full stokes modeling of marine ice sheets influence of the grid size using the finite element code elmer we show that the full stokes modeling of the ice sheet ice shelf transition we propose can give consistent predictions of grounding line migration like other marine ice sheet models our approach is highly sensitive to the chosen mesh resolution however with a grid size down to 5 km in the vicinity of the grounding line predictions start to be robust because 1 whatever the grid size 5 km the steady state grounding line position is sensibly the same 6 km standard deviation and 2 with a grid size refinement in the vicinity of the grounding line 200 m the steady state solution is independent of the applied perturbation in fluidity provided this perturbation remains monotonic

climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

a continuum mechanical formulation for shallow polythermal ice sheets this paper is concerned with a new theoretical approach to model grounded ice sheets in three dimensions these are considered as polythermal i e there will be regions with temperatures below the

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

modeling the evolution of antarctic ice sheet over the last 420 000 years implications for altitude changes in the vostok region a new thermomechanical three dimensional model designed to simulate the evolution of the antarctic ice sheet over long time periods is presented this model incorporates the various types of ice flow found in antarctica relatively slow inland ice flow that is essentially due to ice deformation fast ice flow in the regions with ice streams and ice shelf flow by coupling these three types of flow it is possible to predict grounding line migration simulations covering four glacial interglacial cycles have been conducted by forcing this model with a temperature record from vostok and a sea level record from marine cores our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the antarctic ice volume on the other hand the altitude of the ice sheet surface at vostok is driven by accumulation rate variations the amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the antarctic plateau

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

21st century evolution of greenland outlet glacier velocities earlier observations on several of greenland s outlet glaciers starting near the turn of the 21st century indicated rapid annual scale and large 100 increases in glacier velocity combining data from several satellites we produce a decade long 2000 to 2010 record documenting the ongoing velocity evolution of nearly all 200 of greenland s major outlet glaciers revealing complex spatial and temporal patterns changes on fast flow marine terminating glaciers contrast with steady velocities on ice shelf terminating glaciers and slow speeds on land terminating glaciers regionally glaciers in the northwest accelerated steadily with more variability in the southeast and relatively steady flow elsewhere intraregional variability shows a complex response to regional and local forcing observed acceleration indicates that sea level rise from greenland may fall well below proposed upper bounds

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

a new high precision borehole temperature logging system used at gisp2 greenland and taylor dome antarctica we describe a high precision 0 1 1 0 mk borehole temperature bt logging system developed at the united states geological survey usgs for use in remote polar regions we discuss calibration operational and data processing procedures and present an analysis of the measurement errors the system is modular to facilitate calibration procedures and field repairs by interchanging logging cables and temperature sensors measurements can be made in either shallow air filled boreholes or liquid filled holes up to 7 km deep data can be acquired in either incremental or continuous logging modes the precision of data collected by the new logging system is high enough to detect and quantify various thermal effects at the milli kelvin level to illustrate this capability we present sample data from the 3 km deep borehole at gisp2 greenland and from a 130m deep air filled hole at taylor dome antarctica the precision of the processed gisp2 continuous temperature logs is 0 25 0 34 mk while the accuracy is estimated to be 4 5 mk the effects of fluid convection and the dissipation of the thermal disturbance caused by drilling the borehole are clearly visible in the data the precision of the incremental taylor dome measurements varies from 0 11 to 0 32 mk depending on the wind strength during the experiments with this precision we found that temperature fluctuations and multi hour trends in the bt measurements correlate well with atmospheric pressure changes

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

ice flow in greenland for the international polar year 2008 2009 a digital representation of ice surface velocity is essential for a variety of glaciological geologic and geophysical analyses and modeling here we present a new reference comprehensive high resolution digital mosaic of ice motion in greenland assembled from satellite radar interferometry data acquired during the international polar year 2008 to 2009 by the envisat advanced synthetic aperture radar asar the advanced land observation system alos s phase array l band sar palsar and the radarsat 1 sar that covers 99 of the ice sheet in area the best mapping performance is obtained using alos palsar data due to higher levels of temporal coherence at the l band frequency but c band frequency sar data are less affected by the ionosphere the ice motion map reveals various flow regimes ranging from patterned enhanced flow into a few large glaciers in the cold low precipitation areas of north greenland to diffuse enhanced flow into numerous narrow fast moving glaciers in the warmer high precipitation sectors of northwest and southeast greenland we find that the 100 fastest glaciers v 800m yr drain 66 of the ice sheet in area marine terminating glaciers drain 88 of greenland and basal sliding motion dominates internal deformation over more than 50 of the ice sheet this view of ice sheet motion provides significant new constraints on ice flow modeling copyright 2012 by the american geophysical union

