marine ice sheet stability we examine the stability of two dimensional marine ice sheets in steady state the dynamics of marine ice sheets is described by a viscous thin film model with two stefan type boundary conditions at the moving boundary or grounding line that marks the transition from grounded to floating ice one of these boundary conditions constrains ice thickness to be at a local critical value for flotation which depends on depth to bedrock at the grounding line the other condition sets ice flux as a function of ice thickness at the grounding line depending on the shape of the bedrock multiple equilibria may be possible using a linear stability analysis we confirm a long standing heuristic argument that asserts that the stability of these equilibria is determined by a simple mass balance consideration if an advance in the grounding line away from its steady state position leads to a net mass gain the steady state is unstable and stable otherwise this also confirms that grounding lines can only be stable in positions where bedrock slopes downwards sufficiently steeply

continued retreat of thwaites glacier west antarctica controlled by bed topography and ocean circulation the amundsen sea sector is experiencing the largest mass loss glacier acceleration and grounding line retreat in antarctica enhanced intrusion of circumpolar deep water onto the continental shelf has been proposed as the primary forcing mechanism for the retreat here we investigate the dynamics and evolution of thwaites glacier with a novel fully coupled ice ocean numerical model we obtain a significantly improved agreement with the observed pattern of glacial retreat using the coupled model coupled simulations over the coming decades indicate a continued mass loss at a sustained rate uncoupled simulations using a depth dependent parameterization of sub ice shelf melt significantly overestimate the rate of grounding line retreat compared to the coupled model as the parameterization does not capture the complexity of the ocean circulation associated with the formation of confined cavities during the retreat bed topography controls the pattern of grounding line retreat while oceanic thermal forcing impacts the rate of grounding line retreat the importance of oceanic forcing increases with time as thwaites grounding line retreats farther inland

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

west antarctic ice sheet retreat in the amundsen sea driven by decadal oceanic variability mass loss from the amundsen sea sector of the west antarctic ice sheet has increased in recent decades suggestive of sustained ocean forcing or an ongoing possibly unstable response to a past climate anomaly lengthening satellite records appear to be incompatible with either process however revealing both periodic hiatuses in acceleration and intermittent episodes of thinning here we use ocean temperature salinity dissolved oxygen and current measurements taken from 2000 to 2016 near the dotson ice shelf to determine temporal changes in net basal melting a decadal cycle dominates the ocean record with melt changing by a factor of about four between cool and warm extremes via a nonlinear relationship with ocean temperature a warm phase that peaked around 2009 coincided with ice shelf thinning and retreat of the grounding line which re advanced during a post 2011 cool phase these observations demonstrate how discontinuous ice retreat is linked with ocean variability and that the strength and timing of decadal extremes is more influential than changes in the longer term mean state the nonlinear response of melting to temperature change heightens the sensitivity of amundsen sea ice shelves to such variability possibly explaining the vulnerability of the ice sheet in that sector where subsurface ocean temperatures are relatively high

a 308 year record of climate variability in west antarctica 1 we present a new stable isotope record from ellsworth land which provides a valuable 308 year record 1702 2009 of climate variability from coastal west antarctica climate variability at this site is strongly forced by sea surface temperatures and atmospheric pressure in the tropical pacific and related to local sea ice conditions the record shows that this region has warmed since the late 1950s at a similar magnitude to that observed in the antarctic peninsula and central west antarctica however this warming trend is not unique more dramatic isotopic warming and cooling trends occurred in the mid nineteenth and eighteenth centuries suggesting that at present the effect of anthropogenic climate drivers at this location has not exceeded the natural range of climate variability in the context of the past 300 years

response of marine terminating glaciers to forcing time scales sensitivities instabilities and stochastic dynamics recent observations indicate that many marine terminating glaciers in greenland and antarctica are currently retreating and thinning potentially due to long term trends in climate forcing in this study we describe a simple two stage model that accurately emulates the response to external forcing of marine terminating glaciers simulated in a spatially extended model the simplicity of the model permits derivation of analytical expressions describing the marine terminating glacier response to forcing we find that there are two time scales that characterize the stable glacier response to external forcing a fast time scale of decades to centuries and a slow time scale of millennia these two time scales become unstable at different thresholds of bed slope indicating that there are distinct slow and fast forms of the marine ice sheet instability we derive simple expressions for the approximate magnitude and transient evolution of the stable glacier response to external forcing which depend on the equilibrium glacier state and the strength of nonlinearity in forcing processes the slow response rate of marine terminating glaciers indicates that current changes at some glaciers are set to continue and accelerate in coming centuries in response to past climate forcing and that the current extent of change at these glaciers is likely a small fraction of the future committed change caused by past climate forcing finally we find that changing the amplitude of natural fluctuations in some nonlinear forcing processes such as ice shelf calving changes the equilibrium glacier state

