The dark side of snow – understanding surface melting of the Greenland Ice Sheet

Overview

The Earth’s glaciers are melting at unprecedented rates. Greenland’s km-thick ice sheet is no exception – with huge implications for global sea levels. The surface melting rate of Greenland’s ice sheet has been accelerating over the past decades. This is attributed to two factors: larger snow grains due to atmospheric warming; and snow impurities, such as algae and airborne particles [1,2]. These impurities lower the albedo of the ice, thereby affecting the amount of solar radiation being reflected back into the atmosphere. To date, most research on this topic has treated the surface of the snow or ice as a plane surface – an appropriate assumption in atmospheric sciences, given that the behaviour of the boundary between snow/ice and atmosphere is what feeds into large-scale models. Thus, experimental work carried out to date focuses on the albedo of the snow/ice surface [3] rather than the energy transfer within the snow/ice column [4]. Little research effort has gone into understanding processes, on mm to dm scales, In short: what happens to all the solar energy that enters the ice rather than the energy that is reflected.
Although both grain growth and snow impurities affect the snow pack’s albedo in the same way, the effect on the snow pack itself is entirely different: larger grains result in greater snow transparency with respect to sunlight while dust makes snow darker and more opaque, leading to heating of the near-surface layer, causing melting, with increasing amounts of dark residue preferentially accumulating on the surface, further accelerating melting. This forms a positive feedback with sunlight absorbed by dark particles increasing snow/ice temperatures at depth – a phenomenon called the solid state greenhouse effect [5].

Project scope
The aim of this project is to understand the solid state greenhouse effect in contaminated snow. We want to quantify the interaction between snow grain size, aerosol impurities, temperature and the melting process in order to provide important input parameters for models of ice sheet retreat in Greenland and elsewhere.
The key science questions of this project are:
1. How do impurities contribute to textural changes of snow packs and how do they influence melting and (compaction) sintering?
2. What is the feedback of dark surface residue? Does it accelerate melting by absorbing more sunlight? Or does it slow melting by preventing sunlight from heating deeper layers of the snow pack?
3. How effective is seasonal self-cleaning by re-accumulation?

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Image Captions

Greenland Ice Sheet Summit Station.
Public domain. Credit: Peter West, NSF

Methodology

This project will use a combination of laboratory experiments, fieldwork and numerical modelling.
You will run experiments on artificial snow in the Stirling Planetary Ices Laboratory (SPIL), making snow and mixing it with contaminants to produce samples representative of the snow found in a glacial environment. The samples will then be exposed to sunlight under controlled conditions. Light penetration, temperatures, melt rate, density, porosity and mass loss will be observed in real time whilst microstructure will be investigated at the beginning and end of each experiment. Stirling University is part of the EuroPlanet Reserarch Infrastructure, providing accesss to field sites in Greenland. A field trip to the Greenland Ice Sheet will provide an opportunity to sample the Greenland snow pack in the field, establishing important baseline environmental conditions for laboratory experiments.

Project Timeline

Year 1

The experimental work of this project will take approximately 30 months to complete. 12 months are allocated to disseminating the results through journal papers and conference presentations as well as writing the thesis.
In year 1, after establishing the theoretical foundations of the project and a few scoping experiments in the laboratory, you will travel to Greenland to characterise the physical properties of the near-surface layers of the ice sheet.

Year 2

In year 2, you will replicate the in situ conditions (temperature, light irradiation, contamination) in the laboratory. Further field sampling on alpine glaciers will test whether the in situ conditions investigated in year 1 are globally applicable or whether the parameter space needs to be expanded.
Experiments of the relevant parameter space will continue until the middle of year 3, when documentation and dissemination of the results will take over.

Year 3

Experiments of the relevant parameter space will continue until the middle of year 3, when documentation and dissemination of the results will take over.

Year 3.5

Continuation of thesis writing and publication.

Training
& Skills

The studentship will provide an opportunity to build an interdisciplinary research career in environmental science and climate studies.
Skills development during fieldwork will include training in sampling techniques; experiment design and analytical techniques will be acquired during laboratory work. Data manipulation and quantitative analysis skills will be improved with the help of modelling software.

References & further reading

[1] Dumont et al., Nature Geoscience 7, 2180 (2014) DOI: 10.1038/NGEO2180.[2] Lutz, S. et al., Nature Communications 7, 11968 (2016) DOI: 10.1038/ncomms11968.[3] Hadley, O. L. and Kirchstetter, T. W. Nature Climate Change 2, 437-440 (2012) DOI: 10.1038/NCLIMATE1433[4 Beaglehole, D et al., JGR Atmospheres 103(D8), 8849-8857 (1998) DOI: 10.1029/97JD03604[5] Kaufmann E & Hagermann A. Icarus, 252, 144-149 (2015). DOI: 10.1016/j.icarus.2015.01.007

https://www.theguardian.com/environment/2014/jul/05/dark-snow-speeding-glacier-melting-rising-sea-levels
https://www.rollingstone.com/interactive/feature-greenland-melting/

Further Information

axel.hagermann@stir.ac.uk – https://www.stir.ac.uk/people/479719

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