sensitivity experiments for the antarctic ice sheet with varied sub ice shelf melting rates ice sheet modelling is an important tool for predicting the possible response of ice sheets to climate change in the past and future an established ice sheet model is sicopolis simulation code for polythermal ice sheets and for this study the previously grounded ice only model was complemented by an ice shelf module the new version of sicopolis is applied to the antarctic ice sheet driven by standard forcings defined by the searise sea level response to ice sheet evolution community effort a crucial point for simulations into the future is to obtain reasonable initial conditions by a palaeoclimatic spin up which we carry out over 125 000 years from the eemian until today we then carry out a set of experiments for 500 years into the future in which the surface temperature and precipitation are kept at their present day distributions while sub ice shelf melting rates between 0 and 200 m a 1 are applied these simulations show a significant but not catastrophic sensitivity of the ice sheet grounded ice volumes decrease with increasing melting rates and the spread of the results from the zero to the maximum melting case is similar to 0 65 m s l e metres sea level equivalent after 100 years and similar to 2 25 m s l e after 500 years

volume loss from antarctic ice shelves is accelerating the floating ice shelves surrounding the antarctic ice sheet restrain the grounded ice sheet flow thinning of an ice shelf reduces this effect leading to an increase in ice discharge to the ocean using 18 years of continuous satellite radar altimeter observations we have computed decadal scale changes in ice shelf thickness around the antarctic continent overall average ice shelf volume change accelerated from negligible loss at 25 64 cubic kilometers per year for 1994 2003 to rapid loss of 310 74 cubic kilometers per year for 2003 2012 west antarctic losses increased by 70 in the past decade and earlier volume gain by east antarctic ice shelves ceased in the amundsen and bellingshausen regions some ice shelves have lost up to 18 of their thickness in less than two decades

sustained increase in ice discharge from the amundsen sea embayment west antarctica from 1973 to 2013 we combine measurements of ice velocity from landsat feature tracking and satellite radar interferometry and ice thickness from existing compilations to document 41 years of mass flux from the amundsen sea embayment ase of west antarctica the total ice discharge has increased by 77 since 1973 half of the increase occurred between 2003 and 2009 grounding line ice speeds of pine island glacier stabilized between 2009 and 2013 following a decade of rapid acceleration but that acceleration reached far inland and occurred at a rate faster than predicted by advective processes flow speeds across thwaites glacier increased rapidly after 2006 following a decade of near stability leading to a 33 increase in flux between 2006 and 2013 haynes smith pope and kohler glaciers all accelerated during the entire study period the sustained increase in ice discharge is a possible indicator of the development of a marine ice sheet instability in this part of antarctica key points sustained ase mass flux increase 77 since 1973 thwaites accelerated by 33 during the last 6 years speed changes are pervasive and rapid major implications for ice flow modeling 2014 american geophysical union all rights reserved

why marine ice sheet model predictions may diverge in estimating future sea level rise 1 despite major recent efforts marine ice sheet models aiming at predicting future mass loss from ice sheets still suffer from uncertainties with respect to grounding line migration a recent model intercomparison provided tools to test how models treat grounding line dynamics in a three dimensional setting here we use these tools to address to what extent differences in mass loss occur according to the approximation to the stokes equations describing marine ice sheet flow used we find that models that neglect components of vertical shearing in the force budget wrongly estimate ice sheet mass loss by 50 over century time scales when compared to models that solve the full stokes system of equations models that only include horizontal stresses also misrepresent velocities and ice shelf geometry suggesting that interactions between the grounded ice sheet and the ocean will also be modeled incorrectly based on these findings we strongly advise the use of high order models to compute reliable projections of ice sheet contribution to sea level rise