glacial interglacial variation in organic carbon burial on the slope of the n w european continental margin 48 50 n the sources and fate of organic carbon corg were examined at two sites on the goban spur and a single site within the porcupine sea bight on the n w european slope between 48 n and 50 n here we present data on the sedimentary corg concentration its 13c 12c composition 13corg and corg n of 224 total and 119 fine fraction 63 m samples as a time series running from the instantaneous marker horizon heinrich layer 2 25 5 ka bp through the last glacial isotope stage 2 and interglacial isotope stage 1 to the present the significance of the n w european slope as a depositional sink for corg has declined in the past twenty thousand years with flux values decreasing two to three fold from the late glacial 60 70 mg c cm 2 ka 1 to low late holocene values significant shifts are seen in the 13corg signature of both the total and fine fractions between the present interglacial and last glacial the open slope of the goban spur is characterised by relatively low values over the last glacial with minimum values for the total sediment of around 24 5 occurring at ca 18 ka bp and generally increasing values during the holocene with the most recent samples having values of ca 19 2 to 20 2 in the porcupine sea bight a different relationship is evident with samples from the last glacial maximum being generally higher while values from the holocene are lower with a minimum of 23 3 occurring just below the mixed layer similar trends are observed in the fine fraction albeit with a different dynamic range paired analysis of 13corg and corg n are consistent with the organic matter being a climatically forced mixture from c3 photosynthetic terrigenous and marine organic matter sources an increased influence of terrestrial corg is suggested on the glacial goban spur and holocene porcupine sea bight and vice versa we suggest that this may be explained by modern wind driven flushing of terrigenous carbon across the armorican and celtic shelves to the n w and thence into porcupine sea bight during the last glacial increased strength of westerly winds created a residual current regime driving material to the s w where it was transported in vigorous tidal off shelf flushing to the upper slope followed by slope current transfer to the goban spur much of this material was presumably delivered by the great channel river to the outer shelf thus leading to enhanced flux of terrestrial corg on the goban spur the influence of a freshwater plume from meltwater discharge of the european ice sheet in this region is supported by foramininiferal 18o data from which a palaeo salinity reconstruction shows a greatest negative anomaly of 2 55 that is s 35 at the time of maximum terrestrial corg content in the sedimentary material in addition to showing that surface waters over goban spur were relatively fresh throughout the glacial our results also reveal variability in surface salinity on shorter timescales

the response of ice sheets to climate variability west antarctic ice sheet loss is a significant contributor to sea level rise while the ice loss is thought to be triggered by fluctuations in oceanic heat at the ice shelf bases ice sheet response to ocean variability remains poorly understood using a synchronously coupled ice ocean model permitting grounding line migration this study evaluates the response of an ice sheet to periodic variations in ocean forcing resulting oscillations in grounded ice volume amplitude is shown to grow as a nonlinear function of ocean forcing period this implies that slower oscillations in climatic forcing are disproportionately important to ice sheets the ice shelf residence time offers a critical time scale above which the ice response amplitude is a linear function of ocean forcing period and below which it is quadratic these results highlight the sensitivity of west antarctic ice streams to perturbations in heat fluxes occurring at decadal time scales

relationship between tidewater glacier calving velocity and water depth at the calving front an analysis of the relationship between iceberg calving rates and water depth has been completed for 22 tidewater glaciers a linear relationship provides reasonable accuracy with a correlation coefficient of 0 85 for all tidewater glaciers examined whether they be polar or temperate the polar glaciers have a slightly lower calving rate for a given water depth this relationship indicates a lower calving rate for water depths over 50 m than determined by brown and others 1982 it is based only on glaciers or ice streams and cannot be applied to ice shelves

