Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctic Ice Sheet Mass Balance – 4 Decades 1979-2017

Rignot et al. used updated drainage inventory, thickness of the ice, and ice velocity data to calculate the discharge at the grounding line of 176 basins that drain into the Antarctic Ice Sheet from 1979 to 2017. They compared their results with a surface mass balance model in order to deduce the ice sheet mass balance. They found that the total mass loss increased from 40 ± 9Gt/y in 1979-1990 to 50 ± 14 Gt/y in 1989-2000, 166 ± 18 Gt/y in 1999-2009, and 252 ± 26 Gt/y in 2009-2017. In 2009-2017 the Amundsen/Bellingshausen Sea Sectors, West Antarctica, dominated the mass loss (159 ± 8 Gt/y), Wilkes Land, East Antarctica (51 ± 13 Gt/y), and West and Northeast Antarctic Peninsula (42 ± 5 Gt/y). The contribution from Antarctica to sea level rise averaged 3.6 ± 0.5 mm per decade with a cumulative 14 ± 2.0 mm per decade since 1979, which included 6.9 ± 0.6 mm from West Antarctica, 4.4 ± 0.9 mm from East Antarctica, and 2.5 ± 0.4 mm from the Antarctic Peninsula (i.e., East Antarctica is a major participant in the mass loss). The mass loss during the entire period was concentrated in areas that are closest to warm, salty, subsurface, circumpolar deep water (CDW), which is consistent with enhanced polar westerlies pushing CDW towards Antarctica where it melts the floating ice shelves, destabilises the glaciers, and raises sea level.

The total Antarctic ice volume translates into a sea level equivalent (SLE) of 57.2 m (Fretwell et al., 2013). The annual input of mass from snowfall is 2,100 Gt/y, which excludes ice shelves, equivalent to 5.8 mm fluctuation in global sea level (Wessem et al., 2013). The accumulation of snowfall in the interior should balance surface ablation (wind transport and sublimation) and the discharge of ice along the edge of the Southern Ocean, in a state of mass balance. Almost half of land ice that crosses the grounding line and discharges into the ocean to form the floating ice shelves melts when it makes contact with the ocean, the remaining half breaking up and detaches in the form of icebergs (Rignot et al., 2013; Liu et al., 2015).

It has been shown by recent observations that there is mass loss into the ocean of mass along the periphery as a result of enhanced flow of its glaciers, at a rate that has been increasing over time, though there is no long-term change in the accumulation of snowfall over time in the interior, i.e., Antarctica contributes to sea level rise (SLR) mainly by changes in the dynamics of ice (Van de Berg, 2006; Velicogna, Sutterley & van den Broeke, 2014; Rignot et al., 2008). Estimation of mass balance of the ice sheet has been by various techniques:

i)                   The component method, which involves comparing the accumulation of snowfall over the interior basins with the discharge by glaciers of ice across the grounding line, where the ice begins to float on the ocean and detaches from the bed, at a high resolution of 100 m to 1 km;

ii)                The altimetry method, which measures the elevation changes of the ice over the entire ice sheet and converts them into mass changes by the assumption of a change in density at intermediate resolution (1 to 10 km);

iii)              The gravity method, which measures directly the relative change in mass on a monthly basis, within centimetres per year, though at low resolution (333 km).

The techniques have been compared (Rignot et al., 2011; Shepherd et al., 2012) and found to yield reconciled numbers for assessments across the ice sheet for the time periods 1992-2011 and 1992-2017, with the exception of East Antarctica, where the uncertainties that remain.

In this paper Rignot et al. present the results from the component method updated to 2017, or 4 decades of observations. They used improved annual time series of the velocity of the ice sheet, updated the thickness of the ice, modelled reconstruction of surface mass balance (SMB), revised drainage inventories, and high resolution of topography in order to assess the continental discharge of ice of 18 regions that include a total of 176 basins, plus the surrounding islands. The entire period of reconstruction of the surface mass balance by regional climate models is covered by the period of the study. They derived the mass balance of the ice sheet for 1979-2017, the acceleration of the mass loss of ice on a decadal time scale, the partitioning of the processes for the surface mass balance and ice dynamics, the contribution made by various regions to the total mass budget, and the implications of the results for the future contribution of Antarctica to SLR.

