Happy New Year 2024 (and sorry for the forum being offline some hours) /DM
Significant changes have taken place on Denman Glacier and Shackleton Ice Shelf, which hold a 149-cm SLE (basin C-C′). Denman sped up 16% since the 1970s and the ice shelf sped up by 33% in 1957–1996 and 43% in 1957–2016. The glacier is 10% out of balance. Its neighbor Scott decelerated by 16% in 1957–1996 and 22% in 2000–2008 and sped up by 18% in 2016.
We find that the glacier grounding line experiences a complex pattern of migration with several kilometers retreat at its center, in contrast to a small retreat of the neighboring glaciers, e.g. Scott glacier. The floating section of the glacier experiences vigorous ice melt in contact with the ocean, which suggests the presence of Circumpolar Deep Water (CDW). However, there is no historical oceanographic data near the glacier. The marked increase in ice shelf velocity observed in recent decades could result from the grounding line migration associated with enhanced ice shelf melt. Alternatively, it can be symptomatic of a complex interaction between the fast-moving glacier tongue (Shackleton Ice Shelf) and the surrounding slower moving ice shelves; similarly, to the case of Stancomb-Wills Ice Shelf or Thwaites Ice Shelf.
Some time ago the Denman Glacier ice tongue broke off but remained embedded in the Shackleton ice shelf. That tongue fragment has recently calved a large part.
Denman Glacier is one of the largest in East Antarctica, with a catchment that contains an ice volume equivalent to 1.5 m of global sea-level and which sits in the Aurora Subglacial Basin (ASB). Geological evidence of this basin’s sensitivity to past warm periods,combined with recent observations showing that Denman’s ice speed is accelerating, and its grounding line is retreating along a retrograde slope, have raised the prospect that it could contribute to near-future sea-level rise.The recent changes in the Denman system are important because Denman’s grounding line447 currently rests on a retrograde slope which extends 50 km into its basin (Morlighem et al.,448 2019; Brancato et al., 2020), suggesting clear potential for marine ice sheet instability. Given449 the large catchment size, it has potential to make globally significant contributions to mean sea450 level rise in the coming decades (1.49 m; Morlighem et al., 2020). Crucial to assessing the451 magnitude of any future sea level contributions is improving our understanding of regional452 oceanography, and determining whether the observed changes at Denman are the consequence453 of a longer-term ocean warming. This is in addition to monitoring and understanding the454 potential impact of any future changes in the complex Shackleton/Denman ice shelf system.455 In a wider context our results add to the growing body of evidence that some major East456 Antarctic outlet glaciers, with multi-meter sea-level equivalent catchments have responded to457 changes in ocean-climate forcing over the past 100 years and, therefore, will be sensitive to458 projected future warming.
Mt Sandow nunatak (alt 1380m) 450km west of Casey Station; almost enveloped by the Denman Glacier flowing from the East Antarctic Ice Sheet to the Shackleton Ice Shelf 150km away. Discovered by Mawson’s Western Base Party led by Frank Wild in 1912, pic Greg Barras @AusAntarctic
AbstractDenman Glacier, which drains a marine-based sector of the East Antarctic Ice Sheet with an ice volume equivalent to 1.5 m of global sea level rise, has accelerated and undergone grounding line retreat in recent decades. A deep trough and retrograde bed slope inward of the grounding line leave this glacier prone to marine ice sheet instability. The ocean heat flux to the ice shelf cavity is a critical factor determining the susceptibility of the glacier to unstable retreat. Profiling float observations show modified Circumpolar Deep Water as warm as −0.16°C reaches a deep trough extending beneath the Denman Ice Tongue. The ocean heat transport (0.77 ± 0.35 TW) is sufficient to drive high rates of basal melt (70.8 ± 31.5 Gt y−1), consistent with rates inferred from glaciological observations. These results suggest the Denman Glacier is potentially at risk of unstable retreat triggered by transport of warm water to the ice shelf cavity.Plain Language SummaryThe Denman Glacier is a vast river of ice that drains the East Antarctic Ice Sheet. The Denman holds a volume of ice equivalent to 1.5 m of global sea level rise, so changes in the glacier could have a large impact on future sea level rise. The vulnerability of the Denman Glacier to melting by warm ocean waters has been difficult to assess because very few oceanographic observations have been collected in the region. We use new profiling float measurements to show warm water reaches a deep trough that extends inland beneath the glacier, exposing the base of the ice to ocean-driven melting. We estimate that the amount of warm water entering the cavity is sufficient to melt 70.8 billion tons of ice each year. These observations suggest that the Denman Glacier is potentially at risk from unstable retreat driven by warm water flowing into the cavity and melting the ice from below.4.Discussion and Conclusions....................... The observation of warm water in the deep trough adjacent to the DIT is reminiscent of the Totten Ice Shelf, where strong inflow of warm water was found to carry sufficient heat transport to explain high rates of basal melt inferred from satellite data (Rintoul et al., 2016). The similarity between the two systems extends further. The inflow of water warmer than −1.0°C is smaller at Denman (138 ± 65 mSv) than at Totten (220 ± 70 mSv), but the transport-weighted temperature is higher (−0.48°C at Denman vs. −0.81°C at Totten), resulting in ocean heat transports and meltwater production rates that agree within error bars (ocean heat flux sufficient to form 2.3 ± 1.0 mSv of meltwater at Denman, compared to 2.8 ± 0.9 mSv at Totten). The float measurements confirm that the Denman Glacier, like the Totten Glacier, is exposed to ocean heat transport sufficient to drive high rates of basal melt (70.8 ± 31.5 Gt y−1), as inferred from independent glaciological observations.........................Estimates of the melt rate near the grounding line of the Denman Glacier are higher than for other glaciers in East Antarctica and rival melt rates of rapidly thinning glaciers in West Antarctica. Melt rates at the grounding line of 45 ± 4 m a−1 between 2011 and 2014 (Brancato et al., 2020) and 50 m a−1 between 2010 and 2018 (Liang et al., 2021) imply strong ocean thermal forcing. ..............................Glaciological observations have documented a long-term acceleration of the Denman Glacier. For exam-ple, Rignot et al. (2019) estimated a 16% increase in flow speed of the glacier since the 1970's, while Miles et al. (2021) found accelerations of 17 ± 4% (grounded portion) and 36 ± 5% (floating portion) between 1972 and 2017. For the recent short period between 2017 and 2021, no acceleration was found (Thompson et al., 2021). The grounding line of the Denman Glacier retreated by 5.4 ± 0.3 km between 1996 and 2017–2018 (Brancato et al., 2020). The changes in glacial flow may reflect an increase in ocean heat transport to the base of the glacier and/or ice dynamics (i.e., ice tongue thinning, changes in ice tongue structure following calving, or release from pinning points) (Miles et al., 2021; Rignot et al., 2019). While changes in ocean heat transport cannot be assessed with available observations, recent studies have documented warming of waters off East Antarctica that may have increased ocean heat transport to the continental shelf (e.g., Herraiz-Borreguero & Naveira Garabato, 2022; Yamazaki et al., 2021).
This is a story about a curious seal, a wayward robot and a gigantic climate change disaster that may be waiting to happen.