the effect of buttressing on grounding line dynamics determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice sheet models here we investigate the role of lateral shear and ice shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets we derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non local ice shelf properties application of these results to a laterally confined version of the mismip 1a experiment shows that the boundary condition at the ice shelf front i e the calving law is a major control on the location and stability of the grounding line in the presence of buttressing allowing for both stable and unstable grounding line positions on downwards sloping beds these results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice shelf properties in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed consequently the marine ice sheet instability hypothesis may not apply to buttressed marine ice sheets

active diagenetic formation of metal rich layers in n e atlantic sediments sediment cores from the porcupine abyssal plain exhibit an indurated layer 0 5 3 cm thick at depths of approximately 50 cm this is some 15 20 cm below the glacial holocene transition as interpreted by radiocarbon dating and the palaeontological criteria of ruddiman and mcintyre 1981 the layer is forming currently at the oxic post oxic boundary in the sediments as revealed by pore water data o2 and no 3 are present in solution above the layer while fe2 mn2 po3 4 and nh 4 are present in solution below and all these species show concentration gradients indicating fluxes into the layer these data are consistent with the hypothesis for the initiation and sustained formation of such layers proposed by wilsonet al 1986a b the elements mn ni co fe p v cu zn and u are all enriched to varying degrees in the vicinity of the layer some differential stratification of these elements in the vertical consistent with a redox control is observed at one site with a 0 5 cm layer with mn ni and co above fe p v and cu in the layer and u below at another site the metal rich layer has higher fe and p concentrations and is more indurated here all enrichments except co are contained within a single layer sample 3 cm thick

an observationally validated theory of viscous flow dynamics at the ice shelf calving front an analytical theory is developed for ice flow velocity in a boundary layer couplet at the calving front the theory has simple quantitative characteristics that relate ice front velocity to thickness strain rate and shelf width matching one set of empirically derived relationships alley and others 2008 and implying that these relationships predict ice velocity rather than calving rate the two boundary layers are where longitudinal and transverse flow fields change from the interior flow to patterns consistent with the calving front stress condition numerical simulations confirm the analytical theory the quantitative predictions of the theory have low sensitivity to unmeasured parameters and to shelf plan aspect ratio while its robustness arises from its dependence on the scale invariance of the governing equations the theory provides insights into calving the stability of ice shelf calving fronts the stability of the grounding line of laterally resisted ice streams and also suggests that the calving front is an instructive dynamical analogue to the grounding line

the response of glaciers to climatic persistence 1 attribution of past glacier fluctuations to changes in the climate requires character2 izing glacier mass balance variability the available observational records which are 3 relatively short are consistent with random fluctuations uncorrelated in time plus 4 an anthropogenic trend however longer records of other climate variables suggest 5 that in fact there is a degree of temporal persistence associated with internal that 6 is unforced climate variability and that it varies with location and climate variable 7 therefore it is likely that climatic persistence does exist for mass balance but that 8 records are too short to confirm its presence or establish its magnitude with conven9 tional statistical tests extending previous work we explore the impact of potential 10 climatic persistence on glacier length fluctuations we use a numerical model and a 11 newly developed analytical model to establish that even a degree of persistence so 12

the transient response of ice shelf melting to ocean change abstractidealized modeling studies have shown that the melting of ice shelves varies as a quadratic function of ocean temperature however this result is the equilibrium response derived from steady ice ocean simulations subjected to a fixed ocean forcing this study considers instead the transient response of melting using unsteady simulations subjected to forcing conditions that are oscillated with a range of periods the results show that the residence time of water in the subice cavity offers a critical time scale when the forcing varies slowly period of oscillation residence time the cavity is fully flushed with forcing anomalies at all stages of the cycle and melting follows the equilibrium response when the forcing varies rapidly period residence time multiple cold and warm anomalies coexist in the cavity cancelling each other in the spatial mean and thus inducing a relatively steady melt rate this implies that all ice shelves have a maximum frequency of ocean variability that can

