twenty first century warming of a large antarctic ice shelf cavity by a redirected coastal current the redirection of warm water under the filchner ronne ice shelf during the second half of this century could cause the ice shelf base to melt at a rate 20 times higher than at present warm ocean currents are known to erode ice shelves from below but changes in currents can be forced by many different mechanisms leading to uncertain outcomes this study highlights the vulnerability to climate change of a small antarctic coastal region which has potentially severe consequences for the mass balance of a large antarctic ice shelf hellmer et al use climate modelling to show that the projected loss of sea ice in the weddell sea east of the antarctic peninsula leads to an increase in wind stress which in turn accelerates a warm ocean current far underneath the vast filchner ronne ice shelf the authors predict that the increased warmth could increase melt by a factor of 20 with possible consequences for ice stream dynamics in the east antarctic ice sheet the antarctic ice sheet loses mass at its fringes bordering the southern ocean at this boundary warm circumpolar water can override the continental slope front reaching the grounding line1 2 through submarine glacial troughs and causing high rates of melting at the deep ice shelf bases3 4 the interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice mass loss here we show that a redirection of the coastal current into the filchner trough and underneath the filchner ronne ice shelf during the second half of the twenty first century would lead to increased movement of warm waters into the deep southern ice shelf cavity water temperatures in the cavity would increase by more than 2 degrees celsius and boost average basal melting from 0 2 metres or 82 billion tonnes per year to almost 4 metres or 1 600 billion tonnes per year our results which are based on the output of a coupled ice ocean model forced by a range of atmospheric outputs from the hadcm35 climate model suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern weddell sea due to thinning of the formerly consolidated sea ice cover the projected ice loss at the base of the filchner ronne ice shelf represents 80 per cent of the present antarctic surface mass balance6 thus the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of antarctic ice streams and ice shelves and global sea level rise

the role of lithospheric flexure in the landscape evolution of the wilkes subglacial basin and transantarctic mountains east antarctica reconstructions of the bedrock topography of antarctica since the eocene oligocene boundary ca 34 ma provide important constraints for modelling antarctic ice sheet evolution this is particularly important in regions where the bedrock lies below sea level since in these sectors the overlying ice sheet is thought to be most susceptible to past and future change here we use 3d flexural modelling to reconstruct the evolution of the topography of the wilkes subglacial basin wsb and transantarctic mountains tam in east antarctica we estimate the spatial distribution of glacial erosion beneath the east antarctic ice sheet and restore this material to the topography which is also adjusted for associated flexural isostatic responses we independently constrain our post 34 ma erosion estimates using offshore sediment stratigraphy interpretations our reconstructions provide a better defined topographic boundary condition for modelling early east antarctic ice sheet history we show that the majority of glacial erosion and landscape evolution occurred prior to 14 ma which we interpret to reflect more dynamic and erosive early ice sheet behaviour in addition we use closely spaced 2d flexural models to test previously proposed hypotheses for a flexural origin of the tam and wsb the pre 34 ma topography shows lateral variations along the length of the tam and wsb that cannot be explained by uniform flexure along the front of the tam we show that some of these variations may be explained by additional flexural uplift along the south western flank of the wsb and the rennick graben in northern victoria land

testing the sensitivity of the east antarctic ice sheet to southern ocean dynamics past changes and future implications the stability of antarctic ice sheets and their potential contribution to sea level under projected future warming remains highly uncertain the last interglacial 135 000 116 000 years ago provides a potential analogue with global temperatures 2uc higher and rates of sea level rise 5 6m ka 1 leading to sea levels 6 6 9 4m higher than present the source s of this sea level rise remain fiercely debated here we report a series of independent model simulations exploring the effects of migrating southern hemisphere westerlies shws on southern ocean circulation and antarctic ice sheet dynamics we suggest that southerly shifts in winds may have significantly impacted the sub polar gyres inducing pervasive warming 0 2 0 8uc in the upper 1200m adjacent to sectors of the east antarctic ice sheet eais which due to their geometries and connectivity to the southern ocean are highly sensitive to ocean forcing we conclude that the eais potentially made a substantial hitherto unsuspected contribution to interglacial sea levels and given 21st century projections in the southern annular mode and associated shw migration we highlight how pervasive circum antarctic warming may threaten eais stability copyright 2013 john wiley sons ltd

