rapid disintegration of the wordie ice shelf in response to atmospheric warming the breaking up of ice shelves around the antarctic peninsula has been cited1 as a sign that a dangerous warming is beginning in antarctica here we present satellite images showing the disintegration of the wordie ice shelf which lies off the west coast of the antarctic peninsula fig 1 fracture either in the form of surface crevasses or rifts extending to the bottom of the ice shelf has been responsible for iceberg calving and weakening the central region of the ice shelf these fracture processes which led to retreat of the ice front were apparently enhanced by the presence of increased amounts of melt water resulting from a warming trend recorded in mean annual air temperatures in marguerite bay if this warming trend continues other nearby ice shelves on the antarctic peninsula may be at risk but substantial additional warming would be required before similar processes could initiate breakup of the ross and filchner ronne ice shelves which help stabilize the west antarctic ice sheet

retreat of northern margins of george vi and wilkins ice shelves antarctic peninsula the george vi and wilkins ice shelves are considered at risk of disinteg ration due to a regional atmospheric warming trend on the antarctic peninsula retreat of the northern margin of the george vi ice shelf has been observed previously but the wilkins ice shelf was thought to be stable we investigated the positions of the northern fronts of these shelves from the literature and looked for changes on 1974 landsat and 1992 and 1995 european remote sensing satellite ers synthetic aperture radar images our investigation shows that the northern george vi ice shelf lost a total of 906 km 2 between 1974 and 1992 and an additional 87 km2 by 1995 the northern margin of the wilkins ice shelf lost 796 km2 between 1990 and 1992 and another 564 km2 between 1992 and 1995 armadas of tabular icebergs were visible in front of this shelf in the ers images these two ice shelves mark the southern most documented conspicuous retreat of ice shelf margins

breakup and conditions for stability of the northern larsen ice shelf antarctica the breakup of ice shelves has been widely regarded as an indicator of climate change1 with observations around the antarctic peninsula having shown a pattern of gradual retreat associated with regional atmospheric warming and increased summer melt and fracturing processes2 3 4 5 6 7 8 9 the rapid collapse of the northernmost section of the larsen ice shelf larsen a over a few days in january 1995 indicated that after retreat beyond a critical limit ice shelves may disintegrate rapidly here we use a finite element numerical model that treats ice as a continuum without fracture10 to examine the breakup history2 between 1986 and 1997 of the two northern sections of larsen ice shelf larsen a and larsen b from which we establish stability criteria for ice shelves analysis of various ice shelf configurations reveals characteristic patterns in the strain rates near the ice front which we use to describe the stability of the ice shelf on larsen a only the initial and final ice front configurations show a stable pattern larsen b at present exhibits a stable pattern but if the ice front were to retreat by a further few kilometres it too is likely to enter an irreversible retreat phase

long melt seasons on ice shelves of the antarctic peninsula an analysis using satellite based microwave emission measurements we have examined the record of melt season duration on the antarctic peninsula using two techniques for detecting the presence of a melt signal in microwave emission time series covering the period 1978 2000 we have obtained similar estimates of melt season length using the cross polarized gradient ratio xpgr technique and calibrations previously applied in greenland and a technique which detects the jump in emission caused by melt without using a sensor and frequency dependent threshold value the close correspondence between results from the two techniques on peninsula ice shelves suggests that the xpgr analysis can be used over the length of the time series the results show that the long melt seasons of 1992 93 and several later years were exceptional occurrences on the northern parts of the larsen ice shelf these melt seasons were followed by disintegration events supporting a possible cause and effect relationship

