Abstract
AbstractRefreezing of meltwater in firn is a major component of Greenland ice-sheet's mass budget, but in situ observations are rare. Here, we compare the firn density and total ice layer thickness in the upper 15 m of 19 new and 27 previously published firn cores drilled at 15 locations in southwest Greenland (1850–2360 m a.s.l.) between 1989 and 2019. At all sites, ice layer thickness covaries with density over time and space. At the two sites with the earliest observations (1989 and 1998), bulk density increased by 15–18%, in the top 15 m over 28 and 21 years, respectively. However, following the extreme melt in 2012, elevation-detrended density using 30 cores from all sites decreased by 15 kg m−3 a−1 in the top 3.75 m between 2013 and 2019. In contrast, the lowest elevation site's density shows no trend. Thus, temporary build-up in firn pore space and meltwater infiltration capacity is possible despite the long-term increase in Greenland ice-sheet melting.
Highlights
Greenland ice-sheet’s mass balance was close to zero for about two decades before the 1990s (1972–1990, Mouginot and others, 2019), and possibly longer (1961–1990, van den Broeke and others, 2016)
Most of it infiltrates into the subsurface snow and firn layers and refreezes at varying depths depending on the amount, snow/firn layer thickness and temperature
We present 19 shallow firn cores drilled in the percolation zone in southwest Greenland between 2017 and 2019, extending the period covered by previously published cores (Kameda and others, 1995; Machguth and others, 2016; Graeter and others, 2018)
Summary
Greenland ice-sheet’s mass balance was close to zero for about two decades before the 1990s (1972–1990, Mouginot and others, 2019), and possibly longer (1961–1990, van den Broeke and others, 2016). Since the 2000s, increased surface melt and runoff caused the surface mass balance to be the largest contributor to the total mass loss (55 ± 5% between 1998 and 2018, Mouginot and others, 2019; 68% between 2009 and 2012, Enderlin and others, 2014). Most of this meltwater forms in the ablation zone where it runs off in channels on and under the ice sheet (van den Broeke and others, 2016). Concurrent with the increase in surface melt, an increase in refreezing starting in the mid-1990s is seen in both regional climate models (e.g. Noël and others, 2017), and in situ observations in shallow (10–20 m) firn cores (Machguth and others, 2016; Graeter and others, 2018; Vandecrux and others, 2019)
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