inversion of basal friction in antarctica using exact and incomplete adjoints of a higher order model basal friction beneath ice sheets remains poorly characterized and yet is a fundamental control on ice mechanics here we use a complete map of surface velocity of the antarctic ice sheet to infer the basal friction over the entire continent by combining these observations with a three dimensional thermomechanical higher order ice sheet numerical model from the ice sheet system model open source software we demonstrate that inverse methods can be readily applied at the continental scale with appropriate selections of cost function and of scheme of regularization at a spatial resolution as high as 3 km along the coastline we compare the convergence of two descent algorithms with the exact and incomplete adjoints to show that the incomplete adjoint is an excellent approximation the results reveal that the driving stress is almost entirely balanced by the basal shear stress over 80 of the ice sheet the basal friction coefficient which relates basal friction to basal velocity is however significantly heterogeneous it is low on fast moving ice and high near topographic divides areas with low values extend far out into the interior along glacier and ice stream tributaries almost to the flanks of topographic divides suggesting that basal sliding is widespread beneath the antarctic ice sheet 2013 american geophysical union all rights reserved

dependence of century scale projections of the greenland ice sheet on its thermal regime observations show that the greenland ice sheet has been losing mass at an increasing rate over the past few decades which makes it a major contributor to sea level rise here we use a threedimensional higher order ice flow model adaptive mesh refinement and inverse methods to accurately reproduce the present day ice flow of the greenland ice sheet we investigate the effect of the ice thermal regime on 1 basal sliding inversion and 2 projections over the next 100 years we show that steady state temperatures based on present day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century scale projections are weakly sensitive to small changes in the initial temperature field compared with changes in atmospheric conditions or basal sliding we conclude that although more englacial temperature measurements should be acquired to validate the models and a better estimation of geothermal heat flux is needed it is reasonable to use steady state temperature profiles for short term projections as external forcings remain the main drivers of the changes occurring in greenland 2014 publishing technology

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

observation and analysis of ice flow in the largest greenland ice stream we have applied satellite radar interferometry methods to map the velocity field of the recently discovered northeast greenland ice stream we have used these data in conjunction with ice thickness and accumulation data to determine that the ice stream is in balance within the measurement errors we used control methods to invert a finite element model of ice sheet flow constrained by the data to infer the ice stream s basal shear stress distribution our results reveal that flow in a section of the downstream end has much in common with the streaming flow of the ross ice streams of west antarctica e g a weak bed and fast flow in the presence of low driving stresses for several hundred kilometers along the middle of the ice stream the basal shear stress balances the driving stress in the upstream area where the ice stream is first visible in the velocity data the bed appears to be weak which may contribute to the initiation of the ice stream

initial results of the searise numerical experiments with the models sicopolis and icies for the greenland ice sheet searise sea level response to ice sheet evolution is a us led multi model community effort to predict the likely range of the contribution of the greenland and antarctic ice sheets to sea level rise over the next few hundred years under global warming conditions the japanese ice sheet modelling community is contributing to searise with two large scale dynamic thermodynamic models sicopolis and icies here we discuss results for the greenland ice sheet obtained using both models under the forcings surface temperature and precipitation scenarios defined by the searise effort a crucial point for meaningful simulations into the future is to obtain initial conditions that are close to the observed state of the present day ice sheet this is achieved by proper tuning during model spin up from the last glacial interglacial cycle to today experiments over 500 years indicate that both models are more sensitive exhibit a larger rate of ice sheet mass loss to future climate warming based on the a1b emission scenario than to a doubling in the basal sliding speed ice sheet mass loss varies between the two models by a factor of 2 for sliding experiments and a factor of 3 for climate warming experiments highlighting the importance of improved constraints on the parameterization of basal sliding and surface mass balance in ice sheet models

a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes 1 a new three dimensional thermomechanically coupled ice sheet model is developed contrary to the majority of three dimensional ice sheet models shallow ice approximation higher order stress gradients longitudinal and transverse are accounted for in the force balance equations the horizontal velocity field is determined from the force balance equations in their derivative form elliptical equation using the method presented by pattyn 2000 2002a the model is solved on a regular grid in the horizontal and an irregular grid in the vertical and is numerically stable basic experiments include the european ice sheet modeling initiative eismint benchmarks for largescale ice sheet models and a comparison with the saito blatter ice sheet model including higher order stress gradients saito et al 2003 detailed calculations of ice flow over three dimensional bedrock perturbations showed the validity of the higher order solution the model is capable of simulating the evolution of an ice stream within the ice sheet and shows important aspects of observed ice stream features such as the surface flattening and the importance of side drag the simulation of the ice flow over a subglacial lake results in a flattening of the surface a local velocity increase over the lake and a deviation of the ice flow from the main flow direction features which are also observed at lake vostok antarctica index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3230 mathematical geophysics numerical solutions keywords higher order ice sheet model thermomechanical ice stream subglacial lake citation pattyn f a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes j geophys res 108 b8 2382 doi 10 1029 2002jb002329 2003

