rapid circulation of warm subtropical waters in a major glacial fjord in east greenland the recent rapid increase in mass loss from the greenland ice sheet is primarily attributed to an acceleration of outlet glaciers oceanographic data obtained in summer 2008 show that subtropical waters that reside year round in the shelf ocean off greenland continuously enter a large glacial fjord in east greenland and contribute to melting at the glacier terminus the recent rapid increase in mass loss from the greenland ice sheet1 2 is primarily attributed to an acceleration of outlet glaciers3 4 5 one possible cause of this acceleration is increased melting at the ice ocean interface6 7 driven by the synchronous warming8 9 10 of subtropical waters offshore of greenland however because of the lack of observations from greenland s glacial fjords and our limited understanding of their dynamics this hypothesis is largely untested here we present oceanographic data collected in sermilik fjord east greenland by ship in summer 2008 and from moorings our data reveal the presence of subtropical waters throughout the fjord these waters are continuously replenished through a wind driven exchange with the shelf where they are present all year the temperature and renewal of these waters indicate that they currently cause enhanced submarine melting at the glacier terminus key controls on the melting rate are the volume and properties of the subtropical waters on the shelf and the patterns of along shore winds suggesting that the glaciers acceleration has been triggered by a combination of atmospheric and oceanic changes our measurements provide evidence for a rapid advective pathway for the transmission of oceanic variability to the ice sheet margins

role of model initialization for projections of 21st century greenland ice sheet mass loss model simulations of the greenland ice sheet contribution to 21st century sea level rise are performed with a state of the art ice sheet model parallel ice sheet model pism the climate forcing fields are obtained from the european union s seventh framework programme project ice2sea in which three regional climate models are used to dynamically downscale two scenarios a1b and e1 from two general circulation models echam5 and hadcm3 to assess the sensitivity of the projections to the model initial state four initialization methods are applied in these experiments the simulated contribution to sea level rise by 2100 ranges from an equivalent of 0 2 to 6 8 cm the largest uncertainties arise from different formulations of the regional climate models 0 8 3 9 cm and applied scenarios 0 65 1 9 cm but an important source of uncertainty is the initialization method 0 1 0 8 cm these model simulations do not account for the recently observed acceleration of ice streams and consequent thinning rates the changing ice discharge that may result from the spatial and temporal variability of ocean forcing or the feedback occurring between ice sheet elevation changes and climate forcing thus the results should be considered the lower limit of greenland ice sheet contributions to sea level rise until such processes have been integrated into large scale ice sheet models

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

3 d inversion of mt data for imaging deformation fronts in nw poland the pomerania region northwest part of poland occupies a significant position where the largest european tectonic boundary is situated this is the area of the contact between the east european craton eec and the paleozoic platform pp and it is known as the trans european suture zone tesz the tesz was formed during paleozoic time as a consequence of the collision of several crustal units and it extends from the black sea in the southeast to the british isles in the northwest it is a region of key importance for our understanding of the tectonic history of europe previous magnetotelluric mt results based on 2 d inverse modeling show that the contact zone is of lithospheric discontinuity character and there are distinct differences in geoelectric structures between the precambrian eec transitional zone tesz and the younger pp the presence of a significant conductor at mid and lower crustal depths was also shown thus the main aim of the research presented here was to obtain detailed 3 d images of electrical conductivity in the crust and upper mantle and its regional distribution below the tesz in the northwest part of poland to accomplish this task we applied the latest 3 d inversion codes which allowed us to get more realistic model geometries additionally to confirm and complement the study the horizontal magnetic tensor hmt analysis was realized this method gives us an opportunity to efficiently locate the position of well conducting structures as a result we obtain a clearer three dimensional model of conductivity distribution where highly conductive rock complexes appear which we tentatively connected to deformation fronts

