circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

freshening of the ross sea during the late 20th century ocean measurements in the ross sea over the past four decades one of the longest records near antarctica reveal marked decreases in shelf water salinity and the surface salinity within the ross gyre these changes have been accompanied by atmospheric warming on ross island ocean warming at depths of 300 meters north of the continental shelf a more negative southern oscillation index and thinning of southeast pacific ice shelves the freshening appears to have resulted from a combination of factors including increased precipitation reduced sea ice production and increased melting of the west antarctic ice sheet

on the wind forcing of bottom pressure variability at amsterdam and kerguelen islands southern indian ocean 1 bottom pressure records from amsterdam and kerguelen islands southern indian ocean were collected between 1986 and 1989 and interpreted previously in terms of volume transport fluctuations between the islands vassie et al 1994 since a substantial portion of the mean transport of the antarctic circumpolar current acc flows between the islands it was assumed that acc transport variability was the predominant signal being measured and comparisons with acc variability at other longitudes were made here we examine the relationship between atmospheric forcing and observed bottom pressure variability we find that kerguelen bottom pressure is forced predominantly by wind stress curl over the kerguelen plateau for periods between around 5 days and 1 year a weaker correlation with circumpolar eastward winds is observed but is seen not to be causal amsterdam pressure shows a similar local response to curl for high frequencies 5 10 days but at longer periods is sensitive to curl over the sub basin between kerguelen plateau the southeast indian ridge and antarctica this is the region of long wavelength sea level variability observed by chao and fu 1995 and interpreted in terms of a geostrophic resonance by webb and de cuevas 2002a altimetric studies confirm the usefulness of the amsterdam location in monitoring this mode we conclude that the amsterdam kerguelen pressure difference is not a useful measure of acc transport variability and that previous agreement found between variability observed here and other locations is largely fortuitous instead the series separately reflect different important dynamical modes in the southern indian ocean

antarctic bottom water from the adelie and george v land coast east antarctica 140 149 e 1 we report on observations of dense shelf water overflows and antarctic bottom water aabw formation along the continental margin of the adelie and george v land coast between 140 e and 149 e vertical sections and bottom layer water mass properties sampled during two rvib nathaniel b palmer hydrographic surveys nbp00 08 december 2000 january 2001 and nbp04 08 october 2004 describe the spreading of cold dense shelf water on the continental slope and rise from two independent source regions the primary source region is the adelie depression exporting high salinity dense shelf water through the adelie sill at 143 e an additional eastern source region of lower salinity dense shelf water from the mertz depression is identified for the first time from bottom layer properties northwest of the mertz sill and mertz bank 146 e 148 e that extend as far as the buffon channel 144 75 e in summer regional analysis of satellite derived ice production estimates over the entire region from 1992 to 2005 suggests that up to 40 of the total ice production for the region occurs over the mertz depression and therefore this area is likely to make a significant contribution to the total dense shelf water export concurrent time series from bottom mounted microcats and adcp instruments from the mertz polynya experiment april 1998 to may 1999 near the adelie sill and on the upper continental slope 1150 m and lower continental rise 3250 m to the north describe the seasonal variability in downslope events and their interaction with the ambient water masses the critical density for shelf water to produce aabw is examined and found to be 27 85 kg m 3 from the adelie depression and as low as 27 80 kg m 3 from the mertz depression this study suggests previous dense shelf water export estimates based on the flow through the adelie sill alone are conservative and that other regions around east antarctica with similar ice production to the mertz depression could be contributing to the total aabw in the australian antarctic basin

acceleration of the contribution of the greenland and antarctic ice sheets to sea level rise 1 ice sheet mass balance estimates have improved substantially in recent years using a variety of techniques over different time periods and at various levels of spatial detail considerable disparity remains between these estimates due to the inherent uncertainties of each method the lack of detailed comparison between independent estimates and the effect of temporal modulations in ice sheet surface mass balance here we present a consistent record of mass balance for the greenland and antarctic ice sheets over the past two decades validated by the comparison of two independent techniques over the last 8 years one differencing perimeter loss from net accumulation and one using a dense time series of time variable gravity we find excellent agreement between the two techniques for absolute mass loss and acceleration of mass loss in 2006 the greenland and antarctic ice sheets experienced a combined mass loss of 475 158 gt yr equivalent to 1 3 0 4 mm yr sea level rise notably the acceleration in ice sheet loss over the last 18 years was 21 9 1 gt yr2 for greenland and 14 5 2 gt yr2 for antarctica for a combined total of 36 3 2 gt yr2 this acceleration is 3 times larger than for mountain glaciers and ice caps 12 6 gt yr2 if this trend continues ice sheets will be the dominant contributor to sea level rise in the 21st century