spatial and seasonal effects of temperature variability in a positive degree day glacier surface mass balance model this doctoral dissertation presents a study of the glacial history of the north american cordillera using numerical ice sheet modelling calibrated against field evidence this area characterized by the steep topography of several mountain ranges separated by large inter montane depressions was once covered by a large scale ice mass the former cordilleran ice sheet because of the irregular topography on which the ice sheet formed geological studies have often had only local or regional relevance thus leaving the cordilleran ice sheet least understood among pleistocene ice sheets in terms of its extent volume and dynamics here i present numerical simulations that allow quantitative reconstructions of the former ice sheet evolution based on approximated physics of glacier flow these simulations show that the geometry of the last glacial maximum cordilleran ice sheet was largely controlled by sharp contrasts in regional temperature precipitation and daily temperature variability associated with the presence of mountain ranges however this maximum stage appears short lived and out of balance with contemporaneous climate during most of the simulated last glacial cycle the north american cordillera is characterized by an intermediate state of glaciation including isolated glaciers and ice caps covering major mountain ranges the largest of which is located over the skeena mountains the numerically modelled cordilleran ice sheet appears in constant imbalance with evolving climate conditions while the complexity of this transient response transcends that encapsulated in two dimensional conceptual models of ice sheet growth and decay this thesis demonstrates the potential of numerical ice sheet modelling to inform on ice sheet history and former climate conditions over a glacial cycle given that ice sheet models can be calibrated against field constraints

a variational approach to ice stream flow ice sheets are susceptible to the formation of ice streams or narrow bands of fast flowing ice whose high velocities are caused by rapid sliding at the contact between ice and the underlying bed based on recent geophysical work which has shown that the sliding motion of ice streams may be described by a coulomb friction law we investigate how the location of ice streams depends on the geometry of an ice sheet and on the mechanical properties of the underlying bed more generally this problem is relevant to the flow of thin films with coulomb or solid friction laws applied at their base by analogy with friction problems in elasticity we construct a variational formulation for the free boundary between ice streams where bed failure occurs and the surrounding ice ridges where there is little or no sliding this variational problem takes the form of a non coercive variational inequality and we show that solutions exist provided a force and moment balance condition is satisfied in that case solutions are also unique except under certain specialized circumstances which are unlikely to arise for a real ice sheet further we show how the variational formulation of the ice flow problem can be exploited to calculate numerical solutions and to simulate the effect of changing ice geometry and bed friction on the location and velocities of streaming flow lastly we study the effect of ice shelf buttressing on the flow of ice streams whose spatial extent is determined by the yield stress distribution of the bed in line with previous studies of ice shelf buttressing we find that the removal of an ice shelf can cause an ice stream feeding the ice shelf to speed up considerably which underlines the important role ice shelves may play in controlling the dynamics of marine ice sheets

potential sea level rise from antarctic ice sheet instability constrained by observations large parts of the antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine ice sheet instability misi a self sustaining retreat of the grounding line triggered by oceanic or atmospheric changes there is growing evidence that misi may be underway throughout the amundsen sea embayment ase which contains ice equivalent to more than a metre of global sea level rise if triggered in other regions the centennial to millennial contribution could be several metres physically plausible projections are challenging numerical models with sufficient spatial resolution to simulate grounding line processes have been too computationally expensive to generate large ensembles for uncertainty assessment and lower resolution model projections rely on parameterizations that are only loosely constrained by present day changes here we project that the antarctic ice sheet will contribute up to 30 cm sea level equivalent by 2100 and 72 cm by 2200 95 quantiles where the ase dominates our process based statistical approach gives skewed and complex probability distributions single mode 10 cm at 2100 two modes 49 cm and 6 cm at 2200 the dependence of sliding on basal friction is a key unknown nonlinear relationships favour higher contributions results are conditional on assessments of misi risk on the basis of projected triggers under the climate scenario a1b ref 9 although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss we find that contributions are restricted by a combination of these constraints calibration with success in simulating observed ase losses and low assessed risk in some basins our assessment suggests that upper bound estimates from low resolution models and physical arguments up to a metre by 2100 and around one and a half by 2200 are implausible under current understanding of physical mechanisms and potential triggers