marine ice sheet instability amplifies and skews uncertainty in projections of future sea level rise sea level rise may accelerate significantly if marine ice sheets become unstable if such instability occurs there would be considerable uncertainty in future sea level rise projections due to imperfectly modeled ice sheet processes and unpredictable climate variability in this study we use mathematical and computational approaches to identify the ice sheet processes that drive uncertainty in sea level projections using stochastic perturbation theory from statistical physics as a tool we show mathematically that the marine ice sheet instability greatly amplifies and skews uncertainty in sea level projections with worst case scenarios of rapid sea level rise being more likely than best case scenarios of slower sea level rise we also perform large ensemble simulations with a state of the art ice sheet model of thwaites glacier a marine terminating glacier in west antarctica that is thought to be unstable these ensemble simulations indicate that the uncertainty solely related to internal climate variability can be a large fraction of the total ice loss expected from thwaites glacier we conclude that internal climate variability alone can be responsible for significant uncertainty in projections of sea level rise and that large ensembles are a necessary tool for quantifying the upper bounds of this uncertainty

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

sea level as a stabilizing factor for marine ice sheet grounding lines climate change could potentially destabilize marine ice sheets which would affect projections of future sea level rise1 2 3 4 specifically an instability mechanism5 6 7 8 has been predicted for marine ice sheets such as the west antarctic ice sheet that rest on reversed bed slopes whereby ice sheet thinning or rising sea level leads to irreversible retreat of the grounding line however existing analyses of this instability mechanism have not accounted for deformational and gravitational effects that lead to a sea level fall at the margin of a rapidly shrinking ice sheet9 10 11 here we present a suite of predictions of gravitationally self consistent sea level change following grounding line migration our predictions vary the initial ice sheet size and also consider the contribution to sea level change from various subregions of the simulated ice sheet using these results we revisit a canonical analysis of marine ice sheet stability5 and demonstrate that gravity and deformation induced sea level changes local to the grounding line contribute a stabilizing influence on ice sheets grounded on reversed bed slopes we conclude that accurate treatments of sea level change should be incorporated into analyses of past and future marine ice sheet dynamics

timing of late quaternary productivity pulses in the panama basin and implications for atmospheric co2 high resolution percent corg and 18oforam records obtained from panama basin core atlantis ii 54 25pc and additional data from nearby core p7 show that enhanced burial of organic carbon has characterized every major glacial period for the last 500 kyr in that area both corg concentration and mass accumulation rate profiles exhibit a sawtooth pattern with maxima occurring typically in the later stages of glacial periods comparison with dust records suggests that the carbon accumulation rate profile reflects both the upwelling history and a variable rate of iron input during the late quaternary the sawtooth character may derive from increased wind velocities and rates of upwelling during glacials which are indirectly related to ice volume sarnthein et al 1988 the rapid decline in export production at the end of glacials in the equatorial pacific may be attributed to the retreat of ice sheets thus reduced wind velocities and upwelling coupled with a coincident decline in atmospheric dust load and or delivery rate the corg accumulation rate profiles do not correlate well with atmospheric co2 records for example atmospheric co2 was already at a minimum 40 kyr ago when production in the panama basin began increasing dramatically commensurate with an increase in global dust levels using the relationship between the degree of photosynthetic fractionation and the concentration of free co2 in the surface ocean postulated by popp et al 1989 13corg measurements made on core p7 show that panama basin surface waters have been supplying co2 to the atmosphere continually for at least the last 50 kyr there is no evidence for a flux of co2 into the surface ocean in this area at any time during this period despite the higher production if the panama basin cores are representative of the eastern and central equatorial pacific then these observations weaken the influence on co2 drawdown postulated for increased glacial productivity at low latitudes

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

a slip law for glaciers on deformable beds slip of marine terminating ice streams over beds of deformable till is responsible for most of the contribution of the west antarctic ice sheet to sea level rise flow models of the ice sheet and till bedded glaciers elsewhere require a law that relates slip resistance slip velocity and water pressure at the bed we present results of experiments in which pressurized ice at its melting temperature is slid over a water saturated till bed steady state slip resistance increases with slip velocity owing to sliding of ice across the bed but above a threshold velocity till shears at its rate independent coulomb strength these results motivate a generalized slip law for glacier flow models that combines processes of hard bedded sliding and bed deformation