the northern sector of the last british ice sheet maximum extent and demise abstract strongly divided opinion has led to competing apparently contradictory views on the timing extent flow configuration and decay mechanism of the last british ice sheet we review the existing literature and reconcile some of these differences using remarkable new sea bed imagery this bathymetric data provides unprecedented empirical evidence of confluence and subsequent separation of the last british and fennoscandian ice sheets critically it also allows a viable pattern of ice sheet disintegration to be proposed for the first time covering the continental shelf around the northern united kingdom extensive echosounder data reveals striking geomorphic evidence in the form of tunnel valleys and moraines relating to the former british and fennoscandian ice sheets the pattern of tunnel valleys in the northern north sea basin and the presence of large moraines on the west shetland shelf coupled with stratigraphic evidence from the witch ground basin all suggest that at its maximum extent a grounded ice sheet flowed from se to nw across the northern north sea basin terminating at the continental shelf edge the zone of confluence between the british and much larger fennoscandian ice sheets was probably across the northern orkney islands with fast flowing ice in the fair isle channel focusing sediment delivery to the rona and foula wedges this period of maximum confluent glaciation c 30 25 ka bp was followed by a remarkable period of large scale ice sheet re organisation we present evidence suggesting that as sea level rose a large marine embayment opened in the northern north sea basin as far south as the witch ground basin forcing the two ice sheets to decouple rapidly along a north south axis east of shetland as a result both ice sheets rapidly adjusted to new quasi stable margin positions forming a second distinct set of moraines c 24 18 ka bp the lobate overprinted morphology of these moraines on the mid shelf west of orkney and shetland indicates that the re organisation of the british ice sheet was extremely dynamic probably dominated by a series of internally forced readvances critically much of the ice in the low lying north sea basin may have disintegrated catastrophically as decoupling progressed in response to rising sea levels final stage deglaciation was marked by near shore ice streaming and increasing topographic control on ice flow direction punctuated retreat of the british ice sheet continued until c 16 ka bp when following the north atlantic iceberg discharge event heinrich 1 ice was situated at the present day coastline in nw scotland

grounding line migration from 1992 to 2011 on petermann glacier north west greenland we use satellite radar interferometry to investigate changes in the location of the petermann glacier grounding line between 1992 and 2011 the grounding line location was identified in 17 quadruple difference interferograms produced from european remote sensing ers 1 2 data the most extensive time series assembled at any ice stream to date there is close agreement 20 6 cm between vertical displacement of the floating ice shelf and relative tide amplitudes simulated by the arctic ocean dynamics based tide model 5 aodtm 5 arctic tide model over the 19 a period the groundling line position varied by 470 m on average with a maximum range of 7 0 km observed on the north east margin of the ice stream although the mean range 2 8 km and variability 320 m of the grounding line position is considerably lower if the unusually variable north east sector is not considered our observations demonstrate that large isolated movements cannot be precluded thus sparse temporal records should be analysed with care the grounding line migration observed on petermann glacier is not significantly correlated with time r 2 0 22 despite reported ice shelf thinning and episodes of large iceberg calving which suggests that unlike other ice streams on the south west margin of the greenland ice sheet petermann glacier is dynamically stable

northern larsen ice shelf antarctica further retreat after collapse changes of larsen ice shelf antarctica and the surrounding glaciers after its collapse in 1995 were investigated using satellite radar imagery with emphasis onchanges in the glaciers which previously nourished the ice shelf north of seal nunataks the large glaciers retreated several kilometres inland of the previousgrounding line the velocity field of drygalski glacier the largest glacier in this area was mapped by means of interferograms derived from pairs of ers syntheticaperture radar images from 1995 and 1999 the main part of the glacier showed significant acceleration of flow over these 4 years with an increase of veocity upto three fold at the terminus similar accelerations were observed by means of interferometry on several other grounded glaciers suggesting that the removal ofice shelves could lead to an effect on eustatic sea level for larsen b the northernmost surviving part of larsen ice shelf the retreat of the ice front to october2000 is documented

changes in ice dynamics elevation and mass discharge of dinsmoor bombardier edgeworth glacier system antarctic peninsula abstract the northern antarctic peninsula is one of the fastest changing regions on earth the disintegration of the larsen a ice shelf in 1995 caused tributary glaciers to adjust by speeding up surface lowering and overall increased ice mass discharge in this study we investigate the temporal variation of these changes at the dinsmoor bombardier edgeworth glacier system by analyzing dense time series from various spaceborne and airborne earth observation missions precollapse ice shelf conditions and subsequent adjustments through 2014 were covered our results show a response of the glacier system some months after the breakup reaching maximum surface velocities at the glacier front of up to 8 8 m d in 1999 and a subsequent decrease to 1 5 m d in 2014 using a dense time series of interferometrically derived tandem x digital elevation models and photogrammetric data an exponential function was fitted for the decrease in surface elevation elevation changes in areas below 1000 m a s l amounted to at least 130 15 m between 1995 and 2014 with change rates of 3 15 m a between 2003 and 2008 current change rates 2010 2014 are in the range of 1 7 m a mass imbalances were computed with different scenarios of boundary conditions the most plausible results amount to 40 7 3 9 gt the contribution to sea level rise was estimated to be 18 8 1 8 gt corresponding to a 0 052 0 005 mm sea level equivalent for the period 1995 2014 our analysis and scenario considerations revealed that major uncertainties still exist due to insufficiently accurate ice thickness information the second largest uncertainty in the computations was the glacier surface mass balance which is still poorly known our time series analysis facilitates an improved comparison with grace data and as input to modeling of glacio isostatic uplift in this region the study contributed to a better understanding of how glacier systems adjust to ice shelf disintegration