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

processes of buildup and retreat of the greenland ice sheet sensitivity experiments were conducted to study the influence of several physical processes on the evolution of the greenland ice sheet over the last 250 000 years the experiments were carried out by means of a three dimensional thermodynamical model the mass balance is modeled by a surface energy balance model this means that variations in orbital parameters following from the astronomical milankovitch theory can be incorporated in the ablation calculations it is shown that with regard to the greenland ice sheet variations in shortwave radiation are only of minor importance for the changes in volume over time calculations of the various components of the energy balance under glacial and interglacial conditions show that the changes in absorbed shortwave radiation resulting from changes in orbital parameters are considerably smaller than the variations in the sensible heat flux caused by temperature variations thermodynamics are only of importance for the volume changes of the ice sheet if the climate is on average stable over a long period in between 110 kyr b p and 20 kyr b p the ice volume increased slowly due to the cooling of the ice which resulted in a higher effective viscosity and thus a thicker ice sheet the long response time of the thermodynamics means that the thermodynamics cannot play a crucial role during transition from a glacial to a deglacial period which occur typically within several thousands of years the mass balance height feedback has an important function in the buildup of the ice sheet after a warm period like the eemian without isostatic bedrock compensation the ablation will remain very high in some marginal parts yielding a considerably smaller ice sheet volume for the last glacial period

modelled atmospheric temperatures and global sea levels over the past million years the climate has passed through a series of glacials and interglacials over the past million years but the nature of this cyclicity in terms of temperature ice volume and sea level and the underlying causes are not well known bintanja et al use a new method to deduce a one million year time series of these variables the reconstructed records are much longer than other methods have provided for any of these variables individually the most intense glacial stages were 17 c colder than today and most of the continental ice was present in north america strong cooling in the beginning of glacials was found to precede ice sheet build up these findings may shed light on the causes of ice age cycles marine records of sediment oxygen isotope compositions show that the earth s climate has gone through a succession of glacial and interglacial periods during the past million years but the interpretation of the oxygen isotope records is complicated because both isotope storage in ice sheets and deep water temperature affect the recorded isotopic composition1 2 3 4 5 separating these two effects would require long records of either sea level or deep ocean temperature which are currently not available here we use a coupled model of the northern hemisphere ice sheets6 and ocean temperatures forced to match an oxygen isotope record for the past million years compiled from 57 globally distributed sediment cores to quantify both contributions simultaneously we find that the ice sheet contribution to the variability in oxygen isotope composition varied from ten per cent in the beginning of glacial periods to sixty per cent at glacial maxima suggesting that strong ocean cooling preceded slow ice sheet build up the model yields mutually consistent time series of continental mean surface temperatures between 40 and 80 n ice volume and global sea level we find that during extreme glacial stages air temperatures were 17 1 8 c lower than present with a 120 10 m sea level equivalent of continental ice present

shallow shelf approximation as a sliding law in a thermomechanically coupled ice sheet model 1 the shallow shelf approximation a balance of membrane stresses for ice flow is an effective sliding law for ice sheet modeling our use of it as a sliding law becomes a standard model for ice stream flow when the sliding velocity is large 100 m a 1 and faster following schoof 2006a we describe the basal resistance as plastic till for which the yield stress is given by a mohr coulomb formula pore water pressure is related to basal melt rate the velocity field used in the mass continuity and conservation of energy equations is an average of velocities from the shallow shelf approximation and the nonsliding shallow ice approximation using this scheme our model has realistic time dependent ice streams which exhibit the range of surface velocities seen in actual ice streams we demonstrate the model at high spatial resolution 5 km grid over multiple millenia using its implementation in the parallel ice sheet model numerical experiments show that the entire scheme is stable with respect to many parameter changes some experiments reveal significant ice stream variability in a hypothetical steady climate with characteristic cycles on the order of 1000 years we believe this is the first practical whole ice sheet model with a unified treatment of vertical shear stresses and membrane stresses it is capable of high resolution thermomechanically coupled multimillenia simulations of ice sheets containing ice streams

