| 1. |
- Charalampidis, Charalampos, 1983-, et al.
(author)
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Changing surface-atmosphere energy exchange and refreezing capacity of the lower accumulation area, West Greenland
- 2015
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In: The Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 9:6, s. 2163-2181
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Journal article (peer-reviewed)abstract
- We present 5 years (2009-2013) of automatic weather station measurements from the lower accumulation area (1840 m a.s.l. - above sea level) of the Greenland ice sheet in the Kangerlussuaq region. Here, the summers of 2010 and 2012 were both exceptionally warm, but only 2012 resulted in a strongly negative surface mass budget (SMB) and surface meltwater run-off. The observed run-off was due to a large ice fraction in the upper 10 m of firn that prevented meltwater from percolating to available pore volume below. Analysis reveals an anomalously low 2012 summer-averaged albedo of 0.71 (typically similar to 0.78), as meltwater was present at the ice sheet surface. Consequently, during the 2012 melt season, the ice sheet surface absorbed 28% (213 MJ m-2) more solar radiation than the average of all other years. A surface energy balance model is used to evaluate the seasonal and interannual variability of all surface energy fluxes. The model reproduces the observed melt rates as well as the SMB for each season. A sensitivity analysis reveals that 71% of the additional solar radiation in 2012 was used for melt, corresponding to 36% (0.64 m) of the 2012 surface lowering. The remaining 64% (1.14 m) of surface lowering resulted from high atmospheric temperatures, up to a + 2.6 degrees C daily average, indicating that 2012 would have been a negative SMB year at this site even without the melt-albedo feedback. Longer time series of SMB, regional temperature, and remotely sensed albedo (MODIS) show that 2012 was the first strongly negative SMB year, with the lowest albedo, at this elevation on record. The warm conditions of recent years have resulted in enhanced melt and reduction of the refreezing capacity in the lower accumulation area. If high temperatures continue, the current lower accumulation area will turn into a region with superimposed ice in coming years.
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| 2. |
- Kulessa, Bernd, et al.
(author)
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Seismic evidence for complex sedimentary control of Greenland Ice Sheet flow
- 2017
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In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 3:8
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Journal article (peer-reviewed)abstract
- The land-terminating margin of the Greenland Ice Sheet has slowed down in recent decades, although the causes and implications for future ice flow are unclear. Explained originally by a self-regulating mechanism where basal slip reduces as drainage evolves from low to high efficiency, recent numerical modeling invokes a sedimentary control of ice sheet flow as an alternative hypothesis. Although both hypotheses can explain the recent slowdown, their respective forecasts of a long-term deceleration versus an acceleration of ice flow are contradictory. We present amplitude-versus-angle seismic data as the first observational test of the alternative hypothesis. We document transient modifications of basal sediment strengths by rapid subglacial drainages of supraglacial lakes, the primary current control on summer ice sheet flow according to our numerical model. Our observations agree with simulations of initial postdrainage sediment weakening and ice flow accelerations, and subsequent sediment restrengthening and ice flow decelerations, and thus confirm the alternative hypothesis. Although simulated melt season acceleration of ice flow due to weakening of subglacial sediments does not currently outweigh winter slowdown forced by self regulation, they could dominate over the longer term. Subglacial sediments beneath the Greenland Ice Sheet must therefore be mapped and characterized, and a sedimentary control of ice flow must be evaluated against competing self-regulation mechanisms.
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| 3. |
- Lindbäck, Katrin, et al.
(author)
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Subglacial water drainage, storage, and piracy beneath the Greenland Ice Sheet
- 2015
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:18, s. 7606-7614
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Journal article (other academic/artistic)abstract
- Meltwater drainage across the surface of the Greenland Ice Sheet (GrIS) is well constrained by measurements and modeling, yet despite its critical role, knowledge of its transit through the subglacial environment remains limited. Here we present a subglacial hydrological analysis of a land-terminating sector of the GrIS at unprecedented resolution that predicts the routing of surface-derived meltwater once it has entered the basal drainage system. Our analysis indicates the probable existence of small subglacial lakes that remain undetectable by methods using surface elevation change or radar techniques. Furthermore, the analysis suggests transient behavior with rapid switching of subglacial drainage between competing catchments driven by seasonal changes in the basal water pressure. Our findings provide a cautionary note that should be considered in studies that attempt to relate and infer future response from surface temperature, melt, and runoff from point measurements and/or modeling with measurements of proglacial discharge and ice dynamics.