climatically induced retreat and collapse of northern larsen ice shelf antarctic peninsula the areal changes of the northern larsen ite shelf lis antarctic peninsula between march 1986 and march 1997 have been analyzed based on synthetic aperture radar images of the european remote sensing satellites ers 1 and ers 2 and on landsat images this analysis is complemented by data on ice motion and surface mass balance which have been obtained during several field campaigns since the early 1980s after a period of retreat coinciding with atmospheric warming and with decreasing net accumulation at the surface due to melt losses the two northernmost sections of lis disintegrated almost completely within a few days in january 1995 recent observations of the ice shelf section north of jason peninsula which is presently the northernmost section of lis show increased summer melt and intensification of the rifting processes probably causing accelerated retreat of this section in the near future the retreat and the disintegration event of lis indicate high sensitivity of ice shelves to prolonged perturbations of the mass balance

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

interferometric analysis of the deformation pattern of the northern larsen ice shelf antarctic peninsula compared to field measurements and numerical modeling the motion field of the northern larsen ice shelf antarctic peninsula was analyzed using radar interferometry in combination with field measurements and finite element model calculations the ice shelf between jason peninsula and seal nunataks has been in steady retreat since january 1995 model calculations suggest that the ice shelf is in a stage of irreversible retreat since the last calving events in summer 1998 99 the interferometric analysis is based on synthetic aperture radar data of the tandem mission of european remote sensing satellites ers 1and 2 in austral spring 1995 the phase contributions due to tidal motion were estimated from the vertical displacement at those parts of the grounding zone where the horizontal motion is close to zero in order to separate the vertical and horizontal motion components over the ice shelf satellite derived velocities compare well with the long term field measurements along a transverse and a longitudinal profile the real interferograms and synthetic interferograms calculated from model velocities show reasonable agreement over the main parts of the ice shelf but differ in the boundary zones where the details are not resolved by the model

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

evidence of recent climatic warming on the eastern antarctic peninsula air temperatures at the stations marambio esperanza and matienzo have been analysed to investigate recent climate change on the eastern part of the antarctic peninsula the data are compared with the station orcadas on the south orkney islands the longest record available in antarctica and with faraday on the western coast of the peninsula though the interannual variability is comparatively high and the stations are located in different climatic regimes a pronounced warming trend shows up in all records at marambio a temperature increase of 1 5 c has been observed since the beginning of the record in 1971 this is of similar magnitude as the increase at the station faraday on the west coast with 2 5 c for the longer period since 1945 the steady retreat and collapse of the northern larsen ice shelf coincided with this warming trend of particular importance for the ice shelf mass balance in this region are the summer temperatures which show a statistically significant warming trend at the stations marambio and esperanza the representativity of the summer temperatures of marambio for northern larsen ice shelf is confirmed by intercomparison with the parallel measurements at matienzo which is located on the ice shelf

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

34 year satellite time series to monitor characteristics extent and dynamics of larsen b ice shelf antarctic peninsula a variety of data are used to investigate larsen b which is at present the northernmost section of the larsen ice shelf recently declassified usargon satellite photographs of 1963 kosmos photographs of 1975 landsat images of 1986 1988 and 1990 ers 1 2 sar images from 1992 to1997 radarsat of 1998 and field surveys are used to analyze the areal extent surface characteristics and dynamic behaviour of this ice shelf sectionover more than three decades visible and radar imagery together with field observations are used synergistically to describe the ice shelf morphology including meltwater features and rifts in contrast to the retreat of the ice shelf sections in the north larsen b advanced steadily from 1963 to early1995 when the area decreased significantly due to a major calving event analysis of different satellite images indicates that melting is proceedingfurther south in coincidence with the regional warming trend in addition fracturing processes and rapid development of new rifts are observed associated with recent acceleration of ice motion close to the front all observations indicate that major calving events should be expected for this iceshelf section in the near future

