Ice albedo feedback (global, well-established). Ice albedo is a strong, positive feedback. The ice cover is more reflective than the ocean or continental surface, resulting in the increase of ice producing further cooling. In contrast, reduced ice coverage, caused by higher surface temperatures, increases insolation.
Ice elevation feedback (local, well-established). The thickness of the ice sheet creates a positive feedback with the altitude-based temperature change. As the ice sheet begins to grow, the accumulation rate can be increased as the elevation of the ice field increases and the atmospheric temperature therefore decreases. This feedback, however, takes place on a long time scale as thousands of years may be required for a kilometer growth in the ice sheet.
Disintegrated WAIS (transition phase)
Sea level rise (regional, contested). A strong positive feedback, which is at present contested, is suggested to form by the higher sea level ocean water, which would further undercut the ice sheet and trigger its separation from the bedrock as the grounding line retreats ( Michael Oppenheimer 1998).
Sea-ice / ocean feedback (regional, speculative). A suggested by Bintanja et al. (2013), negative feedback for increase in the sea ice states that the warm CDW protruding onto continental shelves causes basal melt, in particular in the ice shelves which are very sensitive to ocean temperatures. The melt water has lower density than seawater, and thus accumulates in the top layer of the ocean. Upper layer gets fresher and cold halocline stabilizes the ocean, resulting in less mixing between the cold and warm water. The atmosphere can cool and freeze the fresh and cold water more easily, which results in increased sea ice. The negative feedback is therefore created by the cool and fresh surface water from the ice-shelf melt to shield the surface water from the warmer, deeper waters that cause melting of the ice shelves. In this mechanism the sea-ice induced, upper ocean density changes increase the ocean heat flux available to melt sea ice (Zhang 2007).
Ice albedo feedback (global, well-established). Ice albedo is a strong, positive feedback. The ice cover is more reflective than the ocean or continental surface, resulting in the increase of ice producing further cooling. In contrast, reduced ice coverage, caused by higher surface temperatures, increases insolation. Ice elevation feedback (local, well-established). The thickness of the ice sheet creates a positive feedback with the altitude-based temperature change. As the ice sheet begins to grow, the accumulation rate can be increased as the elevation of the ice field increases and the atmospheric temperature therefore decreases. This feedback, however, takes place on a long time scale as thousands of years may be required for a kilometer growth in the ice sheet.
The main external direct drivers that contribute to the shift include:
Oceanic warming: the intrusion of warm ocean water beneath the ice sheet (regional, proposed). The highest melting rates occur where the ice shelves interact with the warmest water (ocean thermal forcing) (Pritchard et al. 2012). 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) have shown that warm subsurface water significantly contributes to the basal ice-shelf melt. Although the upper layers of the Southern Ocean have cooled, the subsurface Southern Ocean has in fact warmed faster than any other part of the world oceans (Robertson et al. 2002). Warming of the CDW causes basal ice-shelf melt, its effectivity depending on how well it can reach the ice shelves (Joughin & Alley 2011). The warm 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). The reasons for the warming of the Southern Ocean are not completely know, but multiple processes, such as increased greenhouse gases in the atmosphere, shift in SAM and changes in currents are suggested.
Sea level rise (global, contested). It has been suggested that the sea level rise would create a positive feedback for increased melting: the ice shelf collapse could start by the intrusion of ocean water between the ice sheet and its ground or by surface melting, and 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). The positive feedback with sea level changes driving further retreat is also stated to be unlikely by other studies, doubting that the sea level changes, either from non-WAIS sources or the WAIS, will drive future WAIS loss (Alley et al. 2007; Gomez et al. 2010).
Atmospheric warming (regional, speculative/well-established). The estimated atmospheric temperature increase required for the WAIS to reach the summer time melting point would be ca 5-8°C of local surface atmospheric temperatures and less for ocean warming (Oppenheimer & Alley 2004). In addition, atmospheric warming (well-established) affects oceanic temperatures contributing to the ocean thermal forcing (Pritchard et al. 2012). It has also been suggested that because higher surface temperatures (global warming) might reduce the sea ice formation, it might actually lessen basal melt rates due to reduction in dense high-salinity waters produced by the sea ice formation (Nicholls 1997).
The main external indirect drivers that contribute to the shift include:
Wind forcing (regional, well-established) affects oceanic temperatures contributing to the ocean thermal forcing (Pritchard et al. 2012).
Ocean upwelling (regional, well-established) affects oceanic temperatures contributing to the ocean thermal forcing (Pritchard et al. 2012).
Tides and long period waves (regional, speculative). Tides and long-period waves cause mixing of shallow waters beneath the ice shelf, contributing to the sub-ice shelf melting (Joughin & Alley 2011).
Changes in stratospheric ozone (regional, speculative). The Antarctic ozone hole can affect oceanic and atmospheric circulation. One explanation to the increased sea ice extent is that the change in stratospheric ozone causes altered circulation (Turner et al. 2009).
