recent mass changes of glaciers in the russian high arctic 1 glaciers and ice caps are known to contribute significantly to present day sea level rise but there are still glaciated regions where little is known about modern changes in glacier mass one of these regions is the russian high arctic archipelagos which has a total glaciated area of 51 500 km2 we have assessed the glacier mass budget of this region for a 6 year period between october 2003 and october 2009 using independent icesat laser altimetry and grace gravimetry over this period we found that the archipelagos have lost ice at a rate of 9 1 2 0 gt a 1 which corresponds to a sea level contribution of 0 025 mm a 1 approximately 80 of the ice loss came from novaya zemlya with the remaining 20 coming from franz josef land and severnaya zemlya meteorological records of temperature and precipitation for the period 1980 2009 suggest that the recent climatic mass budget is not substantially different from the longer term trend

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

distinct patterns of seasonal greenland glacier velocity predicting greenland ice sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms we present a 5 year record of seasonal velocity measurements for 55 marine terminating glaciers distributed around the ice sheet margin along with ice front position and runoff data sets for each glacier among glaciers with substantial speed variations we find three distinct seasonal velocity patterns one pattern indicates relatively high glacier sensitivity to ice front position the other two patterns are more prevalent and appear to be meltwater controlled these patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient distributed hydrologic networks to efficient channelized drainage while others do not the difference may be determined by meltwater availability which in some regions may be influenced by perennial firn aquifers our results highlight the need to understand subglacial meltwater availability on an ice sheet wide scale to predict future dynamic changes

anisotropy induction logging the apparent tensor anisotropic conductivity of the formations around a borehole can be calculated from triaxial induction logging data in particular analysis of the late time transient coupling of triaxial coincident loops yields a simple method to determine the principle axes and conductivities

on coincident loop transient electromagnetic induction logging abstractcoincident loop transient induction wireline logging is examined as the borehole analog of the well known land and airborne time domain electromagnetic em method the concept of whole space late time apparent resistivity is modified from the half space version commonly used in land and airborne geophysics and applied to the coincident loop voltages produced from various formation borehole and invasion models given typical tool diameters off time measurements with such an instrument must be made on the order of nanoseconds to microseconds much more rapidly than for surface methods departure curves of the apparent resistivity for thin beds calculated using an algorithm developed to model the transient response of a loop in a multilayered earth indicate that the depth of investigation scales with the bed thickness modeled resistivity logs are comparable in accuracy and resolution with standard frequency domain focused induction logs however if measurement times are longer than a few mic

force budget i theory and numerical methods a practical method is developed for calculating stresses and velocities at depth using field measurements of the geometry and surface velocity of glaciers to do this it is convenient to partition full stresses into lithostatic and resistive components the horizontal gradient in vertically integrated lithostatic stress is the driving stress and it describes the horizontal action of gravity the horizontal resistive stress gradients describe the reactions resistive stresses are simply related to deviatoric stresses and hence to strain rates through a constitutive relation a numerical scheme can be used to calculate stresses and velocities from surface velocities and slope and from ice thickness there is no mathematical requirement that the variations in these quantities be small

potential antarctic ice sheet retreat driven by hydrofracturing and ice cliff failure abstract geological data indicate that global mean sea level has fluctuated on 103 to 106 yr time scales during the last 25 million years at times reaching 20 m or more above modern if correct this implies substantial variations in the size of the east antarctic ice sheet eais however most climate and ice sheet models have not been able to simulate significant eais retreat from continental size given that atmospheric co2 levels were relatively low throughout this period here we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model data conflict in response to atmospheric and ocean temperatures typical of past warm periods floating ice shelves may be drastically reduced or removed completely by increased oceanic melting and by hydrofracturing due to surface melt draining into crevasses ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing and may fail structurally if stresses exceed the ice yield strength producing rapid retreat incorporating these mechanisms in our ice sheet model accelerates the expected collapse of the west antarctic ice sheet to decadal time scales and also causes retreat into major east antarctic subglacial basins producing 17 m global sea level rise within a few thousand years the mechanisms are highly parameterized and should be tested by further process studies but if accurate they offer one explanation for past sea level high stands and suggest that antarctica may be more vulnerable to warm climates than in most previous studies