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

role of transition zones in marine ice sheet dynamics 1 it has been suggested that rapid thinning of the west antarctic ice sheet is due to increased melting under ice shelves caused by a gradual ocean warming shepherd et al 2004 payne et al 2004 showed that such melting could lead to an acceleration of grounded ice flow in this paper we analyze the response of a marine ice sheet to different perturbations near the grounding line using a numerical ice sheet model that takes into account longitudinal stress coupling and grounding line migration at subgrid precision based on a novel technique results show that stress transmission or longitudinal coupling across the grounding line plays a decisive role the grounding line migration is a function of the length scale over which the basal conditions change from frozen to the bed to floating the transition zone we demonstrate that thinning of the ice shelf due to bottom melting has a negligible effect on the grounded ice mass only perturbations at the grounding line or reduction in buttressing of the ice shelf substantially thins the grounded ice sheet inclusion of lateral drag does not alter these results qualitatively marine ice sheets with large transition zones such as ice streams seem highly sensitive to such perturbations compared to ice sheets with small transition zones such as an abrupt ice sheet ice shelf junction

dynamic in stability of thwaites glacier west antarctica 1 thwaites glacier west antarctica has the potential to directly contribute 1 m to sea level and currently is losing mass and thinning rapidly here we report on regional results for the sea level response to ice sheet evolution searise experiments and investigate the impact of i spatial resolution within existing data sets ii grounding zone processes and iii till rheology on the dynamics of this outlet glacier in addition to the searise data sets we use detailed aerogeophysical and satellite data from thwaites glacier as input to a coupled ice stream ice shelf ocean plume model that includes oceanic influences across a several kilometers wide grounding zone suggested by new high resolution data our results indicate that the ice tongue provides limited stability and that while future atmospheric warming will likely add mass to the surface of the glacier strong ice stream stabilization on bedrock highs narrower than the length of the grounding zone may be ephemeral if circulating waters substantially reduce basal resistance and enhance melting beneath grounded ice within this zone however we find that stability is significantly enhanced by effectively plastic till beds accurate projections of future sea level change relies on correct understanding of the till rheology as well as local basal processes near the grounding line

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

parameterization of the annual surface temperature and mass balance of antarctica this study entails the parameterization by means of a linear multiple regression analysis of the annual surface temperature and mass balance of antarctica the analysis was performed for the entire ice cap as well as for three separate regions ice shelves elevation less than 200 m the interior elevation above 1500 m and the escarpment region in between it was found that temperature can be parameterized very well in terms of elevation and latitude the latitudinal gradient on the ice shelves can be explained by the super adiabatic lapse rate along the surface and latitudinal temperature gradient in the interior assuming adiabatic descent of air in the inversion layer from the interior region towards the coast and an axisymmetric spreading over the ice shelves the surface mass balance can be parameterized reliably only in the interior where it has a strong positive correlation with the saturation vapour pressure of the free atmosphere and a significant correlation with the shape of the dome the convex shape of the dome contributes to the mass balance by inducing subsidence of the relatively moist air of the free atmosphere into the inversion layer this results in precipitation as radiative cooling in the inversion exceeds adiabatic warming an estimate is made of the annual horizontal and vertical advective velocities in the free atmosphere above the interior based on regression results and a physical analysis of the precipitation processes in this region a temperature sensitivity analysis was performed for the current mass balance distribution for a 1 k rise in surface temperature the regression estimate of the increase in accumulation on the grounded ice sheet is equivalent to a rate of sea level lowering of 0 2 mm a 1 this is about 30 less than estimates based on the current mass balance perturbated by the increase in saturation vapour pressure of the free atmosphere

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

marine ice sheet profiles and stability under coulomb basal conditions the behavior of marine terminating ice sheets such as the west antarctic ice sheet is of interest due to the possibility of rapid grounding line retreat and consequent catastrophic loss of ice critical to modeling this behavior is a choice of basal rheology where the most popular approach is to relate the ice sheet velocity to a power law function of basal stress recent experiments however suggest that near grounding line tills exhibit coulomb friction behavior here we address how coulomb conditions modify ice sheet profiles and stability criteria the basal rheology necessarily transitions to coulomb friction near the grounding line due to low effective stresses leading to changes in ice sheet properties within a narrow boundary layer ice sheet profiles taper off towards a flatter upper surface compared with the power law case and basal stresses vanish at the grounding line consistent with observations in the coulomb case the grounding line ice flux also depends more strongly on flotation ice thickness which implies that ice sheets are more sensitive to climate perturbations furthermore with coulomb friction the ice sheet grounds stably in shallower water than with a power law rheology this implies that smaller perturbations are required to push the grounding line into regions of negative bed slope where it would become unstable these results have important implications for ice sheet stability in a warming climate

