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The Cryosphere An interactive open-access journal of the European Geosciences Union
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
19 May 2017
Review status
This discussion paper is under review for the journal The Cryosphere (TC).
Dark ice dynamics of the south-west Greenland Ice Sheet
Andrew J. Tedstone1, Jonathan L. Bamber1, Joseph M. Cook2, Christopher J. Williamson1, Xavier Fettweis3, Andrew J. Hodson2, and Martyn Tranter1 1Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol, UK
2Department of Geography, University of Sheffield, Winter Street, Sheffield, UK
3Laboratory of Climatology, Department of Geography, University of Liège, Liège, Belgium
Abstract. Runoff from the Greenland Ice Sheet (GrIS) has increased in recent years due largely to declining albedo and enhanced surface melting. Some of the largest declines in GrIS albedo have occurred in the ablation zone of the south-west sector and are associated with the development of 'dark' ice surfaces. Field observations at local scales reveal that a variety of light-absorbing impurities (LAIs) can be present on the surface, ranging from inorganic particulates, to cryoconite materials and ice algae. Meanwhile, satellite observations show that the areal extent of dark ice has varied significantly between recent successive melt seasons. However, the processes that drive such large inter-annual variability in dark ice extent remain essentially unconstrained. At present we are therefore unable to project how the albedo of bare-ice sectors of the GrIS will evolve, causing uncertainty in the projected sea level contribution from the GrIS over the coming decades.

Here we use MODIS satellite imagery to examine dark ice dynamics on the south-west GrIS each year from 2000 to 2016. We quantify dark ice in terms of its annual extent, duration, intensity and timing of first appearance. Not only does dark ice extent vary significantly between years, but so too does its duration (from 0 % to > 80 % of June–July–August, JJA), intensity and the timing of its first appearance. Comparison of dark ice dynamics with potential meteorological drivers from the regional climate model MAR reveals that the JJA sensible heat flux, the number of positive minimum-air-temperature days and the timing of bare ice appearance are significant inter-annual synoptic controls.

We use these findings to identify the surface processes which are most likely to explain recent dark ice dynamics.We suggest that whilst the spatial distribution of dark ice is best explained by outcropping of particulates from ablating ice, these particulates alone do not drive dark ice dynamics. Instead, they may enable the growth of pigmented ice algal assemblages which cause visible surface darkening, but only when the climatological pre-requisites of liquid meltwater presence and sufficient photosynthetically-active radiation fluxes are met. Further field studies are required to fully constrain the processes by which ice algae growth proceeds and the apparent dependency of algae growth on melt-out particulates.

Citation: Tedstone, A. J., Bamber, J. L., Cook, J. M., Williamson, C. J., Fettweis, X., Hodson, A. J., and Tranter, M.: Dark ice dynamics of the south-west Greenland Ice Sheet, The Cryosphere Discuss., doi:10.5194/tc-2017-79, in review, 2017.
Andrew J. Tedstone et al.
Andrew J. Tedstone et al.
Andrew J. Tedstone et al.


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Short summary
The bare-ice albedo of the south-west Greenland ice sheet varies dramatically between melt seasons. Darkening by inorganic particulates, cryoconite hole processes and ice algae can occur, but the reasons for inter-annual albedo variability are unclear. We use satellite imagery to examine dark ice dynamics, and climate model outputs to find likely climatological controls. Melt-out particulates best explain the spatial extent of dark ice, but the darkening itself is likely due to ice algae growth.
The bare-ice albedo of the south-west Greenland ice sheet varies dramatically between melt...