extensive liquid meltwater storage in firn within the greenland ice sheet surface melt water from the greenland ice sheet can become trapped in firn delaying its journey to the sea radar and ice core observations provide direct evidence of a perennial aquifer in the firn layer in southern greenland that represents a potentially significant contribution to the greenland mass budget

mass balance of east antarctic glaciers and ice shelves from satellite data thevelocityandmassdischarge of ninemajoreast antarcticglaciersnot draining into the ross or filchner ronne ice shelves is investigated using interferometric syntheticapertureradar insar datafromtheeuropeanremote sensingsatellite1and2 ers 1 2 andradarsat 1 theglaciersare david ninnis mertz totten scott denman lambert shiraseandstancomb wills insarisusedtolocatetheirgroundinglinewithpre cision ice velocity is measured with either insar or a speckle tracking technique ice thickness is deduced from prior determined ice shelf elevation assuming hydrostatic equi librium mass fluxes are calculated both at the grounding line and at a flux gate located downstream the grounding line flux is compared to a mass input calculated from snow accumulation to deduce the glacier mass balance the calculation is repeated at the flux gate downstream of the grounding line to estimate the average bottom melt rate of the ice shelf under steady state conditions the main results are 1 grounding lines are found several tens of km upstream of prior identified positions not because of a recent ice sheet retreatbutbecause of the inadequacy of prior determined grounding line positions 2 no gross imbalance between outflow and inflow is detected on the nine glaciersbeing investi gated with anuncertainty of 10 20 prior determined largely positive mass imbalances wereduetoanincorrectlocalizationofthegroundingline 3 highratesofbottommelting 24 7 micea 1 are inferred neargrounding zones where ice reaches the deepestdraft a few glaciers exhibit lower bottom melt rates 4 7 micea 1 bottom melting however appearsto be a major source of mass loss on antarctic ice shelves

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

assessing the ability of numerical ice sheet models to simulate grounding line migration grounding line migration is a key process affecting the stability of marine ice sheets such as the west antarctic ice sheet wais grounding line motion is often included in numerical models simulating the past and future evolution of the wais however little attention has been paid to the numerical consistency of these models the aim of this paper is to assess the ability of simple versions of existing marine ice sheet models to simulate grounding line migration in particular we investigate the response of the grounding line to external forcing and the sensitivity of the models predictions to their numerics and the mechanical coupling between ice sheet and shelf from the model comparison there is no consensus on how the grounding line should react to changes in boundary conditions a crucial finding is the strong dependency of models using a fixed grid on numerical details such as the horizontal grid size this implies that we should be very wary about grounding line predictions from such models including mechanical coupling at the grounding line does not seem to change the qualitative behavior of the models this suggests that the way the grounding line is treated in marine ice sheet models dominantly determines the grounding line dynamics we find that models that employ a moving grid to explicitly track the grounding line do not share many of the deficiencies of the fixed grid models we conclude that at present no reliable model of the grounding line is available and further model development is urgently needed

laurentide ice sheet instability during the last deglaciation the factors leading to the full retreat of ice sheets during deglaciation are debated numerical modelling suggests that the laurentide ice sheet retreated only after a threshold for warming and radiative forcing was passed in the holocene

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

weak bed control of the eastern shear margin of thwaites glacier west antarctica recent acceleration and thinning of thwaites glacier west antarctica motivates investigation of the controls upon and stability of its present ice flow pattern its eastern shear margin separates thwaites glacier from slower flowing ice and the southern tributaries of pine island glacier troughs in thwaites glacier s bed topography bound nearly all of its tributaries except along this eastern shear margin which has no clear relationship with regional bed topography along most of its length here we use airborne ice penetrating radar data from the airborne geophysical survey of the amundsen sea embayment antarctica agasea to investigate the nature of the bed across this margin radar data reveal slightly higher and rougher bed topography on the slower flowing side of the margin along with lower bed reflectivity however the change in bed reflectivity across the margin is partially explained by a change in bed roughness from these observations we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties given the potential for future increases in ice flux farther downstream the eastern shear margin may be vulnerable to migration however there is no evidence that this margin is migrating presently despite ongoing changes farther downstream

static grounding lines and dynamic ice streams evidence from the siple coast west antarctica 1 grounding line dynamics are key to our understanding of marine ice sheets such as the west antarctic ice sheet wais we present an accurate and rapid method for locating grounding lines using glas icesat laser altimetry data we then apply this technique to the siple coast region of the wais our technique exploits the high surface slope at the grounding line relative to the adjacent flat ice streams and ice shelf the technique is validated by ground based observations comparisons between the current grounding line and earlier measured locations demonstrate that the siple coast s grounding line has been largely static over the last two decades this is important as the ice streams have been undergoing considerable variations over this period we conclude that ice stream flow speeds can vary while the grounding line remains stable we suggest that grounding lines may migrate abruptly and then stabilize rather than undergoing continuous retreat

constructing improved decadal records of antarctic ice shelf height change from multiple satellite radar altimeters abstract antarctica s ice shelves the floating extensions of the ice sheet exert an important dynamic constraint on the flow of ice from the grounded ice sheet to the ocean and therefore on changes in global sea level thinning of an ice shelf reduces its ability to restrain the ice discharge from the grounded ice sheet however our understanding of how ice shelf processes couple ice sheet changes to climate variability is still rudimentary in part this is due to the brevity and low temporal resolution of surveys of ice shelf thickness relative to the broad range of time scales on which ice sheet mass fluctuates here we present improved procedures to construct 18 year 1994 2012 time series of antarctic ice shelf surface height at high spatial resolution 30 km by merging data from three overlapping satellite radar altimeter missions ers 1 ers 2 and envisat we apply an averaging scheme to enhance the signal to noise ratio of height changes over the floating ice shelves and extract low order polynomial trends using a robust approach regularized regression with cross validation that accounts for both bias and variance in the fit we construct formal confidence intervals by bootstrap resampling of the residuals of the fit the largest source of height error arises from the interaction of the radar signal with the snow and firn surface on annual time scales changes in surface and sub surface scattering and radar penetration lead to apparent height changes that are larger than the true surface height change arising from densification our 18 year time series of surface height provide an insight into how ice shelves respond to the changing atmospheric and oceanic conditions our methods could also be applied to grounded portions of the ice sheets both in antarctica and greenland

satellite radar interferometry for monitoring ice sheet motion application to an antarctic ice stream satellite radar interferometry sri provides a sensitive means of monitoring the flow velocities and grounding line positions of ice streams which are indicators of response of the ice sheets to climatic change or internal instability the detection limit is about 1 5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction the grounding line detected by tidal motions where the ice goes afloat can be mapped at a resolution of 0 5 kilometer the sri velocities and grounding line of the rutford ice stream antarctica agree fairly well with earlier ground based data the combined use of sri and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior

tide corrected flow velocity and mass balance of campbell glacier tongue east antarctica derived from interferometric sar abstract accurate measurement of ice flow velocity of floating glaciers is required to estimate ice mass balance from volume flux thinning thickening basal melting freezing and surface accumulation ablation we derived a tide corrected ice velocity map and mass budget of campbell glacier tongue cgt in east antarctica by using 14 cosmo skymed one day tandem differential interferometric sar dinsar pairs obtained from january to november 2011 the vertical tidal deflection of cgt was estimated and removed from the dinsar images by using a tide deflection ratio map generated by double differential insar ddinsar method we then generated averaged ice flow velocity v map and its standard deviation v map ice flow velocity increased from the upper part of the grounding line 0 20 m d 1 to the seaward edge of cgt 0 67 m d 1 with v less than 0 04 m d 1 in the main stream of cgt the eastern part of cgt flows slower than the western part because it is grounded along the flow line and thus experiences severe basal drag flux mass balance fmb i e ice thickness change by volume flux divergence of cgt was obtained by combining the v map and an ice thickness value of 340 18 m estimated by the icesat glas data we used mass conservation assumption in which total mass balance tmb 6 29 1 37 m a 1 observed by icesat glas data is attributed to fmb basal mass balance bmb and surface mass balance smb 0 24 0 02 m a 1 mass loss in the freely floating zone of cgt is mainly caused by basal melting bmb 144 5 39 9 mt a 1 while thinning by volume flux fmb 67 2 8 9 mt a 1 is relatively small in the hinge zone of cgt mass change is contributed to fmb of 147 3 25 2 mt a 1 and bmb of 15 3 39 8 mt a 1 however basal freezing derived for the hinge zone may be erroneous as a result of the constant ice thickness assumption extrapolated from the freely floating zone

