antarctic ice sheet melting in the southeast pacific the first oceanographic measurements across a deep channel beneath the calving front of pine island glacier reveal a sub ice circulation driven by basal melting of 10 12 m yr 1 a salt box model described here gives a melt rate similar to that of ice balance and numerical models 5 50 times higher than averages for the george vi and ross ice shelves melting is fueled by relatively warm circumpolar deep water that floods the deep floor of the amundsen and bellingshausen sea continental shelves reaching the deep draft of this floating glacier a revised melt rate for ice shelves in the southeast pacific sector raises circumpolar ice shelf melting to 756 gt yr 1 given prior estimates of surface accumulation and iceberg calving this suggests that the antarctic ice sheet is currently losing mass to the ocean

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

reconstruction of land and freshwater palaeoenvironments near the eocene oligocene boundary southern england mammalian assemblages in the bembridge limestone formation of late eocene age headon hill isle of wight england indicate habitats ranging from open woodland to closed forest distinctive lower and upper mammalian faunas reflect different fauna1 provinces probably in response to climatic fluctuations that foreshadowed the terminal eocene event in order to improve our understanding of these patterns we have examined a variety of other palaeoenvironmental indicators from this section these include palynological organic matter pom plant macrofossils non mammalian faunas organic geochemistry and stable isotopes the evidence shows that the depositional setting was a tranquil shallow freshwater lake with a brief lagoonal interval however evidence for habitats surrounding the lake is contradictory emphasizing the necessity for multidisciplinary approaches to palaeoenvironmental reconstruction the mammal faunas give unequivocal evidence for woodland or forest yet apart from some of the land snails there is no other indication of the presence of trees furthermore according to the mammalian evidence the lake was bordered by distinctive vegetation at different times with closed forest woodland during mar1 deposition and open woodland during black mud deposition but there are no parallel fluctuations in other biotic elements

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

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

modelling circumpolar deep water intrusions on the amundsen sea continental shelf antarctica results are presented from an isopycnic coordinate model of ocean circulation in the amundsen sea focusing on the delivery of circumpolar deep water cdw to the inner continental shelf around pine island bay the warmest waters to reach this region are channeled through a submarine trough accessed via bathymetric irregularities along the shelf break temporal variability in the influx of cdw is related to regional wind forcing easterly winds over the shelf edge change to westerlies when the amundsen sea low migrates west and south in winter spring this drives seasonal on shelf flow while inter annual changes in the wind forcing lead to inflow variability on a decadal timescale a modelled period of warming following low cdw influx in the late 1980 s and early 1990 s coincides with a period of observed thinning and acceleration of pine island glacier

sensitivity of 21st century sea level to ocean induced thinning of pine island glacier antarctica 1 pine island glacier pig antarctica is rapidly losing mass supporting arguments that it may play a major role in 21st century sea level rise yet this glacier s quantitative contribution to sea level based on theoretical and computational models is poorly known we have developed a basin scale glaciological model to examine the sensitivity of pig to a range of environmental forcings while oceanic melt likely played the leading role in recent thinning and retreat we find that the particular grounding line geometry with an extended ice plain in the 1990s made it susceptible to such forcing our model further indicates that while the rate of grounding line retreat should diminish soon the glacier s mass loss may continue at rates similar to or moderately elevated from the present while substantial our model derived maximum rate of 2 7 cm century is considerably smaller than previous heuristically derived bounds on the sea level contribution

antarctic ice shelf thickness from satellite radar altimetry ice shelf thickness is an important boundary condition for ice sheet and sub ice shelf cavity modelling it is required near the grounding line to calculate the ice flux used to determine ice sheet mass balance by comparison with the upstream accumulation in this mass budget approach the accuracy of the ice thickness is one of the limiting factors in the calculation we present a satellite retrieval of the ice thickness for all antarctic ice shelves using satellite radar altimeter data from the geodetic phases of the european remote sensing satellite ers 1 during 1994 95 supplemented by icesat data for regions south of the ers 1 latitudinal limit surface elevations derived from these instruments are interpolated on to regular grids using kriging and converted to ice thicknesses using a modelled firn density correction the availability of a new spatial variable firn density correction significantly reduces the error in ice thickness as this was previously the dominant error source comparison to airborne data shows good agreement particularly when compared to soar casertz data on the largest ice shelves biases range from 13 0 m for areas where the assumption of hydrostatic equilibrium breaks down to 53 4 m in regions where marine ice may be present