rapid collapse of northern larsen ice shelf antarctica in january 1995 4200 square kilometers of the northern larsen ice shelf antarctic peninsula broke away radar images from the ers 1 satellite complemented by field observations showed that the two northernmost sections of the ice shelf fractured and disintegrated almost completely within a few days this breakup followed a period of steady retreat that coincided with a regional trend of atmospheric warming the observations imply that after an ice shelf retreats beyond a critical limit it may collapse rapidly as a result of perturbated mass balance

regional and local factors in attenuation modelling hong kong case study seismic attenuation behaviour is controlled by a large number of wave modification mechanisms the characteristics of some of these mechanisms are specific to a local area whilst the remainder can be generalised to the entire seismic region factors representing these mechanisms are often not resolved a new attenuation modelling approach is demonstrated in this paper using hong kong as a case study to evaluate individual regional and local wave modification factors shear wave velocity swv information for the four prevalent geological formations found in hong kong was first obtained a at shallow depths from instrumented boreholes b at depths of up to 100 200 m from measurements using the microtremor spatial auto correlation spac technique c at depths of up to 1 5 km from the monitoring of quarry blasts and d at depths from 1 5 to 8 km in the hard basement rock layers from results of seismological refraction surveys the upper crust amplification factor calculated from the four modelled rock swv profiles was then combined with predicted attenuation parameters to determine the upper crust modification factor filter function incorporating the local wave modification characteristics associated with hong kong geological formations such functions may then be combined with the regional attenuation characteristics in that part of the south china region a seismic attenuation model was developed by combining the upper crust modification factor with the regional source function of intra plate earthquakes based on stochastic simulations the ground shaking model developed from the presented methodology is supported by the comparison with macro seismic data of seven historical earthquake events affecting hong kong

2001 2009 elevation and mass losses in the larsen a and b embayments antarctic peninsula we investigate the elevation and mass balance response of tributary glaciers following the loss of the larsen a and b ice shelves antarctic peninsula in 1995 and 2002 respectively our study uses modis imagery to track ice extent and aster and spot5 digital elevation models dems plus atm and icesat laser altimetry to track elevation changes spanning the period 2001 09 the measured larsen b tributary glaciers hektoria green evans punchbowl jorum and crane lost up to 160 m in elevation during 2001 06 and thinning continued into 2009 elevation changes were small for the more southerly flask and leppard glaciers which are still constrained by a larsen b ice shelf remnant in the northern embayment continued thinning of 3 m a 1 on drygalski glacier 14 years after the larsen a ice shelf disintegrated suggests that mass losses for the exposed larsen b tributaries will continue for years into the future grounded ice volume losses exceed 13 km 3 for crane glacier and 30 km 3 for the hektoria green evans glaciers the combined mean loss rate for 2001 06 is at least 11 2 gt a 1 our values differ significantly from published mass budget based estimates for these embayments but are a reasonable fraction of grace derived rates for the region 40 gt a 1

soft sediment deformation structures as indicators of tectono volcanic activity during evolution of a lacustrine basin a case study from the upper triassic ordos basin china abstract the ordos basin central china is a large scale late triassic intracratonic lacustrine basin and in which the upper triassic yanchang formation comprises interbedded sandstones mudstones and tuffs representing deposits of a complete lacustrine life cycle based on observation of extensive well cores various types of soft sediment deformation structures have been identified in the lacustrine deposits of the southwest basin including deformed lamination bedding load casts and flame structures pseudonodules microfaults autoclastic brecciation sand dykes dyke sill complexes and sandy and muddy slumps liquefaction and fluidization of unconsolidated permeable sediments together with brittle fracturing of semi consolidated sediments are the major deformation mechanisms that generated these structures both the scale and prevalence of the deformed sediments as well as the geological setting of the lacustrine basin suggest that seismic events synsedimentary tectonic activity were the main trigger to initiate deformation processes although sedimentary overloading could also have been a minor localized trigger many volcanic tuffs preserved in both the primary undisturbed deposits and deformed sediments serve as support for synsedimentary tectono volcanic activities that induced deformation in these lacustrine sediments the deformation structures usually form vertical clusters of several different types representing active syndepositional tectonic periods stratigraphic correlation shows three such deformation clusters to be widespread in the area detailed herein and likely recorded three major tectonic periods during deposition a thick extensive oil shale with various deformation structures also reveals another active tectonic period these four intervals coincide well with the transition of the sedimentary architecture in the area indicating four phases of most intense tectonic periods and their influence on lacustrine evolution