timing and pathways of east antarctic ice sheet retreat abstract a lack of reliable chronology often leaves antarctic ice sheet reconstructions incomplete despite being the most heavily investigated region of the antarctic continental shelf the post last glacial maximum lgm record of marine based ice sheet dynamics in the ross sea has remained largely elusive and at odds with fringing terrestrial records of ice sheet decay issues with radiocarbon dating stem primarily from poorly preserved carbonate contamination by glacially recycled carbon and insufficient consideration for glacial geomorphic context and sedimentary facies we assess nearly 70 newly acquired and more than 200 published radiocarbon ages in the context of a consistent sedimentary facies and geomorphic framework to interpret the timing of the local glacial maximum and subsequent retreat of the east antarctic ice sheet eais in the ross sea and potential causes of retreat compound specific radiocarbon ages help to provide good constraints on open marine onset and we demonstrate that with proper corrections and considerations acid insoluble organic aio radiocarbon ages can be reliable in open marine sediments and carbonate ages can be reliable in grounding zone proximal sediments we find that the ice flowing through pennell and joides troughs remained at maximum extent for approximately 10 000 years before retreating at 15 1 and 13 cal ka bp respectively an ice shelf covered the outer continental shelf until 9 5 cal ka bp after which it collapsed in joides trough creating deep iceberg furrows retreat through pennell trough was more episodic marked by grounding zone wedges that backstep upstream and onto lateral banks creating deglacial emergent ice rises as ice retreated from 9 to 4 cal ka bp there was considerable reorganization of drainage and at least one episode of readvance through southern joides trough that began drawdown of terrestrial ice near ross island at 7 8 cal ka bp the ice shelf appears to have retreated to a near present configuration by 2 cal ka bp however due to inconsistencies in calendar ages grounding line position must be inferred from relative chronology based on glacial landforms which suggests grounded ice retreated nearly to drygalski ice tongue and lingered atop crary bank where it was maintained by ice from david glacier our reconstruction further suggests that the eais in the western ross sea was not a discernible contributor to meltwater pulse 1a but likely contributed to eustatic sea level rise well through the middle perhaps even late holocene

east antarctic ice sheet most vulnerable to weddell sea warming models predict considerable spatial variability in the magnitude of future climate change around antarctica suggesting that some sectors of the continent may be more affected by these changes than others furthermore the geometry of the bedrock topography underlying the east and west antarctic ice sheets together with regional differences in ice thickness mean that certain ice drainage basins may respond more or less sensitively to environmental forcings here we use an ensemble of idealized climates to drive ice sheet simulations that explore regional and continental scale thresholds allowing us to identify a hierarchy of catchment vulnerabilities based on differences in long term catchment averaged ice loss considering this hierarchy in the context of recent observations and climate scenarios forecast for 2100 ce we conclude that the majority of future ice loss from east antarctica both this century and over subsequent millennia will likely come from the recovery subglacial basin in the eastern weddell sea

shallow shelf approximation as a sliding law in a thermomechanically coupled ice sheet model 1 the shallow shelf approximation a balance of membrane stresses for ice flow is an effective sliding law for ice sheet modeling our use of it as a sliding law becomes a standard model for ice stream flow when the sliding velocity is large 100 m a 1 and faster following schoof 2006a we describe the basal resistance as plastic till for which the yield stress is given by a mohr coulomb formula pore water pressure is related to basal melt rate the velocity field used in the mass continuity and conservation of energy equations is an average of velocities from the shallow shelf approximation and the nonsliding shallow ice approximation using this scheme our model has realistic time dependent ice streams which exhibit the range of surface velocities seen in actual ice streams we demonstrate the model at high spatial resolution 5 km grid over multiple millenia using its implementation in the parallel ice sheet model numerical experiments show that the entire scheme is stable with respect to many parameter changes some experiments reveal significant ice stream variability in a hypothetical steady climate with characteristic cycles on the order of 1000 years we believe this is the first practical whole ice sheet model with a unified treatment of vertical shear stresses and membrane stresses it is capable of high resolution thermomechanically coupled multimillenia simulations of ice sheets containing ice streams