ice and ocean processes in the bellingshausen sea antarctica 1 in the vicinity of the antarctic peninsula observations show diminishing sea ice and a rapid warming of atmosphere and ocean these changes have led to the collapse of ice shelves and retreat acceleration and thinning of inland ice however ocean observations in the center of the nearby bellingshausen sea are spatially and temporally coarse in this study ocean and sea ice models forced by atmospheric reanalyses are used to investigate this data gap by simulating processes in the bellingshausen sea for the years 1979 2007 the model suggests flow features in the region and predicts the basal melting of local ice shelves modeled ocean conditions are found to be less variable than in the nearby amundsen sea which is situated closer to foci of annual and interannual atmospheric variability melt rates beneath george vi ice shelf are investigated in detail concluding that the ice shelf may have been melting out of balance and therefore thinning for decades the melt rate contains significant interannual variability that the model links to variation in sea ice conditions offshore of the southern end of the ice shelf this stands in contrast to the amundsen sea where models suggest that ice shelf melting is controlled by variable transport of circumpolar deep water onto the continental shelf

oceanic heat transport onto the amundsen sea shelf through a submarine glacial trough glaciers which drain the west antarctic ice sheet wais into the amundsen sea are accelerating and thinning rapidly these observations have been attributed to the regional oceanography whereby heat contained within circumpolar deep water cdw drives the basal melting of floating glaciers on the basis of new data we calculate that 2 8 terra watts 1012 of oceanic heat flow onto the continental shelf and toward the glaciers via a submarine glacial trough this is enough to account for most of the basal melting in the entire region suggesting the ocean is supplying an excess of heat toward the antarctic continent

variability of basal melt beneath the pine island glacier ice shelf west antarctica observations from satellite and airborne platforms are combined with model calculations to infer the nature and efficiency of basal melting of the pine island glacier ice shelf west antarctica by ocean waters satellite imagery shows surface features that suggest ice shelf wide changes to the ocean s influence on the ice shelf as the grounding line retreated longitudinal profiles of ice surface and bottom elevations are analyzed to reveal a spatially dependent pattern of basal melt with an annual melt flux of 40 5 gt a 1 one profile captures a persistent set of surface waves that correlates with quasi annual variations of atmospheric forcing of amundsen sea circulation patterns establishing a direct connection between atmospheric variability and sub ice shelf melting ice surface troughs are hydrostatically compensated by ice bottom voids up to 150 m deep voids form dynamically at the grounding line triggered by enhanced melting when warmer than average water arrives subsequent enlargement of the voids is thermally inefficient 4 or less compared with an overall melting efficiency beneath the ice shelf of 22 residual warm water is believed to cause three persistent polynyas at the ice shelf front seen in landsat imagery landsat thermal imagery confirms the occurrence of warm water at the same locations

numerical modeling of ocean ice interactions under pine island bay s ice shelf 1 a two dimensional numerical model is used to simulate the dynamics of buoyant meltwater rich plumes flowing beneath the ice shelf occupying much of pine island bay west antarctica recent studies have shown that this ice shelf along with all others fringing the amundsen sea is thinning rapidly in the model both the coriolis effect and subshelf topography are important in controlling plume dynamics and the spatial distribution of ice melt melt is concentrated in a narrow zone within 20 km of the grounding line where steep subshelf slopes and access to warm ambient water allow melt rates to exceed 100 m yr 1 the plume generated by entrainment of ambient water into the meltwater in these areas is guided by the topography of the ice shelf underside and exits the ice shelf at three distinct outflow locations melt rates generated along the course of the plume are higher approximately 2 5 than rates elsewhere the model suggests that the observed ice shelf thinning rates could have resulted from a hypothetical instantaneous 0 25 c warming of the ambient water entrained by the plume a context for this value is provided by the 40 year warming trend documented by jacobs et al 2002 for circumpolar deep water in the nearby ross sea

observations of change in the southern ocean the southern ocean has been in a state of disequilibrium with its atmosphere and cryosphere during recent decades ocean station and drifting float observations have revealed rising temperatures in

the role of meltwater advection in the formulation of conservative boundary conditions at an ice ocean interface abstract upper boundary conditions for numerical models of the ocean are conventionally formulated under the premise that the boundary is a material surface in the presence of an ice cover such an assumption can lead to nonconservative equations for temperature salinity and other tracers the problem arises because conditions at the ice ocean interface differ from those in the water beneath advection of water with interfacial properties into the interior of the ocean therefore constitutes a tracer flux neglect of which induces a drift in concentration that is most rapid for those tracers having the lowest diffusivities if tracers are to be correctly conserved either the kinematic boundary condition must explicitly allow advection across the interface or the flux boundary condition must parameterize the effects of both vertical advection and diffusion in the boundary layer in practice the latter alternative is often implemented although this is rarely done for all tracers