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

antarctic bottom water warming and freshening contributions to sea level rise ocean freshwater budgets and global heat gain abstractfreshening and warming of antarctic bottom water aabw between the 1980s and 2000s are quantified assessing the relative contributions of water mass changes and isotherm heave the analysis uses highly accurate full depth ship based conductivity temperature depth measurements taken along repeated oceanographic sections around the southern ocean fresher varieties of aabw are present within the south pacific and south indian oceans in the 2000s compared to the 1990s with the strongest freshening in the newest waters adjacent to the antarctic continental slope and rise indicating a recent shift in the salinity of aabw produced in this region bottom waters in the weddell sea exhibit significantly less water mass freshening than those in the other two southern basins however a decrease in the volume of the coldest deepest waters is observed throughout the entire southern ocean this isotherm heave causes a salinification and warming on isobaths from the bottom up to the shallow potential tem

large multidecadal salinity trends near the pacific antarctic continental margin abstract ocean temperature and salinity measurements on and near the antarctic continental shelf in the southwest pacific sector are evaluated for evidence of temporal change shelf water in the southwest ross sea has declined in salinity by 0 03 decade 1 from 1958 to 2008 while its temperatures have increased in proportion to the influence of salinity on the sea surface freezing point modified deep water intrusions that reach the central ross ice shelf have freshened at a similar rate and cooled by 0 5 c since the late 1970s salinity has decreased by 0 08 decade 1 in the westward coastal and slope front currents consistent with increased melting of continental ice upstream in the amundsen sea overturning of those near surface waters during winter sea ice formation and mixing across the slope front is sufficient to account for the 5 decade shelf water salinity change a strong correlation between the freshening and change in the southern annular mode index suggests a link with the large scale atmosp

closing the sea level rise budget with altimetry argo and grace 1 an analysis of the steric and ocean mass components of sea level shows that the sea level rise budget for the period january 2004 to december 2007 can be closed using corrected and verified jason 1 and envisat altimetry observations of total sea level upper ocean steric sea level from the argo array and ocean mass variations inferred from grace gravity mission observations we find that the sum of steric sea level and the ocean mass component has a trend of 1 5 1 0 mm a over the period in agreement with the total sea level rise observed by either jason 1 2 4 1 1 mm a or envisat 2 7 1 5 mm a within a 95 confidence interval

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

twentieth century global mean sea level rise is the whole greater than the sum of the parts confidence in projections of global mean sea level rise gmslr depends on an ability to account for gmslr during the twentieth century there are contributions from ocean thermal expansion mass loss from glaciers and ice sheets groundwater extraction and reservoir impoundment progress has been made toward solving the enigma of twentieth century gmslr which is that the observed gmslr has previously been found to exceed the sum of estimated contributions especially for the earlier decades the authors propose the following thermal expansion simulated by climate models may previously have been underestimated because of their not including volcanic forcing in their control state the rate of glacier mass loss was larger than previously estimated and was not smaller in the first half than in the second half of the century the greenland ice sheet could have made a positive contribution throughout the century and groundwater depletion and reservoir impoundment which are of opposite sign may have been approximately equal in magnitude it is possible to reconstruct the time series of gmslr from the quantified contributions apart from a constant residual term which is small enough to be explained as a long term contribution from the antarctic ice sheet the reconstructions account for the observation that the rate of gmslr was not much larger during the last 50 years than during the twentieth century as a whole despite the increasing anthropogenic forcing semiempirical methods for projecting gmslr depend on the existence of a relationship between global climate change and the rate of gmslr but the implication of the authors closure of the budget is that such a relationship is weak or absent during the twentieth century

warming and freshening in the abyssal southeastern indian ocean abstract warming and freshening of abyssal waters in the eastern indian ocean between 1994 95 and 2007 are quantified using data from two closely sampled high quality occupations of a hydrographic section extending from antarctica northward to the equator these changes are limited to abyssal waters in the princess elizabeth trough and the australian antarctic basin with little abyssal change evident north of the southeast indian ridge as in previous studies significant cooling and freshening is observed in the bottom potential temperature salinity relations in these two southern basins in addition analysis on pressure surfaces shows abyssal warming of about 0 05 c and freshening of about 0 01 practical salinity scale 1978 pss 78 in the princess elizabeth trough and warming of 0 1 c with freshening of about 0 005 in the abyssal australian antarctic basin these 12 yr differences are statistically significant from zero at 95 confidence intervals over the bottom few to several hundred decibars of t

