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West Antarctic Ice Sheet collapse

Main Contributors:

Johanna Yletyinen

Other Contributors:

Garry Peterson

Summary

Indication exists for a possible regime shift of collapsed West Antarctic Ice Sheet (WAIS) due to the warming climate. As the atmosphere and oceans warm as a result of global warming, ice sheets are predicted to shrink in size, resulting in raised sea level. The WAIS is a marine ice sheet, surrounded by floating ice shelves with the main part of the sheet below sea-level (Oppenheimer 1998). It is considered to be capable of past and future collapses bringing about several meters sea level rise (Mercer 1978; Oppenheimer & Alley 2004). The two WAIS regimes consist of the intact ice sheet and disintegrated WAIS. The global warming-induced future WAIS collapse could cause a sea level rise of approximately 3-5 meters with significant societal and economic impacts. Marine fauna that is adapted to sea ice dynamics would be directly impacted through habitat changes, food web interaction alterations and shifts in marine isotherms (Rogers et al. 2012; Clarke et al. 2007). Many uncertainties remain about the mechanisms of the WAIS system, drivers of the observed change and future scenarios. It is suggested that the warming of the oceanic deep water currently causes significant basal melting and thinning of the ice sheet. A basin-scale ice model study, published in 2014, provides strong evidence that the collapse has already begun (Joughin et al. 2014.)

Drivers

Key direct drivers

  • Global climate change

Land use

  • Conservation
  • Tourism
  • Land use impacts are primarily off-site (e.g. dead zones)

Impacts

Ecosystem type

  • Polar

Key Ecosystem Processes

  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Fisheries

Regulating services

  • Climate regulation

Cultural services

  • Recreation
  • Knowledge and educational values

Human Well-being

  • Livelihoods and economic activity
  • Social conflict

Key Attributes

Typical spatial scale

  • Sub-continental/regional

Typical time scale

  • Centuries

Reversibility

  • Irreversible (on 100 year time scale)

Evidence

  • Models
  • Paleo-observation
  • Contemporary observations

Confidence: Existence of RS

  • Contested – Reasonable evidence both for and against the existence of RS

Confidence: Mechanism underlying RS

  • Speculative – Mechanisms have been proposed, but little evidence as yet

Links to other regime shifts

Alternate regimes

Intact WAIS (full glacial or modern interglacial)

Antarctica is divided into two large ice sheets by the Transantarctic Mountains. A large part of Antarctic ice mass consists of the WAIS, which lies on a marine sediment-draped bed many hundreds of meters below sea level (Joughin & Alley 2011; Bamber et al. 2009). Compared to the East Antarctic ice sheet, the WAIS has rapid ice flow and discharge (Anderson et al. 2002). Ice shelves, which form over the water and float permanently attached to the landmass, serve as buttresses for the inland ice streams of the ice sheets (permanent layer of ice covering land), and are vulnerable to environmental forcing. Ice volume is stable when the water entering and leaving the system are in balance; the net accumulation (mainly snow) balances losses (ice calving, basal melting at the ice sheet margins) (Ivins 2009). The stability of the marine ice sheet is created between local basal restraint of the inland ice and longitudinal stretching of the ice shelves (Joughin & Alley 2011; Ivins 2009), meaning that the marine-based bottom topography, geometry of the interior part and stability of the buttressing ice shelves (Joughin & Alley 2011) contribute to maintaining the regime of the intact WAIS. The Southern Ocean marine environment generally rich in diversity, whereas the Antarctic terrestrial environment is low in diversity and missing many taxonomic groups (Rogers et al. 2012). In this regime humans have harvested large marine mammals, fish and krill, the latter of which is in direct competition with higher trophic level species (summer breeding colonies) (Rogers et al. 2012). At the present the WAIS appears to be thinning and the extent of sea ice is increasing.