rate of mass loss across the instability threshold for thwaites glacier determines rate of mass loss for entire basin author s waibel ms hulbe cl jackson cs martin df abstract 2018 american geophysical union all rights reserved rapid change now underway on thwaites glacier tg raises concern that a threshold for unstoppable grounding line retreat has been or is about to be crossed we use a high resolution ice sheet model to examine the mechanics of tg self sustained retreat by nudging the grounding line just past the point of instability we find that by modifying surface slope in the region of the grounding line the rate of the forcing dictates the rate of retreat even after the external forcing is removed grounding line retreats that begin faster proceed more rapidly because the shorter time interval for the grounding line to erode into the grounded ice sheet means relatively thicker ice and larger driving stress upstream of the boundary retreat is sensitive to short duration re advances associated with reduced external forcing where the bathymetry allows regrounding even when an instability is invoked

the relationship between sedimentary organic carbon isotopic composition and organic biomarker compound concentration abstract a study of organic biomarker compounds which could serve as tracers of terrigenous and marine sedimentary organic matter sources was performed on samples from a 208 7 m hydraulic piston core hole dsdp hole 619 from the hemipelagic pigmy basin in the northern gulf of mexico organic carbon normalized concentrations of total long chain c37 c39 alkenones and some individual c27 c29 desmethyl sterols were determined to be useful proportional indicators tracers of preserved marine and terrigenous organic carbon respectively the alkenones whose only known source is marine phytoplankton of the class prymnesiophyceae generally occurred in higher concentrations in interglacial isotope stages 1 and 5a b than in the intervening glacial stages sterols c27 c29 apparently of a dominantly terrigenous origin occurred in lower concentrations during interglacial stages than in glacial stages tracers of both terrigenous and marine organic matter appear to be affected by the differential diagenetic alteration of the biomarker corg ratios as indicated by a simple first order kinetic model the lack of any desmethylor 4a methylsterol which is linearly related to the proportion of marine sedimentary organic matter as scaled by 13corg indicates that either 1 1 sedimentary diagenesis has obscured the biomarker corg vs 13corg record 2 2 phytoplanktonic assemblage changes caused variations in the biomarker corg ratio of the primary input preferential preservation of terrigenous sterols may result in a biased sedimentary record of sterol input which could be misinterpreted as indicating solely terrigenous sterol sources a simple model which characterizes the effects of sedimentary diagenesis on the relationship between corg normalized biomarker ratios and 13corg demonstrates the potential problems of long term differential diagenetic skewing on those tracer records

terrestrial organic carbon contributions to sediments on the washington margin elemental and stable carbon isotopic compositions and biomarker concentrations were determined in sediments from the columbia river basin and the washington margin in order to evaluate geochemical approaches for quantifying terrestrial organic matter in marine sediments the biomarkers include an homologous series of long chain n alkanes derived from the surface waxes of higher plants phenolic and hydroxyalkanoic compounds produced by cuo oxidation of two major vascular plant biopolymers lignin and cutin all marine sediments including samples collected from the most remote sites in cascadia basin showed organic geochemical evidence for the presence of terrestrial organic carbon using endmember values for the various biomarkers determined empirically by two independent means we estimate that the terrestrial contribution to the washington margin is 60 for shelf sediments 30 for slope sediments and decreases further to 15 in basin sediments results from the same geochemical measurements made with depth in gravity core 6705 7 from cascadia seachannel suggest that our approach to assess terrestrial organic carbon contributions to contemporary deposits on the washington margin can be applied to the study of sediments depositing in this region since the last glacial period

a numerical model of interactions between a polar ice stream and the ocean application to ice stream e west antarctica a time dependent numerical model has been developed to study the dynamics of a polar ice stream grounded below sea level the model was applied to ice stream e one of the two most active ice streams draining the ross sea sector of the west antarctic ice sheet the retarding force or back stress from the floating ross ice shelf needed to hold the model grounding line in dynamic equilibrium was found to compare closely with retarding force mapped from field data by earlier investigators this implies that the grounding line of ice stream e is in fact close to dynamic equilibrium neither advancing nor retreating rapidly if the ice stream and its catchment area are approximately in mass balance sensitivity tests showed that the model ice stream is more sensitive to changes in back stress from the ice shelf than to changes in the accumulation rate changes in back stress can be caused by changes in the average thickness of the ice shelf the model was used to simulate retreat of the ice stream from the edge of the continental shelf in the ross sea during the holocene period of rising sea level the sea floor was assumed to be isostatically depressed but to remain rigid during retreat the effect of the assumed retreat history of the ross ice shelf on the computed timing of grounding line retreat was investigated by considering several alternative ice shelf histories the results support the conclusion of earlier investigators that the ross ice shelf caused the grounding line to stop retreating near its present position after extensive retreat during the holocene in addition the timing of retreat of the calving front of the ross ice shelf exerted major influence on the timing of retreat of the ice stream grounding line

