Abstract
Abstract. Runoff from the Greenland Ice Sheet (GrIS) has increased in recent years due largely to changes in atmospheric circulation and atmospheric warming. Albedo reductions resulting from these changes have amplified 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 interannual 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 in the future, 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 interannual 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 prerequisites 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.
Highlights
Overall mass losses from the Greenland Ice Sheet (GrIS) have increased substantially since the early 1990s (Rignot and Kanagaratnam, 2006; Rignot et al, 2011; Shepherd et al, 2012)
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
We examine the extent to which interannual variations in dark ice dynamics are controlled by prevailing seasonal meteorological and climatological conditions and how they could drive surface darkening through three potential processes: (1) inorganic particulate deposition or redistribution, (2) cryoconite hole processes and (3) growth of ice algal assemblages
Summary
Overall mass losses from the Greenland Ice Sheet (GrIS) have increased substantially since the early 1990s (Rignot and Kanagaratnam, 2006; Rignot et al, 2011; Shepherd et al, 2012). The average rate of mass loss increased from 34 Gt yr−1 during 1992–2001 to 215 Gt yr−1 during 2002–2011 (Sasgen et al, 2012). During 1991–2015 the GrIS lost mass at a rate equivalent to approximately 0.47 ± 0.23 mm yr−1 of sea level rise, with a peak contribution in 2012 of 1.2 mm (van den Broeke et al, 2016). Tedstone et al.: Dynamics of Greenland dark ice increased surface runoff, with only 32 % of the total loss in this period attributable to solid ice discharge (Enderlin et al, 2014). It is essential to understand the processes which control surface melting in order to be able to quantify the contribution of the GrIS to sea level rise over the coming century
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