improved estimation of the mass balance of glaciers draining into the amundsen sea sector of west antarctica from the cecs nasa 2002 campaign in november december 2002 a joint airborne experiment by centro de estudios cientificos and nasa flew over the antarctic ice sheet to collect laser altimetry and radio echo sounding data over glaciers flowing into the amundsen sea a p 3 aircraft on loan from the chilean navy made four flights over pine island thwaites pope smith and kohler glaciers with each flight yielding 1 5 2 hours of data the thickness measurements reveal that these glaciers flow into deep troughs which extend far inland implying a high potential for rapid retreat interferometric synthetic aperture radar data insar and satellite altimetry data from the european remote sensing satellites ers 1 2 show rapid grounding line retreat and ice thinning of these glaciers using the new thickness data we have re evaluated glacier fluxes and the present state of mass balance which was previously estimated using ice thicknesses deduced largely from inversion of elevation data assuming hydrostatic equilibrium the revised total ice discharge of 241 5 km 3 a 1 exceeds snow accumulation by 81 17 km 3 a 1 of ice equivalent to a sea level rise of 0 21 0 04 mm a 1 this magnitude of ice loss is too large to be caused by atmospheric forcing and implies dynamic thinning of the glaciers this is confirmed by ice flow acceleration observed with insar we attribute the flow acceleration and ice thinning to enhanced bottom melting of the ice shelves by a warmer ocean which reduces buttressing of the glaciers and in turn accelerates them out of balance

rapid climate driven changes in outlet glaciers on the pacific coast of east antarctica multidecadal trends in the terminus position of outlet glaciers along the margin of the east antarctic ice sheet reveal three epochal patterns of glacier retreat or advance consistent with a rapid and coherent response to air temperature and sea ice trends it is widely known that glaciers in west antarctica and greenland have been discharging ice into the oceans in response to warming in recent years and that sea levels have risen as a result by contrast we hear little about the much larger east antarctic ice sheet which has been viewed as being less vulnerable to climate change owing to its location in an extremely cold climate chris stokes and colleagues have used satellite imagery to map a large sample of glaciers along the pacific coast of east antarctica and show that in fact they are responding to decadal climate variability in concert with climate glaciers retreated in the 1970s and 1980s advanced in the 1990s and approximately split between advance and retreat since the 2000s the authors conclude that parts of the world s largest ice sheet may be more responsive to external forcing than previously recognized observations of ocean terminating outlet glaciers in greenland and west antarctica1 2 3 4 5 6 indicate that their contribution to sea level is accelerating as a result of increased velocity thinning and retreat7 8 9 10 11 thinning has also been reported along the margin of the much larger east antarctic ice sheet1 but whether glaciers are advancing or retreating there is largely unknown and there has been no attempt to place such changes in the context of localized mass loss7 9 or climatic or oceanic forcing here we present multidecadal trends in the terminus position of 175 ocean terminating outlet glaciers along 5 400 kilometres of the margin of the east antarctic ice sheet and reveal widespread and synchronous changes despite large fluctuations between glaciers linked to their size three epochal patterns emerged 63 per cent of glaciers retreated from 1974 to 1990 72 per cent advanced from 1990 to 2000 and 58 per cent advanced from 2000 to 2010 these trends were most pronounced along the warmer western south pacific coast whereas glaciers along the cooler ross sea coast experienced no significant changes we find that glacier change along the pacific coast is consistent with a rapid and coherent response to air temperature and sea ice trends linked through the dominant mode of atmospheric variability the southern annular mode we conclude that parts of the world s largest ice sheet may be more vulnerable to external forcing than recognized previously

glaciology ice sheet advance in antarctica reliable forecasts of sea level rise depend on accurately modelling the dynamics of polar ice sheets a numerical framework that better reflects ice sheet basal drag adds greater realism to such models

flow of glaciar moreno argentina from repeat pass shuttle imaging radar images comparison of the phase correlation method with radar interferometry high resolution radar images of glaciar moreno argentina acquired by the shuttle imaging radar c sir c on 9 and 10 october 1994 at 24 cm wavelength l band are utilized to map the glacier velocity both interferometrically and using the phase correlation method the precision of the interferometric ice velocities is 1 8 cm d 1 6 m a 1 1 the phase correlation method measures ice velocity with a precision of 14 cm d 1 50 m a 1 with image data at a 6 m sample spacing acquired 1 day apart averaged strain rates are measured with a precision of 10 4 d 1 at a 240 m sample spacing with the phase correlation method and 10 5 d 1 with radar interferometry the phase correlation method is less precise than radar interferometry but it performs better in areas of rapid flow is more robust to temporal changes in glacier scattering and measures the glacier velocity in two dimensions with only one image pair using this technique we find that glaciar moreno flows at 400 m a 1 in the terminal valley and 800 m a 1 at the calving front in agreement with velocities recorded a decade ago assuming steady state flow conditions the vertical strain rates measured by sir c are combined with prior data on mass ablation to estimate the glacier thickness and ice discharge the calculated discharge is 0 6 0 2 km3 ice a 1 at 300 m elevation and 1 1 0 2 km3 ice a 1 at the equilibrium line elevation 1150 m which yields a balance accumulation of 6 1m ice a 1

deep glacial troughs and stabilizing ridges unveiled beneath the margins of the antarctic ice sheet the antarctic ice sheet has been losing mass over past decades through the accelerated flow of its glaciers conditioned by ocean temperature and bed topography glaciers retreating along retrograde slopes that is the bed elevation drops in the inland direction are potentially unstable while subglacial ridges slow down the glacial retreat despite major advances in the mapping of subglacial bed topography significant sectors of antarctica remain poorly resolved and critical spatial details are missing here we present a novel high resolution and physically based description of antarctic bed topography using mass conservation our results reveal previously unknown basal features with major implications for glacier response to climate change for example glaciers flowing across the transantarctic mountains are protected by broad stabilizing ridges conversely in the marine basin of wilkes land east antarctica we find retrograde slopes along ninnis and denman glaciers with stabilizing slopes beneath moscow university totten and lambert glacier system despite corrections in bed elevation of up to 1 km for the latter this transformative description of bed topography redefines the high and lower risk sectors for rapid sea level rise from antarctica it will also significantly impact model projections of sea level rise from antarctica in the coming centuries a high resolution update of antarctic bed topography using mass conservation reveals broad stabilizing ridges for glaciers flowing across the transantarctic mountains and stabilizing slopes beneath moscow university totten and lambert glacier system

north and northeast greenland ice discharge from satellite radar interferometry ice discharge from north and northeast greenland calculated from satellite radar interferometry data of 14 outlet glaciers is 3 5 times that estimated from iceberg production the satellite estimates obtained at the grounding line of the outlet glaciers differ from those obtained at the glacier front because basal melting is extensive at the underside of the floating glacier sections the results suggest that the north and northeast parts of the greenland ice sheet may be thinning and contributing positively to sea level rise