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| 4. |
- Mikkelsen, Andreas Bech, et al.
(author)
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Extraordinary runoff from the Greenland ice sheet in 2012 amplified by hypsometry and depleted firn retention
- 2016
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In: The Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 10:3, s. 1147-1159
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Journal article (peer-reviewed)abstract
- It has been argued that the infiltration and retention of meltwater within firn across the percolation zone of the Greenland ice sheet has the potential to buffer up to similar to 3.6aEuro-mm of global sea-level rise (Harper et al., 2012). Despite evidence confirming active refreezing processes above the equilibrium line, their impact on runoff and proglacial discharge has yet to be assessed. Here, we compare meteorological, melt, firn stratigraphy and discharge data from the extreme 2010 and 2012 summers to determine the relationship between atmospheric forcing and melt runoff at the land-terminating Kangerlussuaq sector of the Greenland ice sheet, which drains into the Watson River. The 6.8aEuro-km(3) bulk discharge in 2012 exceeded that in 2010 by 28aEuro-%, despite only a 3aEuro-% difference in net incoming melt energy between the two years. This large disparity can be explained by a 10aEuro-% contribution of runoff originating from above the long-term equilibrium line in 2012 caused by diminished firn retention. The amplified 2012 response was compounded by catchment hypsometry; the disproportionate increase in area contributing to runoff as the melt-level rose high into the accumulation area. Satellite imagery and aerial photographs reveal an extensive supraglacial network extending 140aEuro-km from the ice margin that confirms active meltwater runoff originating well above the equilibrium line. This runoff culminated in three days with record discharge of 3100aEuro-m(3)aEuro-s(-1) (0.27aEuro-GtaEuro-d(-1)) that peaked on 11 July and washed out the Watson River Bridge. Our findings corroborate melt infiltration processes in the percolation zone, though the resulting patterns of refreezing are complex and can lead to spatially extensive, perched superimposed ice layers within the firn. In 2012, such layers extended to an elevation of at least 1840aEuro-m and provided a semi-impermeable barrier to further meltwater storage, thereby promoting widespread runoff from the accumulation area of the Greenland ice sheet that contributed directly to proglacial discharge and global sea-level rise.
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| 5. |
- Shackleton, Calvin, et al.
(author)
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Subglacial water storage and drainage beneath the Fennoscandian and Barents Sea ice sheets
- 2018
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In: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 201, s. 13-28
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Journal article (peer-reviewed)abstract
- Subglacial hydrology modulates how ice sheets flow, respond to climate, and deliver meltwater, sediment and nutrients to proglacial and marine environments. Here, we investigate the development of subglacial lakes and drainage networks beneath the Fennoscandian and Barents Sea ice sheets over the Late Weichselian. Utilizing an established coupled climate/ice flow model, we calculate high-resolution, spatio-temporal changes in subglacial hydraulic potential from ice sheet build-up (similar to 37 ka BP) to complete deglaciation (similar to 10 ka BP). Our analysis predicts up to 3500 potential subglacial lakes, the largest of which was 658 km(2), and over 70% of which had surface areas <10 km(2), comparable with subglacial lake size distributions beneath the Antarctic Ice Sheet. Asynchronous evolution of the Fennoscandian Ice Sheet into the flatter relief of northeast Europe affected patterns of subglacial drainage, with up to 100 km(3) more water impounded within subglacial lakes during ice build-up compared to retreat. Furthermore, we observe frequent fill/drain cycles within clusters of subglacial lakes at the onset zones and margins of ice streams that would have affected their dynamics. Our results resonate with mapping of large subglacial channel networks indicative of high-discharge meltwater drainage through the Gulf of Bothnia and central Barents Sea. By tracking the migration of meltwater drainage outlets during deglaciation, we constrain locations most susceptible to focussed discharge, including the western continental shelf-break where subglacial sediment delivery led to the development of major trough mouth fans. Maps of hydraulic potential minima that persist throughout the Late Weichselian reveal potential sites for preserved subglacial lake sediments, thereby defining useful targets for further field investigation.
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