Abstract

The firn layer that covers 90 % of the Greenland ice sheet (GrIS) plays an important role in determining the response of the ice sheet to climate change. Meltwater can percolate into the firn layer and refreeze at greater depths, thereby temporarily preventing mass loss. However, as global warming leads to increasing surface melt, more surface melt may refreeze in the firn layer, thereby reducing the capacity to buffer subsequent episodes of melt. This can lead to a tipping point in meltwater runoff. It is therefore important to study the evolution of the Greenland firn layer in the past, present and future. In this study, we present the latest version of our firn model, IMAU-FDM (Firn Densification Model), with an application to the GrIS. We improved the density of freshly fallen snow, the dry-snow densification rate and the firn's thermal conductivity using recently published parameterizations and by calibrating to an extended set of observations of firn density, temperature and liquid water content at the GrIS. Overall, the updated model settings lead to higher firn air content and higher 10 m firn temperatures, owing to a lower density near the surface. The effect of the new model settings on the surface elevation change is investigated through three case studies located at Summit, KAN-U and FA-13. Most notably, the updated model shows greater inter- and intra-annual variability in elevation and an increased sensitivity to climate forcing.

Highlights

  • 15 Firn, the layer of compressed snow that represents the transitional stage between seasonal snow and ice in the accumulation zone of glaciers, strongly influences the climate response of mountain glaciers, ice caps and ice sheets

  • As global warming leads to increasing surface melt, more surface melt may refreeze in the 5 firn layer, thereby reducing the capacity to buffer subsequent episodes of melt

  • Vertical profiles of observed firn density from ice cores vary in depth from 9.6 to 150.8 m and have been drilled between 1952 and 2018 in the framework of the Program for Arctic Regional Climate Assessment (PARCA; McConnell et al (2000); Mosley-Thompson et al (2001); Hanna et al (2006); Banta and McConnell (2007)), the Arctic Circle Traverses (ACT, Box et al (2013)) and the EGIG line (Harper et al (2012b)), Das 1 and Das 2 (e.g. from Hanna et al (2006)) and several other cores were retrieved from the SUMup database (SUrface Mass 70 balance and snow depth on sea ice working grouP), (Koenig et al (2013); Koenig and Montgomery (2019))

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Summary

Introduction

15 Firn, the layer of compressed snow that represents the transitional stage between seasonal snow and ice in the accumulation zone of glaciers, strongly influences the climate response of mountain glaciers, ice caps and ice sheets. As melt rates increase further in response to global warming, firn pore space is increasingly taken up by refrozen meltwater, degrading the efficiency of the refreezing process until at some point it collapses This is happening to Greenland’s marginal ice caps since the mid 1990s, accelerating mass loss and initiating their irreversible demise in the coming centuries (Noël et al (2017)). With a depth of up to 80 m (Kuipers Munneke et al (2015a)), Vandecrux et al (2019b) estimated that the GrIS firn layer contains a total of 26800 ± 1840 km of air This is equivalent to more than 60 times the total annual (1961-1990 average) volume of GrIS meltwater production (Van den Broeke et al (2016)), this reduces to a factor of ∼1-4 if only pore space in the 30 percolation zone is considered (Harper et al (2012b)).

Observations
IMAU-FDM
Dry snow densification rate
Thermal conductivity
Meltwater percolation, retention and refreezing
Firn thickness and elevation change
Model initialisation
Pilot application to firn-induced surface elevation change
Summit
Findings
425 References
Full Text
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