potential antarctic ice sheet retreat driven by hydrofracturing and ice cliff failure abstract geological data indicate that global mean sea level has fluctuated on 103 to 106 yr time scales during the last 25 million years at times reaching 20 m or more above modern if correct this implies substantial variations in the size of the east antarctic ice sheet eais however most climate and ice sheet models have not been able to simulate significant eais retreat from continental size given that atmospheric co2 levels were relatively low throughout this period here we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model data conflict in response to atmospheric and ocean temperatures typical of past warm periods floating ice shelves may be drastically reduced or removed completely by increased oceanic melting and by hydrofracturing due to surface melt draining into crevasses ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing and may fail structurally if stresses exceed the ice yield strength producing rapid retreat incorporating these mechanisms in our ice sheet model accelerates the expected collapse of the west antarctic ice sheet to decadal time scales and also causes retreat into major east antarctic subglacial basins producing 17 m global sea level rise within a few thousand years the mechanisms are highly parameterized and should be tested by further process studies but if accurate they offer one explanation for past sea level high stands and suggest that antarctica may be more vulnerable to warm climates than in most previous studies

subglacial geology and geomorphology of the pensacola pole basin east antarctica the east antarctic ice sheet eais is underlain by a series of low lying subglacial sedimentary basins the extent geology and basal topography of these sedimentary basins are important boundary conditions governing the dynamics of the overlying ice sheet this is particularly pertinent for basins close to the grounding line wherein the eais is grounded below sea level and therefore potentially vulnerable to rapid retreat here we analyze newly acquired airborne geophysical data over the pensacola pole basin ppb a previously unexplored sector of the eais using a combination of gravity magnetic and ice penetrating radar data we present the first detailed subglacial sedimentary basin model for the ppb radar data reveal that the ppb is defined by a topographic depression situated 500 m below sea level gravity and magnetic depth to source modeling indicate that the southern part of the basin is underlain by a sedimentary succession 2 3 km thick this is interpreted as an equivalent of the beacon supergroup and associated ferrar dolerites that are exposed along the margin of east antarctica however we find that similar rocks appear to be largely absent from the northern part of the basin close to the present day grounding line in addition the eastern margin of the basin is characterized by a major geological boundary and a system of overdeepened subglacial troughs we suggest that these characteristics of the basin may reflect the behavior of past ice sheets and or exert an influence on the present day dynamics of the overlying eais

early last interglacial ocean warming drove substantial ice mass loss from antarctica the future response of the antarctic ice sheet to rising temperatures remains highly uncertain a useful period for assessing the sensitivity of antarctica to warming is the last interglacial lig 129 to 116 ky which experienced warmer polar temperatures and higher global mean sea level gmsl 6 to 9 m relative to present day lig sea level cannot be fully explained by greenland ice sheet melt 2 m ocean thermal expansion and melting mountain glaciers 1 m suggesting substantial antarctic mass loss was initiated by warming of southern ocean waters resulting from a weakening atlantic meridional overturning circulation in response to north atlantic surface freshening here we report a blue ice record of ice sheet and environmental change from the weddell sea embayment at the periphery of the marine based west antarctic ice sheet wais which is underlain by major methane hydrate reserves constrained by a widespread volcanic horizon and supported by ancient microbial dna analyses we provide evidence for substantial mass loss across the weddell sea embayment during the lig most likely driven by ocean warming and associated with destabilization of subglacial hydrates ice sheet modeling supports this interpretation and suggests that millennial scale warming of the southern ocean could have triggered a multimeter rise in global sea levels our data indicate that antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice climate feedbacks that further amplify warming

limited retreat of the wilkes basin ice sheet during the last interglacial the response of the east antarctic ice sheet to global warming represents a major source of uncertainty in sea level projections thinning of the east antarctic george v and sabrina coast ice cover is currently taking place and regional ice sheet instability episodes might have been triggered in past warm climates however the magnitude of ice retreat in the past can not yet be quantitatively derived from paleo proxy records alone we propose that a runaway retreat of the george v coast grounding line and subsequent instability of the wilkes basin ice sheet would either leave a clear imprint on the water isotope composition in the talos dome region or prohibit a talos dome ice core record from the last interglacial altogether testing this hypothesis our ice sheet model simulations suggest that wilkes basin ice sheet retreat remained relatively limited during the last interglacial and provide a constraint on last interglacial east antarctic grounding line stability