increased ice losses from antarctica detected by cryosat 2 we use 3 years of cryosat 2 radar altimeter data to develop the first comprehensive assessment of antarctic ice sheet elevation change this new data set provides near continuous 96 coverage of the entire continent extending to within 215 km of the south pole and leading to a fivefold increase in the sampling of coastal regions where the vast majority of all ice losses occur between 2010 and 2013 west antarctica east antarctica and the antarctic peninsula changed in mass by 134 27 3 36 and 23 18 gt yr 1 respectively in west antarctica signals of imbalance are present in areas that were poorly surveyed by past missions contributing additional losses that bring altimeter observations closer to estimates based on other geodetic techniques however the average rate of ice thinning in west antarctica has also continued to rise and mass losses from this sector are now 31 greater than over the period 2005 2010

exsolution of ca amphibole from glaucophane and the miscibility gap between sodic and calcic amphiboles transmission and analytical electron microscopy tem aem of glaucophane from glaucophane ca amphibole bearing blueschist and eclogite from two vermont localities ecologite brook and tillotson peak and one california locality cazadero has revealed the first evidence from exsolution for the miscibility gap between sodie and calcic amphiboles the tillotson peak samples and the cazadero samples contain coarsegrained glaucophane actinolite pairs while the eclogite brook samples contain coexisting glaucophane and actinolitic hornblende ca rich glaucophanes contain abundant fine scale lamellae of ca rich amphibole these lamellae are usually oriented near 100 and overline 2 01 high resolution tem hrtem images show them to be coherent the exsolution lamellae are so narrow beyond the resolution of aem that their true ca contents are obscured by analytical contributions from the surrounding host the aem data suggest that the lamellae are either winchite or actinolite depending on the true ca concentration in most cases the exsolution lamellae have very curved interfaces and show variable orientation this is attributed to the close similarity of unit cell parameters for the two amphiboles three dimensional optimal phase boundary calculations using eplag fleet 1982 show that the observed 100 and overline 2 01 orientations are consistent with the minimization of area strain between the two lattices along the interface some samples show evidence for incipient exsolution in the form of homogeneously distributed fine scale precipitates these results suggest that ca rich glaucophanes from other glaucophane actinolite assemblages may be exsolved at the tem scale the coexisting amphiboles from eclogite brook also have been studied using the electron microprobe emp the compositional gap defined by the eclogite brook pairs is consistent with previously reported results but shows a wider break along the glaucophane actinolite pseudobinary join suggesting very limited solid solution up to temper atures of about 500 550 c the glaucophanes are relatively poor in ca except for one anomalous grain containing ca amphibole lamellae ferric iron estimated by normalization to fixed cation numbers is strongly partitioned into the actinolitic hornblende and the glaucophanes are very poor in the riebeckite component

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

on the short term grounding zone dynamics of pine island glacier west antarctica observed with cosmo skymed interferometric data using radar satellite data from the italian cosmo skymed csk constellation and the german tandem x formation we present comprehensive measurements of the bi weekly grounding line dynamics of pine island glacier west antarctica from august to december 2015 the one day repeat cycle of csk reveals tidally induced grounding line migration on the scale of kilometers and extensive seawater intrusion within the grounding zone which significantly exceeds that predicted for a stiff bed but are consistent with that calculated for a deformable bed the deformable bed also explains the continuous draining filling of subglacial lakes proximal to the grounding line after correction for oceanic tides we estimate a retreat rate for 2011 2015 of 0 3 km yr at the glacier center and 0 5 km yr on the sides which is three times slower than for 1994 2011 1 2 km yr at the center we attribute the decrease in retreat rate to colder ocean conditions in 2012 2013 relative to 2000 2011

rapid sea level rise along the antarctic margins in response to increased glacial discharge melting of the antarctic ice sheet and ice shelves reduces the salinity of the coastal seas satellite and model data suggest that the freshwater discharge has also caused coastal antarctic sea level to rise by about 2 mm yr 1 more than the regional mean

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

tidal modulation of ice shelf buttressing stresses ocean tides influence the flow of marine terminating glaciers observations indicate that the large fortnightly variations in ice flow at rutford ice stream in west antarctica originate in the floating ice shelf we show that nonlinear variations in ice shelf buttressing driven by tides can produce such fortnightly variations in ice flow these nonlinearities in the tidal modulation of buttressing stresses can be caused by asymmetries in the contact stress from migration of the grounding line and bathymetric pinning points beneath the ice shelf using a simple viscoelastic model we demonstrate that a combination of buttressing and hydrostatic stress variations can explain a diverse range of tidal variations in ice shelf flow including the period phase and amplitude of flow variations observed at rutford and bindschadler ice streams

the last glaciation of bear peninsula central amundsen sea embayment of antarctica constraints on timing and duration revealed by in situ cosmogenic 14c and 10be dating abstract ice streams in the pine island thwaites region of west antarctica currently dominate contributions to sea level rise from the antarctic ice sheet predictions of future ice mass loss from this area rely on physical models that are validated with geological constraints on past extent thickness and timing of ice cover however terrestrial records of ice sheet history from the region remain sparse resulting in significant model uncertainties we report glacial geological evidence for the duration and timing of the last glaciation of hunt bluff in the central amundsen sea embayment a multi nuclide approach was used measuring cosmogenic 10be and in situ 14c in bedrock surfaces and a perched erratic cobble bedrock 10be ages 118 144 ka reflect multiple periods of exposure and ice cover not continuous exposure since the last interglacial as had previously been hypothesized in situ 14c dating suggests that the last glaciation of hunt bluff did not start until 21 1 5 8 ka probably during the last glacial maximum and finished by 9 6 0 9 ka at the same time as ice sheet retreat from the continental shelf was complete thickening of ice at hunt bluff most likely post dated the maximum extent of grounded ice on the outer continental shelf flow re organisation provides a possible explanation for this with the date for onset of ice cover at hunt bluff providing a minimum age for the timing of convergence of the dotson and getz tributaries to form a single palaeo ice stream this is the first time that timing of onset of ice cover has been constrained in the amundsen sea embayment

the location of the grounding zone of evans ice stream antarctica investigated using sar interferometry and modelling evans ice stream west antarctica has five tributaries and a complex grounding zone the grounding zone of evans ice stream between the landward and seaward limits of tidal flexing was mapped using sar interferometry the width of the mapped grounding zone was compared with that derived from an elastic beam model and the tidal height changes derived from interferometry were compared with the results of a tidal model results show that in 1994 and 1996 the evans grounding zone was located up to 100 km upstream of its location in the bedmap dataset the grounding line of evans ice stream is subjected to 5 m vertical tidal forcing which would clearly affect ice stream flow

ice flow reorganization in west antarctica 2 5 kyr ago dated using radar derived englacial flow velocities we date a recent ice flow reorganization of an ice divide in the weddell sea sector west antarctica using a novel combination of inverse methods and ice penetrating radars we invert for two dimensional ice flow within an ice divide from data collected with a phase sensitive ice penetrating radar while accounting for the effect of firn on radar propagation and ice flow by comparing isochronal layers simulated using radar derived flow velocities with internal layers observed with an impulse radar we show that the divide s internal structure is not in a steady state but underwent a disturbance potentially implying a regional ice flow reorganization 2 5 1 8 2 9 kyr b p our data are consistent with slow ice flow in this location before the reorganization and the ice divide subsequently remaining stationary these findings increase our knowledge of the glacial history of a region that lacks dated constraints on late holocene ice sheet retreat and provides a key target for models that reconstruct and predict ice sheet behavior

rapid grounding line migration induced by internal ice stream variability observations indicate that the grounding line position of west antarctica is sensitive to both forced and unforced ice stream variabilities this study endeavors to characterize and understand unforced ice stream variability and associated grounding line migration we use a flowline ice stream model with an undrained plastic bed lateral shear stresses and a stretched grid refined in the grounding zone this model exhibits parameter space structure and hysteresis behavior similar to simpler ice stream models low prescribed temperature at the ice surface or weak geothermal heating produces thermal oscillations between active and stagnant phases as in previous spatially resolved ice flow models thermal activation propagates as an activation wave this model s fine resolution of the grounding zone allows for accurate simulations of transient unforced grounding line migration upstream of the grounding zone horizontal grid spacing of 1 km is required to accurately resolve activation waves activation waves induce the grounding line to migrate over 100 km at a rate that can exceed 1 km yr this is followed during the active phase by retreat which then continues for the duration of the stagnant phase grounding line retreat is the result of a negative mass balance near the grounding line but is not necessarily associated with negative mass balance for the entire ice stream in our simulations of internal variability the novel approach and experiments described in this study show that there can be large excursions in grounding line position in the absence of either external forcing or retrograde slopes