Natural climatic variability (global/regional, well-established). Natural variation in the SAM (Southern Annular Mode, high-latitude mode, also called the Antarctic Oscillation) affects temperature and winds (Thompson & Solomon 2002). Tropical atmospheric and oceanic conditions, such as El Nino, can also have an impact on high latitudes. The SAM can shift in response to anthropogenic forcing, for instance ozone hole development. It is suggested that the shift to a positive SAM has increased the production of the coastal sea ice.
Slow internal system changes that contribute to the regime shift include:
Ice stream flow changes (regional, speculative). In the past slowdowns and deviations from flow directions have taken place. Switching on and off ice streams may have some effect on grounding line retreat. For instance a small degree of widening of an ice stream may produce large changes in speed. A shear heating could yield a positive feedback (ice stream widens, increase in margin shear heating, further widening and speedup) or a stabilizing feedback in some locations (ice-stream thinning steepens basal temperature, thus conducts more heat away from the bed than is supplied through the geothermal heat flux and friction from sliding, causing basal freezing to exceed the supply of meltwater from other areas). The resulting withdrawal of water from basal till can alter till porosity, and that way change the sensitivity of till to water content. (Joughin & Richard B. Alley 2011 and references therein)
Thermally driven cycling of ice streams (regional, speculative). The sensitivity of till to water content may produce a thermal cycling of ice streams, in which thinning causes basal freezing that stops fast flow. The stagnation results in thickening, which traps geothermal heat, increase melting and reduce till strength to reactive the ice stream and repeat the cycle. (Joughin & Richard B. Alley 2011 and references therein)
Surface melting and ponded water (regional, contested). Surface melting and ponded water may contribute to the ice shelf breakup through hydrofracturing of crevasses (Mercer 1978; Doake et al. 1997; Scambos et al. 2000). Yet it has been stated that the role of surface melt is insignificant at the present, as average summer surface temperatures over most of the WAIS are below freezing (Fyke et al. 2010). The annual mass gain from snowfall is discharged on the ocean (Jacobs et al. 1992).
Glaciers flowing into Amundsen Sea (local, speculative). The impact of the glaciers flowing to the Amundsen Sea on the behavior of ice shelf and glaciers themselves is still uncertain but possible (Thomas et al. 2004). They are suggested to affect the dynamics of the icescape since the glaciers have experienced ground line retreat, thinning and acceleration (Thomas et al. 2004).
Subsurface meltwater (regional, speculative). Subsurface melt of ice shelves may contribute to southern ocean surface cooling and meltwater-induced sea ice expansion (Bintanja et al. 2013). Melt water 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, and the atmosphere cools the upper 100 m more easily. The relatively cold and fresh surface waters can then freeze even more easily. (Southern Ocean exhibits warming of the deeper layers and cooling of the upper layer) (Bintanja et al. 2013).)
Summary of Drivers
|#||Driver (Name)||Type (Direct, Indirect, Internal, Shock)||Scale (local, regional, global)||Uncertainty (speculative, proposed, well-established)|
|1||Increased ocean temperature (warming of CDW)||Direct||Regional||Proposed|
|2||Sea level rise||Direct||Regional/global||Proposed|
|3||Surface melting, ponded water||Internal||Regional||Speculative|
|4||Subsurface melt water (for sea-ice extent||Internal||Regional||Speculative|
|9||Tides and long-period waves||Indirect||Regional||Speculative|
|10||Changes in stratospheric ozone||Indirect||Regional||Speculative|
|11||Natural climatic variability||Indirect||Regional/global||Well-established|
|12||Ice-stream flow changes||Internal||Regional||Speculative|
|13||Thermally driven cycling of ice streams||Internal||Regional||Speculative|
Threshold of global warming. Stating a tipping point to the ice sheet disintegrations is complex due to temperature changes, oceanic changes and the internal behavior of the ice sheet. A tipping point for the WAIS collapse is likely, but scientific community is still very uncertain about the exact temperature, and it has even been said that for such a complex system, the concept of tipping point may be virtually useless. Suggested threshold has ranged at 1-5°C of global warming (O'Reilly & ACOS 2013; see also M. Oppenheimer & R.B. Alley 2004).
Oceanic warming (regional/global, speculated). Increase in the CDW ocean temperature has been identified as the key mechanism for the melting and thinning of the ice shelves. Counteracting human-induced oceanic warming would require acting against the climate change. However, in two studies published in 2014 (Joughin Smith & Medley; NASA in press) state that the collapse is basically unstoppable and stabilization might be possible only if CDW receded.
Ecosystem service impacts
Humans have mainly used the Antarctica for conservation, research, tourism, and commercial fishing. Cultural ecosystem services include possibly increased tourism, increased accessibility and the societal losses caused by the sea level rise.