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

numerical simulations of major ice streams in western dronning maud land antarctica under wet and dry basal conditions we study the presence and effect of subglacial water on the motion of the inland ice in western dronning maud land a full stokes model including three routing schemes for a thin film of subglacial water and a modification of a weertman type sliding relation to account for higher sliding velocities under wet basal conditions were used to perform 200 ka spin up simulations on a 2 5 km grid subsequent 30 ka simulations with wet and dry basal conditions were analysed for the effects of sliding on the thermal regime and velocities the occurrence of the major ice streams in this area is mainly controlled by the ice and bedrock geometry smaller glaciers only appear as pronounced individual glaciers when subglacial water is taken into account the thermal regime is affected by creep instabilities produced by an ice rheology including a microscopic water content leading to a cyclic behaviour on millennial time scales of the ice flow and occurrence of temperate ice at the base

nd isotopic evolution of the precambrian mantle abstract the most convincing examples of proterozoic oceanic crust assemblages occur in the circum superior fold belt of quebec nd isotope analyses of the chukotat and hellancourt basalts from this belt constrain the e nd of the early proterozoic depleted mantle reservoir at 4 to 6 along with data for phanerozoic ophiolites and modern morb the proterozoic basalts infer a constant rate of evolution of the depleted mantle at a rate of 2 2 e nd units per ga recalculation of sm nd isotopic data for archaean greenstone belts to their most reasonable pb pb or u pb ages suggests significantly greater isotopic heterogeneity in the archaean presumed uncontaminated tholeiites and al undepleted komatiites fall within the extrapolated growth limits for the proterozoic phanerozoic depleted mantle while al depleted komatiites adk in early archaean greenstone belts have significantly higher e nd the latter require derivation from a source whose isotopic composition evolved at 2 4 times the rate of the al undepleted komatiite and tholeiite source the isotopic heterogeneities noted for the archaean indicate a compositionally layered mantle by the late archaean the adk source was either isolated by declining geothermal gradients or had become remixed with the overlying mantle giving rise to the present depleted mantle reservoir

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

high tide of the warm pliocene implications of global sea level for antarctic deglaciation we obtained global sea level eustatic estimates with a peak of 22 m higher than present for the pliocene interval 2 7 3 2 ma from backstripping in virginia united states new zealand and enewetak atoll north pacifi c ocean benthic foraminiferal 18 o values and mg ca 18 o estimates statistical analysis indicates that it is likely 68 confi dence interval that peak sea level was 22 5 m higher than modern and extremely likely 95 that it was 22 10 m higher than modern benthic foraminiferal 18 o values appear to require that the peak was 20 21 m our estimates imply loss of the equivalent of the greenland and west antarctic ice sheets and some volume loss from the east antarctic ice sheet and address the longstanding controversy concerning the pliocene stability of the east antarctic ice sheet

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

widespread rapid grounding line retreat of pine island thwaites smith and kohler glaciers west antarctica from 1992 to 2011 we measure the grounding line retreat of glaciers draining the amundsen sea sector of west antarctica using earth remote sensing ers 1 2 satellite radar interferometry from 1992 to 2011 pine island glacier retreated 31 km at its center with most retreat in 2005 2009 when the glacier ungrounded from its ice plain thwaites glacier retreated 14 km along its fast flow core and 1 to 9 km along the sides haynes glacier retreated 10 km along its flanks smith kohler glaciers retreated the most 35 km along its ice plain and its ice shelf pinning points are vanishing these rapid retreats proceed along regions of retrograde bed elevation mapped at a high spatial resolution using a mass conservation technique that removes residual ambiguities from prior mappings upstream of the 2011 grounding line positions we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin 2014 american geophysical union all rights reserved