transient electromagnetic responses in seafloor with triaxial anisotropy summary electrical anisotropy of young oceanic crust at mid ocean ridges is detectable by observation of the rate and geometry of the diffusion of electromagnetic fields the anisotropy in electrical properties arises from the presence of conductive seawater in an interconnected network of mostly ridge parallel cracks in this paper we first justify the choice of a triaxial model to represent young oceanic crust with three distinct electrical conductivities in the vertical strike and spreading directions we then present an algorithm to calculate the transient electromagnetic responses generated by an electric dipole source over such a triaxially anisotropic seafloor we show that if the transient passages are measured with three distinct electric dipole dipole configurations it is possible to discern all three unknown conductivities independently of each other

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

subglacial hydrology and ice stream margin locations fast flowing ice streams in west antarctica are separated from the nearly stagnant ice in the adjacent ridge by zones of highly localized deformation known as shear margins it is presently uncertain what mechanisms control the location of shear margins and possibly allow them to migrate in this paper we show how subglacial hydrological processes can select the shear margin location leading to a smooth transition from a slipping to a locked bed at the base of an ice stream our study uses a two dimensional thermomechanical model in a cross section perpendicular to the direction of flow we confirm that the intense straining at the shear margins can generate large temperate regions within the deforming ice assuming that the melt generated in the temperate ice collects in a drainage channel at the base of the margin we show that a channel locally decreases the pore pressure in the subglacial till therefore the basal shear strength just outside the channel assuming a coulomb plastic rheology can be substantially higher than that inferred under the majority of the stream results show that the additional basal resistance produced by the channel lowers the stress concentrated on the locked portion of the bed matching the model to surface velocity data we find that shear margins are stable when the slipping to locked bed transition occurs less than 500 m away from a channel operating at an effective pressure of 200 kpa and for a hydraulic transmissivity equivalent to a basal water film of order 0 2 mm thickness

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

kamb ice stream flow history and surge potential a basal zone tens of meters thick of debris laden ice was observed in kamb ice stream west antarctica using a video camera lowered into boreholes made by hot water drilling the debris content varies sometimes abruptly forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction in most parts the concentration of debris is low a few percent by weight with particles often mud clots dispersed in a matrix of clear ice the nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of kamb ice stream including processes leading up to the termination of its streaming behavior and possible reactivation

bayou corne louisiana sinkhole precursory deformation measured by radar interferometry catastrophic sinkholes are formed through the collapse of natural or human made subterranean caverns and are common in areas with evaporite and carbonate rock despite their danger advance warning of these events is rare we report a measurement of precursory surface deformation of as much as 260 mm derived with interferometric synthetic aperture radar insar and evident over a month before surface collapse at the site of the bayou corne louisiana usa sinkhole that formed in august 2012 data collected by the airborne uninhabited aerial vehicle sar uavsar instrument were used for the study analysis of data acquired from two flight tracks with near opposing imaging geometries reveal a deformation pattern consistent with compressive loading at the surface due to loss of support from a subterranean cavity collapse related to texas brine oxy geismar well 3 the precursor deformation was nearly entirely horizontal i e oriented along the surface and manifested as movement of surface material toward the location where the sinkhole later formed the sinkhole formed in the area with the largest gradient in surface strain but did not cover the full extent of the precursory deformation detected with radar this work suggests that insar data collected operationally for hazard monitoring could in some cases identify sinkhole development before surface collapse and decrease subsequent danger to people and property

rapid dynamic activation of a marine based arctic ice cap we use satellite observations to document rapid acceleration and ice loss from a formerly slow flowing marine based sector of austfonna the largest ice cap in the eurasian arctic during the past two decades the sector ice discharge has increased 45 fold the velocity regime has switched from predominantly slow 101 m yr to fast 103 m yr flow and rates of ice thinning have exceeded 25 m yr at the time of widespread dynamic activation parts of the terminus may have been near floatation subsequently the imbalance has propagated 50 km inland to within 8 km of the ice cap summit our observations demonstrate the ability of slow flowing ice to mobilize and quickly transmit the dynamic imbalance inland a process that we show has initiated rapid ice loss to the ocean and redistribution of ice mass to locations more susceptible to melt yet which remains poorly understood

