Greenland meltwater storage in firn limited by near-surface ice formation

Machguth, Horst; MacFerrin, Michael; As, Dirk van; Box, Jason E.; Charalampidis, Charalampos; Colgan, William; Fausto, Robert S.; Meijer, H. A. J.; Mosley‐Thompson, Ellen; Wal, R. S. W. van de

doi:10.1038/nclimate2899

Cited by 212 publications

(325 citation statements)

References 32 publications

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“…The PDD melt potential is related to surface melt through a coefficient of 0.005 m of melt per degree day (Pollard and DeConto, 2012a). Although more complex schemes are often used, taking into account refreezing of percolating meltwater in the snow pack and melting of superimposed ice with different melt coefficients (Huybrechts and de Wolde, 1999), which is also confirmed by recent observations (Machguth et al, 2016), surface melt is rather limited for the present-day Antarctic ice sheet. Surface mass balance is then the sum of the different components, i.e.ȧ = P − S, where S = 0.005× PDD is the surface melt rate.…”

Section: Atmospheric and Ocean Forcingsupporting

confidence: 52%

Sea-level response to melting of Antarctic ice shelves on multi-centennial timescales with the fast Elementary Thermomechanical Ice Sheet model (f.ETISh v1.0)

2017

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Abstract. The magnitude of the Antarctic ice sheet's contribution to global sea-level rise is dominated by the potential of its marine sectors to become unstable and collapse as a response to ocean (and atmospheric) forcing. This paper presents Antarctic sea-level response to sudden atmospheric and oceanic forcings on multi-centennial timescales with the newly developed fast Elementary Thermomechanical Ice Sheet (f.ETISh) model. The f.ETISh model is a vertically integrated hybrid ice sheet-ice shelf model with vertically integrated thermomechanical coupling, making the model twodimensional. Its marine boundary is represented by two different flux conditions, coherent with power-law basal sliding and Coulomb basal friction. The model has been compared to existing benchmarks.Modelled Antarctic ice sheet response to forcing is dominated by sub-ice shelf melt and the sensitivity is highly dependent on basal conditions at the grounding line. Coulomb friction in the grounding-line transition zone leads to significantly higher mass loss in both West and East Antarctica on centennial timescales, leading to 1.5 m sea-level rise after 500 years for a limited melt scenario of 10 m a −1 under freely floating ice shelves, up to 6 m for a 50 m a −1 scenario. The higher sensitivity is attributed to higher ice fluxes at the grounding line due to vanishing effective pressure.Removing the ice shelves altogether results in a disintegration of the West Antarctic ice sheet and (partially) marine basins in East Antarctica. After 500 years, this leads to a 5 m and a 16 m sea-level rise for the power-law basal sliding and Coulomb friction conditions at the grounding line, respectively. The latter value agrees with simulations by DeConto and Pollard (2016) over a similar period (but with different forcing and including processes of hydrofracturing and cliff failure).The chosen parametrizations make model results largely independent of spatial resolution so that f.ETISh can potentially be integrated in large-scale Earth system models.

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Section: Atmospheric and Ocean Forcingsupporting

confidence: 52%

Sea-level response to melting of Antarctic ice shelves on multi-centennial timescales with the fast Elementary Thermomechanical Ice Sheet model (f.ETISh v1.0)

2017

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“…These changes are highly significant considering that typical values for total firn air content in the dry snow zone range between 20 and 25 m . Recent research has shown that warm summers can generate thick ice layers that prevent meltwater from reaching the deeper pores, further reducing the meltwater buffering capacity (De la Peña et al, 2015;Machguth et al, 2016).…”

Section: Discussionmentioning

confidence: 99%

“…This can be achieved through statistical/dynamical downscaling in combination with targeted in situ observations. Examples of important processes that are poorly or not at all represented in current models are interactive snow/ice darkening by future enhanced dust/black carbon deposition or microbiological processes (Stibal et al, 2012), and sub-, supra-and englacial hydrology, including vertical and horizontal flow of meltwater in firn or over ice lenses (De la Peña et al, 2015;Machguth et al, 2016). Other emerging research topics of GrIS melt climate are the impact of atmospheric circulation changes on Greenland melt (Hanna et al, 2013a(Hanna et al, , 2014McLeod and Mote, 2016;Tedesco et al, 2013), the impact of rain on ice sheet motion (Doyle et al, 2015), the effect of liquid water clouds on the surface energy balance and melt (Bennartz et al, 2013;Van Tricht et al, 2016) and the increased role of turbulent heat exchange during strong melting episodes over the margins of the GrIS .…”