bed elevation of jakobshavn isbrae west greenland from high resolution airborne gravity and other data jakobshavn isbrae west greenland which holds a 0 6 m sea level volume equivalent has been speeding up and retreating since the late 1990s interpretation of its retreat has been hindered by difficulties in measuring its ice thickness with airborne radar depth sounders here we employ high resolution helicopter borne gravity data from 2012 to reconstruct its bed elevation within 50 km of the ocean margin using a three dimensional inversion constrained by fjord bathymetry data offshore and a mass conservation algorithm inland we find the glacier trough to be asymmetric and several 100 m deeper than estimated previously in the lower part from 1996 to 2016 the grounding line migrated at 0 6 km yr from 700 m to 1100 m depth upstream the bed drops to 1600 m over 10 km then slowly climbs to 1200 m depth in 40 km jakobshavn isbrae will continue to retreat along a retrograde slope for decades to come

lower satellite gravimetry estimates of antarctic sea level contribution a new model of glacial isostatic adjustment used in conjunction with grace satellite data suggests that ice loss from antarctica is contributing 0 19 millimetres per year to global mean sea level which is substantially less than previous grace based estimates estimates for recent mass loss from antarctica vary widely making it difficult to calculate any subsequent contributions to sea level rise one prominent approach uses the grace gravity recovery and climate experiment satellites to detect changes in gravity but the final estimates are dependent on the modelled response of the continent to changes in ice loading over time and to changes in lithospheric elastic rebound matt king and colleagues apply a new model of this glacial isostatic adjustment to the grace data and estimate antarctic mass loss between 2002 and 2010 to be about 69 gigatonnes per year much lower than previous estimates and equivalent to an annual sea level rise of about 0 2 millimetres recent estimates of antarctica s present day rate of ice mass contribution to changes in sea level range from 31 gigatonnes a year gt yr 1 ref 1 to 246 gt yr 1 ref 2 a range that cannot be reconciled within formal errors3 time varying rates of mass loss2 4 5 6 contribute to this but substantial technique specific systematic errors also exist3 in particular estimates of secular ice mass change derived from gravity recovery and climate experiment grace satellite data are dominated by significant uncertainty in the accuracy of models of mass change due to glacial isostatic adjustment7 8 gia here we adopt a new model of gia developed from geological constraints which produces gia rates systematically lower than those of previous models and an improved fit to independent uplift data9 after applying the model to 99 months from august 2002 to december 2010 of grace data we estimate a continent wide ice mass change of 69 18 gt yr 1 0 19 0 05 mm yr 1 sea level equivalent this is about a third to a half of the most recently published grace estimates2 5 which cover a similar time period but are based on older gia models plausible gia model uncertainties and errors relating to removing longitudinal grace artefacts destriping confine our estimate to the range 126 gt yr 1 to 29 gt yr 1 0 08 0 35 mm yr 1 sea level equivalent we resolve 26 independent drainage basins and find that antarctic mass loss and its acceleration is concentrated in basins along the amundsen sea coast outside this region we find that west antarctica is nearly in balance and that east antarctica is gaining substantial mass

grounding line retreat of pope smith and kohler glaciers west antarctica measured with sentinel 1a radar interferometry data we employ sentinel 1a c band satellite radar interferometry data in terrain observation with progressive scans mode to map the grounding line and ice velocity of pope smith and kohler glaciers in west antarctica for the years 2014 2016 and compare the results with those obtained using earth remote sensing satellites ers 1 2 in 1992 1996 and 2011 we observe an ongoing rapid grounding line retreat of smith at 2 km yr 40 km since 1996 an 11 km retreat of pope 0 5 km yr and a 2 km readvance of kohler since 2011 the variability in glacier retreat is consistent with the distribution of basal slopes i e fast along retrograde beds and slow along prograde beds we find that several pinning points holding dotson and crosson ice shelves disappeared since 1996 due to ice shelf thinning which signal the ongoing weakening of these ice shelves overall the results indicate that ice shelf and glacier retreat in this sector remain unabated

grounding line retreat of the west antarctic ice sheet from inner pine island bay ice loss from the marine based potentially unstable west antarctic ice sheet wais contributes to current sea level rise and may raise sea level by 3 3 m or even 5 m in the future over the past few decades glaciers draining the wais into the amundsen sea embayment ase have shown accelerated ice flow rapid thinning and fast retreat of the grounding line gl however the long term context of this ice loss is poorly constrained limiting our ability to accurately predict future wais behavior here we present a new chronology for wais retreat from the inner continental shelf of the eastern ase based on radiocarbon dates from three marine sediment cores the ages document a retreat of the gl to within 100 km of its modern position before ca 10 000 calibrated cal yr b p this early deglaciation is consistent with ages for gl retreat from the western ase our new data demonstrate that in contrast to the ross sea wais retreat from the ase shelf was largely complete by the start of the holocene our results further suggest either slow gl retreat from the inner ase shelf throughout the holocene or that any episodes of fast gl retreat must have been short lived thus today s rapid retreat may be exceptional during the holocene and may originate in recent changes in regional climate ocean circulation or ice sheet dynamics

a method of repeat photoclinometry for detecting kilometer scale ice sheet surface evolution small scale glaciological processes can drive large scale ice sheet behavior but remain underreported due to a paucity of surface elevation measurements in remote polar regions satellite images provide a relatively long record of spatially dense surface observations which allow us to investigate changes in ice sheet topography on the spatial scales of 1 10 km inferring surface topography from satellite images is an established technique but in previous efforts strict requirements for illumination conditions and image quality have led to a great quantity of discarded data relaxing quality requirements and fitting linear trends to the time series of image derived surface topography allow inclusion of more total signal and enable blending data from multiple platforms as a proof of concept we combine 121 moderate resolution imaging spectroradiometer images to develop a 250 m resolution map of surface elevation change at totten glacier antarctica achieving a 1 sigma uncertainty of 0 22 textrm m textrm a 1 for a 15 year period our method of repeat photoclinometry agrees with repeat laser altimetry while revealing clear patterns of surface elevation change associated with ice advection channelized ice shelf basal melt subglacial lake activity and possible grounding line migration

remote sensing of glacier and ice sheet grounding lines a review abstract the precise positioning and long term monitoring of the grounding line forming the boundary between grounded and floating ice of marine ice sheets and tidewater glaciers is critical for assessing ice sheet glacier stability ice sheet glacier mass balance calculations and numerical ice modelling however mapping of the grounding line is a challenging task since it is a subglacial and transient feature remote sensing techniques do not map the grounding line directly but locate surface features in the grounding zone that serve as proxies for the true grounding line position the large variety of methods products and publications as well as an inconsistent use of terminology additionally complicate the topic we present the first detailed review of existing remote sensing techniques and data for grounding line mapping benefits and limitations of the different techniques and data sources are discussed and illustrated with examples we identify a present tradeoff between accuracy and both spatial coverage and temporal resolution of the derived grounding line products rather new methods like sentinel 1 sar synthetic aperture radar differential range offset tracking drot and pseudo crossover radar altimetry pcra offer the potential for recent measurements and to fill gaps in some areas nevertheless there is a high demand of new spaceborne radar sensors that provide time series of very short repeat pass data to enable regular grounding line monitoring with most accurate differential sar interferometry dinsar on a continental scale