ice loss from the east antarctic ice sheet during late pleistocene interglacials understanding ice sheet behaviour in the geological past is essential for evaluating the role of the cryosphere in the climate system and for projecting rates and magnitudes of sea level rise in future warming scenarios1 4 although both geological data5 7 and ice sheet models3 8 indicate that marine based sectors of the east antarctic ice sheet were unstable during pliocene warm intervals the ice sheet dynamics during late pleistocene interglacial intervals are highly uncertain3 9 10 here we provide evidence from marine sedimentological and geochemical records for ice margin retreat or thinning in the vicinity of the wilkes subglacial basin of east antarctica during warm late pleistocene interglacial intervals the most extreme changes in sediment provenance recording changes in the locus of glacial erosion occurred during marine isotope stages 5 9 and 11 when antarctic air temperatures11 were at least two degrees celsius warmer than pre industrial temperatures for 2 500 years or more hence our study indicates a close link between extended antarctic warmth and ice loss from the wilkes subglacial basin providing ice proximal data to support a contribution to sea level from a reduced east antarctic ice sheet during warm interglacial intervals while the behaviour of other regions of the east antarctic ice sheet remains to be assessed it appears that modest future warming may be sufficient to cause ice loss from the wilkes subglacial basin studies of an antarctic marine sediment core suggest that the east antarctic ice sheet retreated in the vicinity of the wilkes subglacial basin during extended warm periods of the late pleistocene when temperatures were similar to those predicted to occur within this century

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

formation of monazite and rhabdophane at the expense of allanite during alpine low temperature retrogression of metapelitic basement rocks from crete greece microprobe data and geochronological implications abstract accessory lree bearing minerals have been investigated in a sequence of palaeozoic amphibolite facies metapelites in eastern crete greece which were overprinted by alpine low t metamorphism 300 c the series is dominated by al ca rich metapelite layers which contain palaeozoic allanite secondary alpine monazite with low y contents only in some quartz rich layers of metapelites which are allanite free is primary palaeozoic monazite to be found in such special variants alpine monazite or rhabdophane appear to be absent and the lrees remained trapped in the palaeozoic monazite not even the outermost rims of the old monazites experienced alteration during the alpine retrograde metamorphism and preserve undisturbed palaeozoic th u pb ages this observations made in this study confirm other recent work according to which in many cases monazite and not allanite is the stable lree phase under low grade metamorphic conditions rocks with primary allanite contents can be considered particularly fertile for the production of monazite at low metamorphic grades in contrast rocks without allanite but with primary monazite are unfertile for the growth of a secondary monazite generation a composite lithologically variable basement with a low temperature overprint will therefore often include both remnant and new monazite although the two monazite generations will commonly occur in different lithologies and rarely together in the same samples therefore if a cunning sampling strategy is applied monazite geochronology is capable of yielding the ages of both primary and secondary events in such settings

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

mantle convection modeling with viscoelastic brittle lithosphere numerical methodology and plate tectonic modeling the earth s tectonic plates are strong viscoelastic shells which make up the outermost part of a thermally convecting predominantly viscous layer brittle failure of the lithosphere occurs when stresses are high in order to build a realistic simulation of the planet s evolution the complete viscoelastic brittle convection system needs to be considered a particle in cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain dependent yield stress this is applied to a plate deformation problem numerical accuracy is demonstrated relative to analytic benchmarks and the characteristics of the method are discussed

modeling thermodynamic ice ocean interactions at the base of an ice shelf abstract models of ocean circulation beneath ice shelves are driven primarily by the heat and freshwater fluxes that are associated with phase changes at the ice ocean boundary their behavior is therefore closely linked to the mathematical description of the interaction between ice and ocean that is included in the code an hierarchy of formulations that could be used to describe this interaction is presented the main difference between them is the treatment of turbulent transfer within the oceanic boundary layer the computed response to various levels of thermal driving and turbulent agitation in the mixed layer is discussed as is the effect of various treatments of the conductive heat flux into the ice shelf the performance of the different formulations that have been used in models of sub ice shelf circulation is assessed in comparison with observations of the turbulent heat flux beneath sea ice formulations that include an explicit parameterization of the oceanic boundary layer give results that