Antarctic Peninsula

In Basin I-I”, In West Graham Land glaciers the ice discharge rate increased by 12% in 1993-2003 (Pritchard & Vaughan, 2007) and 8% in 2003-2016. In 1973-1993 Rignot could not quantify the loss though the glaciers were retreating at that time (Cook et al., 2016). In East Graham Land, uncertainty in thickness, speed and SMB to estimate mass balance, though the balance discharge is only 3.3 Gt/y, therefore it is likely mass loss is to be less than that. The northern Larsen A glaciers, located further to the south lost 1 Gt/y following the collapse of the ice shelf in 1995, though only after a short period (Seehaus et al., 2001). Contrasting with this, the mass loss of 4 Gt/y of the Drygalski Glacier has continued from 1995 to the present. The Larsen B glaciers accelerated until 2002 when the ice shelf collapsed and were still losing 8.4 Gt/y in 2017 (Rignot et al., 2004; Rott et al., 2014). It was estimated by Rignot et al. that there was a small dynamic loss from the Larsen C glaciers of 1 Gt/y and negligible loss from Larsen D-G glaciers in basin I”-J.

In the west, the glaciers that feed the Wordie Ice Shelf (1.3-cm SLE) lost 1 to 2 Gt/y in 1979-2003, which increased to 8 Gt/y in 2017, together with an acceleration that was detected over the entire drainage. There is a small loss from the Wilkins Ice shelf as the glaciers melt completely at their grounding line and do not increase in speed (Padman et al., 2012).

Discussion

The Amundsen Sea Embayment and the Bellingshausen Sea sectors in west Antarctica, Wilks Land in East Antarctica, and the western Antarctic Peninsula and Larsen A and B sectors, dominate the mass loss of Antarctica. The glacier changes are widespread and synchronous in the Amundsen Sea Embayment and the Bellingshausen Sea. They have been attributed to the intrusion of warm, salty, circumpolar deep water (CDW) on the continental shelf (Jenkins et al., 2016; Alley et al., 2015), which melts the ice shelves vigorously, reduces the buttressing of the glaciers allowing them to flow faster. On the Western Peninsula the presence of circumpolar deep water has been documented (Moffat, Owens & Beardsley, 2009; Martinson & McKee, 2012), Amundsen Sea Embayment (Jacobs et al., 2013) and the Bellingshausen Sea Embayment (Zhang Jacobs et al., 2013) and Getz (Wahlin et al., 2010) but not on Sulzberger and Ross (Jacobs et al., 2013), which exhibit low melt of the ice shelf and no mass loss. Rignot et al. suggest that the western limit of the influence of the circumpolar deep water on ice shelves in West Antarctica, as it experiences mass balance of near zero and melt rates of ice shelves that are low. In front of the Nickerson Ice Shelf the sea floor is shallower than further to the east and must block the access of warm circumpolar deep water. Similarly, no evidence has been found of the presence of Circumpolar Deep Water in the Larsen C-G sectors (Nicholls, Makinson & Venables, 2012) and mass loss was only small.

In Amundsen Sea Embayment, Bellingshausen Sea, Wilkes and Western Peninsula the mass loss has been increasing since the 1970s. According to Rignot et al. this evidence is consistent with the polar contraction of the westerlies that force more Circumpolar deep water onto the continental shelf by Ekman transport, which reaches the glaciers through deep troughs that have been carved on the sea floor by former ice streams (Spence Jacobs et al., 2013, 2017), melts the ice shelves, and destabilises the glaciers. In the Amundsen Sea Embayment decadal oscillations modulate the transfer of ocean heat and subsequent loss from the glacier, which explains the higher loss in 2002-2009 which was followed by lower loss in 2010-2016 (Greene Jacobs et al., 2017; Dutrieux et al., 2014). Rignot et al. found, however, that in the 1970s the ASE was in near balance, which contradicts the hypothesis that in the 1940s an instability developed that lasted until the 1970s (Smith et al., 2017). They concluded that the rapid loss that has occurred recently is unique over the last several decades.