Sea level rise impacts for the coastal areas are submergence or increased flooding, increased erosion, ecosystem changes, increased salinization and forced displacement of coastal population and economy (Nicholls et al. 2011). The ice loss from the WAIS is at present equivalent to 0.28 to 0.56 mm yr-1 sea level rise with growing rate for the past two decades (Shepherd & Wingham 2007; Velicogna & Wahr 2006; Rignot et al. 2011). Although the time scale of several meters' sea level rise from WAIS is highly uncertain, it could happen within a millennium, 300 years being the worst case scenario (R. Thomas et al. 2004). If or when the WAIS deglaciation happens, it will raise the sea level up to ca 3 meters creating huge impact on global infrastructure and society (Bamber et al. 2009). There is no agreement among the experts on the likelihood or rate of the rapid collapse (O'Reilly & ACOS 2013). The economic consequences depend on the time-scale: playing out over a century, the WAIS collapse would damage many coastal communities whereas occurring over a millennium would enable development of an appropriate risk-mitigation strategy (Dowdeswell et al. 2008; Mercer 1978; Joughin & Alley 2011).
The absence of the WAIS would leave broad, deep seaways (Joughin & Alley 2011) which would naturally deepen towards the ice-sheet interior. Increased vessel access to the more southern locations in the conditions where westerly winds would have become stronger (Marshall et al. 2006) may have implications on the safety of ocean navigation (Anon 2009). Tourism, the largest commercial activity in the Antarctic, is predicted to increase and extent in season with reduced sea ice thickness (Anon 2009).
The region of the WAIS turning to open ocean gives rise to changes associated with sea ice presence. Changes in the extent of the sea ice affect the regulating ecosystem services of the Antarctica, but at present the uncertainty of predictions is high. The extent of the ice has a role in the global climate system due to albedo effect and as an important driver of the global ocean circulation. The albedo effect depends on the extent of high albedo snow cover (e.g. Massom & Stammerjohn 2010). Sea ice also has an insulating effect on the ocean: heat that would be given up to the atmosphere remains trapped on the upper layer of the ocean (Anon 2009).
Seasonal brine rejection from sea ice formation and freshwater pulses from ice melt are key determinants for the upper ocean freshwater budgets and formation of cold, dense oxygen-rich Antarctic bottom water (AABW), which is a significant driver of global ocean circulation (Massom & Stammerjohn 2010). Salt added to the ocean in sea ice formation is denser than seawater and sinks toward the bottom, where it may accumulate in depressions and eventually mix. This is important for the formation of Antarctic Bottom Water and global ocean conveyor belt (redistribution of heat and maintenance of climate system) (Bindoff et al. 2000). The characteristics, seasonality and dynamics of sea ice affect the magnitude of these phenomena (Massom & Stammerjohn 2010) and the stability may be disturbed by increasing melting or disintegration of the WAIS.
Changes in the ocean circulation, sea ice coverage, biological activity and temperature will also affect the CO2 uptake of the Southern Ocean, but the mechanisms for future changes are not clear yet, because the data on the carbon uptake in the Southern Ocean is sparse and magnitude of CO2 uptake is still heavily disputed (Caldeira & Duffy 2000; Le Quéré et al. 2007; Sarmiento et al. 1998).
The Southern Ocean has a rich biodiversity and its provisioning ecosystem services are affected by altered habitats. In biogeochemical cycles of the Southern Ocean, the sea ice impacts the structure and dynamics of marine ecosystems (e.g. life cycles of organisms), especially the trophic levels that are adapted to its presence, seasonality and properties (Massom & Stammerjohn 2010; Arrigo 2002; Brierley & Thomas 2002; Thomas et al. 2010; Eicken et al. 1995; Moline et al. 2008; Tynan 2010), e.g. light availability, as substrate for algal biomass, habitat and barrier to air-breathing predators, as well as a barrier separating animals from their food source, for instance Adélie and Emperor penguins (see e.g. Anon 2009). The impact of the changes in sea ice on the biomass and extent of marine species is a very important issue for management and conservation. There already is evidence of species shifts, urging for precautionary approach in conservation while more research is undertaken (Anon 2009). Because of the extreme conditions, fishing in the Southern Ocean is expensive and difficult, and is done is smaller scale than in many other seas. Fisheries management and accessibility changes due to sea ice extent, for instance there might be an increased access for legal and illegal vessels to the more southern locations for longer periods (Anon 2009). This might be a threat to ecosystems since winter ice has usually provided a relief from the fishing pressure.
Summary of Ecosystem Service impacts on different User Groups
||References (if available)|
|Feed, Fuel and Fibre Crops|
|Wild Food & Products|
|Air Quality Regulation|
|Soil Erosion Regulation|
|Pest & Disease Regulation|
|Protection against Natural Hazards|
|Cognitive & Educational||+/-|
|Spiritual & Inspirational|
Uncertainties and unresolved issues
Several uncertainties remain, largely due to sparse data and the complexity and large size of the system, on the Antarctic dynamics and processes. Large-scale modeling of the WAIS requires a model that is able to combine the flow regimes of grounded and floating ice, and allow simulations to circa 105 years or more (Pollard & DeConto 2009). The lack of quantitative information on the role of ongoing glacial readjustments, the size of WAIS and buttressing ice shelves in previous interglacials as contributing factors, and the role of surrounding ice shelves for the WAIS stability contribute to the uncertaintites as well as interactions between different parts of Antarctica. The mechanisms driving the regime shift, regional warming, response of ice streams and the rate of melting remain uncertain as well as changes in the CDW temperature, location or flow.