warming of the polar water layer in disko bay and potential impact on jakobshavn isbrae a number of recent studies have shown enhanced retreat of tidewater glaciers over much of southern and western greenland one of the fastest retreats has occurred at jakobshavn isbrae with the rapid retreat linked to the arrival of relatively warm and saline irminger water along the west coast of greenland similar links to changes in ocean water masses on the coastal shelf of greenlandwere also seen on the east coast this study presents hydrographic data from disko bay additionally revealing that there was also a significant warming of the cold polar water entering disko bay from the mid to late 1990s onward this layer which lies at a depth of 30 200m warmedby 18 28c the heat content of the polar water layer increased by a factor of 3 6 for the post 1997 period compared to the period prior to 1990 the heat content in the west greenland irminger water layer between the same periods increased only by a factor of 2 but contained more total heat the authors suggest that the changes in the polar water layer are related to circulation changes in baffin bay

greenland ice sheet higher nonlinearity of ice flow significantly reduces estimated basal motion in times of warming in polar regions the prediction of ice sheet discharge is of utmost importance to society because of its impact on sea level rise in simulations the flow rate of ice is usually implemented as proportional to the differential stress to the power of the exponent n 3 this exponent influences the softness of the modeled ice as higher values would produce faster flow under equal stress we show that the stress exponent which best fits the observed state of the greenland ice sheet equals n 4 our results which are not dependent on a possible basal sliding component of flow indicate that most of the interior northern ice sheet is currently frozen to bedrock except for the large ice streams and marginal ice ice in the polar ice sheets flows towards the oceans under its own weight knowing how fast the ice flows is of crucial importance to predict future sea level rise the flow has two components 1 internal shearing flow of ice and 2 basal motion which is sliding along the base of ice sheets especially when the ice melts at this base to determine the first component we need to know how soft the ice is by considering the flow velocities at the surface of the northern greenland ice sheet and calculating the stresses that cause the flow we determined that the ice is effectively softer than is usually assumed previous studies indicated that the base of the ice is thawed in large parts up to about 50 of the greenland ice sheet our study shows that that is probably overestimated because these studies assumed ice to be harder than it actually is our new assessment reduces the area with basal motion and thus melting to about 6 13 in the greenland study area

global ice volume variations through the last glacial cycle simulated by a 3 d ice dynamical model abstract a coupled ice sheet ice shelf bedrock model was run at 20 km resolution to simulate the evolution of global ice cover during the last glacial cycle the mass balance model uses monthly mean temperature and precipitation as input and incorporates the albedo mass balance feedback the model is forced by the grip and vostok derived temperature records and by sea level and solar radiation variations which are distributed spatially and seasonally model simulations show that at the last glacial maximum lgm a volume of ice representing about 65 m of sea level equivalent is stored on the eurasian continent whereas north america stores 42 m the other regions greenland tibet south america and antarctica contribute about 17 m to global sea level lowering at the lgm compared to present day sensitivity experiments show that the ice volumes of the eurasian and north american ice sheets are particularly sensitive to summer temperature changes finally the timing and mechanism of grounded ice formation and expansion for the last glacial were studied for the eurasian and north american continents ice sheet formation begins at 118 kyr bp in the barents and kara sea region over baffin island and in the high cordilleran range following a 4 c cooling alternating regions of ice sheet expansion by direct snowfall ice flow and grounding of ice shelves can be identified these relate to specific geomorphological traces and may as such help in model validation

the shallow ice approximation for anisotropic ice formulation and limits ice sheet flow modeling is generally based on the shallow ice approximation sia developed for isotropic ice we extend this approximation to anisotropic ice and check its validity for both the isotropic and anisotropic cases by comparing the results of up to second order sia with those obtained from a model solving the full set of mechanical equations the theory is developed for a constitutive relation describing newtonian and non newtonian behavior but numerical results are shown only for a newtonian behavior the results are compared for plane flow of isothermal ice under steady state conditions sia gives an excellent representation of the newtonian flow of an isotropic ice sheet and a good description for the anisotropic case the zero order approximation is sufficient to describe ice flow precisely over flat bedrock for the anisotropic and isotropic cases even close to an ice divide for uneven bedrock the second order sia gives excellent results for isotropic ice and acceptable results for anisotropic ice finally we quantify the error introduced by using an enhancement factor to represent the anisotropy when longitudinal stresses are taken into account