The low ice shelf melt rates and limited mass loss of the glaciers of the Abbot Ice Shelf are explained by the finding that it floats on the seafloor which is above the depth of the CDW, >400-700 m (Cochran et al., 2014), whereas the high loss of Fox and Ferrigno is consistent with efficient transport of CDW through the Belgica Trough (Bingham et al., 2012). Similarly, it was found by Rignot et al. that the largest loss and acceleration was occurring on the western sector of Getz, which is more exposed to incoming CDW than the eastern sector (Jacobs et al., 2013). In this sector the high mass losses are the result of CDW that is nearly undiluted, manifest with high ice shelf melt rates (Rignot et al., 2013). Areas that are the farthest from CDW, such as the Ross and Filchner Ice Shelves, conversely, are stable, exhibiting no loss. There are a number of areas that are potentially exposed to CDW that do not melt rapidly, e.g., Wilkins and Nickerson, as was stated earlier, most likely because the depth of the seafloor is too shallow to allow the access of warm CDW to grounding lines.

The mass balance numbers of Rignot et al. are within errors of the IMBIE-2 multisensor assessment for the years

1992-2017 (Shepherd et al., 2018) for West Antarctica (-83.7 ± 8 Gt/y) versus the Peninsula (28.3 ± 1 Gt/y) versus (-20 ± 15 Gt/y),

though the overall losses are higher for Antarctica (168.9 ± 5 Gt/y for 1992-2017 versus 109 ± 56 Gt/y)

because it was reported by Rignot et al. there was a loss for East Antarctica (-57.0 ± 2 Gt/y) versus a gain with a large uncertainty in the IMBIE-2 assessment (+5 ± 46 Gt/y).

The estimate by Rignot et al. is affected by uncertainties in the thickness of the ice and Surface Mass Balance (SMB), though when estimating the decadal trends the errors are low. According to Rignot et al. uncertainties would be reduced further by improved Surface Mass Balance models and additional ice thickness data in East Antarctica. Large uncertainties in translating changes in volume into mass changes, especially in East Antarctica does, however, affect the IMBIE-2 altimetry estimate. According to Rignot et al. decades of altimetry data will be required to detect changes in surface mass balance at the 10% level (5cm/decade), with accumulation at the level at 5 cm/y on the high plateau (basin B-C). Similarly, residual uncertainties in correction for the glacial isostatic adjustment (GIA), will affect the IMBIE-2 estimate of gravity, especially in East Antarctica. Revisions of the GIA correction of the order of 10-50 Gt/y would be enough to reconcile the gravity Recovery and Climate Experiment results with the mass balance numbers of Rignot et al.

The dynamic loss from Wilkes Land, East Antarctica, is an emerging result. The study detected a mass loss in the last few years (Shen et al., 2018; Gardner et al., 2017), as well as over the entire period, with even higher losses occurring in the 1980s. The degradation of major ice shelves during that time period corroborates this evolution: In the 1970s Cook lost half of its ice shelf (Miles, Stokes & Jamieson, 2017), in years of low sea ice Frost/Holmes disintegrate regularly (Miles, Stokes & Jamieson, 2017), and Conger/Glenzer, Shackleton and West ice shelves underwent large retreats between 1960 and the early 1980s (Young & Gibson, 2007) that were not compensated by a readvance over subsequent decades.

