An explanation for the dark region in the western melt zone of the Greenland ice sheet

Wientjes, I. G. M.; Oerlemans, J.

doi:10.5194/tc-4-261-2010

Cited by 89 publications

(111 citation statements)

References 22 publications

(18 reference statements)

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“…During this time period, GLASS albedo values are as low as 0.30, lower than that of bare ice (i.e. 0.45), consistent with in situ measured values of dirty ice (Wientjes and Oerlemans, 2010;Bøggild et al, 2010). Figure 4 shows the spatial distribution of MAR and GLASS mean JJA albedo for the year 2010 over an area centred on the dark band in southwest Greenland, as well as the time series of GLASS albedo averaged over the same ice-covered area contained within the region identified by the black rectangle in Fig.…”

Section: Drivers: Surface Grain Size and Bare Icesupporting

confidence: 56%

“…Mean summer albedo from GLASS decreased over this area between 2005 and 2012 from ∼ 0.6 to ∼ 0.45 (vs. a decrease simulated by MAR of 0.075). Minimum summer albedo across all years averaged over the region is ∼ 0.4, but dips close to ∼ 0.3 in 2010, a value consistent with dirty bare ice, as shown in previous studies (Wientjes and Oerlemans, 2010;Wientjes et al, 2011;Bøggild et al, 2010). We hypothesize that the discrepancy along this dark band between MAR and GLASS albedo values is likely due to trends in the concentrations of LAI in the snow and ice in this region, which are not currently captured by the model.…”

Section: Drivers: Surface Grain Size and Bare Icementioning

confidence: 61%

“…Since MAR does not account for the presence of surface LAI, and because the impact of LAI is mostly in the UV and visible portion of the spectrum, we suggest that another mechanism explaining the difference of −0.017 decade −1 between the MAR and GLASS visible albedo trends is associated with increasing mixing ratios of LAI in surface snow and ice on some parts of the GrIS. As we pointed out, this could be due to a combination of increased exposure of dirty ice with ablation (Wientjes and Oerlemans, 2010;Bøggild et al, 2010), to enhanced melt consolidation with warming (e.g. Doherty et al, 2013), or to increased deposition of LAI from the atmosphere.…”

Section: Discussionmentioning

confidence: 99%

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The darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)

et al. 2016

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Abstract. The surface energy balance and meltwater production of the Greenland ice sheet (GrIS) are modulated by snow and ice albedo through the amount of absorbed solar radiation. Here we show, using space-borne multispectral data collected during the 3 decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02 decade −1 between 1996 and 2012. Over the same period, albedo modelled by the Modèle Atmosphérique Régionale (MAR) also shows a decrease, though at a lower rate (∼ −0.01 decade −1 ) than that obtained from space-borne data. We suggest that the discrepancy between modelled and measured albedo trends can be explained by the absence in the model of processes associated with the presence of light-absorbing impurities. The negative trend in observed albedo is confined to the regions of the GrIS that undergo melting in summer, with the drysnow zone showing no trend. The period 1981-1996 also showed no statistically significant trend over the whole GrIS. Analysis of MAR outputs indicates that the observed albedo decrease is attributable to the combined effects of increased near-surface air temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, and to trends in light-absorbing impurities (LAI) on the snow and ice surfaces. Neither aerosol models nor in situ and remote sensing observations indicate increasing trends in LAI in the atmosphere over Greenland. Similarly, an analysis of the number of fires and BC emissions from fires points to the absence of trends for such quantities. This suggests that the apparent increase of LAI in snow and ice might be related to the exposure of a "dark band" of dirty ice and to increased consolidation of LAI at the surface with melt, not to increased aerosol deposition. Albedo projections through to the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies averaged over the whole ice sheet lower by 0.08 in 2100 than in 2000, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in modelled melting (as seen in hindcasts) and because the model albedo scheme does not currently include the effects of LAI, which have a positive feedback on albedo decline through increased melting, grain growth, and darkening.

