Further to my prior Reply #1188 relating the Jakobshavn to the Byrd response to ice-bed uncoupling (including from an influx of basal meltwater), Hughes et al 2016 present a state-of-the-art discussion of such matters on the progression/acceleration of marine glacial flow/retreat with continued global warming. The authors also relate their findings to other critical marine glaciers in both Greenland and Antarctica. The two attached images relate ice-bed uncoupling for Jakobshavn and to the Jakobshavn Effect, respectively.
Hughes, T., Sargent, A., Fastook, J., Purdon, K., Li, J., Yan, J.-B., and Gogineni, S.: Sheet, stream, and shelf flow as progressive ice-bed uncoupling: Byrd Glacier, Antarctica and Jakobshavn Isbrae, Greenland, The Cryosphere, 10, 193-225, doi:10.5194/tc-10-193-2016, 2016
http://www.the-cryosphere.net/10/193/2016/Abstract. The first-order control of ice thickness and height above sea level is linked to the decreasing strength of ice-bed coupling along flowlines from an interior ice divide to the calving front of an ice shelf. Uncoupling progresses as a frozen bed progressively thaws for sheet flow, as a thawed bed is progressively drowned for stream flow, and as lateral and/or local grounding vanish for shelf flow. This can reduce ice thicknesses by 90 % and ice elevations by 99 % along flowlines. Original work presented here includes (1) replacing flow and sliding laws for sheet flow with upper and lower yield stresses for creep in cold overlying ice and basal ice sliding over deforming till, respectively, (2) replacing integrating the Navier–Stokes equations for stream flow with geometrical solutions to the force balance, and (3) including resistance to shelf flow caused by lateral confinement in a fjord and local grounding at ice rumples and ice rises. A comparison is made between our approach and two approaches based on continuum mechanics. Applications are made to Byrd Glacier in Antarctica and Jakobshavn Isbrae in Greenland.
Our ice-sheet modeling approach is based on the first-order dependence of ice-sheet thickness on the strength of ice-bed coupling, such that ice 3000m high and 4000m thick at an interior ice divide can lower to 100m high and 1000m thick when ice margins become afloat, and lower further to 30m high and 300m thick at the front of calving ice shelves, a 99% reduction of ice elevations, all due to reduced ice-bed coupling. We began by quantifying ice-bed uncoupling as an increase in thawed fraction f of the bed for sheet flow, of floating fraction phi of ice for stream flow, and of unbuttressed fraction phiO of ice for shelf flow. Our treatment is holistic in the sense it provides smooth transitions from sheet flow to stream flow to shelf flow for steady-state conditions along surface flowlines.
We compared our treatment for ice sheets with two treatments based on continuum mechanics, one by Schoof and
Hindmarsh (2010) and one by Pattyn (2003). All three treatments avoided flow “laws” and sliding “laws” of dubious reliability for sheet flow. We substituted respective upper and lower yield stresses applied to cold ice over a frozen bed and to temperate ice sliding over bedrock and/or deforming till for sheet flow, with cold ice above temperate basal ice in ice streams and ice shelves. Schoof and Hindmarsh (2010) introduced “slip” and “no-slip” interfaces at the bed linked to separate deviator stress tensors that can be applied to sheet, stream, and shelf flow. Pattyn (2003) reduced basal drag as a frozen bed thaws. His approach can also be applied to sheet, stream, and shelf flow.
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Our results for both Byrd Glacier and Jakobshavn Isbrae are compatible with basal buoyancy fraction phiB = phiphiO in Table 2 used to quantify a hypothetical “life cycle” of ice streams. The product of fraction phi linked to ice-bed uncoupling and fraction phiO linked to ice-shelf unbuttressing is maximized when surface meltwater floods the bed under an ice stream, and when its buttressing ice shelf shelf disintegrates. Hughes (1986) postulated these two processes, augmented by other processes, are sufficient to collapse marine portions of an ice sheet, and to that extent contribute to Termination of glaciation cycles lasting approximately 90 000 years during the Quaternary Ice Age in which we now live. He called this the Jakobshavn Effect because all the processes were active for Jakobshavn Isbrae. Contributing processes include additional surface melting when crevasses are ubiquitous, analyzed by Pfeffer and Bretherton (1987), warm ocean water entering Jakobshavn Isfjord, reported by Holland
et al. (2008) and restricted flow of outlet glaciers in curving and branching fjords like Jakobsahvn Isfjord (Pfeffer
et al., 2008).
We conclude the Jakobshavn Effect may have a long-term impact in Greenland if global warming allows these processes to migrate northward, causing successive ice streams to surge, thereby completing their life cycles. Some processes are already appearing in ice streams draining the east, west, and northwest parts of the Greenland Ice Sheet (Rignot and Kanagaratnam, 2006). Schoof (2010) shows how ongoing acceleration and thinning of Jakobshavn Isbrae reported by Joughin et al. (2014) could continue for a century. Various paths can be taken by phi and phiO in Table 2 during a life cycle, including reversals, as documented by Engelhardt and Kamb (2013) for Kamb Ice Stream. Hughes (2011) used Table 2 to determine where major Antarctic ice streams are in their life cycles today. When the Jakobshavn Effect is nearly simultaneous for many ice streams, Table 2 can be used to identify stadials and interstadials within Quaternary glaciation cycles, and to account for Terminations of cycles, all linked to global sea level (Denton et al., 1986) and the Jakobshavn Effect.
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These ice streams bracket ice-bed uncoupling ranging from no surface meltwater lubricating the bed and a massive buttressing ice shelf for Byrd Glacier to massive surface meltwater lubricating the bed and an ice shelf that has recently disintegrated for Jakobshavn Isbrae.
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Today, Byrd Glacier has low values of phi and phiO, but both values are substantially higher for Jakobshavn Isbrae. For Byrd Glacier we temporarily increased phi when two subglacial lakes at its head drained rapidly in 2006–2007 (Stearns and others, 2008). For Jakobshavn Isbrae, we set phiO = 1 when its buttressing ice shelf suddenly disintegrated in 2002 (Thomas, 2004).
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Warming in high polar latitudes can, in principle, trigger a succession of positive feedback mechanisms called
the Jakobshavn Effect (Hughes, 1986). Buoyancy fraction phiB combines the two dominant mechanisms: reduced icebed coupling when surface meltwater floods the bed under an ice stream and reduced ice-shelf buttressing when an ice shelf disintegrates beyond the ice stream. For Greenland, the Jakobshavn Effect would move northward along the east and west coasts, affecting all calving ice streams. For Antarctica, it would affect the northernmost ice streams, which are primarily in East Antarctica, but also ice streams entering the Pine Island Bay polynya in West Antarctica (Hughes, 1987, 2011; Pingree et al., 2011)."