Rignot et al. had incomplete information of the presence of CDW in this sector of East Antarctica (Silvano et al., 2016). It was found by a recent survey that modified CDW was present in front of the Totten Glacier (Rintoul et al., 2016). A prograde slope for the first 50 m protects the Totten Glacier from rapid retreat into a deep marine basin (Li et al., 2016) and is therefore regarded as being at low risk of developing a marine instability in the near future. Contrasting with this, the Denman Glacier is grounded on a ridge with a steep retrograde slope immediately upstream. There are no oceanographic data near the glacier, though the ice shelf experiences high melt rates, which suggest the presence of modified CDW. Enhanced intrusion of CDW may be revealed by the recent acceleration of the ice shelf and an increase in ice shelf melt or may result from complex interactions between the portion of the Shackleton Ice Shelf that is moving fast and the surrounding ice shelf that is moving more slowly, as for the Stancomb-Wills Glacier (Khazendar, Rignot & Larour, 2009).

To the east of the Denman Glacier, troughs that are occupied by Ninnis and Cook Glaciers have the most potential for rapid retreat if the glaciers can be accessed by CDW. The West Ice Shelf, to the west of the Denman Glacier, appears to be stable, or possibly changing only slowly, which suggests that CDW does not have easy access to the cavity, though the bathymetry beneath and in front of the ice shelf is not known.

In sum, the northern sector of West Antarctica is rapidly losing mass and has the potential to entrain the progressive collapse of a large share of West Antarctica and its 5.1 m Sea Level Equivalent (SLE). The loss of ice is 2-3 times slower in Wilkes Land, East Antarctica, though this sector holds an equally large, multi-metre SLE.

The cumulative contribution to sea level from East Antarctica is not far behind that of West Antarctica over the last 4 decades, i.e., East Antarctica is a major participant in the mass loss from Antarctica in spite of the recent rapid mass loss from West Antarctica. Observations of Rignot et al. challenge the traditional view that the East Antarctica Ice Sheet is stable, and therefore immune to change. That closer attention should be paid to East Antarctica is an immediate consequence.

Rignot et al. suggest that in future decades it is likely that SLR will come from sea level rise from Antarctica will originate from the same general areas, which have been found to be nearest to the sources of warm circumpolar deep water and therefore sensitive directly to strengthening and retreat of polar westerlies towards Antarctica that bring more circumpolar deep water in contact with the glaciers. The glaciers will feel less resistance to flow, accelerate, and contribute to sea level rise, as the rate of ice shelf melt increases.

Conclusions

Rignot et al. used improved mapping of thickness, and time series of velocity and surface mass balance to present 4 decades of mass balance in Antarctica, that revealed a mass loss over the entire period and in parts of Antarctica closest to known or suspected sources of circumpolar deep water from observations of high ice shelf melt rates, ocean temperature, or based on ocean model output products. According to Rignot et al. this evolution of glaciers and surrounding ice shelves is consistent with a strengthening of the westerlies that results from a rise of greenhouse gases and depletion of ozone that bring more circumpolar deep water onto the continental shelf. Rignot et al. noted that the Wilkes Land sector of East Antarctica has been a major contributor to sea level rise over the last 40 years, with larger losses in the 1980s, while the mass loss from the Peninsula and West Antarctica have been well documented and reported elsewhere. These sectors are all close to circumpolar deep water and undergo high ice shelf melt rates. They posit that as the enhanced intrusion of circumpolar deep water is the root cause of the mass loss of the Amundsen Sea Embayment and the West Peninsula, a similar situation is occurring in Wilkes Land, were data that are novel and sustained oceanographic data are critically needed. It is suggested by mass balance surveys combined with the prior mass balance assessment of Rignot et al. that the sector between the Cook/Ninnis Glaciers and West ice shelves may be exposed to circumpolar deep water and could contribute to multi-metre sea level rise with climate warming that is unabated.

Sources & Further reading

Rignot, E., et al. (2019). "Four decades of Antarctic Ice Sheet mass balance from 1979–2017." Proceedings of the National Academy of Sciences 116(4): 1095-1103.