retention of greenland runoff by refreezing implications for projected future sea level change melting produced at the surface of subfreezing permeable firn does not freely percolate but instead refreezes near its point of origin raising the firn temperature and decreasing the pore space if sufficient meltwater is introduced the firn will warm to 0 c and subsequent water will fill the remaining pore space without freezing only after the residual water content of the firn is exceeded will water flow through the firn and possibly escape as runoff this process prevents summer melt on high arctic glaciers and ice caps from escaping in its entirety and it must be accounted for in modeling glacier runoff on the basis of surface energy balance a model is presented here which describes in a simple way the transient process of infiltration refreezing and runoff in a future warming climate the model is applied to greenland for which predictions of runoff induced sea level rise that do not consider the refreezing process could be as much as 5 0 cm too high over 150 years

episodic discrete and distributed deformation consequences and controls in a thrust culmination from the central pyrenees abstract the pic de port vieux culmination is a 0 5 km long thrust culmination in the footwall of the gavarnie thrust in the central pyrenees the culmination comprises a set of blind thrusts that imbricate a 50 m thick mesozoic cover sequence comprising triassic red beds and upper cretaceous limestone sandwiched between underlying hercynian basement and the overlying palaeozoic strata of the gavarnie thrust sheet an intense cleavage developed within the culmination associated with the oblique tip lines of the thrusts as they climbed from the basement into the cover strata detailed mapping shows that the distributed strain associated with the fault tips locally re activated earlier folded thrusts creating upper detachments to the system and produced a set of new overstepping isolated faults the main detachment occurred along the roof of the culmination using the back limbs of folds in the roof thrust the movement on these detachments can be demonstrated to have been episodic with periods of fault slip producing splays that were then folded during periods when distributed deformation was dominant the main control on this episodic deformation is modelled as the result of feedback between increase in fluid pressure associated with inelastic porosity reduction during cleavage development and fluid pressure bleed off associated with fault zone dilatancy in the culmination

greenland ice sheet contribution to 21st century sea level rise as simulated by the coupled cesm2 1 cism2 1 the greenland ice sheet gris mass balance is examined with an earth system ice sheet model that interactively couples the gris to the broader earth system the simulation runs from 1850 to 2100 with historical and ssp5 8 5 forcing by the mid 21st century the cumulative gris contribution to global mean sea level rise slr is 23 mm during the second half of the 21st century the surface mass balance becomes negative in all drainage basins with an additional slr contribution of 86 mm the annual mean gris mass loss in the last two decades is 2 7 mm sea level equivalent sle year 1 the increased slr contribution from the surface mass balance 3 1 mm sle year 1 is partly offset by reduced ice discharge from thinning and retreat of outlet glaciers the southern gris drainage basins contribute 73 of the mass loss in mid century but 55 by 2100 as surface runoff increases in the northern basins

effect of isostasy on dynamical ice sheet modeling a case study for eurasia 1 isostatic adjustment is an important process when modeling dynamical ice sheet evolution a number of different types of earth model are used to model this process ranging from models with local or elastic lithospheres with a single relaxation time to account for temporal behavior to self gravitating spherically symmetric models of the crust mantle system we performed sensitivity tests to assess the dynamical response of ice sheets to isostatic adjustment the resulting ice sheet geometries depend on the response times of the system including the growth rate of ice sheets versus the relaxation time of the earth differences in lithospheric strength cause large differences in the modeled ice sheets with the largest differences in isostatic adjustment in the vicinity of the ice margins in addition we compared predictions based on an often used flexural model with an elastic plate and a single relaxation time to a spherically symmetric self gravitating viscoelastic approach no flexural model was able to reproduce the viscoelastic response completely due to the fact that the models have an intrinsically different spatial and temporal response when applied to a full transient glacial cycle in eurasia even the best fitting flexural model underestimated the ice volume at the lgm by 30 percent our results indicate that the commonly used value of 3000 years for the relaxation time in flexural models is too short for eurasia by at least 3000 years