pliocene deglacial event timelines and the biogeochemical response offshore wilkes subglacial basin east antarctica abstract significantly reduced ice coverage in greenland and west antarctica during the warmer than present pliocene could account for 10 m of global mean sea level rise any sea level increase beyond this would require contributions from the east antarctic ice sheet eais previous studies have presented low resolution geochemical evidence from the geological record suggesting repeated ice advance and retreat in low lying areas of the eais such as the wilkes subglacial basin however the rates and mechanisms of retreat events are less well constrained here we present orbitally resolved marine detrital sediment provenance data paired with ice rafted debris and productivity proxies during three time intervals from the middle to late pliocene at iodp site u1361a offshore of the wilkes subglacial basin our new data reveal that pliocene shifts in sediment provenance were paralleled by increases in marine productivity while the onset of such changes was marked by peaks in ice rafted debris mass accumulation rates the coincidence of sediment provenance and marine productivity change argues against a switch in sediment delivery between ice streams and instead suggests that deglacial warming triggered increased rates of iceberg calving followed by inland retreat of the ice margin timescales from the onset of deglaciation to an inland retreated ice margin within the wilkes subglacial basin are on the order of several thousand years this geological evidence corroborates retreat rates determined from ice sheet modeling and a contribution of 3 to 4 m of equivalent sea level rise from one of the most vulnerable areas of the east antarctic ice sheet during interglacial intervals throughout the middle to late pliocene

repeated large scale retreat and advance of totten glacier indicated by inland bed erosion climate variations cause ice sheets to retreat and advance raising or lowering sea level by metres to decametres the basic relationship is unambiguous but the timing magnitude and sources of sea level change remain unclear in particular the contribution of the east antarctic ice sheet eais is ill defined restricting our appreciation of potential future change several lines of evidence suggest possible collapse of the totten glacier into interior basins during past warm periods most notably the pliocene epoch causing several metres of sea level rise however the structure and long term evolution of the ice sheet in this region have been understood insufficiently to constrain past ice sheet extents here we show that deep ice sheet erosion enough to expose basement rocks has occurred in two regions the head of the totten glacier within 150 kilometres of today s grounding line and deep within the sabrina subglacial basin 350 550 kilometres from this grounding line our results based on icecap aerogeophysical data demarcate the marginal zones of two distinct quasi stable eais configurations corresponding to the modern scale ice sheet with a marginal zone near the present ice sheet margin and the retreated ice sheet with the marginal zone located far inland the transitional region of 200 250 kilometres in width is less eroded suggesting shorter lived exposure to eroding conditions during repeated retreat advance events which are probably driven by ocean forced instabilities representative ice sheet models indicate that the global sea level increase resulting from retreat in this sector can be up to 0 9 metres in the modern scale configuration and exceeds 2 metres in the retreated configuration

resolution dependent performance of grounding line motion in a shallow model compared with a full stokes model according to the mismip3d intercomparison making confident statements about the evolution of an ice sheet shelf system with a numerical model requires the capability to reproduce the migration of the grounding line here we show that the shallow ice approximation shallow shelf approximation hybrid type parallel ice sheet model pism with its recent improvements is capable of modeling the grounding line motion in a perturbed ice sheet shelf system the model is set up according to the three dimensional marine ice sheet model intercomparison project mismip3d and simulations are carried out across a broad range of spatial resolutions using 1 a linear interpolation of the grounding line with locally interpolated basal friction and 2 an improved driving stress computation across the grounding line the reversibility of the grounding line i e its retreat after an advance forced by a local perturbation of basal resistance is captured by the model even at medium and low resolutions x 10 km the transient model response is qualitatively similar to that of higher order models but reveals a higher initial sensitivity to perturbations on very short timescales our findings support the application of pism to the antarctic ice sheet from regional up to continental scales and on relatively low spatial resolutions

eddy mediated transport of warm circumpolar deep water across the antarctic shelf break the antarctic slope front asf modulates ventilation of the abyssal ocean via the export of dense antarctic bottom water aabw and constrains shoreward transport of warm circumpolar deep water cdw toward marine terminating glaciers along certain stretches of the continental shelf particularly where aabw is exported density surfaces connect the shelf waters to the middepth circumpolar deep water offshore offering a pathway for mesoscale eddies to transport cdw directly onto the continental shelf using an eddy resolving process model of the asf the authors show that mesoscale eddies can supply a dynamically significant transport of heat and mass across the continental shelf break the shoreward transport of surface waters is purely wind driven while the shoreward cdw transport is entirely due to mesoscale eddy transfer the cdw flux is sensitive to all aspects of the model s surface forcing and geometry suggesting that shoreward eddy heat transport may be localized to favorable sections of the continental slope