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Section: Drivers: Surface Grain Size and Bare Icesupporting

confidence: 56%

Section: Drivers: Surface Grain Size and Bare Icementioning

confidence: 61%

Section: Discussionmentioning

confidence: 99%

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The darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)

et al. 2016

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“…and higher). Melting of dust-rich glacier ice causes the dark band in the middle ablation zone (Wientjes and Oerlemans, 2010). Strong interactions between surface meltwater production and ice dynamics have been observed along the K-transect ( Van de Wal et al, 2008;Shepherd et al, 2009).…”

Section: Aws Datamentioning

confidence: 99%

The seasonal cycle and interannual variability of surface energy balance and melt in the ablation zone of the west Greenland ice sheet

2011

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Abstract. We present the seasonal cycle and interannual variability of the surface energy balance (SEB) in the ablation zone of the west Greenland ice sheet, using seven years (S9) at distances of 6, 38 and 88 km from the ice sheet margin. The hourly AWS data are fed into a model that calculates all SEB components and melt rate; the model allows for shortwave radiation penetration in ice and time-varying surface momentum roughness. Snow depth is prescribed from albedo and sonic height ranger observations. Modelled and observed surface temperatures for non-melting conditions agree very well, with RMSE's of 0.97-1.26 K. Modelled and observed ice melt rates at the two lowest sites also show very good agreement, both for total cumulative and 10-day cumulated amounts. Melt frequencies and melt rates at the AWS sites are discussed. Although absorbed shortwave radiation is the most important energy source for melt at all three sites, interannual melt variability at the lowest site is driven mainly by variability in the turbulent flux of sensible heat. This is explained by the quasi-constant summer albedo in the lower ablation zone, limiting the influence of the melt-albedo feedback, and the proximity of the snow free tundra, which heats up considerably in summer.Correspondence to: M. R. van den Broeke (m.r.vandenbroeke@uu.nl)

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“…However, Wientjes and Oerlemans (2010) showing the dark region in the ablation zone of West Greenland, adapted from Wientjes and Oerlemans (2010). S is Sukkertoppen Iskappe, J is Jakobshavn Isbrae, I is Inugpait Qûat, and R is Russell glacier, the starting point of the K-transect (dashed line).…”

Section: Introductionmentioning

confidence: 99%

Dust from the dark region in the western ablation zone of the Greenland ice sheet

et al. 2011

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Abstract.A dark region tens of kilometres wide is located in the western ablation zone of the Greenland ice sheet. The dark appearance is caused by higher amounts of dust relative to the brighter surroundings. This dust has either been deposited recently or was brought to the surface by melting of outcropping ice. Because the resulting lower albedos may have a significant effect on melt rates, we analysed surface dust on the ice, also called cryoconite, from locations in the dark region as well as locations from the brighter surrounding reference ice with microscopic and geochemical techniques to unravel its composition and origin. We find that (part of) the material is derived from the outcropping ice, and that there is little difference between dust from the dark region and from the reference ice. The dust from the dark region seems enriched in trace and minor elements that are mainly present in the current atmosphere because of anthropogenic activity. This enrichment is probably caused by higher precipitation and lower melt rates in the dark region relative to the ice marginal zone. The rare earth elemental ratios of the investigated material are approximately the same for all sites and resemble Earth's average crust composition. Therefore, the cryoconite probably does not contain volcanic material. The mineralogical composition of the dust excludes Asian deserts, which are often found as provenance for glacial dust in ice cores, as source regions. Consequently, the outcropping dust likely has a more local origin. Finally, we find cyanobacteria and algae in the cryoconite. Total Organic Carbon accounts for up to 5 weight per cent Correspondence to: I. G. M. Wientjes (i.g.m.wientjes@uu.nl) of the cryoconite from the dark region, whereas dust samples from the reference ice contain only 1 % or less. This organic material is likely formed in situ. Because of their high light absorbency, cyanobacteria and the organic material they produce contribute significantly to the low albedo of the dark region.

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An explanation for the dark region in the western melt zone of the Greenland ice sheet

Cited by 89 publications

References 22 publications

The darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)

The darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)

The seasonal cycle and interannual variability of surface energy balance and melt in the ablation zone of the west Greenland ice sheet

Dust from the dark region in the western ablation zone of the Greenland ice sheet

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