synchronous retreat and acceleration of southeast greenland outlet glaciers 2000 06 ice dynamics and coupling to climate a large portion of the recent increase in the rate of mass loss from the greenland ice sheet is from increased outlet glacier discharge along its southeastern margin while previous investigations of the region s two largest glaciers suggest that acceleration is a dynamic response to thinning and retreat of the calving front it is unknown whether this mechanism can explain regional acceleration and what forcing is responsible for initiating rapid thinning and retreat we examine seasonal and interannual changes in ice front position surface elevation and flow speed for 32 glaciers along the southeastern coast between 2000 and 2006 while substantial seasonality in front position and speed is apparent nearly all the observed glaciers show net retreat thinning and acceleration with speed up corresponding to retreat the ratio of retreat to the along flow stress coupling length is proportional to the relative increase in speed consistent with typical ice flow and sliding laws this affirms that speed up results from loss of resistive stress at the front during retreat which leads to along flow stress transfer large retreats were often preceded by the formation of a flat or reverse sloped surface near the front indicating that subsequent retreats were influenced by the reversed bed slope many retreats began with an increase in thinning rates near the front in the summer of 2003 a year of record high coastal air and sea surface temperatures this anomaly was driven in part by recent warming suggesting that episodes of speed up and retreat may become more common in a warmer climate

glaciological investigations in the crete area central greenland a search for a new deep drilling site the results of the 1984 85 post gisp campaigns in central greenland are presented eight ice cores were obtained some spanning up to 360 years we present i geographical positions and elevations at the drill sites

moulin density controls drainage development beneath the greenland ice sheet this work was funded through a uk natural environment research council doctoral training grant lcag 133 a bowring junior research fellowship st catharine s college cambridge and a leverhulme newton trust early career fellowship all awarded to a f b i j h was supported by a marie curie fp7 career integration grant within the 7th european union framework programme

sensitivity of greenland ice sheet projections to model formulations physically based projections of the greenland ice sheet contribution to future sea level change are subject to uncertainties of the atmospheric and oceanic climatic forcing and to the formulations within the ice flow model itself here a higher order three dimensional thermomechanical ice flow model is used initialized to the present day geometry the forcing comes from a high resolution regional climate model and from a flowline model applied to four individual marine terminated glaciers and results are subsequently extended to the entire ice sheet the experiments span the next 200 years and consider climate scenario sres a1b the surface mass balance smb scheme is taken either from a regional climate model or from a positive degree day pdd model using temperature and precipitation anomalies from the underlying climate models our model results show that outlet glacier dynamics only account for 6 18 of the sea level contribution after 200 years confirming earlier findings that stress the dominant effect of smb changes furthermore interaction between smb and ice discharge limits the importance of outlet glacier dynamics with increasing atmospheric forcing forcing from the regional climate model produces a 14 31 higher sea level contribution compared to a pdd model run with the same parameters as for ipcc ar4

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

the dynamic response of the greenland and antarctic ice sheets to multiple century climatic warming new calculations were performed to investigate the combined response of the greenland and antarctic ice sheets to a range of climatic warming scenarios over the next millennium use was made of fully dynamic 3d thermomechanic ice sheet models which were coupled to a two dimensional climate model the experiments were initialized with simulations over the last two glacial cycles to estimate the present evolution and were subsequently forced with temperature scenarios resulting from greenhouse emission scenarios which assume equivalent co2 increases of two four and eight times the present 1990 a d value by the year 2130 a d and a stabilization after that the calculations brought to light that during the next century short term effect the background evolution trend would dominate the response of the antarctic ice sheet but would be negligible for the greenland ice sheet on that timescale the greenland and antarctic ice sheets would roughly balance one another for the middle scenario similar to the ipcc96 is92a scenario with respective contributions to the worldwide sea level stand on the order of about 610 cm on the longer term however both ice sheets would contribute positively to the worldwide sea level stand and the most important effect would come from melting on the greenland ice sheet sensitivity experiments highlighted the role of ice dynamics and the height massbalance feedback on the results it was found that ice dynamics cannot be neglected for the greenland ice sheet not even on a century timescale but becomes only important for antarctica on the longer term the latter is related to an increased outflow of ice into the ice shelves and to the grounding line retreat of the west antarctic ice sheet which are both found to be sensitive to basal melting below ice shelves and the effective viscosity of the ice shelves stretching parameters to their limits yielded a combined maximum rate of sea level rise of 85 cm century21 of which 60 cm would originate from the greenland ice sheet alone