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

glacier motion estimation using sar offset tracking procedures two image to image patch offset techniques for estimating feature motion between satellite synthetic aperture radar sar images are discussed intensity tracking based on patch intensity cross correlation optimization and coherence tracking based on patch coherence optimization are used to estimate the movement of glacier surfaces between two sar images in both slant range and azimuth direction the accuracy and application range of the two methods are examined in the case of the surge of monacobreen in northern svalbard between 1992 and 1996 offset tracking procedures of sar images are an alternative to differential sar interferometry for the estimation of glacier motion when differential sar interferometry is limited by loss of coherence i e in the case of rapid and incoherent flow and of large acquisition time intervals between the two sar images in addition an offset tracking procedure in the azimuth direction may be combined with differential sar interferometry in the slant range direction in order to retrieve a two dimensional displacement map when sar data of only one orbit configuration are available

bed reflectivity beneath inactive ice streams in west antarctica radio echo sounding res techniques are used to examine spatial changes inbed reflectivityacrossrelictice streams inwest antarctica measurementsfrom adjacent interstream ridgesare usedto correctthe measuredpower returned fromthebed for attenuation and losses due to geometric spreading scattering and absorption res measurements near boreholes drilled on ice stream c isc indicate high coefficients of bed reflectivity r40 1 in locations where the bed wasthawed and boreholes connected to thebasalwatersystem andlow reflectivitycoefficients r50 02 atlocationsthat were frozen and not connected intermediate values of bed reflectivity were measured at locations where the connection to the basal water systemwas weak measurements across four relictmargins show thatbedreflectivity usuallyjumpsfromlow tohighvaluesseveral kilometers inside the outermost buried crevasses we interpret this to be a transition from frozento thawedbasal conditions anddiscuss implications of these observations

dynamic patterns of ice stream flow in a 3 d higher order ice sheet model with plastic bed and simplified hydrology 1 predicting ice sheet mass balance is challenging because of the complex flow of ice streams to address this issue we have coupled a three dimensional higher order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress the model was tested for its sensitivity to regional water availability first with an assumed undrained bed the ice stream oscillates between active and stagnant phases solely as a result of thermodynamic feedbacks occurring at the ice till interface however the velocity amplitude decreases over time as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing ice sheet mode second we assume that the till is able to assimilate water from a hypothetical regional hydrological system this leads to significantly different long term behavior as a continuously oscillating ice stream mode is maintained the extra water incorporated in the till leads to higher velocities triggering stronger thermodynamic feedbacks between the ice and till layer results also suggest that fast flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase similarly till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system underscoring the need to accurately describe the coupling between ice dynamics basal conditions and regional subglacial hydrology in ice sheet models

the glaciers climate change initiative methods for creating glacier area elevation change and velocity products glaciers and their changes through time are increasingly obtained from a wide range of satellite sensors due to the often remote location of glaciers in inaccessible and high mountain terrain satellite observations frequently provide the only available measurements furthermore satellite data provide observations of glacier characteristics that are difficult to monitor using ground based measurements thus complementing the latter in the glaciers cci project of the european space agency esa three of these characteristics are investigated in detail glacier area elevation change and surface velocity we use a data from optical sensors to derive glacier outlines b digital elevation models from at least two points in time c repeat altimetry for determining elevation changes and d data from repeat optical and microwave sensors for calculating surface velocity for the latter the two sensor types provide complementary information in terms of spatio temporal coverage while c and d can be generated mostly automatically a and b require the intervention of an analyst largely based on the results of various round robin experiments multi analyst benchmark studies for each of the products we suggest and describe the most suitable algorithms for product creation and provide recommendations concerning their practical implementation and the required post processing for some of the products area velocity post processing can influence product quality more than the main processing algorithm