ephemeral grounding as a signal of ice shelf change interferometric synthetic aperture radar insar observations of ice shelf tidal deformation reveal the wide transition between grounded and floating ice as well as local areas where the ice shelf is only grounded at low tide a condition that we call ephemeral grounding ephemeral grounding creates a subtle local disturbance on the vertical motion field of the ice shelf surface in response to changes in occanic tide which is detected with millimetric precision using insar these ice shelf features are however not expected to produce a noticeable disturbance on the ice shelf velocity field to illustrate the influence of ephemeral grounding on ice shelf creep flow we use a finite element model in which ephemeral grounding is incorporated through a variable basal friction coefficient the results show that while ice rises permanently grounded areas have a pronounced influence on the ice shelf velocity field areas of ephemeral grounding have a vanishingly small influence what is thus of most interest is the capacity for observations of ephemeral grounding to reveal subtle changes in ice shelf thickness over time we discuss an example in the thwaites glacier area west antarctica where multi year data show how ice rises become ephemeral grounding and subsequently disappear this result is consistent with the grounding line retreat and ice thinning of thwaites glacier

tidal motion ice velocity and melt rate of petermann gletscher greenland measured from radar interferometry over a floating glacier ice tongue or an ice shelf the glacier motion measured by a single repeat pass radar interferogram is difficult to analyze because the long term steady motion of the ice is intermixed with its cyclic downward motion induced by tidal forcing multiple interferograms and a quadruple difference technique are necessary to separate the tidal signal from the long term steady motion of the ice an example of application of this technique is given here using ers 1 radar images of petermann gletscher a major outlet glacier of northern greenland tidal displacements are measured with 5 mm statistical noise the long term ice velocity is measured with a precision of 1 m a 1 the inferred tidal displacements agree well with model predictions from a fixed elastic beam with an elastic damping factor of 0 47 0 01 km 1 the hinge line is mapped with a precision of 20 80 m combining the interferometric ice velocities with ice thickness data the glacier ice discharge is calculated at and below the hinge line at the hinge line the ice flux is 12 1 1 km3 a 1 at the ice front calf ice production is only 0 59 km3 a 1 meaning that 95 of the ice that crosses the grounding line melts before it reaches the calving front assuming steady state conditions the melt rate of the glacier tongue averages 12 1 m a 1 with peak values exceeding 20 m a 1 near the hinge line this high melt rate cannot be accommodated by surface ablation alone only about 2 3 m a 1 and is attributed to pronounced basal melting of the ice tongue basal melting often assumed to be negligible in greenland is the dominant process of mass release from the floating section of petermann gletscher

ice streams in the laurentide ice sheet identification characteristics and comparison to modern ice sheets this paper presents a comprehensive review and synthesis of ice streams in the laurentide ice sheet lis based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed the inventory includes 117 ice streams which have been identified based on a variety of evidence including their bedform imprint large scale geomorphology topography till properties and ice rafted debris in ocean sediment records despite uncertainty in identifying ice streams in hard bedrock areas it is unlikely that any major ice streams have been missed during the last glacial maximum laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets large ice streams had extensive onset zones and were fed by multiple tributaries and where ice drained through regions of high relief the spacing of ice streams shows a degree of spatial self organisation which has hitherto not been recognised topography exerted a primary control on the location of ice streams but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock and where networks of ice streams switched direction repeatedly and probably over short time scales as the ice sheet retreated onto its low relief interior several ice streams show no correspondence with topography or underlying geology perhaps facilitated by localised build up of pressurised subglacial meltwater they differed from most other ice stream tracks in having much lower length to width ratios and have no modern analogues there have been very few attempts to date the initiation and cessation of ice streams but it is clear that ice streams switched on and off during deglaciation rather than maintaining the same trajectory as the ice margin retreated we provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30 of the margin was drained by ice streams at the lgm similar to that for present day antarctic ice sheets but this decreases to 15 and 12 at 12 cal ka bp and 10 cal ka bp respectively the extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate or internal ice dynamical feedbacks unrelated to climate or both is largely unknown and represents a key area for future work to address

four decade record of pervasive grounding line retreat along the bellingshausen margin of west antarctica changes to the grounding line where grounded ice starts to float can be used as a remotely sensed measure of ice sheet susceptibility to ocean forced dynamic thinning constraining this susceptibility is vital for predicting antarctica s contribution to rising sea levels we use landsat imagery to monitor grounding line movement over four decades along the bellingshausen margin of west antarctica an area little monitored despite potential for future ice losses we show that 65 of the grounding line retreated from 1990 to 2015 with pervasive and accelerating retreat in regions of fast ice flow and or thinning ice shelves venable ice shelf confounds expectations in that despite extensive thinning its grounding line has undergone negligible retreat we present evidence that the ice shelf is currently pinned to a sub ice topographic high which if breached could facilitate ice retreat into a significant inland basin analogous to nearby pine island glacier

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

subglacial bathymetry and sediment layer distribution beneath the pine island glacier ice shelf west antarctica modeled using aerogravity and autonomous underwater vehicle data

early last interglacial greenland ice sheet melting and a sustained period of meridional overturning weakening a model analysis of the uncertainties proxy data suggest that the last interglacial lig 130 116 ka bp climate was characterized by higher temperatures a partially melted greenland ice sheet gis and a changed atlantic meridional overturning circulation amoc notwithstanding the uncertainties in lig palaeoclimatic reconstructions this setting potentially provides an opportunity to evaluate the relation between gis melt and the amoc as simulated by climate models however first we need to assess the extent to which a causal relation between early lig gis melt and the weakened amoc is plausible with a series of transient lig climate simulations with the loveclim earth system model we quantify the importance of the major known uncertainties involved in early lig gis melt scenarios based on this we construct a specific scenario that is within the parameter space of uncertainties and show that it is physically consistent that early lig gis melting kept the amoc weakened notwithstanding this scenario is at the extreme end of the parameter space assuming that proxy based reconstructions of early lig amoc weakening offer a realistic representation of its past state this indicates that either 1 the amoc weakening was caused by other forcings than early lig gis melt or 2 the early lig amoc was less stable than indicated by our simulations and a small amount of gis melt was sufficient to keep the amoc in the weak state of a bi stable regime we argue that more intensive research is required because of the high potential of the early lig to evaluate model performance in relation to the amoc response to gis melt

ocean access to a cavity beneath totten glacier in east antarctica totten glacier has the largest thinning rate in east antarctica a derivation of the sea floor bathymetry reveals entrances to the ice cavity beneath the glacier that could allow deep warm water to enter and enhance basal melting

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

grounding line variability and subglacial lake drainage on pine island glacier antarctica we produced a 6 year time series of differential tidal displacement for pine island ice shelf antarctica using speckle tracking methods applied to fine resolution terrasar x data these results reveal that the main grounding line has maintained a relatively steady position over the last 6 years following the speedup that terminated in 2009 in the middle of the shelf there are grounded spots that migrate downstream over the 6 year record examination of high resolution digital elevation models reveals that these grounded spots form where deep keels thickness anomalies advect over an approximately flow parallel bathymetric high maintaining intermittent contact with the bed these data sets also reveal several subsidence and uplift events associated with subglacial lake drainages in the fast flowing region above the grounding line although these drainages approximately double the rate of subglacial water flow over periods of a few weeks they have no discernible effect on horizontal flow speed

hinge line migration of petermann gletscher north greenland detected using satellite radar interferometry the synthetic aperture radar interferometry technique is used to detect the migration of the limit of tidal flexing or hinge line of the floating ice tongue of petermann gletscher a major outlet glacier of north greenland the hinge line is detected automatically from differential interferograms using a model fitting technique based on an elastic beam theory the statistical noise of the model fit is less than 3 mm and the hinge line is mapped with a precision of 30 m following automatic registration of multidate image data to a precision of 5 m hinge line migration is subsequently detected with a precision of 40 m in the horizontal plane across the glacier width the results show that the hinge line of petermann gletscher migrates back and forth with tide by 70 m in excellent agreement with the migration calculated from ocean tides predicted by a tidal model combined with the glacier surface and basal slope measured by an ice sounding radar superimposed on the short term hinge line migration due to tide we detect a hinge line retreat of 270 m in 3 87 years which varies across the glacier width by 120 m the retreat suggests glacier thinning at a rate of 78 35 cm ice a 1 coincidentally an analysis of ice volume fluxes indicates that the hinge line ice flux of petermann gletscher exceeds its balance flux by 0 88 1 km3 ice a 1 which in turn implies glacier thinning at 83 95 cm ice a 1 in the glacier lower reaches both methods therefore suggest that petermann gletscher is currently losing mass to the ocean

