Evidence of meltwater retention within the Greenland ice sheet

A K Rennermalm,M R Van Den Broeke,R R Forster,J E Box,S E Moustafa,D Van As,L C Smith,V W Chu

doi:10.5194/tc-7-1433-2013

Abstract

Abstract. Greenland ice sheet mass losses have increased in recent decades with more than half of these attributed to surface meltwater runoff. However, the magnitudes of englacial storage, firn retention, internal refreezing and other hydrologic processes that delay or reduce true water export to the global ocean remain less understood, partly due to a scarcity of in situ measurements. Here, ice sheet surface meltwater runoff and proglacial river discharge between 2008 and 2010 near Kangerlussuaq, southwestern Greenland were used to establish sub- and englacial meltwater storage for a small ice sheet watershed (36–64 km2). This watershed lacks significant potential meltwater storage in firn, surface lakes on the ice sheet and in the proglacial area, and receives limited proglacial precipitation. Thus, ice sheet surface runoff not accounted for by river discharge can reasonably be attributed to retention in sub- and englacial storage. Evidence for meltwater storage within the ice sheet includes (1) characteristic dampened daily river discharge amplitudes relative to ice sheet runoff; (2) three cold-season river discharge anomalies at times with limited ice sheet surface melt, demonstrating that meltwater may be retained up to 1–6 months; (3) annual ice sheet watershed runoff is not balanced by river discharge, and while near water budget closure is possible as much as 54% of melting season ice sheet runoff may not escape to downstream rivers; (4) even the large meltwater retention estimate (54%) is equivalent to less than 1% of the ice sheet volume, which suggests that storage in en- and subglacial cavities and till is plausible. While this study is the first to provide evidence for meltwater retention and delayed release within the Greenland ice sheet, more information is needed to establish how widespread this is along the Greenland ice sheet perimeter.

Highlights

Greenland ice sheet mass losses derive from increased ice discharge and meltwater runoff since the early 1990s (Rignot et al, 2008)
The lower bounds for winter discharge (QLOW) and runoff from the smallest watershed realization (W3) can produce a match between cumulative ice sheet runoff and river discharge over the three-year period (Fig. 4)
(W1 and W2); (2) ice sheet runoff is less than river discharge at the end of the three-year period suggesting an englacial water source

Summary

Introduction

Greenland ice sheet mass losses derive from increased ice discharge and meltwater runoff since the early 1990s (Rignot et al, 2008). Increases in meltwater runoff are estimated to be twice as large as cumulative ice discharge anomalies between 2000 and 2008 (Van den Broeke et al, 2009a). Continued mass loss from the Greenland ice sheet has the potential to raise global sea levels by 9 ± 4 cm by 2050 (Rignot et al, 2011) and between 17 and 54 cm by 2100 (Pfeffer et al, 2008) These estimates project that 13–58 % of this increased mass loss occurs as a result of declining surface mass balance (Pfeffer, 2008; Rignot et al, 2011). The wide range in this fraction can partly be attributed to uncertainties associated with quantifying the link between surface mass balance and ice dynamics (Bindschadler et al, 2013), and surface meltwater retention (e.g., Rennermalm et al, 2013)

Methods

Results

Discussion

Conclusion