the randolph glacier inventory a globally complete inventory of glaciers the randolph glacier inventory rgi is a globally complete collection of digital outlines of glaciers excluding the ice sheets developed to meet the needs of the fifth assessment of the intergovernmental panel on climate change for estimates of past and future mass balance the rgi was created with limited resources in a short period priority was given to completeness of coverage but a limited uniform set of attributes is attached to each of the 198 000 glaciers in its latest version 3 2 satellite imagery from 1999 2010 provided most of the outlines their total extent is estimated as 726 800 34 000 km 2 the uncertainty about 5 is derived from careful single glacier and basin scale uncertainty estimates and comparisons with inventories that were not sources for the rgi the main contributors to uncertainty are probably misinterpretation of seasonal snow cover and debris cover these errors appear not to be normally distributed and quantifying them reliably is an unsolved problem combined with digital elevation models the rgi glacier outlines yield hypsometries that can be com bined with atmospheric data or model outputs for analysis of the impacts of climatic change on glaciers the rgi has already proved its value in the generation of significantly improved aggregate estimates of glacier mass changes and total volume and thus actual and potential contributions to sea level rise

thermal effects on the location of ice stream margins west antarctic ice streams move rapidly over beds that are thawed by heat generated by the motion intervening zones of slow moving ice are probably freezing at their bases where heat generation is absent heat is also generated in the shear zone between the slow and fast moving ice through modeling of coupled heat and mass flow we show that the position of the boundary between freezing and melting at the bed is unstable for an otherwise morphologically uniform bed above a threshold speed in the ice stream heat generation in the shear zone allows melting beneath the slow ice and possible widening of the stream below this threshold heat generation at the edge of the thawed zone is insufficient to maintain melting leading to freezing and narrowing of the stream the threshold speed appears to be a function of the temperature at the base of slow ice relative to the melting point and is also highly sensitive to the inflow of colder ice from the slow zone for environmental conditions typical of west antarctica the threshold speed is about 100 m yr 1 in the absence of inflow and increases to typical ice stream speeds 500 m yr 1 for inflow speeds of a few meters per year thus the ice streams can be in a delicate thermal balance with the potential of melting outward or freezing inward rapidly moving ice streams could expand unless blocked by bed properties that would prevent fast motion even when there is melting at the bed

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

the possible transition from glacial surge to ice stream on vavilov ice cap an edited version of this paper was published by agu copyright 2019 american geophysical union

mass balance of polar ice sheets recent advances in the determination of the mass balance of polar ice sheets show that the greenland ice sheet is losing mass by near coastal thinning and that the west antarctic ice sheet with thickening in the west and thinning in the north is probably thinning overall the mass imbalance of the east antarctic ice sheet is likely to be small but even its sign cannot yet be determined large sectors of ice in southeast greenland the amundsen sea embayment of west antarctica and the antarctic peninsula are changing quite rapidly as a result of processes not yet understood

glaciological advances made with interferometric synthetic aperture radar spaceborne interferometric synthetic aperture radar insar techniques for measuring ice flow velocity and topography have developed rapidly over the last decade and a half revolutionizing the study of ice dynamics spaceborne interferometry has contributed to major progress in many areas of glaciological study by providing the first comprehensive measurements of ice stream flow velocity over the major outlets of greenland and antarctica revealing that ice stream and outlet glacier flow can change rapidly months to years improving understanding of several ice sheet and ice shelf processes providing velocity for flux gate based mass balance assessment mapping flow of mountain glaciers and capturing the geomorphic traces of past ice flow we review the basic technique development the measurement characteristics and the extensive set of results yielded by these measurements