Section: Discussionmentioning

confidence: 99%

“…During 1991-2015, the refrozen fraction averaged 41 %, down from 44 % in 1961-1990, signalling a decrease in the retention efficiency of the GrIS firn layer. In RACMO2.3 this is mainly caused by a decrease in firn pore space , but in reality this effect is exacerbated by the formation of impenetrable ice lenses during warm summers, preventing the surface meltwater from reaching the deeper firn layers and using the full retention potential (De la Peña et al, 2015;Machguth et al, 2016). Figure 4 clearly demonstrates the large interannual variability of the major GrIS SMB components P tot , ME and RU.…”

Section: Temporal Smb Variabilitymentioning

confidence: 99%

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On the recent contribution of the Greenland ice sheet to sea level change

et al. 2016

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Abstract. We assess the recent contribution of the Greenland ice sheet (GrIS) to sea level change. We use the mass budget method, which quantifies ice sheet mass balance (MB) as the difference between surface mass balance (SMB) and solid ice discharge across the grounding line (D). A comparison with independent gravity change observations from GRACE shows good agreement for the overlapping period 2002-2015, giving confidence in the partitioning of recent GrIS mass changes. The estimated 1995 value of D and the 1958-1995 average value of SMB are similar at 411 and 418 Gt yr −1 , respectively, suggesting that ice flow in the mid1990s was well adjusted to the average annual mass input, reminiscent of an ice sheet in approximate balance. Starting in the early to mid-1990s, SMB decreased while D increased, leading to quasi-persistent negative MB. About 60 % of the associated mass loss since 1991 is caused by changes in SMB and the remainder by D. The decrease in SMB is fully driven by an increase in surface melt and subsequent meltwater runoff, which is slightly compensated by a small (< 3 %) increase in snowfall. The excess runoff originates from lowlying (< 2000 m a.s.l.) parts of the ice sheet; higher up, increased refreezing prevents runoff of meltwater from occurring, at the expense of increased firn temperatures and depleted pore space. With a 1991-2015 average annual mass loss of ∼ 0.47 ± 0.23 mm sea level equivalent (SLE) and a peak contribution of 1.2 mm SLE in 2012, the GrIS has recently become a major source of global mean sea level rise.

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“…Density peaks have been shown to vary in depth along ice-core transects, likely due to smallscale microstructure differences (e.g. Machguth et al, 2016).…”

Section: Deriving Accumulation Rates From Snow Radar and Uncertaintiesmentioning

confidence: 99%

Annual Greenland accumulation rates (2009–2012) from airborne snow radar

et al. 2016

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Abstract. Contemporary climate warming over the Arctic is accelerating mass loss from the Greenland Ice Sheet through increasing surface melt, emphasizing the need to closely monitor its surface mass balance in order to improve sea-level rise predictions. Snow accumulation is the largest component of the ice sheet's surface mass balance, but in situ observations thereof are inherently sparse and models are difficult to evaluate at large scales. Here, we quantify recent Greenland accumulation rates using ultra-wideband (2-6.5 GHz) airborne snow radar data collected as part of NASA's Operation IceBridge between 2009 and 2012. We use a semiautomated method to trace the observed radiostratigraphy and then derive annual net accumulation rates for 2009-2012. The uncertainty in these radar-derived accumulation rates is on average 14 %. A comparison of the radarderived accumulation rates and contemporaneous ice cores shows that snow radar captures both the annual and longterm mean accumulation rate accurately. A comparison with outputs from a regional climate model (MAR) shows that this model matches radar-derived accumulation rates in the ice sheet interior but produces higher values over southeastern Greenland. Our results demonstrate that snow radar can efficiently and accurately map patterns of snow accumulation across an ice sheet and that it is valuable for evaluating the accuracy of surface mass balance models.

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Greenland meltwater storage in firn limited by near-surface ice formation

Cited by 212 publications

References 32 publications

Sea-level response to melting of Antarctic ice shelves on multi-centennial timescales with the fast Elementary Thermomechanical Ice Sheet model (f.ETISh v1.0)

Sea-level response to melting of Antarctic ice shelves on multi-centennial timescales with the fast Elementary Thermomechanical Ice Sheet model (f.ETISh v1.0)

On the recent contribution of the Greenland ice sheet to sea level change

Annual Greenland accumulation rates (2009–2012) from airborne snow radar

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