figure 1: Properties of a single conduit.
Figure 2: Steady-state drainage systems.
Figure 3: Idealized seasonal evolution of the drainage system.
Figure 4: Temporal variations in water input.

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Acknowledgements

Thanks to G. Clarke, T. Creyts, G. Flowers, I. Hewitt, R. Hindmarsh, M. Jellinek and V. Radić for comments on the manuscript, and to C. Koziol and M. Jaffrey for discussions. Financial support was provided by the Canada Research Chairs Program, NSERC Discovery Grant 357193-08 and the Canadian Foundation for Climate and Atmospheric Science through the Polar Climate Stability Network.

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Correspondence to Christian Schoof.

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Supplementary information

Supplementary Information

This file contains Supplementary information and Data, supplementary Figures 1-12 with legends and additional references. (PDF 1861 kb)

Supplementary Movie 1

Formation of a subglacial drainage net - Spontaneous formation of a drainage network at melt input rate m = 10 cm day^(-1), remaining parameter values given in Supplementary Information. Panel a) shows conduit size, panel b) shows effective pressure distribution, panel c) shows mean effective pressure against time, with red dot indicating current time. (MPG 2433 kb)

Supplementary Movie 2

Drainage realignment (1) - Realignment of drainage system from a steady state with melt input rate m = 10 cm day^(-1), changed to reduced melt input m = .66 cm day^(-1). Remaining parameter values given in Supplementary Information. Panel a) shows conduit size, panel b) shows effective pressure distribution, panel c) shows mean effective pressure against time, with red dot indicating current time. (MPG 2617 kb)

Supplementary Movie 3

Drainage realignment (2) - Realignment of drainage system from a steady state with melt input rate m = 10 cm day^(-1), changed to reduced melt input m = 1 cm day^(-1). Remaining parameter values given in Supplementary Information. Panel a) shows conduit size, panel b) shows effective pressure distribution, panel c) shows mean effective pressure against time, with red dot indicating current time. (MPG 4688 kb)

Supplementary Movie 4

Oscillatory water input (1) - Response of drainage system to variable water input, starting with steady state with melt input rate m = 10 cm day^(-1). Remaining parameter values given in Supplementary Information. Panel a) shows conduit size, panel b) shows effective pressure distribution. Black line in panel c) shows mean effective pressure against time, with red dot indicating current time, blue line shows rate of water supply. (MPG 1423 kb)

Supplementary Movie 5

Oscillatory water input (2) - Response of drainage system to variable water input, starting with steady state with melt input rate m = 10 cm day^(-1). Remaining parameter values given in Supplementary Information. Panel a) shows conduit size, panel b) shows effective pressure distribution. Black line in panel c) shows mean effective pressure against time, with red dot indicating current time, blue line shows rate of melt supply. (MPG 872 kb)

Supplementary Movie 6

Localized water input - Response of drainage system to variable water input, starting with steady state with melt input rate m = 10 cm day^(-1). Remaining parameter values given in Supplementary Information. Panel a) shows conduit size, panel b) shows effective pressure distribution. Red dots indicate location of water input events. Water input is locally raised to 50 times the background rate, mimicking for instance a lake drainage event. (MPG 581 kb)

Supplementary Movie 7

Seasonal cycle - Response of drainage system to seasonal cycle. Remaining parameter values given in supplementary information. Panel a shows conduit size, panel b shows effective pressure distribution. Black line in panel c shows mean effective pressure against time, with red dot indicating current time, blue line shows rate of water supply. (MPG 1413 kb)

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Schoof, C. Ice-sheet acceleration driven by melt supply variability. Nature 468, 803–806 (2010). https://doi.org/10.1038/nature09618

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