antarctic ice dynamics in warm climates a geological record reveals that the aurora sector of the antarctic ice sheet showed contrasting responses to past periods of atmospheric warmth the findings might help to predict the ice sheet s response to modern warming a geological record reveals that the aurora sector of the antarctic ice sheet showed contrasting responses to past periods of atmospheric warmth the findings might help to predict the ice sheet s response to modern warming

the antarctic slope current in a changing climate the antarctic slope current asc is a coherent circulation feature that rings the antarctic continental shelf and regulates the flow of water towards the antarctic coastline the structure and variability of the asc influences key processes near the antarctic coastline that have global implications such as the melting of antarctic ice shelves and water mass formation that determines the strength of the global overturning circulation recent theoretical modeling and observational advances have revealed new dynamical properties of the asc making it timely to review earlier reviews of the asc focused largely on local classifications of water properties of the asc s primary front here we instead provide a classification of the current s frontal structure based on the dynamical mechanisms that govern both the along slope and cross slope circulation these two modes of circulation are strongly coupled similar to the antarctic circumpolar current highly variable motions such as dense overflows tides and eddies are shown to be critical components of cross slope and cross shelf exchange but understanding of how the distribution and intensity of these processes will evolve in a changing climate remains poor due to observational and modeling limitations results linking the asc to larger modes of climate variability such as el nino show that the asc is an integral part of global climate an improved dynamical understanding of the asc is still needed to accurately model and predict future antarctic sea ice extent the stability of the antarctic ice sheets and the southern ocean s contribution to the global carbon cycle

antarctic ice sheet response to sudden and sustained ice shelf collapse abumip author s sun s pattyn f simon eg albrecht t cornford s calov r dumas c gillet chaulet f goelzer h golledge nr greve r hoffman mj humbert a kazmierczak e kleiner t leguy gr lipscomb wh martin d morlighem m nowicki s pollard d price s quiquet a seroussi h schlemm t sutter j van de wal rsw winkelmann r zhang t abstract copyright the author s 2020 antarctica s ice shelves modulate the grounded ice flow and weakening of ice shelves due to climate forcing will decrease their buttressing effect causing a response in the grounded ice while the processes governing ice shelf weakening are complex uncertainties in the response of the grounded ice sheet are also difficult to assess the antarctic buttressing model intercomparison project abumip compares ice sheet model responses to decrease in buttressing by investigating the end member scenario of total and sustained loss of ice shelves although unrealistic this scenario enables gauging the sensitivity of an ensemble of 15 ice sheet models to a total loss of buttressing hence exhibiting the full potential of marine ice sheet instability all models predict that this scenario leads to multi metre 1 12 m sea level rise over 500 years from present day west antarctic ice sheet collapse alone leads to a 1 91 5 08 m sea level rise due to the marine ice sheet instability mass loss rates are a strong function of the sliding friction law with plastic laws cause a further destabilization of the aurora and wilkes subglacial basins east antarctica improvements to marine ice sheet models have greatly reduced variability between modelled ice sheet responses to extreme ice shelf loss e g compared to the searise assessments

interhemispheric ice sheet synchronicity during the last glacial maximum the timing of the last maximum extent of the antarctic ice sheets relative to those in the northern hemisphere remains poorly understood we develop a chronology for the weddell sea sector of the east antarctic ice sheet that combined with ages from other antarctic ice sheet sectors indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of northern hemisphere ice sheets surface climate forcing of antarctic mass balance would probably cause an opposite response whereby a warming climate would increase accumulation but not surface melting our new data support teleconnections involving sea level forcing from northern hemisphere ice sheets and changes in north atlantic deep water formation and attendant heat flux to antarctic grounding lines to synchronize the hemispheric ice sheets