antarctic grounding line mapping from differential satellite radar interferometry 1 the delineation of an ice sheet grounding line i e the transition boundary where ice detaches from the bed and becomes afloat in the ocean is critical to ice sheet mass budget calculations numerical modeling of ice sheet dynamics ice ocean interactions oceanic tides and subglacial environments here we present 15 years of comprehensive high resolution mapping of grounding lines in antarctica using differential satellite synthetic aperture radar interferometry dinsar data from the earth remote sensing satellites 1 2 ers 1 2 radarsat 1 and 2 and the advanced land observing system alos palsar for years 1994 to 2009 dinsar directly measures the vertical motion of floating ice shelves in response to tidal oceanic forcing with millimeter precision at a sample spacing better than 50 m simultaneously over areas several 100 km wide in contrast with earlier methods that detect abrupt changes in surface slope in satellite visible imagery or altimetry data on stagnant and slow moving areas we find that breaks in surface slope are reliable indicators of grounding lines but on most fast moving glaciers and ice streams our dinsar results reveal that prior mappings have positioning errors ranging from a few km to over 100 km a better agreement is found with icesat s data also based on measurements of vertical motion but with a detection noise one order of magnitude larger than with dinsar overall the dinsar mapping of antarctic grounding lines completely redefines the coastline of antarctica

on the retrieval of internal temperature of antarctica ice sheet by using smos observations abstract internal temperature is an essential parameter for understanding ice sheet dynamics glaciological models provide estimations of temperature profiles over antarctica and few boreholes are also available but at present no measurement exists at the scale of the whole continent the analysis of passive l band observations from the soil moisture and ocean salinity smos satellite shows that thanks to the high penetration depth i e up to 1500 m it is possible to infer information on in depth glaciological properties of the ice sheet including temperature in this study the temperature profile is retrieved from smos observations using jointly glaciological and emission models the developed methodology is valid in the inner part of antarctica where the ice sheet is almost stable i e its velocity is limited to 10 m yr 1 this analysis points out that in several cases differences are observed between retrieved temperature profiles and those predicted by glaciological models in particular some geophysical parameters namely the geothermal heat flux and the mean annual accumulation need to be modified with respect to their prior values in order to simulate smos brightness temperatures results also clearly show that the reliability of the retrieved profile in depth decreases with increasing ice thickness due to the limited penetration of microwaves in the ice the obtained results prove the capability of l band 1 4 ghz passive microwave sensors for investigating the internal temperature of the ice sheet

evidence for rapid retreat and mass loss of thwaites glacier west antarctica thwaites glacier the second largest ice stream in west antarctica drains an area of 166 500 2000 km2 which accumulates 55 5 gt a 1 or 60 6 km3 ice a 1 into the amundsen sea unrestrained by an ice shelf using interferometric synthetic aperture radar insar data collected by the european remote sensing satellites ers 1 and 2 in 1996 an output flux of 71 7 gt a 1 or 77 8 km3 ice a 1 is estimated at the grounding line where ice thickness is deduced from hydrostatic equilibrium a similar flux 70 7 gt a 1 or 76 8 km3 ice a 1 is obtained at a gate located 20 km upstream where ice thickness was measured in 1978 by ice sounding radar total accumulation in between the two gates is 1 6 gt a 1 or 1 8 km3 ice a 1 ice discharge therefore exceeds mass accumulation by 30 15 and thwaites glacier must be thinning and retreating at present the insar data show that the glacier floating ice tongue exerts no back pressure on the inland ice calves into tabular icebergs along a significant fraction of its grounding line and has a grounding line thickness which exceeds a prior calculated limit for stability glacier thinning is confirmed at the coast by the detection of a 1 4 0 2 km retreat of its grounding line between 1992 and 1996 with insar which implies 3 2 0 6 m ice a 1 thinning at the glacier center and less near the sides these results complement the decimeter scale annual surface lowering observed with satellite radar altimetry several hundred km inland of the grounding line the magnitude of ice thinning estimated at the coast however rules out temporal changes in accumulation as the explanation for surface lowering ice thinning must be due to changes in ice flow

the relationship between sticky spots and radar reflectivity beneath an active west antarctic ice stream isolated areas of high basal drag or sticky spots are important and poorly understood features in the force balance and dynamics of west antarctic ice streams characterizing sticky spots formed by thin or drying subglacial till using ice penetrating radar is theoretically possible as high radar bed returned power brp is commonly related to an abundance of free water at the ice bed interface provided losses from englacial attenuation can be estimated in this study we use airborne radar data collected over evans ice stream to extract brp profiles and test the sensitivity of brp to the adopted englacial attenuation correction we analyse 11 20 km profiles in four fast flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions in the majority of profiles we note that the increase in basal drag is accompanied by a decrease in brp and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence a comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation and our findings generally support previous studies that advocate a modelling approach

the sensitivity of the atlantic meridional overturning circulation to enhanced freshwater discharge along the entire eastern and western coast of greenland the possible responses of the atlantic meridional overturning circulation amoc to increased freshwater discharge along the greenland coast has become an issue of growing concern given the increasing rate of the greenland ice sheet gris melting a recent model study suggested a weakened amoc of about 13 30 when a freshwater anomaly of 0 1 sv 1 sv 106 m3 s 1 was released along the entire greenland coast during the late twentieth century 1965 2000 in this study we use a fully coupled climate model to examine the sensitivity of amoc to a similar amount of freshwater forcing but released separately along the eastern the western and the entire greenland coast our results show that in all three cases there is a general weakening of the amoc mainly due to a reduced formation of labrador sea water moreover when additional freshwater is released along the eastern coast of greenland the amoc weakens more compared to the other two cases the different degree of convective mixing reduction in the irminger sea is the main reason for the spread in amoc responses in the three freshwater hosing experiments compared to the other two experiments the eastern coast experiment shows a relative warming in the labrador sea and the generation of a negative greenland tip jet like wind pattern anomaly these anomalies lead to a weaker convective mixing in the southern irminger sea and result subsequently in less formation of the simulated upper labrador sea water ulsw in the eastern coast experiment this study therefore highlights a potential important role for ulsw formation in determining the sensitivity of the amoc in response to large gris melting

tide deflection of campbell glacier tongue antarctica analyzed by double differential sar interferometry and finite element method abstract a glacier tongue the terminus of a glacier projecting out from the coastline and floating over the ocean experiences vertical deflection due to the fluctuation of ocean tide accurate analysis of the tidal deflection of a glacier tongue is necessary to understand the elastic properties and dynamics of glacial ice we extracted the tidal deflection of campbell glacier tongue cgt one of the fast flowing glacier tongues in east antarctica by using 120 double differential interferometric sar ddinsar images from the 16 cosmo skymed one day tandem interferometric sar image pairs obtained during 2010 and 2011 a map of tide deflection ratio map defined as the ratio of the tidal deflection over tide height was generated by pixel based linear regression between the ddinsar derived tidal deflection and the tidal variation predicted by various tide models such as tpxo7 1 fes2004 cats2008a and ross inv after the inverse barometer effect correction we also generated the maps of r 2 root mean square error rmse and intercept of the linear regression model we confirmed that the horizontal flow of the campbell glacier is steady over the whole data acquisition period due to the fact that values were close to zero over the grounded part of the glacier ross inv provided the most reliable map with the highest r 2 0 97 and the smallest rmse 0 3 cm beyond the hinge zone than other tide models the spatial variation of the tidal response of the whole cgt was clearly defined from the map the grounding line retreated by 0 3 1 5 km in comparison with that in 1996 defined by rignot et al 2011a the ice thickness of cgt was estimated to be 397 71 m from icesat surface elevation data the young s modulus of cgt was found to be 0 5 1 3 gpa 0 8 gpa at an ice thickness of 397 m by simulating the in the hinge zone using a 2 dimensional finite element method