Disintegrated WAIS (extreme interglacial)

Changed internal dynamics, such as complicated ice stream flow changes or natural climatic and oceanic forcing, create instability in the WAIS. Disintegration (collapse) of the WAIS means the nonlinear process, in which the ice sheet would flow at increasing rates into the ocean as its buttressing ice shelves diminish (O'Reilly & ACOS 2013). In the absence of the WAIS, the area would be covered by a broad open sea, punctuated by islands (Scherer 1991). The changes are occurring now and the collapse of the WAIS appears to already be underway, but large uncertainty remains on the timing (Joughin et al. 2014; Joughin & Alley 2011). The absence of the WAIS would affect local ecosystems adapted to the presence of ice, and due to the sea level rise, all global coastal ecosystems and coastal human habitats. Marine fauna adapted to the sea ice dynamics would be directly impacted through habitat changes and shifts in marine isotherms, and the changes could cascade to the higher trophic levels, and alter food webs (Rogers et al. 2012; Clarke et al. 2007). The absence of WAIS would leave broad, deep seaways (Joughin & Alley 2011), increasing accessibility for fishing vessels and tourists.

Drivers and causes of the regime shift

Climatic and oceanic forces appear to cause the WAIS retreats. Geologic evidence shows that the Antarctic ice sheet has shrunk in the past when global temperatures were warmer, for instance during the late Quaternary period (see e.g. Barnes & Hillenbrand 2010). There have been times when parts of the WAIS have been lost to the ocean, causing raised sea levels (Ivins 2009). According to the model by Pollard and DeConto (2009), a collapse from the modern conditions could occur when sub-ice ocean melting increase from 0.1 to 2 m yr-1 under shelf interiors, and from 5 to 10 m yr-1 near exposed shelf edges. The mechanisms leading to the WAIS collapse are still unclear. It has been suggested that the WAIS collapse could start by the intrusion of ocean water (i.e. sea level rise) between the ice sheet and the ground. As the grounding line retreats, a strong positive feedback would be triggered when ocean water undercuts the ice sheet and causes further separation from the bedrock (Oppenheimer 1998). This feedback with sea level changes driving further retreat is contested by other studies (Alley et al. 2007; Gomez et al. 2010). At present, main cause for the thinning of the WAIS ice mass is suggested to be the intrusion of relatively warm ocean water beneath the ice. Ice shelves are very sensitive to ocean temperature changes (Turner & Overland 2009) and several studies (see e.g. Price et al. 2008; Thoma et al. 2008, Joughin et al. 2014) have shown that increased transport of warm subsurface water significantly contributes to the basal ice-shelf melt. The warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs (Bintanja et al. 2013) and produces large melt rates at the regions where it is able to access the sub-ice-shelf cavities (Jacobs et al. 1992). Although the upper layers of Southern Ocean have cooled, the subsurface sea has in fact warmed (Robertson et al. 2002). The reasons for the warming of the Southern Ocean are not completely know. It is probably caused by multiple processes, such as increased greenhouse gases in the atmosphere, shifts in Southern Annular Mode (SAM, a high-latitude mode, also called Antarctic Oscillation) and changes in sea currents.

Other, less direct contributors to the regime shift found in literature are hydrofracturing of crevasses by surface melting water, changes in landscape (glacier behavior), wind forcing, ocean upwelling, tides and long period waves, changes in stratospheric ozone affecting atmospheric and ocean circulation, and intensification and southward migration of the Southern Ocean Westerlies due to the positive trend in the SAM (e.g. Thompson & Solomon 2002; Goosse et al. 2008; Turner et al. 2009; Joughin et al. 2014).

How the regime shift works

In the intact WAIS regime there is variability in the system depending on the natural climate variation, but the rate and scale of the changes in ice mass are not as remarkable as in the second regime, and the ice volume is maintained by equal net balance. The WAIS icescape is at the present characterized by decrease in ice mass and increased sea ice extent. The most recent research indicates that the WAIS is changing significantly and rapidly, and at an accelerating rate (O'Reilly & ACOS 2013; Abram et al. 2013; Joughin & Alley 2011; Thomas et al. 2004; Velicogna & Wahr 2006). West Antarctica is currently warming, and has been stated to be one of the most rapidly warming regions on Earth (Bromwich et al. 2012; Steig et al. 2013).