tsunami and infragravity waves impacting antarctic ice shelves the responses of the ross ice shelf ris to the september 16 2015 8 3 mw chilean earthquake tsunami 75 s period and to oceanic infragravity ig waves 50 300 s period were recorded by a broadband seismic array deployed on the ris from november 2014 to november 2016 here we show that tsunami and ig generated signals within the ris propagate at gravity wave speeds 70 m s as water ice coupled flexural gravity waves ig band signals show measureable attenuation away from the shelf front the response of the ris to chilean tsunami arrivals is compared with modeled tsunami forcing to assess ice shelf flexural gravity wave excitation by very long period vlp 300 s gravity waves displacements across the ris are affected by gravity wave incident direction bathymetry under and north of the shelf and water layer and ice shelf thicknesses horizontal displacements are typically about 10 times larger than vertical displacements producing dynamical extensional motions that may facilitate expansion of existing fractures vlp excitation is continuously observed throughout the year with horizontal displacements highest during the austral winter with amplitudes exceeding 20 cm because vlp flexural gravity waves exhibit no discernable attenuation this energy must propagate to the grounding zone both ig and vlp band flexural gravity waves excite mechanical perturbations of the ris that likely promote tabular iceberg calving consequently affecting ice shelf evolution understanding these ocean excited mechanical interactions is important to determine their effect on ice shelf stability to reduce uncertainty in the magnitude and rate of global sea level rise

where glaciers meet water subaqueous melt and its relevance to glaciers in various settings glacier change is ubiquitous but the fastest and largest magnitude changes occur in glaciers that terminate in water this includes the most rapidly retreating glaciers and also several advancing ones often in similar regional climate settings furthermore water terminating glaciers show a large range in morphology particularly when ice flow into ocean water is compared to that into freshwater lakes all water terminating glaciers share the ability to lose significant volume of ice at the front either through mechanical calving or direct melt from the water in contact here we present a review of the subaqueous melt process we discuss the relevant physics and show how different physical settings can lead to different glacial responses we find that subaqueous melt can be an important trigger for glacier change it can explain many of the morphological differences such as the existence or absence of floating tongues subaqueous melting is influenced by glacial runoff which is largely a function of atmospheric conditions this shows a tight connection between atmosphere oceans and lakes and glaciers subaqueous melt rates even if shown to be large should always be discussed in the context of ice supply to the glacier front to assess its overall relevance we find that melt is often relevant to explain seasonal evolution can be instrumental in shifting a glacier into a different dynamical regime and often forms a large part of a glacier s mass loss on the other hand in some cases melt is a small component of mass loss and does not significantly affect glacier response

glacier surge after ice shelf collapse the possibility that the west antarctic ice sheet will collapse as a consequence of ice shelf disintegration has been debated for many years this matter is of concern because such an event would imply a sudden increase in sea level evidence is presented here showing drastic dynamic perturbations on former tributary glaciers that fed sections of the larsen ice shelf on the antarctic peninsula before its collapse in 1995 satellite images and airborne surveys allowed unambiguous identification of active surging phases of boydell sjogren edgeworth bombardier and drygalski glaciers this discovery calls for a reconsideration of former hypotheses about the stabilizing role of ice shelves

interferometric estimation of three dimensional ice flow using ascending and descending passes satellite radar interferometry sri provides an important new tool for determining ice flow velocity interferometric measurements made from a single track direction are sensitive only to a single component of the three component velocity vector observations from along three different track directions would allow the full velocity vector to be determined a north south looking synthetic aperture radar sar could provide these observations over large portions of the globe but not over large areas of the polar ice sheets the authors develop and demonstrate a technique that allows the three component velocity vector to be estimated from data acquired along two track directions ascending and descending under a surface parallel flow assumption this technique requires that there are accurate estimates of the surface slope which are also determined interferometrically to demonstrate the technique the authors estimate the three component velocity field for the ryder glacier greenland their results are promising although they do not have yet ground truth data with which to determine the accuracy of their estimates