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

iceberg calving from the amery ice shelf east antarctica we investigate the iceberg calving cycle of the amery ice shelf ais east antarctica using evidence acquired between 1936 and 2000 the most recent major iceberg calving event occurred between late 1963 and early 1964 when a large berg totalling about 10000 km2 in area broke from the ice front the rate of forward advance of the ice front is presently 1300 1400 m a 1 at this rate of advance based on the present ice front position from recent radarsat imagery it would take 20 25 years to attain the 1963 pre calve position suggesting that the ais calving cycle has a period of approximately 60 70 years two longitudinal parallel to flow rifts approximately 25 km apart at the ais front are observed in satellite imagery acquired over the last 14 years these rifts have formed at suture zones in the ice shelf where neighbouring flow bands have separated in association with transverse spreading the rifts were 15 km rift a and 26 km rift b in length in september 2000 and will probably become the sides of a large tabular iceberg 25 km 25 km a transverse perpendicular to flow fracture visible at the upstream end of rift a in 1996 had propagated 6 km towards rift b by september 2000 when it meets rift b the iceberg will calve a satellite image acquired in 1962 shows an embayment of this size in the ais front hence we deduce that this calving pattern also occurred during the last calving cycle and therefore that the calving behaviour of the ais apparently follows a regular pattern

could the last interglacial constrain projections of future antarctic ice mass loss and sea level rise deep uncertainty in future antarctic ice sheet mass loss makes it challenging to produce robust projections of sea level rise previous studies used peak last interglacial period lig 129 116 ka

revisiting antarctic ice loss due to marine ice cliff instability predictions for sea level rise this century due to melt from antarctica range from zero to more than one metre the highest predictions are driven by the controversial marine ice cliff instability mici hypothesis which assumes that coastal ice cliffs can rapidly collapse after ice shelves disintegrate as a result of surface and sub shelf melting caused by global warming but mici has not been observed in the modern era and it remains unclear whether it is required to reproduce sea level variations in the geological past here we quantify ice sheet modelling uncertainties for the original mici study and show that the probability distributions are skewed towards lower values under very high greenhouse gas concentrations the most likely value is 45 centimetres however mici is not required to reproduce sea level changes due to antarctic ice loss in the mid pliocene epoch the last interglacial period or 1992 2017 without it we find that the projections agree with previous studies all 95th percentiles are less than 43 centimetres we conclude that previous interpretations of these mici projections over estimate sea level rise this century because the mici hypothesis is not well constrained confidence in projections with mici would require a greater range of observationally constrained models of ice shelf vulnerability and ice cliff collapse by better quantifying uncertainties for marine ice cliff instability future antarctic ice loss is predicted to be much lower than previously estimated

marine ice sheet dynamics part 1 the case of rapid sliding marine ice sheets are continental ice masses resting on bedrock below sea level their dynamics are similar to those of land based ice sheets except that they must couple with the surrounding floating ice shelves at the grounding line where the ice reaches a critical flotation thickness in order to predict the evolution of the grounding line as a free boundary two boundary conditions are required for the diffusion equation describing the evolution of the grounded ice thickness by analogy with stefan problems one of these conditions imposes a prescribed ice thickness at the grounding line and arises from the fact that the ice becomes afloat the other condition must be determined by coupling the ice sheet to the surrounding ice shelves here we employ matched asymptotic expansions to study the transition from ice sheet to ice shelf flow for the case of rapidly sliding ice sheets our principal results are that the ice flux at the grounding line in a two dimensional ice sheet is an increasing function of the depth of the sea floor there and that ice thicknesses at the grounding line must be small compared with ice thicknesses inland these results indicate that marine ice sheets have a discrete set of steady surface profiles if they have any at all and that the stability of these steady profiles depends on the slope of the sea floor at the grounding line

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