global contraction of antarctic bottom water between the 1980s and 2000s abstracta statistically significant reduction in antarctic bottom water aabw volume is quantified between the 1980s and 2000s within the southern ocean and along the bottom most southern branches of the meridional overturning circulation moc aabw has warmed globally during that time contributing roughly 10 of the recent total ocean heat uptake this warming implies a global scale contraction of aabw rates of change in aabw related circulation are estimated in most of the world s deep ocean basins by finding average rates of volume loss or gain below cold deep potential temperature surfaces using all available repeated hydrographic sections the southern ocean is losing water below 0 c at a rate of 8 2 2 6 106 m3 s 1 this bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near surface recovery apparently through a southward surge of circumpolar deep water from the north to the north smaller losses of bottom waters are see

impact of the 1997 98 enso on upper ocean characteristics in marguerite bay western antarctic peninsula 1 a year round sequence of hydrographic casts is used to trace the evolution of the upper ocean waters in marguerite bay western antarctic peninsula wap between 1998 and 2002 winter 1998 was anomalous showing an unusually deep mixed layer that became progressively more saline until spring reaching salinities as high as 34 0 the remnant of this mixed layer the winter water ww was the deepest and most saline observed atmospheric and cryospheric conditions were anomalous during winter 1998 at both local and regional scales locally we observed low sea ice concentrations high air temperatures and a high frequency of northerly winds these are the local manifestations of the strong enso event of 1997 1998 that was then rapidly weakening at the regional scale this enso produced significant anomalies in the sea ice distribution throughout the amundsen bellingshausen sea area and a large scale low pressure anomaly over the southeast pacific was seen to be responsible for the warm northerly winds we use a coupled mixed layer ice production model to investigate the enso driven forcings for the anomalous ocean conditions observed in winter 1998 this reveals that ice production is the main control on upper ocean stratification and that the deep saline mixed layer in 1998 was forced by anomalous sea ice conditions on spatial scales larger than purely local we conclude that the near coastal hydrography along the peninsula shows a profound response to enso with atmospheric and cryospheric forcings both implicated

existence of a marginal basin within the circum superior belt geochemical evidence from the churchill superior boundary in manitoba canada mafic volcanic rocks incorporated within the churchill superior boundary zone at ospwagan lake moak lake assean lake and the fox river belt constitute part of the circum superior belt major and trace element data have been obtained from mafic and ultramafic volcanic rocks from four localities the data provide a means of comparing and contrasting the geochemistry of the rocks from the portion of the circum superior belt surrounding the archean superior province in manitoba with other units from elsewhere in the belt and with rocks from the adjacent proterozoic churchill province major element geochemical data are consistent with the rocks having originally formed in a oceanic environment trace element and rare earth element data suggest that the environment was a marginal basin al2o3 tio2 ratios indicate that the magmas were derived from a mantle source by small degrees of partial melting high field strength element patterns show that the mantle source region giving rise to the rocks was relatively uniform in composition trace element mid ocean ridge basalt morb and chondrite normalized patterns indicate similarities in alkali element enrichment for the rocks at asean and ospwagan lakes virtually all the rocks have ti zr nb y and p levels morb which is consistent with the mantle source region having undergone a small degree of partial melting zr p ratios for the rocks from the fox river belt are consistently lower than those from the other localities and the assean lake rocks show slight enrichment in light rare earth elements which may suggest local compositional variability in the mantle source regions supplying the magmas to the fox river belt and the ospwagan assean and moak lake areas

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

southern ocean response to strengthening winds in an eddy permitting global climate model abstract a global climate model with horizontal resolutions in the ocean ranging from relatively coarse to eddy permitting is used to investigate the resolution dependence of the southern ocean response to poleward intensifying winds through the past and present centuries the higher resolution simulations show poleward migration of distinct ocean fronts associated with a more highly localized near surface temperature response than in the lower resolution simulations the higher resolution simulations also show increasing southward eddy heat transport less high latitude cooling and greater sea ice loss than the lower resolution simulations for all resolutions from relatively coarse to eddy permitting there is poleward migration of the antarctic circumpolar current in the atlantic and the western half of the indian basin finally zonal transports associated with the antarctic circumpolar current are shown to be sensitive to resolution and this is discussed in the context of recent observed change

accelerated west antarctic ice mass loss continues to outpace east antarctic gains a b s t r a c t article history while multiple data sources have confirmed that antarctica is losing ice at an accelerating rate different measurement techniques estimate the details of its geographically highly variable mass balance with different levels of accuracy spatio temporal resolution and coverage some scope remains for methodological improvements using a single data type in this study we report our progress in increasing the accuracy and spatial resolution of time variable gravimetry from the gravity recovery and climate experiment grace we determine the geographic pattern of ice mass change in antarctica between january 2003 and june 2014 accounting for glacio isostatic adjustment gia using the ij05 r2 model expressing the unknown signal in a sparse slepian basis constructed by optimization to prevent leakage out of the regions of interest we use robust signal processing and statistical estimation methods applying those to the latest time series of monthly grace solutions we map antarctica s mass loss in space and time as well as can be recovered from satellite gravity alone ignoring gia model uncertainty over the period 2003 2014 west antarctica has been losing ice mass at a rate of 121 8g t yr and has experienced large acceleration of ice mass losses along the amundsen sea coast of 18 5g t yr 2 doubling the mass loss rate in the past six years the antarctic peninsula shows slightly accelerating ice mass loss with larger accelerated losses in the southern half of the peninsula ice mass gains due to snowfall in dronning maud land have continued to add about half the amount of west antarctica s loss back onto the continent over the last decade we estimate the overall mass losses from antarctica since january 2003 at 92 10 gt yr