timing of recent accelerations of pine island glacier antarctica 1 we have used interferometric synthetic aperture radar insar data and sequential landsat imagery to identify and temporally constrain two acceleration events on pine island glacier pig these two events are separated by a period of at least seven years 1987 1994 the change in discharge between two flux gates indicates that the majority of the increase in discharge associated with the second acceleration originates well inland 80 km from the grounding line an analysis indicates that changes in driving stress consistent with observed thinning rates are sufficient in magnitude to explain much of the acceleration

an interpretation of tsunami earthquake based on a simple dynamic model failure of shallow megathrust earthquake 1 the 2011 tohoku earthquake demonstrated that extremely large seismic slip can occur at shallow plate interfaces in subduction zones the large slip area for the tohoku earthquake included the source region of a tsunami earthquake we performed dynamic rupture simulations using simplified fault models and the mechanism of thermal fluid pressurization we found that small fluctuations of initial shear stress near a trench within 1 mpa lead to differences in seismic moment release greater than two orders of magnitude in addition we performed another experiment varying rupture nucleation sizes and obtained similar acute sensitivities of parameters moderate slip events with trapezoidal flat topped source time functions appear to occupy a transition position between shallow megathrust earthquakes with surface rupture and smaller ordinary earthquakes without surface rupture we interpret this result as representing the differences in interplate slip between shallow megathrust earthquakes tsunami moderate earthquakes and non tsunami ordinary earthquakes on the basis of seismic observations

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

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

modeling ice shelf cavities in a z coordinate ocean general circulation model 1 processes at the ice shelf ocean interface and in particular in ice shelf cavities around antarctica have an observable effect on the solutions of basin scale to global coupled ice ocean models despite this these processes are not routinely represented in global ocean and climate models it is shown that a new ice shelf cavity model for z coordinate models can reproduce results from an intercomparison project of earlier approaches with vertical or isopycnic coordinates as a proof of concept ice shelves are incorporated in a 100 year global integration of a z coordinate model in this simulation glacial meltwater can be traced as far as north as 15 s the observed effects of processes in the ice shelf cavities agree with previous results from a coordinate model notably the increase in sea ice thickness however melt rates are overestimated probably because the parameterization of basal melting does not suit the low resolution of this configuration

inflow of warm circumpolar deep water in the central amundsen shelf the thinning and acceleration of the west antarctic ice sheet has been attributed to basal melting induced by intrusions of relatively warm salty water across the continental shelf a hydrographic section including lowered acoustic doppler current profiler measurements showing such an inflow in the channel leading to the getz and dotson ice shelves is presented here the flow rate was 0 3 0 4 sv 1 sv 10 6 m 3 s 21 and the subsurface heat loss was estimated to be 1 2 1 6 tw assuming that the inflow persists throughout the year it corresponds to an ice melt of 110 130 km 3 yr 21 which exceeds recent estimates of the net ice glacier ice volume loss in the amundsen sea the results also show a 100 150 m thick intermediate water mass consisting of circumpolar deep water that has been modified cooled and freshened by subsurface melting of ice shelves and or icebergs this water mass has not previously been reported in the region possibly because of the paucity of historical data

stronger ocean circulation and increased melting under pine island glacier ice shelf the ice shelf buttressing antarctica s pine island glacier has been melting rapidly observations taken between 1994 and 2009 show that meltwater production has increased by about 50 since 1994 as a result of a stronger circulation below the ice shelf

ice shelf changes in pine island bay antarctica 1947 2000 aerial photographs from 1947 and 1966 satellite optical imagery from 1973 and 1980 and interferometric synthetic aperture radar insar data from 1992 1996 and 2000 are employed to detect ice shelf changes in pine island bay antarctica the front position of the fast flowing central ice shelf did not migrate discernibly over the past 50 years new cracks and rifts appeared in the 1990s however that reveal a major weakening of the ice shelf at the grounding line center the ice shelf thinned 21 m in 8 years the northern slow moving ice shelf also shows signs of decay 1 its calving front is retreating at an accelerating rate and 2 the ice shelf is slowly unpinning from its bedrock anchors these changes are taking place in a region well beyond the temperature dependent limit of viability of ice shelves and hence differ from those observed along the antarctica peninsula they are likely due to a change in oceanic forcing not to a change in air temperature one possibility is that the documented intrusion of warm circumpolar deep water on the continental shelf has increased basal melting compared to that required to maintain the ice shelf in a state of mass balance and that this has triggered a general retreat of ice in this sector