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

ice sheet model sensitivities to environmental forcing and their use in projecting future sea level the searise project ten ice sheet models are used to study sensitivity of the greenland and antarctic ice sheets to prescribed changes of surface mass balance sub ice shelf melting and basal sliding results exhibit a large range in projected contributions to sea level change in most cases the ice volume above flotation lost is linearly dependent on the strength of the forcing combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments suggesting that nonlinear feedbacks are modest our models indicate that greenland is more sensitive than antarctica to likely atmospheric changes in temperature and precipitation while antarctica is more sensitive to increased ice shelf basal melting an experiment approximating the intergovernmental panel on climate change s rcp8 5 scenario produces additional first century contributions to sea level of 22 3 and 8 1 cm from greenland and antarctica respectively with a range among models of 62 and 14 cm respectively by 200 years projections increase to 53 2 and 26 7 cm respectively with ranges of 79 and 43 cm linear interpolation of the sensitivity results closely approximates these projections revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice sheet response to complicated forcings over 200 years can be linearized

aeolian sediment flux decay non linear behaviour on developing deflation lag surfaces wind tunnel simulations of the effect of non erodible roughness elements on sediment transport show that the flux ratio q qs shear velocity u and roughness density are co dependent variables initially the sediment flux is enhanced by kinetic energy retention in relatively elastic collisions that occur at the roughness element surfaces but at the same time the rising surface coverage of the immobile elements reduces the probability of grain ejection a zone of strong shearing stress develops within 0 03 to 0 04 m of the rough bed because of a relative straightening of velocity profiles which are normally convex with saltation drag this positive influence on fluid entrainment is opposed by declining shear stress partitioned to the sand bed similarly because the free stream velocity uf is fixed while u increases velocity at height z and particle momentum gain from the airstream decline leading eventually to lower numbers of particles ejected on average at each impact when the ratio of the element basal area to frontal area is approximately equal to 3 5 secondary flow effects appear to become significant so that the dimensionless aerodynamic roughness parameter z0 h and shear stress on the exposed sand bed ts decrease it is at this point that grain supply to the airstream and saltation drag appear to be significantly reduced thereby intensifying the reduction in u the zone of strong fluid shear near the bed dissipates

earthquakes series preceding very long period seismic signals observed during the 2000 miyakejima volcanic activity 1 unusual earthquakes series were observed one to two hours before a very long period vlp seismic pulse that happened once or twice a day during the 2000 activity of miyakejima volcano japan the series on july 11 and 12 have particularly a clear feature the time intervals of the earthquakes in the series decrease at a constant rate in the manner of geometric progression the maximum amplitude of each earthquake is initially almost constant but linearly decreases with time from few minutes before the occurrence of a vlp pulse hypocenters of these earthquakes are below the south western region of the crater and near the sea level in depth we interpret these earthquakes series as repetition of a gradual stress accumulation and release under the variable critical stress level the critical stress level is first constant but linearly decreases later

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

changes in the dynamics of marine terminating outlet glaciers in west greenland 2000 2009 1 recent changes in the dynamics of greenland s marine terminating outlet glaciers indicate a rapid and complex response to external forcing despite observed ice front retreat and recent geophysical evidence for accelerated mass loss along greenland s northwestern margin it is unclear whether west greenland glaciers have undergone the synchronous speed up and subsequent slow down as observed in southeastern glaciers earlier in the decade to investigate changes in west greenland outlet glacier dynamics and the potential controls behind their behavior we derive time series of front position surface elevation and surface slope for 59 marine terminating outlet glaciers and surface speeds for select glaciers in west greenland from 2000 to 2009 using these data we look for relationships between retreat thinning acceleration and geometric parameters to determine the first order controls on glacier behavior our data indicate that changes in front positions and surface elevations were asynchronous on annual time scales though nearly all glaciers retreated and thinned over the decade we found no direct relationship between retreat acceleration and external forcing applicable to the entire region in regard to geometry we found that following retreat 1 glaciers with grounded termini experienced more pronounced changes in dynamics than those with floating termini and 2 thinning rates declined more quickly for glaciers with steeper slopes overall glacier geometry should influence outlet glacier dynamics via stress redistribution following perturbations at the front but our data indicate that the relative importance of geometry as a control of glacier behavior is highly variable throughout west greenland

simulations of the greenland ice sheet 100 years into the future with the full stokes model elmer ice it is likely that climate change will have a significant impact on the mass balance of the greenland ice sheet contributing to future sea level rise here we present the implementation of the full stokes model elmer ice for the greenland ice sheet which includes a mesh refinement technique in order to resolve fast flowing ice streams and outlet glaciers we discuss simulations 100 years into the future forced by scenarios defined by the searise sea level response to ice sheet evolution community effort for comparison the same experiments are also run with the shallow ice model sicopolis simulation code for polythermal ice sheets we find that elmer ice is 43 more sensitive exhibits a larger loss of ice sheet volume relative to the control run than sicopolis for the ice dynamic scenario doubled basal sliding but 61 less sensitive for the direct global warming scenario based on the a1 b moderate emission scenario for greenhouse gases the scenario with combined a1b global warming and doubled basal sliding forcing produces a greenland contribution to sea level rise of 15cm for elmer ice and 12cm for sicopolis over the next 100 years