acceleration of jakobshavn isbrae triggered by warm subsurface ocean waters observations over past decades show a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning in 1997 this switch is associated with a doubling in glacier velocity hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were triggered by the arrival of relatively warm water originating from the irminger sea observations over the past decades show a rapid acceleration of several outlet glaciers in greenland and antarctica1 one of the largest changes is a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning2 in 1997 associated with a doubling in glacier velocity3 suggested explanations for the speed up of jakobshavn isbrae include increased lubrication of the ice bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers4 and weakening and break up of the floating ice tongue that buttressed the glacier5 here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were instead triggered by the arrival of relatively warm water originating from the irminger sea near iceland we trace these oceanic changes back to changes in the atmospheric circulation in the north atlantic region we conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets

potential for bias in 21st century semiempirical sea level projections we examine the limitations of a semiempirical model characterized by a sea level projection of 73 cm with rcp4 5 scenario by 2100 calibrating the model with data to 1990 and then simulating the period 1993 2009 produces sea level in close agreement with acceleration in sea level rise observed by satellite altimetry nonradiative forcing contributors such as long term adjustment of greenland and antarctica ice sheets since last glacial maximum abyssal ocean warming and terrestrial water storage may bias model calibration which if corrected for tend to reduce median sea level projections at 2100 by 2 10 cm though this is within the confidence interval we apply the semiempirical approach to simulate individual contributions from thermal expansion and small glacier melting steric sea level projections agree within 3 cm of output from process based climate models in contrast semiempirical simulation of melting from glaciers is 26 cm which is twice large as estimates from some process based models however all process models lack simulation of calving which likely accounts for 50 of small glacier mass loss worldwide furthermore we suggest that changes in surface mass balance and dynamics of greenland ice sheet made contributions to the sea level rise in the early 20th century and therefore are included within the semiempirical model calibration period and hence are included in semiempirical sea level projections by 2100 antarctic response is probably absent from semiempirical models which will lead to a underestimate in sea level rise if as is probable antarctica loses mass by 2100

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

sermeq model produces a realistic upper bound on calving retreat for 155 greenland outlet glaciers the rate of land ice loss due to iceberg calving is a key source of variability among model projections of 21st century sea level rise it is especially challenging to account for mass loss due to

a 3 d model for the antarctic ice sheet a sensitivity study on the glacial interglacial contrast on the longer climatic time scales changes in the elevation and extent of the antarctic ice sheet have an important role in modulating global atmospheric and oceanographic processes and contribute significantly to world wide sea levels in this paper a 3 d time dependent thermomechanical model for the entire ice sheet is presented that is subsequently used to examine the effects of glacial interglacial shifts in environmental boundary conditions on its geometry the model takes into account a coupled ice shelf grounding line dynamics basal sliding and isostatic bed adjustment and considers the fully coupled velocity and temperature fields ice flow is calculated on a fine mesh 40 km horizontal grid size and 10 layers in the vertical for grounded and floating ice and a stress transition zone in between at the grounding line where all stress components contribute in the effective stress in the flow law there is free interaction between ice sheet and ice shelf so that the entire geometry is internally generated a simulation of the present ice sheet reveals that the model is able to yield realistic results a series of sensitivity experiments are then performed in which lower temperatures reduced accumulation rates and lower global sea level stands are imposed either singly or in combination by comparing results of pairs of experiments the effects of each of these environmental changes can be determined in agreement with glacial geological evidence we found that the most pronounced changes show up in the west antarctic ice sheet configuration they appear to be essentially controlled by variations in eustatic sea level whereas typical glacial interglacial changes in temperature and ice deposition rates tend to balance one another these findings support the hypothesis that the antarctic ice sheet basically follows glacial episodes in the northern hemisphere by means of sea level teleconnections grounding occurs more readily in the weddell sea than in the ross sea and long time scales appear to be involved it may take up to 30 40000 years for these continental shelf areas to become completely grounded after an initial stepwise perturbation in boundary conditions according to these reconstructions a steady state antarctic ice sheet may contribute some 16 m to global sea level lowering at maximum glaciation