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

recent loss of floating ice and the consequent sea level contribution we combine new and published satellite observations and the results of a coupled ice ocean model to provide the first estimate of changes in the quantity of ice floating in the global oceans and the consequent sea level contribution rapid losses of arctic sea ice and small antarctic ice shelves are partially offset by thickening of antarctic sea ice and large antarctic ice shelves altogether 746 127 km 3 yr 1 of floating ice was lost between 1994 and 2004 a value that exceeds considerably the reduction in grounded ice over the same period although the losses are equivalent to a small 49 8 m yr 1 rise in mean sea level there may be large regional variations in the degree of ocean freshening and mixing ice shelves at the antarctic peninsula and in the amundsen sea for example have lost 481 38 km 3 yr 1

stochastic simulations of regional scale advective transport in fractured rock masses using block upscaled hydro mechanical rock property data summary a method for upscaling coupled hydro mechanical properties in fractured rock from the small metre scale to the large decametre to kilometre scale has been developed and applied within a stochastic modelling framework the method was implemented to assess the impact on estimates of advective travel times and transport pathways in the geosphere arising from uncertainties in the mechanical properties of a fractured host rock a 2d hypothetical geological environment for a deep nuclear waste repository characterized using rock and fracture property data from the site investigations at sellafield uk was used for the assessment hydro mechanical modelling was undertaken to obtain block scale equivalent hydraulic properties for discrete fracture networks described by power law statistics for fracture length and density values of the mechanical properties for the host rock were employed to derive a basic statistical model of the uncertainty in the key fracture property values joint compressive strength jcs and joint roughness coefficient jrc the block upscaling was carried out in two stages first mechanical modelling was undertaken to determine aperture distributions throughout each modelled fracture network using the distinct element code udec the barton bandis model for fracture closure was adopted in udec and each fracture network was modelled under a range of stress conditions with five jcs jrc pairs that reflected the spread of mechanical properties observed in the actual field data second mean block scale hydraulic conductivity tensors were determined for each stress and mechanical property combination by inverting the results from steady state hydraulic models for the mechanically controlled fracture networks finally a monte carlo simulation experiment was performed on a 2d profile model of the geosphere populated by sampling from the block upscaled hydraulic results particle tracking was applied to each realisation of the flow field in the geosphere to study the range of advective travel times from the repository to the seabed biosphere the simulated travel times were found to depend significantly on the rock and fracture mechanical properties with breakthrough times at the seabed ranging between 4000 years and 2 560 000 years the spread of travel times in the simulation results demonstrate the significance of both hydro mechanical rock properties and their spatial distribution the very low joint compressive strength jcs of the uppermost formation and the assumed lateral uniformly distributed mechanical and hydraulic parameters of this formation are observed to provide the dominant control on the particle travel times the present study suggests that observations of hydro mechanical property data and their spatial distribution can offer useful additional information for developing models of the regional scale hydraulic behaviour of a fractured host rock mass

calving fluxes and basal melt rates of antarctic ice shelves iceberg calving has been assumed to be the dominant cause of mass loss for the antarctic ice sheet with previous estimates of the calving flux exceeding 2 000 gigatonnes per year1 2 more recently the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front3 4 5 so far however no study has reliably quantified the calving flux and the basal mass balance the balance between accretion and ablation at the ice shelf base for the whole of antarctica the distribution of fresh water in the southern ocean and its partitioning between the liquid and solid phases is therefore poorly constrained here we estimate the mass balance components for all ice shelves in antarctica using satellite measurements of calving flux and grounding line flux modelled ice shelf snow accumulation rates6 and a regional scaling that accounts for unsurveyed areas we obtain a total calving flux of 1 321 144 gigatonnes per year and a total basal mass balance of 1 454 174 gigatonnes per year this means that about half of the ice sheet surface mass gain is lost through oceanic erosion before reaching the ice front and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking1 2 7 in addition the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves we find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering8 and enhanced discharge9 we suggest that basal mass loss is a valuable metric for predicting future ice shelf vulnerability to oceanic forcing