a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes 1 a new three dimensional thermomechanically coupled ice sheet model is developed contrary to the majority of three dimensional ice sheet models shallow ice approximation higher order stress gradients longitudinal and transverse are accounted for in the force balance equations the horizontal velocity field is determined from the force balance equations in their derivative form elliptical equation using the method presented by pattyn 2000 2002a the model is solved on a regular grid in the horizontal and an irregular grid in the vertical and is numerically stable basic experiments include the european ice sheet modeling initiative eismint benchmarks for largescale ice sheet models and a comparison with the saito blatter ice sheet model including higher order stress gradients saito et al 2003 detailed calculations of ice flow over three dimensional bedrock perturbations showed the validity of the higher order solution the model is capable of simulating the evolution of an ice stream within the ice sheet and shows important aspects of observed ice stream features such as the surface flattening and the importance of side drag the simulation of the ice flow over a subglacial lake results in a flattening of the surface a local velocity increase over the lake and a deviation of the ice flow from the main flow direction features which are also observed at lake vostok antarctica index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3230 mathematical geophysics numerical solutions keywords higher order ice sheet model thermomechanical ice stream subglacial lake citation pattyn f a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes j geophys res 108 b8 2382 doi 10 1029 2002jb002329 2003

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

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

ice sheet model sensitivities to environmental forcing and their use in projecting future sea level the searise project ten ice sheet models are used to study sensitivity of the greenland and antarctic ice sheets to prescribed changes of surface mass balance sub ice shelf melting and basal sliding results exhibit a large range in projected contributions to sea level change in most cases the ice volume above flotation lost is linearly dependent on the strength of the forcing combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments suggesting that nonlinear feedbacks are modest our models indicate that greenland is more sensitive than antarctica to likely atmospheric changes in temperature and precipitation while antarctica is more sensitive to increased ice shelf basal melting an experiment approximating the intergovernmental panel on climate change s rcp8 5 scenario produces additional first century contributions to sea level of 22 3 and 8 1 cm from greenland and antarctica respectively with a range among models of 62 and 14 cm respectively by 200 years projections increase to 53 2 and 26 7 cm respectively with ranges of 79 and 43 cm linear interpolation of the sensitivity results closely approximates these projections revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice sheet response to complicated forcings over 200 years can be linearized

multidecadal warming of antarctic waters decadal trends in the properties of seawater adjacent to antarctica are poorly known and the mechanisms responsible for such changes are uncertain antarctic ice sheet mass loss is largely driven by ice shelf basal melt which is influenced by ocean ice interactions and has been correlated with antarctic continental shelf bottom water asbw temperature we document the spatial distribution of long term large scale trends in temperature salinity and core depth over the antarctic continental shelf and slope warming at the seabed in the bellingshausen and amundsen seas is linked to increased heat content and to a shoaling of the mid depth temperature maximum over the continental slope allowing warmer saltier water greater access to the shelf in recent years regions of asbw warming are those exhibiting increased ice shelf melt

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

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

insights into spatial sensitivities of ice mass response to environmental change from the searise ice sheet modeling project i antarctica atmospheric oceanic and subglacial forcing scenarios from the sea level response to ice sheet evolution searise project are applied to six three dimensional thermomechanical ice sheet models to assess antarctic ice sheet sensitivity over a 500 year timescale and to inform future modeling and field studies results indicate i growth with warming except within low latitude basins where inland thickening is outpaced by marginal thinning ii mass loss with enhanced sliding with basins dominated by high driving stresses affected more than basins with low surface slope streaming ice and iii mass loss with enhanced ice shelf melting with changes in west antarctica dominating the signal due to its marine setting and extensive ice shelves cf minimal impact in the terre adelie george v oates and victoria land region of east antarctica ice loss due to dynamic changes associated with enhanced sliding and or sub shelf melting exceeds the gain due to increased precipitation furthermore differences in results between and within basins as well as the controlling impact of sub shelf melting on ice dynamics highlight the need for improved understanding of basal conditions grounding zone processes ocean ice interactions and the numerical representation of all three key points sensitivity study of antarctica to atmospheric oceanic and subglacial forcings different sectors of antarctica are vulnerable to the forcings atmospheric forcing lead to a growth but dynamic forcing lead to a mass loss 2013 american geophysical union all rights reserved

reassessment of the potential sea level rise from a collapse of the west antarctic ice sheet theory has suggested that the west antarctic ice sheet may be inherently unstable recent observations lend weight to this hypothesis we reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution we obtain a value for the global eustatic sea level rise contribution of about 3 3 meters with important regional variations the maximum increase is concentrated along the pacific and atlantic seaboard of the united states where the value is about 25 greater than the global mean even for the case of a partial collapse