the greenland ice sheet during the last glacial cycle current ice loss and contribution to sea level rise from a palaeoclimatic perspective abstract during the last interglacial lig period between 130 and 116 thousand years before present ka bp the greenland ice sheet gris was considerably reduced in size contributing to a global mean sea level msl rise of 0 5 4 2 m relative to the present this is not sufficient to explain the 6 9 m msl rise estimated for the lig which implies that a significant contribution to the lig highstand came from antarctica following the lig the gris grew and attained its maximum volume of about 12 m global sea level equivalents sles between 18 and 16 ka bp since then the gris margins have retreated on an order of several hundred km following a general pattern of lagged response to changes in high latitude summer insolation and global greenhouse gas concentrations on shorter timescales and over smaller spatial scales the combination of internal ice sheet dynamics and external climate dynamics has often resulted in a more complex and asynchronous behaviour of the ice sheet margin the gris probably reached its minimum holocene extent around 4 ka bp and modelling studies suggest that it contributed to a rise in global msl of less than 0 2 m relative to the present at this time a period of steady growth followed the holocene minimum and in many areas the ice sheet advanced beyond its present limits during the little ice age i e the last few centuries currently the gris occupies an area of 1 7 10 6 km 2 and features a volume of 2 96 10 6 km 3 which amounts to 7 4 m sle observations show that the rate of mass loss from the gris has been accelerating over the past few decades and ice sheet modelling indicates that we have to go back to the last deglaciation around 10 ka bp in order to find sustained melt rates that were higher than those experienced over the last decade the future development of the gris will have a profound influence on sea level both globally and regionally but there are large uncertainties related to how the gris will respond to future global warming based on a range of modelling studies employing different emission scenarios the gris is expected to contribute about 0 1 0 3 m to global msl rise by the end of the 21st century in a longer time perspective modelling suggests that melting of the gris may replace ocean thermal expansion as the most important factor in future sea level rise potentially contributing with 0 7 2 6 m sle within the next 500 years multi millennial simulations show that the entire ice sheet 7 4 m sle might disappear completely within less than three thousand years under high emission scenarios i e with atmospheric co 2 reaching four times preindustrial levels

mass balance of the greenland ice sheet from 1958 to 2007 we combine estimates of the surface mass balance smb of the greenland ice sheet for years 1958 to 2007 with measurements of the temporal variability in ice discharge d to deduce the total ice sheet mass balance during that time period we find a robust correlation r2 0 83 between anomalies in smb and in d which we use to reconstruct a continuous series of total ice sheet mass balance we find that the ice sheet was losing 110 70 gt yr in the 1960s 30 50 gt yr or near balance in the 1970s 1980s and 97 47 gt yr in 1996 increasing rapidly to 267 38 gt yr in 2007 multi year variations in ice discharge themselves related to variations in smb cause 60 20 more variation in total mass balance smb and therefore dominate the ice sheet mass budget copyright 2008 by the american geophysical union