antarctic ice sheet sensitivity to obliquity forcing enhanced through ocean connections deep sea geological records indicate that antarctic ice sheet growth and decay is strongly influenced by the earth s astronomical variations known as milankovitch cycles and that the frequency of the glacial interglacial cycles changes through time here we examine the emergence of a strong obliquity axial tilt control on antarctic ice sheet evolution during the miocene by correlating the antarctic margin geological records from 34 to 5 million years ago with a measure of obliquity sensitivity that compares the variance in deep sea sediment core oxygen isotope data at obliquity timescales with variance of the calculated obliquity forcing our analysis reveals distinct phases of ice sheet evolution and suggests the sensitivity to obliquity forcing increases when ice sheet margins extend into marine environments we propose that this occurs because obliquity driven changes in the meridional temperature gradient affect the position and strength of the circum antarctic easterly flow and enhance or reduce ocean heat transport across the antarctic continental margin the influence of obliquity driven changes in ocean dynamics is amplified when marine ice sheets are extensive and sea ice is limited our reconstruction of the antarctic ice sheet history suggests that if sea ice cover decreases in the coming decades ocean driven melting at the ice sheet margin will be amplified the sensitivity of the antarctic ice sheet to obliquity increases when ice sheet margins are exposed to the ocean suggests an analysis of sediment core oxygen isotope records

southern ocean temperature records and ice sheet models demonstrate rapid antarctic ice sheet retreat under low atmospheric co2 during marine isotope stage 31 abstract over the last 5 million years the earth s climate has oscillated between warm interglacial and cold glacial states some particularly warm interglacial periods i e super interglacials occurred under low atmospheric co2 and may have featured extensive antarctic ice sheet collapse here we focus on an extreme super interglacial known as marine isotope stage 31 mis31 between 1 085 and 1 055 million years ago and is the subject of intense discussion we reconstructed the first southern ocean and antarctic margin sea surface temperatures ssts from organic biomarkers and used them to constrain numerical ice sheet shelf simulations our ssts indicate that the ocean was on average 5 c 1 2 c warmer in summer than today between 50 s and the antarctic ice margin our most conservative ice sheet simulation indicates a complete collapse of the west antarctic ice sheet wais with additional deflation of the east antarctic ice sheet we suggest the wais retreated because of anomalously high southern hemisphere insolation coupled with the intrusion of circumpolar deep water onto the continental shelf under poleward intensified winds leading to a shorter sea ice season and ocean warming at the continental margin in this scenario the extreme warming we observed likely reflects the extensively modified oceanic and hydrologic system following ice sheet collapse our work highlights the sensitivity of the antarctic ice sheets to minor oceanic perturbations that could also be at play for future changes

initiation and long term instability of the east antarctic ice sheet antarctica s continental scale ice sheets have evolved over the past 50 million years however the dearth of ice proximal geological records limits our understanding of past east antarctic ice sheet eais behaviour and thus our ability to evaluate its response to ongoing environmental change the eais is marine terminating and grounded below sea level within the aurora subglacial basin indicating that this catchment which drains ice to the sabrina coast may be sensitive to climate perturbations here we show using marine geological and geophysical data from the continental shelf seaward of the aurora subglacial basin that marine terminating glaciers existed at the sabrina coast by the early to middle eocene epoch this finding implies the existence of substantial ice volume in the aurora subglacial basin before continental scale ice sheets were established about 34 million years ago subsequently ice advanced across and retreated from the sabrina coast continental shelf at least 11 times during the oligocene and miocene epochs tunnel valleys associated with half of these glaciations indicate that a surface meltwater rich sub polar glacial system existed under climate conditions similar to those anticipated with continued anthropogenic warming cooling since the late miocene resulted in an expanded polar eais and a limited glacial response to pliocene warmth in the aurora subglacial basin catchment geological records from the sabrina coast shelf indicate that in addition to ocean temperature atmospheric temperature and surface derived meltwater influenced east antarctic ice mass balance under warmer than present climate conditions our results imply a dynamic eais response with continued anthropogenic warming and suggest that the eais contribution to future global sea level projections may be under estimated

contribution of antarctica to past and future sea level rise polar temperatures over the last several million years have at times been slightly warmer than today yet global mean sea level has been 6 9 metres higher as recently as the last interglacial 130 000 to 115 000 years ago and possibly higher during the pliocene epoch about three million years ago in both cases the antarctic ice sheet has been implicated as the primary contributor hinting at its future vulnerability here we use a model coupling ice sheet and climate dynamics including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine terminating ice cliffs that is calibrated against pliocene and last interglacial sea level estimates and applied to future greenhouse gas emission scenarios antarctica has the potential to contribute more than a metre of sea level rise by 2100 and more than 15 metres by 2500 if emissions continue unabated in this case atmospheric warming will soon become the dominant driver of ice loss but prolonged ocean warming will delay its recovery for thousands of years