mapping the grounding zone of the ross ice shelf antarctica using icesat laser altimetry we use laser altimetry from the ice cloud and land elevation satellite icesat to map the grounding zone gz of the ross ice shelf antarctica at 491 locations where icesat tracks cross the grounding line gl ice flexure in the gz occurs as the ice shelf responds to short term sea level changes due primarily to tides icesat repeat track analysis can be used to detect this region of flexure since each repeated pass is acquired at a different tidal phase the technique provides estimates for both the landward limit of flexure and the point where the ice becomes hydrostatically balanced we find that the icesat derived landward limits of tidal flexure are in many places offset by several km and up to 60 km from the gl mapped previously using other satellite methods we discuss the reasons why different mapping methods lead to different gl estimates including instrument limitations variability in the surface topographic structure of the gz and the presence of ice plains we conclude that reliable and accurate mapping of the gl is most likely to be achieved when based on synthesis of several satellite datasets

acoustic impedance and basal shear stress beneath four antarctic ice streams the acoustic impedance of the subglacial material beneath 7 2 km profiles on four ice streams in antarctica has been measured using a seismic technique the ice streams span a wide range of dynamic conditions with flow rates of 35 464 in a 1 the acoustic impedance indicates that poorly lithified or dilated sedimentary material is ubiquitous beneath these ice streams mean acoustic impedance across each profile correlates well with basal shear stress and the slipperiness of the bed indicating that acoustic impedance is a good diagnostic not only for the porosity of the subglacial material but also for its dynamic state deforming or non deforming beneath two of the ice streams lodged non deforming and dilated deforming sediment coexist but their distribution is not obviously controlled by basal topography or ice thickness their distribution may be controlled by complex material properties or the deformation history beneath rutford ice stream lodged and dilated sediment coexist and are distributed in broad bands several kilometres wide while on talutis inlet there is considerable variability over much shorter distances this may reflect differences in the mechanism of drainage beneath the ice streams the material beneath the slow moving carlson inlet is probably lodged but unlithified sediment this is consistent with the hypothesis that carlson inlet was once a fast flowing ice stream but is now in a stagnant phase which could possibly be revived by raised basal water content the entire bed beneath fast flowing evans ice stream is dilated sediment

the supply of subglacial meltwater to the grounding line of the siple coast west antarctica recent satellite studies have shown that active subglacial lakes exist under the antarctic ice streams and persist almost to their grounding lines when the lowest lying lakes flood the water crosses the grounding line and enters the sub ice shelf cavity modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice shelf base we examine the spatial and temporal variability in subglacial water supply to the grounding lines of the siple coast ice streams by combining estimates for lake volume change derived from ice cloud and land elevation satellite icesat data with a model for subglacial water transport our results suggest that subglacial outflow tends to concentrate towards six embayments in the siple coast grounding line although mean grounding line outflow is 60m3 s 1 for the entire siple coast maximum local grounding line outflow may temporarily exceed 300 m3 s 1 during the synchronized flooding of multiple lakes in a hydrologic basin variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the ross ice shelf cavity the temporal variability in grounding line outflow results in a net reduction in long term average melt rate but temporary peak melting rates may exceed the long term average by a factor of three

continued slowing of the ross ice shelf and thickening of west antarctic ice streams as part of the ross ice shelf geophysical and glaciological survey riggs ice velocities were measured on the ross ice shelf ris during 1973 78 comparisons of these with velocity estimates at the same locations derived from radarsat synthetic aperture radar sar measurements in 1997 and 2009 show velocity reduction in the southeast quadrant of the ice shelf by almost 200 m a 1 with deceleration rates increasing with time large areas of ice shelf in this region are lightly grounded forming an ice plain that increases local buttressing of the ice streams icesat measurements show this ice plain to be thickening the observed decrease in ice shelf velocities implies a total reduction in the mass of ice flowing into the ris from the west antarctic ice sheet wais by 23 gt a 1 shifting the mass balance of the wais drainage basin from strongly negative in the 1970s to strongly positive in 2009 the resulting decrease in ice advection should lead to ice shelf thinning further seaward of the ice plain this thinning would reduce the lateral drag and back stress of the shelf ice further contributing to thinning through an increase in spreading rate icesat measurements show recent thinning of most of the freely floating ice shelf

rapid ice unloading in the fleming glacier region southern antarctic peninsula and its effect on bedrock uplift rates rapid regional warming in the antarctic peninsula has led to the significant retreat and eventual collapse of several major ice shelves since the 1970s triggering the subsequent acceleration and thinning of their feeding glaciers the wordie ice shelf lying off the west coast of the antarctic peninsula has undergone long term disintegration since the 1960s with a substantial calving event occurring around 1989 followed by continuous steady retreat and its almost complete disappearance the dynamic response of the upstream glaciers to the ice shelf collapse and the response of the solid earth to the associated mass loss are not fully understood to quantify the mass loss from the system we generated a digital elevation model dem using airborne vertical and oblique imagery from 1966 and compared it to a dem derived from 2008 spot data this analysis reveals lowering over that time of approximately 60 m at the front of fleming glacier using icebridge and icesat 2 glas data spanning 2002 2014 we show an increased rate of mean ice surface lowering with rates post 2008 more than twice those of 2002 2008 we use these load change data as a basis for the simulation of viscoelastic solid earth deformation we subtract modeled elastic deformation rates and a suite of modeled viscous rates from gps derived three dimensional bedrock velocities at sites to the south of fleming glacier to infer properties of earth rheology assuming the pre breakup bedrock uplift was positive due to post last glacial maximum lgm ice retreat our viscoelastic corrected gps uplift rates suggest upper mantle viscosities are 2 1019pas and likely 1 1020pas in this region 1 2 orders of magnitude greater than previously found for the northern antarctic peninsula horizontal velocities at the gps site nearest the fleming glacier after the application of elastic and plate tectonic corrections point away from marguerite bay rather than the present glacier front this suggests that horizontal motion in the region reflects the earlier retreat of the glacier system following the lgm compatible with a relatively strong mantle in this region these findings highlight the need for improved understanding of ice load changes in this region through the late holocene in order to accurately model glacial isostatic adjustment

important role for ocean warming and increased ice shelf melt in antarctic sea ice expansion changes in sea ice significantly modulate climate change because of its high reflective and strong insulating nature in contrast to arctic sea ice sea ice surrounding antarctica has expanded1 with record extent2 in 2010 this ice expansion has previously been attributed to dynamical atmospheric changes that induce atmospheric cooling3 here we show that accelerated basal melting of antarctic ice shelves is likely to have contributed significantly to sea ice expansion specifically we present observations indicating that melt water from antarctica s ice shelves accumulates in a cool and fresh surface layer that shields the surface ocean from the warmer deeper waters that are melting the ice shelves simulating these processes in a coupled climate model we find that cool and fresh surface water from ice shelf melt indeed leads to expanding sea ice in austral autumn and winter this powerful negative feedback counteracts southern hemispheric atmospheric warming although changes in atmospheric dynamics most likely govern regional sea ice trends4 our analyses indicate that the overall sea ice trend is dominated by increased ice shelf melt we suggest that cool sea surface temperatures around antarctica could offset projected snowfall increases in antarctica with implications for estimates of future sea level rise