boundary condition of grounding lines prior to collapse larsen b ice shelf antarctica grounding zones where ice sheets transition between resting on bedrock to full floatation help regulate ice flow exposure of the sea floor by the 2002 larsen b ice shelf collapse allowed detailed morphologic mapping and sampling of the embayment sea floor marine geophysical data collected in 2006 reveal a large arcuate complex grounding zone sediment system at the front of crane fjord radiocarbon constrained chronologies from marine sediment cores indicate loss of ice contact with the bed at this site about 12 000 years ago previous studies and morphologic mapping of the fjord suggest that the crane glacier grounding zone was well within the fjord before 2002 and did not retreat further until after the ice shelf collapse this implies that the 2002 larsen b ice shelf collapse likely was a response to surface warming rather than to grounding zone instability strengthening the idea that surface processes controlled the disintegration of the larsen ice shelf

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

pattern of retreat and disintegration of larsen b ice shelf antarctic peninsula the retreat of the larsen b ice shelf antarctic peninsula and the collapse of its northern section are analyzed using satellite images acquired between january 1995 and may 2003 over 1 week during march 2002 after a period of steady retreat since 1995 2300 km 2 of the ice shelf broke up into many small icebergs this rapid collapse occurred at the end of an exceptionally warm summer and after a multi year period of decreasing surface net mass balance ice thinning flow acceleration and widening of rifts the ice shelf area decreased from 11 512 km 2 in january 1995 to 3463 km 2 in march 2002 and 2667 km 2 in april 2003 ers synthetic aperture radar sar images were used to identify ice shelf zones with different surface morphology which generated icebergs of different sizes and shapes the pattern of retreat and break up similar to that of larsen a in 1995 suggests that fracturing enhanced by abundant surface melt played a key role in addition the recent changes of grounded and residual floating ice north of larsen b are analyzed by means of envisat advanced synthetic aperture radar asar images up to summer 2003 showing significant loss of grounded ice upstream of those ice shelf sections which disintegrated in 1995 and 2002

fracture field for large scale ice dynamics recent observations and modeling studies emphasize the crucial role of fracture mechanics for the stability of ice shelves and thereby the evolution of ice sheets here we introduce a macroscopic fracture density field into a prognostic continuum ice flow model and compute its evolution incorporating the initiation and growth of fractures as well as their advection with two dimensional ice flow to a first approximation fracture growth is assumed to depend on the spreading rate only while fracture initiation is defined in terms of principal stresses the inferred fracture density fields compare well with observed elongate surface structures since crevasses and other deep reaching fracture structures have been shown to influence the overall ice shelf dynamics we propose the fracture density field introduced here be used as a measure for ice softening and decoupling of the ice flow in fracture weakened zones this may yield more accurate and realistic velocity patterns in prognostic simulations additionally the memory of past fracture events links the calving front to the upstream dynamics thus the fracture density field proposed here may be employed in fracture based calving parameterizations the aim of this study is to introduce the field and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms

ice loss processes in the seal nunataks ice shelf region from satellite altimetry and imagery the seal nunataks ice shelf snis 743 km 2 in 2013 is an unofficial name for a remnant area between the former larsen a and larsen b ice shelves off the northeastern antarctic peninsula analyses using landsat 7 etm and terra aster images from 2001 to 13 and icesat altimetry from 2003 to 09 show it has retreated and thinned following the larsen a 1995 and larsen b 2002 disintegrations despite some regional cooling and more fast ice since 2008 snis continues to lose ice along its margins and may be losing contact with some nunataks detailed analysis of data from four icesat tracks indicates that ice shelf thinning rates range between 1 9 and 2 7 m a 1 and generally increase from west to east an icesat repeat track crossing the adjacent robertson island shows a mean elevation loss of 1 8 m a 1 two tracks crossing the snis s remaining tributary rogosh glacier show sub meter elevation losses comparing shelf remnant and grounded ice thinning rates implies that basal ocean melting augments snis thinning by 1 m a 1 a rate that is consistent with other estimates of ocean driven shelf thinning in the region

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis