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

holocene deceleration of the greenland ice sheet recent peripheral thinning of the greenland ice sheet is partly offset by interior thickening and is overprinted on its poorly constrained holocene evolution on the basis of the ice sheet s radiostratigraphy ice flow in its interior is slower now than the average speed over the past nine millennia generally higher holocene accumulation rates relative to modern estimates can only partially explain this millennial scale deceleration the ice sheet s dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across nares strait also contributed to this pattern thus recent interior thickening of the greenland ice sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry

greenland glacial history and local geodynamic consequences summary space time reconstructions of the continental ice sheets that existed at last glacial maximum lgm have previously been produced using two entirely independent methodologies one based upon the use of theoretical models of ice sheet accumulation and flow and one based upon the geophysical inversion of relative sea level rsl histories from previously ice covered regions the analyses described in this paper demonstrate the significant advantages that derive from the simultaneous application of both methods to the particular case of greenland we thereby show that the ice 4g reconstruction of the glaciation history of this region from lgm to present which was based upon the geophysical inversion of rsl data alone was reasonably accurate in the peripheral regions where rsl data were available but inaccurate in the interior of the ice sheet which was unconstrained by such information we test the new model of greenland glacial history determined by the simultaneous application of the constraints that derive from ice sheet modelling and the geophysical inversion of rsl data by employing recently published geodetic inferences of mass balance over the entire interior region of the ice sheet and of gps measurements of vertical crustal motion these observations which were not employed to constrain the ice sheet reconstruction provide significant support for the new glacial history for greenland that our analyses have led us to infer

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

insights into spatial sensitivities of ice mass response to environmental change from the searise ice sheet modeling project ii greenland searise greenland

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

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

acceleration of the contribution of the greenland and antarctic ice sheets to sea level rise 1 ice sheet mass balance estimates have improved substantially in recent years using a variety of techniques over different time periods and at various levels of spatial detail considerable disparity remains between these estimates due to the inherent uncertainties of each method the lack of detailed comparison between independent estimates and the effect of temporal modulations in ice sheet surface mass balance here we present a consistent record of mass balance for the greenland and antarctic ice sheets over the past two decades validated by the comparison of two independent techniques over the last 8 years one differencing perimeter loss from net accumulation and one using a dense time series of time variable gravity we find excellent agreement between the two techniques for absolute mass loss and acceleration of mass loss in 2006 the greenland and antarctic ice sheets experienced a combined mass loss of 475 158 gt yr equivalent to 1 3 0 4 mm yr sea level rise notably the acceleration in ice sheet loss over the last 18 years was 21 9 1 gt yr2 for greenland and 14 5 2 gt yr2 for antarctica for a combined total of 36 3 2 gt yr2 this acceleration is 3 times larger than for mountain glaciers and ice caps 12 6 gt yr2 if this trend continues ice sheets will be the dominant contributor to sea level rise in the 21st century

holocene thinning of the greenland ice sheet the nature of the response of the greenland ice sheet gis to climate change is a fundamental question in climate science but the details remain unclear during the holocene climate optimum 6 000 to 9 000 years ago for example many locations in the northern hemisphere showed unusual warmth yet changes in greenland appeared inconsistent vinther et al now standardize the previously disparate seeming ice core records of climate history on the gis during the holocene and find consistent evidence for a stronger climate optimum than was previously recognized together with elevation reduction and marginal ice sheet thinning the response of the greenland ice sheet gis to changes in climate remains uncertain during the holocene climatic optimum an unusually warm period from about 9 000 to 6 000 years ago changes in greenland appear inconsistent with the rest of the northern hemisphere here the greenland holocene temperature history and the evolution of gis surface elevation are extracted from four gis locations the results may resolve this disparity on entering an era of global warming the stability of the greenland ice sheet gis is an important concern1 especially in the light of new evidence of rapidly changing flow and melt conditions at the gis margins2 studying the response of the gis to past climatic change may help to advance our understanding of gis dynamics the previous interpretation of evidence from stable isotopes 18o in water from gis ice cores was that holocene climate variability on the gis differed spatially3 and that a consistent holocene climate optimum the unusually warm period from about 9 000 to 6 000 years ago found in many northern latitude palaeoclimate records4 did not exist here we extract both the greenland holocene temperature history and the evolution of gis surface elevation at four gis locations we achieve this by comparing 18o from gis ice cores3 5 with 18o from ice cores from small marginal icecaps contrary to the earlier interpretation of 18o evidence from ice cores3 6 our new temperature history reveals a pronounced holocene climatic optimum in greenland coinciding with maximum thinning near the gis margins our 18o based results are corroborated by the air content of ice cores a proxy for surface elevation7 state of the art ice sheet models are generally found to be underestimating the extent and changes in gis elevation and area our findings may help to improve the ability of models to reproduce the gis response to holocene climate

geodetic measurements of postglacial adjustments in greenland 1 we analyze data from seven continuous global positioning system gps receivers and one tide gauge all located along the edge of the greenland ice sheet to determine vertical uplift rates we compare our results with predictions based on the ice 5g deglaciation model of peltier 2004 results from the gps receiver at kellyville 1 2 1 1 mm a and from the tide gauge at nuuk 2 2 1 3 mm a indicate that ice 5g overestimates the subsidence rates at those locations by 2 1 and 1 1 mm a respectively kellyville and nuuk are located along the southwestern margin of the greenland ice sheet and the observed negative uplift rates are consistent with independent evidence that the ice margin along the southwestern edge readvanced during the last 8 ka to its current position the ice 5g glaciation deglaciation history includes a readvance between the latitudes of 62 n and 72 n the gps measurements suggest the ice 5g readvance may be too large or mistimed our gps results at qaqortoq located at the southern tip of greenland suggest a secular subsidence rate of 0 3 1 1 mm a while ice 5g predicts an uplift rate of 1 0 mm a ice 5g assumes no ice sheet readvance in south greenland including no readvance of the qassimiut lobe the difference of 1 3 1 1 mm a can tentatively be explained as due to a 33 km readvance of the qassimiut lobe during the last 3 ka for the other gps sites the observed predicted uplift rates are 3 6 1 1 0 1 mm a at thule 0 0 1 1 2 0 mm a at scoresbysund and 0 4 1 1 1 7 mm a at kulusuk for thule kulusuk and scoresbysund the differences between the observed and predicted rates are on the order of 1 3 3 7 mm a though with opposite signs and indicate that ice 5g does not exactly reproduce the correct rebound signal at those locations

strain rate enhancement at dye 3 greenland ice at depth in ice sheets can be softer in bed parallel shear than glen s flow law predicts for example at dye 3 greenland enhancement factors of 3 4 are needed in order to explain the rate of borehole tilting previous authors have identified crystal fabric as the dominant contributor but the role of impurities and crystal size is still incompletely resolved here we use two formulations of anisotropic flow laws for ice azuma s and sachs models to account for the effects of anisotropy and show that the measured anisotropy of the ice at dye 3 cannot explain all the detailed variations in the measured strain rates the jump in enhancement across the holocene wisconsin boundary is larger than expected from the measured fabrics alone dust and soluble ion concentration divided by crystal size correlates well with the residual enhancement indicating that most of the excess deformation may be due to impurities or crystal size while the major features of the deformation at dye 3 are explained by anisotropy and temperature results also suggest that further research into the role of impurities and crystal size is warranted

present day jakobshavn isbrae west greenland close to the holocene minimum extent abstract marine terminating glaciers evolve on millenial timescales in response to changes in oceanic and atmospheric conditions however the relative role of oceanic and atmospheric drivers remains uncertain the evolution of marine terminating glaciers under the warmer than present holocene climate optimum climate can give important insights into the dynamics of ice streams as the climate evolves the early holocene evolution of jakobshavn isbrae from the last glacial maximum extent up to 8 2 ka bp is well constrained by geomorphological studies in the area however the holocene minimum extent of the glacier is unknown here we use a high resolution regional ice sheet model to study the retreat and readvance of jakobshavn isbrae from the mid holocene to the little ice age this model of jakobshavn isbrae accurately tracks the terrestrial ice margin and agrees with available estimates of marine grounding line evolution we find that the holocene minimum extent of both the terrestrial ice margin and the grounding line reached at 6 5 ka bp is close to the present day extent of the glacier we also find that the glacier is currently located close to a tipping point from beyond which readvance would require a longer and more significant cooling than the little ice age we assess the importance of the ocean forcing in explaining the holocene evolution of jakobshavn and find that cooling within the fjord during the mid holocene is critical for the glacier to readvance this finding emphasizes the role of ocean forcing when trying to understand the millenial scale evolution of marine terminating glaciers

tracer transport in the greenland ice sheet three dimensional isotopic stratigraphy abstract polar ice cores can provide both a record of climate history and a sharp test of the performance of numerical ice dynamics models the stratigraphic structure of an ice sheet is an expression of its full depositional and dynamic history and thus presents a greater challenge to computer models than merely matching the contemporary ice thickness and areal extent we describe a coupled model of ice and tracer dynamics that is realized by adding a semi lagrangian tracer transport scheme to a conventional thermomechanical ice dynamics model model skill is demonstrated by using ice core data from the grip site near summit greenland to successfully predict the isotopic stratigraphy of ice cores at other deep drilling sites the success of this effort indicates that when compensated for the effects of ice flow and elevation all the deep cores relate a coherent glacial history over the past 120 000 years according to the simulation results the oldest greenland ice lies beneath the grip gisp2 and northgrip sites although comparably old ice may also be found in north greenland and east central greenland

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

a low angle normal fault in the umbria region central italy a mechanical model for the related microseismicity abstract in the northern apennines the upper crust is thinned by a set of east dipping low angle normal faults lanfs the easternmost and more recent of these lanfs is the altotiberina fault atf located in northern umbria the geometry of the atf has been reconstructed matching surface geology with seismic reflection profiles and borehole data the fault whose average dip is 20 borders the upper pliocene quaternary tiber basin and has a displacement of about 8 km the deeper portion of the atf is located below the axial zone of the northern apennines where the strongest instrumental and historical seismicity is recorded the microseismicity of the region m 1 to evaluate the boundary conditions for the brittle activity of the atf the fault can be reactivated for low values of differential stress 1 3 t 10 mpa and tensile fluid overpressure p f 3 e g v 0 93 in the peculiar situation of the northern apennines the deep emissions of large amounts of co 2 documented in the area can be entrapped in their ascent by structural seals e g atf favouring localised fluid overpressures the impossibility of sustaining p f 3 for wide fault portions counteracted by hydraulic fracturing increased permeability under low effective stress and load weakening behaviour for normal faulting would prevent the nucleation of moderate ruptures along the fault the short lived attainment of p f 3 along small fault portions can account for the microseismic activity located along the atf which occurs on rupture surfaces in the range of 10 1 10 3 km 2

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 flux evolution in fast flowing areas of the greenland ice sheet over the 20th and 21st centuries this study investigates the evolution of greenland ice sheet flux focusing on five of the main fast flowing regions petermann glacier north east greenland ice stream kangerdlugssuaq glacier helheim glacier and jakobshavn glacier in response to 20th and 21st century climate change a hybrid shallow ice and shallow shelf ice sheet model ism is forced with the combined outputs of a set of seven cmip5 models and the regional climate model mar the ism simulates the present day ice velocity pattern topography and surface mass balance smb in good agreement with observations except for the kangerdlugssuaq glacier over the 21st century all the fast flowing areas have exhibited a decrease in ice flux as a result of a negative smb rather than dynamical changes only the fronts of kangerdlugssuaq and helheim glaciers have shown an interannual variability driven by dynamical rather than climate changes finally the results predict a substantial inland ice margin retreat by the end of the 21st century especially along the northern coasts

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

the δ18o record along the greenland ice core project deep ice core and the problem of possible eemian climatic instability over 70 000 samples from the 3029 m long greenland ice core project grip ice core drilled on the top of the greenland ice sheet summit have been analyzed for 8o a highly detailed and continuous 8o profile has thus been obtained and is discussed in terms of past temperatures in greenland we also discuss a three core stacked annual 8o profile for the past 917 years the short term 50 years variability of the annual 8o signal is found to be 1 in the holocene and estimates for the coldest parts of the last glacial are 3 or higher these data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the eemian spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the 8o signal which for the holocene ice is found to be considerably stronger than expected we suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix deconvolution techniques were employed for restoring with great confidence the highly attenuated annual 8o signal in the holocene we confirm earlier findings of dramatic temperature changes in greenland during the last glacial cycle abrupt and strong climatic shifts are also found within the eem sangamon interglaciation which is normally recorded as a period of warm and stable climate in lower latitudes the stratigraphic continuity of the eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile

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

stress state and fluid pressure level along the whipple detachment fault california brittle structures in the upper 200 m of the footwall of the whipple detachment fault are used to characterize the state of stress under which the detachment slipped they are younger than or coeval with chloride brecciation and those that record shear typically are either moderately northeast dipping normal structures or steep northwest striking southwest down structures that we interpret as conjugate shears from this we infer that the maximum principal stress was oriented 55 80 from the detachment while they were active the angle between the conjugate faults averages about 30 indicating failure in the transtensional regime during conjugate faulting the effective least principal stress was nearly equal to the tensile strength the whipple fault probably had a normal coefficient of static friction and it slipped under hydrostatic to only moderately elevated fluid pressure rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength this causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress

water isotope diffusion rates from the northgrip ice core for the last 16 000 years glaciological and paleoclimatic implications abstract a high resolution 0 05 m water isotopic record o 18 is available from the northgrip ice core in this study we look into the water isotope diffusion history as estimated by the spectral characteristics of the o 18 time series covering the last 16 000 years the diffusion of water vapor in the porous medium of the firn pack attenuates the initial isotopic signal predominantly having an impact on the high frequency components of the power spectrum higher temperatures induce higher rates of smoothing and thus the signal can be used as a firn paleothermometer we use a water isotope diffusion model coupled to a steady state densification model in order to infer the temperature signal from the site assuming the accumulation and strain rate history as estimated using the gicc05 layer counted chronology and a dansgaard johnsen ice flow model the temperature reconstruction accurately captures the timing and magnitude of the bolling allerod and younger dryas transitions a holocene climatic optimum is seen between 7 and 9 ky b2k with an apparent cooling trend thereafter our temperature estimate for the holocene climatic optimum points to a necessary adjustment of the ice thinning function indicating that the ice flow model overestimates past accumulation rates by about 10 at 8 ky b2k this result is supported by recent gas isotopic fractionation studies proposing a similar reduction for glacial conditions finally the record presents a climatic variability over the holocene spanning millennial and centennial scales with a profound cooling occurring at approximately 4000 years b2k the new reconstruction technique is able to provide past temperature estimates by overcoming the issues apparent in the use of the classical o 18 slope method it can at the same time resolve temperature signals at low and high frequencies

consistently dated records from the greenland grip gisp2 and ngrip ice cores for the past 104 ka reveal regional millennial scale δ18o gradients with possible heinrich event imprint abstract we present a synchronization of the ngrip grip and gisp2 ice cores onto a master chronology extending back to 104 ka before present providing a consistent chronological framework for these three greenland records the synchronization aligns distinct peaks in volcanic proxy records and other impurity records chemo stratigraphic matching and assumes that these layers of elevated impurity content represent the same instantaneous event in the past at all three sites more than 900 marker horizons between the three cores have been identified and our matching is independently confirmed by 24 new and previously identified volcanic ash tephra tie points using the reference horizons we transfer the widely used greenland ice core chronology gicc05modelext to the two summit cores grip and gisp2 furthermore we provide gas chronologies for the summit cores that are consistent with the gicc05modelext timescale by utilizing both existing and new gas data ch4 concentration and 15n of n2 we infer that the accumulation contrast between the stadial and interstadial phases of the glacial period was 10 greater at summit compared to at ngrip the 18o temperature proxy records from ngrip grip and gisp2 are generally very similar and display synchronous behaviour at climate transitions the 18o differences between summit and ngrip however changed slowly over the last glacial interglacial cycle and also underwent abrupt millennial to centennial scale variations we suggest that this observed latitudinal 18o gradient in greenland during the glacial period is the result of 1 relatively higher degree of precipitation with a pacific signature at ngrip 2 increased summer bias in precipitation at summit and 3 enhanced rayleigh distillation due to an increased source to site distance and a potentially larger source to site temperature gradient we propose that these processes are governed by changes in the north american ice sheet nais volume and north atlantic sea ice extent and or sea surface temperatures sst on orbital timescales and that changing sea ice extent and ssts are the driving mechanisms on shorter timescales finally we observe that maxima in the summit ngrip 18o difference are roughly coincident with prominent heinrich events this suggests that the climatic reorganization that takes place during stadials with heinrich events possibly driven by a southward expansion of sea ice and low ssts in the north atlantic are recorded in the ice core records

sea level changes at the lgm from ice dynamic reconstructions of the greenland and antarctic ice sheets during the glacial cycles abstract new experiments were performed with three dimensional thermomechanical models of the greenland and antarctic ice sheets to simulate their behaviour during the glacial cycles to reconstruct their thickness and extent at the last glacial maximum lgm and to estimate their glacio eustatic contribution to the global sea level stand the calculations used improved ice dynamic and isostatic treatments updated datasets on higher grid resolutions and refined climatic treatments based on newly calibrated transfer functions between ice core records and climatic perturbations results are discussed from a reference run with standard parameters that is compared with available glacial geological observations and from a series of sensitivity experiments focusing on isostatic adjustment thermomechanical coupling climatic forcing mass balance changes and basal melting rates and viscosity changes of antarctic ice shelves for the antarctic ice sheet we find that volume changes are closely linked with grounding line changes of the west antarctic ice sheet at the lgm the grounding line extended close to the continental shelf break almost everywhere ice over central east antarctica was generally thinner than today and varied mainly in accordance with accumulation fluctuations for the greenland ice sheet melting is important only during interglacial periods and the most sensitive period concerns the size of the ice sheet during the eemian at the lgm the greenland ice sheet extended beyond the present coastline to cover at least the inner continental shelf and thinned by up to several hundred meters in central areas for a plausible range of parameters the experiments indicate that at the time of maximum sea level depression 21 kyr bp the antarctic ice sheet contributed 14 18 m to the sea level lowering and the greenland ice sheet 2 3 m significantly less than the older climap reconstructions whereas both ice sheets were at 21 kyr bp close to their maximum extent the experiments also indicate that their maximum volumes were reached only by 16 5 kyr bp greenland and 10 kyr bp antarctica equal to an additional sea level lowering of respectively 0 4 and 3 7 m holocene retreat was essentially complete by 5 kyr bp in greenland but is found to still continue today in west antarctica before reversing to growth during the next millenium the models were found to reproduce gross features of the ice sheet s history since the lgm in reasonably good agreement with available glacial geological data although observational control on ice thickness changes remains very limited

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

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

a sem cathodoluminescence study on weathering of quartz grains in faults central japan summary cathodoluminescence cl of quartz grains or particles was observed using a scanning electron microscope the quartz grains were collected from fault gouge and fault breccia within the atotsugawa fault central japan and the quartz particles were obtained by crushing parent rocks of the fault such as granite and gneiss in an iron bowl in comparison with particles from the parent rocks those from the fault showed the following differences cl inherited from the parent rock cl caused by faulting and cl modified by weathering or alterations of the fault these differences are attributed to the faulting and subsequent weathering the cl observations were in general agreement with secondary electron observations and it is recommended that both methods should be used in parallel

a model of greenland ice sheet deglaciation constrained by observations of relative sea level and ice extent an ice sheet model was constrained to reconstruct the evolution of the greenland ice sheet gris from the last glacial maximum lgm to present to improve our understanding of its response to climate change the study involved applying a glaciological model in series with a glacial isostatic adjustment and relative sea level rsl model the model reconstruction builds upon the work of simpson et al 2009 through four main extensions 1 a larger constraint database consisting of rsl and ice extent data model improvements to the 2 climate and 3 sea level forcing components 4 accounting for uncertainties in non greenland ice the research was conducted primarily to address data model misfits and to quantify inherent model uncertainties with the earth structure and non greenland ice our new model termed huy3 fits the majority of observations and is characterised by a number of defining features during the lgm the ice sheet had an excess of 4 7 m ice equivalent sea level iesl which reached a maximum volume of 5 1 m iesl at 16 5 cal ka bp modelled retreat of ice from the continental shelf progressed at different rates and timings in different sectors southwest and southeast greenland began to retreat from the continental shelf by 16 to 14 cal ka bp thus responding in part to the bolling allerod warm event c 14 5 cal ka bp subsequently ice at the southern tip of greenland readvanced during the younger dryas cold event in northern greenland the ice retreated rapidly from the continental shelf upon the climatic recovery out of the younger dryas to present day conditions upon entering the holocene 11 7 cal ka bp the ice sheet soon became land based during the holocene thermal maximum htm 9 5 cal ka bp air temperatures across greenland were marginally higher than those at present and the gris margin retreated inland of its present day southwest position by 40 60 km at 4 cal ka bp which produced a deficit volume of 0 16 m iesl relative to present in response to the htm warmth our optimal model reconstruction lost mass at a maximum centennial rate of c 103 4 gt yr our results suggest that remaining data model discrepancies are affiliated with missing physics and sub grid processes of the glaciological model uncertainties in the climate forcing lateral earth structure and non greenland ice particularly the north american component finally applying the huy3 greenland reconstruction with our optimal earth model we generate present day uplift rates across greenland due to past changes in the ocean and ice loads with explicit error bars due to uncertainties in the earth structure present day uplift rates due to past changes are spatially variable and range from 3 5 to 7 mm a including earth model uncertainty

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

the effect of a holocene climatic optimum on the evolution of the greenland ice sheet during the last 10 kyr the holocene climatic optimum was a period 8 5 kyr ago when annual mean surface temperatures in greenland were 2 3 c warmer than present day values however this warming left little imprint on commonly used temperature proxies often used to derive the climate forcing for simulations of the past evolution of the greenland ice sheet in this study we investigate the evolution of the greenland ice sheet through the holocene when forced by different proxy derived temperature histories from ice core records focusing on the effect of sustained higher surface temperatures during the early holocene we find that the ice sheet retreats to a minimum volume of 0 15 1 2 m sea level equivalent smaller than present in the early or mid holocene when forcing an ice sheet model with temperature reconstructions that contain a climatic optimum and that the ice sheet has continued to recover from this minimum up to present day reconstructions without a warm climatic optimum in the early holocene result in smaller ice losses continuing throughout the last 10 kyr for all the simulated ice sheet histories the ice sheet is approaching a steady state at the end of the 20th century

insights into spatial sensitivities of ice mass response to environmental change from the searise ice sheet modeling project i antarctica atmospheric oceanic and subglacial forcing scenarios from the sea level response to ice sheet evolution searise project are applied to six three dimensional thermomechanical ice sheet models to assess antarctic ice sheet sensitivity over a 500 year timescale and to inform future modeling and field studies results indicate i growth with warming except within low latitude basins where inland thickening is outpaced by marginal thinning ii mass loss with enhanced sliding with basins dominated by high driving stresses affected more than basins with low surface slope streaming ice and iii mass loss with enhanced ice shelf melting with changes in west antarctica dominating the signal due to its marine setting and extensive ice shelves cf minimal impact in the terre adelie george v oates and victoria land region of east antarctica ice loss due to dynamic changes associated with enhanced sliding and or sub shelf melting exceeds the gain due to increased precipitation furthermore differences in results between and within basins as well as the controlling impact of sub shelf melting on ice dynamics highlight the need for improved understanding of basal conditions grounding zone processes ocean ice interactions and the numerical representation of all three key points sensitivity study of antarctica to atmospheric oceanic and subglacial forcings different sectors of antarctica are vulnerable to the forcings atmospheric forcing lead to a growth but dynamic forcing lead to a mass loss 2013 american geophysical union all rights reserved

determination of the flow properties at dye 3 south greenland by bore hole tilting measurements and perturbation modelling les donnees issues d inclinometrie a dye 3 groenland rendent possible le calcul du parametre de la loi de fluage en fonction de la profondeur ce parametre montre une importante variation lors de la transition holocene pleistocene en triplant sa valeur pour la glace formee au cours de la derniere glaciation les parametres d ecoulement et la topographie du lit sont utilises dans un modele de perturbation pour calculer les ondulations de surface et les taux de deformation

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

substantial contribution to sea level rise during the last interglacial from the greenland ice sheet during the last interglacial period the eemian global sea level was at least three metres and probably more than five metres higher than at present1 2 complete melting of either the west antarctic ice sheet or the greenland ice sheet would today raise sea levels by 6 7 metres but the high sea levels during the last interglacial period have been proposed to result mainly from disintegration of the west antarctic ice sheet3 with model studies attributing only 1 2 m of sea level rise to meltwater from greenland4 5 this result was considered consistent with ice core evidence4 although earlier work had suggested a much reduced greenland ice sheet during the last interglacial period6 here we reconsider the eemian evolution of the greenland ice sheet by combining numerical modelling with insights obtained from recent central greenland ice core analyses our results suggest that the greenland ice sheet was considerably smaller and steeper during the eemian and plausibly contributed 4 5 5 m to the sea level highstand during that period we conclude that the high sea level during the last interglacial period most probably included a large contribution from greenland meltwater and therefore should not be interpreted as evidence for a significant reduction of the west antarctic ice sheet