the effect of a salinity gradient on the dissolution of a vertical ice face we investigate experimentally the effect of stratification on a vertical ice face dissolving into cold salty water we measure the interface temperature ablation velocity and turbulent plume velocity over a range of salinity gradients and compare our measurements with results of similar experiments without a salinity gradient kerr mcconnochie j fluid mech vol 765 2015 pp 211 228 mcconnochie kerr j fluid mech vol 787 2016 pp 237 253 we observe that stratification acts to reduce the ablation velocity interface temperature plume velocity and plume acceleration we define a stratification parameter s n 2 q it phi o that describes where stratification will be important where n is the brunt vaisala frequency q is the height dependent plume volume flux and it phi o is the buoyancy flux per unit area without stratification the relevance of this stratification parameter is supported by our experiments which deviate from the homogeneous theory at approximately s 1 finally we calculate values for the stratification parameter at a number of ice shelves and conclude that ocean stratification will have a significant effect on the dissolution of both the antarctic and greenland ice sheets

cryospheric science vulnerable ice in the weddell sea of the west antarctic ice shelves those in the amundsen sea sector have given the most cause for concern ocean modelling of the weddell sea region together with a detailed survey of the ice bed morphology indicates that this region too may change soon

ice sheet grounding zone stabilization due to till compaction 1 based on modeling motivated by new gps and radio echo sounding surveys a few kilometers long zone of whillans ice stream west antarctica just inland of the grounding line has higher basal shear stress than the ice farther upstream or the freely slipping ice shelf downstream data from this zone show a few meter high upwarp of the surface overlying a large fold extending through all internal radar layers observed flowband modeling shows that the fold can be generated by decreased basal lubrication beneath the upwarp basal topography alone cannot create this fold physical modeling and available data suggest that low amplitude tidal flexure of the ice shelf extends a few kilometers inland downward flexing of this grounded ice from the rising tide would compact subglacial till resulting in higher basal shear stress this result suggests that important processes influencing grounding line stability are not included in modern whole ice sheet models

deglaciation of pope glacier implies widespread early holocene ice sheet thinning in the amundsen sea sector of antarctica abstract the amundsen sea sector of the antarctic ice sheet presently dominates the contribution from antarctica to sea level rise several large ice streams that currently drain the sector have experienced rapid flow acceleration grounding line retreat and thinning during the past few decades however little is known of their longer term millennial scale retreat history despite the reliance of several ice sheet and glacial isostatic adjustment models on such data for improving sea level prediction from this critical region this study investigates the timing and extent of surface lowering of one of those ice streams pope glacier since the last glacial maximum lgm using glacial geological evidence for former ice cover we present a new deglacial chronology for the glacier derived from surface exposure dating of glacially deposited cobbles and ice scoured bedrock from mount murphy and its surrounding peaks cosmogenic 10be exposure ages from 44 erratic cobbles and 5 bedrock samples and in situ 14c exposure ages from one erratic and 8 bedrock samples are predominantly in the range 5 5 16 ka although 10be inheritance from prior exposure is prevalent in some erratics and probably all bedrock samples none of the ages pre date the lgm from these results we infer that the surface of pope glacier lowered by 560 m during the early to mid holocene 9 6 ka at an average rate of 0 13 0 09 0 04 m yr 1 the lowering coincided with a period of enhanced upwelling of warm circumpolar deep water onto the continental shelf in the region a reduction in buttressing facilitated by such upwelling by an ice shelf that is thought to have spanned the embayment until 10 6 cal kyr bp could have triggered simultaneous early holocene thinning of pope glacier and glaciers elsewhere in the amundsen sea embayment

recent contributions of glaciers and ice caps to sea level rise satellite measurements of earth s gravity field show that the mass loss of glaciers and ice caps contributed to sea level rise by approximately 0 4 millimetres per year between 2003 and 2010 the extent to which mass loss from ice covered land areas contributes to sea level rise is not known accurately in part because previous global estimates use different techniques for glaciers ice caps and ice sheets the gravity recovery and climate experiment grace satellite has provided monthly measurements of the global gravity field since 2002 using grace data jacob et al assess regional mass loss between 2003 and 2010 and conclude that mass loss from ice covered land areas contributed almost 1 5 mm per year to the sea level rise estimated mass loss from the high mountains of asia was negligible in contrast to some other reports glaciers and ice caps gics are important contributors to present day global mean sea level rise1 2 3 4 most previous global mass balance estimates for gics rely on extrapolation of sparse mass balance measurements1 2 4 representing only a small fraction of the gic area leaving their overall contribution to sea level rise unclear here we show that gics excluding the greenland and antarctic peripheral gics lost mass at a rate of 148 30 gt yr 1 from january 2003 to december 2010 contributing 0 41 0 08 mm yr 1 to sea level rise our results are based on a global simultaneous inversion of monthly grace derived satellite gravity fields from which we calculate the mass change over all ice covered regions greater in area than 100 km2 the gic rate for 2003 2010 is about 30 per cent smaller than the previous mass balance estimate that most closely matches our study period2 the high mountains of asia in particular show a mass loss of only 4 20 gt yr 1 for 2003 2010 compared with 47 55 gt yr 1 in previously published estimates2 5 for completeness we also estimate that the greenland and antarctic ice sheets including their peripheral gics contributed 1 06 0 19 mm yr 1 to sea level rise over the same time period the total contribution to sea level rise from all ice covered regions is thus 1 48 0 26 mm 1 which agrees well with independent estimates of sea level rise originating from land ice loss and other terrestrial sources6

grounding line retreat of totten glacier east antarctica 1996 to 2013 2015 american geophysical union all rights reserved totten glacier east antarctica a glacier that holds a 3 9 m sea level change equivalent has thinned and lost mass for decades we map its grounding line positions in 1996 and 2013 using differential radar interferometry insar data and develop precise high resolution topographies of its ice surface and ice draft using nasa operation icebridge data insar data and a mass conservation method we detect a 1 to 3 km retreat of the grounding line in 17 years the retreat is asymmetrical along a two lobe pattern where ice is only grounded a few 10 m above sea level or ice plain which may unground further with only modest amounts of ice thinning the pattern of retreat indicates ice thinning of 12 m in 17 years or 0 7 0 1 m yr at the grounding line on average sustained thinning will cause further grounding line retreat but may not be conducive to a marine instability key points grounding line of totten glacier is retreating not as fast as west antarctica retreat pattern explained by the newly inferred bed geometry if ice thinning maintains bed geometry conducive to further retreat

greenland ice sheet mass balance reconstruction part iii marine ice loss and total mass balance 1840 2010 abstractgreenland ice sheet mass loss to the marine environment occurs by some combination of iceberg calving and underwater melting referred to here as marine ice loss lm this study quantifies the relation between lm and meltwater runoff r at the ice sheet scale a theoretical basis is presented explaining how variability in r can be expected to govern much of the lm variability over annual to decadal time scales it is found that r enhances lm through three processes 1 increased glacier discharge by ice warming softening and basal lubrication sliding 2 increased calving susceptibility through undercutting glacier front geometry and reducing ice integrity and 3 increased underwater melting from forcing marine convection applying a semiempirical lm f r parameterization to a surface mass balance reconstruction enables total ice sheet mass budget closure over the 1840 2010 period the estimated cumulative 171 yr net ice sheet sea level contribution is 25 10 mm the rise punctuated by periods

corrigendum to weathering intensity and pedostratigraphy of the middle to upper pleistocene loess palaeosol sequence of wels aschet upper austria quat int 265 2012 142 154

thickening of the ice stream catchments feeding the filchner ronne ice shelf antarctica 1 earlier ice sheet mass balance estimates disagree with satellite altimeter measured thickening of the catchment area that feeds the filchner ronne ice shelf antarctica in an effort to reconcile these differences we generated a new flux gate mass balance estimate using improved elevation and velocity data our results show a 39 26 gton yr thickening whereas earlier flux gate results indicated minor thinning our results are consistent with altimeter derived estimates of elevation change this suggests that altimeter observed thickening on the east antarctic plateau may be the result of some combination of a 20th century accumulation increase and a continuing response to the 50 accumulation increase that began in the early holocene