The possibility for the collapse of the WAIS has been debated in science since the 1970s (Notz 2009). It has been suggested that the WAIS retreats follow glacial and interglacial periods (Clark et al. 2002; Fairbanks 1989; Scherer 1991). The potential of the WAIS to collapse and the mechanisms leading and preventing it are still unclear, but the recent studies argue that the collapse is possible with small changes in the forces (Mercer 1978; Oppenheimer 1998; Vaughan 2008; Schoof 2007; Naish et al. 2009; Pollard & DeConto 2009; Notz 2009). A basin-scale ice-flow model by Joughin et al. (2014) provided strong evidence that the early-stage collapse has already begun. Their model strengthens the argument that the losses are melt-driven and that melt-induced ice-shelf thinning reduces buttressing, creating far greater speedup and retreat through the grounding line retreat. Ice sheet thinning appears to take place through basal melting (i.e. on the underside of the ice shelves) (Pritchard et al. 2012; Joughin & Alley 2011). The highest melting rates occur where the ice shelves interact with the warmest water. Increased atmospheric temperature and ocean temperature (possibly due to several factors such as global climate change, natural climate variation, ozone hole and changes in atmospheric and oceanic circulation) cause intrusion of warmed ocean water beneath the ice shelf of the WAIS. Relatively warm Circumpolar Deep Water (CDW) protrudes into the ice shelves through submarine troughs and produces large melt rates at the ice/ocean interface, decreasing the volume of the ice mass. The estimations of the loss rate vary both regionally and for the whole continent, see for instance King et al. (2012). Although in some regions the ice sheet is thickening, the net balance is negative (e.g. Velicogna & Wahr 2006).

The mass loss of ice is expected to increase both in volume and rate in the future when the warm circumpolar deep water is able to reach further ice shelves (Rignot et al. 2011; Hellmer et al. 2012). Unlike in the Arctic and as an unusual feature in consideration to the global warming, the extent of the Antarctic sea ice has increased in the observed period starting from 1979 (Turner & Overland 2009). Sea-ice expansion may be meltwater-induced (Bintanja et al. 2013). Meltwater from the ice shelves has lower density and thus accumulates in the top ocean layer (Price et al. 2008). This upper layer water gets fresher, and resulting cold halocline reduces the convective mixing, causing the atmosphere to cool and freeze the upper 100 m more easily (Bintanja et al. 2013). The subsurface ocean warming, mass loss of ice due to the basal melt and expanding sea ice may constitute a negative feedback loop (Bintanja et al. 2013; Zhang 2007). The fresh melt water has low density and thus accumulates in the top layer, stabilizing the ocean and resulting in less mixing between cold and warm water (Bintanja et al. 2013). The cold and fresh upper seawater layer is easier to cool by the atmosphere and freezes. This meltwater-induced sea ice expansion hinders the warm deepwater from mixing with the surface water. Another feedback, which is positive but at present contested, has been suggested to be formed by the raised sea level further undercutting the ice sheet and triggering its separation from the bedrock.

Impacts on ecosystem services and human well-being

Humans mainly use the Antarctica for commercial fishing, conservation, research, and tourism. The Antarctic region also plays a remarkable role in the planetary functions. Shifting the regime to that of the fragmented WAIS would affect the regulating ecosystem services of the Antarctica, such as global climate and ocean circulation through ice and snow albedo, CO2 uptake and icescape changes. The Southern Ocean has a rich biodiversity, and fisheries include krill and toothfish. The biogeochemical cycles of the Southern Ocean and the sea ice impact the structure and dynamics of the marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to the presence, seasonality and properties of ice (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010; Clarke et al. 2007; Rogers et al. 2012). Sea level rise may have severe impacts on coastal areas by submergence or increasing flooding and erosion, changing ecosystems, and increasing salinization (Nicholls et al. 2011). Fisheries management and accessibility would change due to the altered sea ice extent (Anon 2009).

The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) and result in forced displacement of coastal population and economy (Nicholls et al. 2011). The economic consequences depend on the time-scale: playing out over a century. The WAIS collapse would damage many coastal communities, whereas occurring over several centruries, the additional time would give possibilities to develop an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011). Increased vessel access to more southern locations may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase due to the extended season with reduced sea ice thickness (Anon 2009). The increased human presence might be a threat to ecosystems, for instance winter ice has usually provided a relief from the fishing pressure.

 

 

 

  

Management options

Two new studies in 2014 state that since the WAIS has reached the early stage collapse phase, the shift is unstoppable (Joughin et al. 2014, NASA in press). Joughin et al. (2014) argue that it is difficult to foresee stabilization of the system unless CDW recedes sufficienty to reduce present level of melting.

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Citation

Johanna Yletyinen, Garry Peterson. West Antarctic Ice Sheet collapse. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-05-11 15:21:00 GMT.
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