the importance of thermodynamics for modeling the volume of the greenland ice sheet two different kinds of ice flow models one two dimensional 2 d and the other three dimensional 3 d have been used to test the importance of the thermodynamic response of the greenland ice sheet the basic difference between the two models is that the thermodynamics are neglected in the two dimensional model whereas these are incorporated in the three dimensional model the 2 d and 3 d models are compared in terms of volume response time and total volume of the ice sheet results indicate that the sawtooth character of the ice volume on glacial interglacial timescales is among other possible reasons a result of the thermodynamic coupling the changes in elevation over the last 130 000 years calculated with the three dimensional version are 230 m for the summit drill site and 190 m for the n grip site the standard deviation of the changes in elevation is 55 m for the summit site and 43 m for the n grip site the present day imbalance is merely a result of the rather constant climate over the last 10 kyr and is not determined by the thermodynamics consequently pleistocene temperatures do still exist in the ice sheet but are not important for volume calculations of the present day ice sheet for short term perturbation experiments in the future the pronounced sensitivity of the mass balance will determine the response of the greenland ice sheet whereas thermodynamics will play only a minor role

the role of membrane like stresses in determining the stability and sensitivity of the antarctic ice sheets back pressure and grounding line motion membrane stresses act along thin bodies which are relatively well lubricated on both surfaces they operate in ice sheets because the bottom is either sliding or is much less viscous than the top owing to stress and heat softening of the basal ice ice streams flow over very well lubricated beds and are restrained at their sides the ideal of the perfectly slippery bed is considered in this paper and the propagation of mechanical effects along an ice stream considered by applying spatially varying horizontal body forces propagation distances depend sensitively on the rheological index and can be very large for ice type rheologies a new analytical solution for ice shelf profiles and grounded tractionless stream profiles is presented which show blow up of the profile in a finite distance upstream at locations where the flux is non zero this is a feature of an earlier analytical solution for a floating shelf the length scale of decay of membrane stresses from the grounding line is investigated through scale analysis in ice sheets such effects decay over distances of several tens of kilometres creating a vertical boundary layer between sheet flow and shelf flow where membrane stresses adjust bounded physically reasonable steady surface profiles only exist conditionally in this boundary layer where bounded steady profiles exist adjacent profile equilibria for the whole ice sheet corresponding to different grounded areas occur neutral equilibrium if no solution in the boundary layer can exist the ice sheet profile must change the conditions for existence can be written in terms of whether the basal rate factor sliding or internal deformation is too large to permit a steady solution the critical value depends extremely sensitively on ice velocity and the back stress applied at the grounding line high ice velocity and high stress both favour the existence of solutions and stability changes in these parameters can cause the steady solution existence criterion to be traversed and the ice sheet dynamics to change a finite difference model which represents both neutral equilibrium and the dynamical transition is presented and preliminary investigations into its numerical sensitivity are carried out evidence for the existence of a long wavelength instability is presented through the solution of a numerical eigenproblem which will hamper predictability

greenland s contribution to global sea level rise by the end of the 21st century the greenland ice sheet holds enough water to raise the global sea level with 7 m over the last few decades observations manifest a substantial increase of the mass loss of this ice sheet both enhanced melting and increase of the dynamical discharge associated with calving at the outlet glacier fronts are contributing to the mass imbalance using a dynamical and thermodynamical ice sheet model and taking into account speed up of outlet glaciers we estimate greenland s contribution to the 21st century global sea level rise and the uncertainty of this estimate boundary fields of temperature and precipitation extracted from coupled climate model projections used for the ipcc fourth assessment report are applied to the ice sheet model we implement a simple parameterization for increased flow of outlet glaciers which decreases the bias of the modeled present day surface height it also allows for taking into account the observed recent increase in dynamical discharge and it can be used for future projections associated with outlet glacier speed up greenland contributes 0 17 cm to global sea level rise by the end of the 21st century this range includes the uncertainties in climate model projections the uncertainty associated with scenarios of greenhouse gas emissions as well as the uncertainties in future outlet glacier discharge in addition the range takes into account the uncertainty of the ice sheet model and its boundary fields