holocene coastal glaciation of alaska holocene uctuations of the three cirque glaciers on the seward peninsula and ve groups of tidewater and land terminating glaciers along the northernmost gulf of alaska provide a proxy record of late holocene climatic change furthermore the movements of the coastal glaciers were relevant to late holocene native american migration the earliest expansion was recorded about 6850 yr bp by hubbard glacier at the head of yakutat bay in the gulf of alaska however its down fjord advance to the bay mouth was delayed until 2700 bp similarly expansions of the icy bay bering and mccarty glaciers occurred near their present termini by 3600 3000 bp compatible with marked cooling and precipitation increases suggested by the alaskan pollen record decrease in glacier activity 2000 bp was succeeded by advances of gulf coastal glaciers between 1500 and 1300 bp correlative with early medieval expansions across the northern hemisphere a medieval optimum encompassing at least a few centuries prior to ad 1200 is recognized by general retreat of land terminating glaciers but not of all tidewater glaciers little ice age advances of land based glaciers many dated with the precision of tree ring cross dating were centered on the middle 13th or early 15th centuries the middle 17th and the last half of the 19th century a d strong synchrony of these events across coastal alaska is evident 2000 elsevier science ltd all rights reserved

variations in alaska tidewater glacier frontal ablation 1985 2013 our incomplete knowledge of the proportion of mass loss due to frontal ablation the sum of ice loss through calving and submarine melt from tidewater glaciers outside of the greenland and antarctic ice sheets has been cited as a major hindrance to accurate predictions of global sea level rise we present a 28 year record 1985 2013 of frontal ablation for 27 alaska tidewater glaciers representing 96 of the total tidewater glacier area in the region calculated from satellite derived ice velocities and modeled estimates of glacier ice thickness we account for cross sectional ice thickness variation long term thickness changes mass lost between an upstream fluxgate and the terminus and mass change due to changes in terminus position the total mean rate of frontal ablation for these 27 glaciers over the period 1985 2013 is 15 11 3 63gta 1 two glaciers hubbard and columbia account for approximately 50 of these losses the regional total ablation has decreased at a rate of 0 14gta 1 over this time period likely due to the slowing and thinning of many of the glaciers in the study area frontal ablation constitutes only 4 of the total annual regional ablation but roughly 20 of net mass loss comparing several commonly used approximations in the calculation of frontal ablation we find that neglecting cross sectional thickness variations severely underestimates frontal ablation

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

geometry and p and s velocity structure of the african anomaly 1 we constrain the geometry and p and s velocity structure of a low velocity anomaly in the lower mantle beneath southern africa we term it the african anomaly on the basis of forward traveltime and waveform modeling of seismic data sampling a great arc across the anomaly from the east pacific rise to the japan sea our collected data set consists of direct s direct p sdiff scs pcp sks and skks phases recorded by three temporary broadband passcal seismic arrays deployed in africa between 1994 and 2002 the tanzania seismic array 1994 1995 the kaapvaal seismic array 1997 1999 and the ethiopia kenya seismic array 2000 2002 for earthquakes occurring in the east pacific rise drake passage south sandwich islands iran hindu kush xinjiang and the japan sea the seismic data provide excellent sampling of the african anomaly in the lower mantle along the specific great arc in order to accurately account for the contributions from the african anomaly we relocate all the events using a global seismic shear velocity tomographic model and seismic data recorded by the global seismographic network and correct for the contributions from the seismic heterogeneities outside the african anomaly the seismic observations suggest that the african anomaly locally extends 1300 km above the core mantle boundary beneath southern africa around 25 n 27 e and exhibits a bell like geometry with both the southwestern and the northeastern flanks dipping toward its center with the lateral dimension of the anomaly increasing with depth the base is about 4000 km wide extending broadly in both the southwestward and the northeastward directions the seismic data can best be explained by a shear velocity structure with average velocity decreases of 5 in the base and 2 to 3 in the mid lower mantle above the base and a compressional velocity structure with a uniform s to p velocity perturbation ratio of 3 1 for the entire african anomaly these geometric and velocity features suggest that the mid lower mantle portion of the african anomaly is an integral component of the very low velocity province and the african anomaly is compositionally distinct and geologically stable

a simple mechanism for irreversible tidewater glacier retreat 1 tidewater glacier retreat is abrupt and typically irreversible the mechanics of abrupt onset are evaluated by considering the behavior of a sliding law that depends inversely upon effective pressure pice pwater for such a law flux can increase with decreasing thickness if the loss of bed traction exceeds the loss of driving stress characteristic values of increased flux and glacier thinning are shown to propagate upstream if ice flux increases with decreasing thickness below an ice thickness threshold determined by the chosen sliding law upstream propagation leads to instability described in earlier literature in which increased flow leads to thinning loss of bed traction and continued acceleration in simplest terms the instability arises when alterations in glacier geometry act to reduce resistive stresses more than driving stresses the effects of diffusion are also analyzed and it is shown that for the form of sliding law considered here diffusion will not necessarily eliminate upstream propagating instabilities application of this theory shows that a transition from stable to unstable conditions occurred at columbia glacier alaska near the time of onset of its retreat in the early 1980s the theory also successfully predicts stable or rapidly retreating states for 12 other alaska tidewater glaciers and its applicability to greenland outlet glaciers is discussed

phase transitions in pyrolite and morb at lowermost mantle conditions implications for a morb rich pile above the core mantle boundary abstract subduction of mid oceanic ridge basalt morb gives rise to strong chemical heterogeneities in the earth s mantle possibly extending down to the core mantle boundary phase relations in both pyrolite and morb compositions are precisely determined at high pressures and temperatures corresponding to lowermost mantle conditions the results demonstrate that the post perovskite phase transition occurs in pyrolite between 116 and 121 gpa at 2500 k while post perovskite and sio2 phase transitions occur in morb at 4 gpa lower pressure at the same temperature theory predicts that these phase changes in pyrolite and morb cause shear wave velocity increase and decrease respectively near the northern margin of the large low shear velocity province in the lowermost mantle beneath the pacific reflections from a negative shear velocity jump near 2520 km depth are followed by reflections from a positive velocity jump 135 to 155 km deeper these negative and positive velocity changes are consistent with the expected phase transitions in a dense pile containing a mixture of morb and pyrolitic material this may be a direct demonstration of the presence of accumulations of subducted morb crust in the deep mantle

closing the mass budget of a tidewater glacier the example of kronebreen svalbard in this study we combine remote sensing in situ and model derived datasets from 1966 to 2014 to calculate the mass balance components of kronebreen a fast flowing tidewater glacier in svalbard

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

structural features and shear velocity structure of the pacific anomaly 1 we constrain structural features and shear velocity structure of a low velocity anomaly in the lower mantle beneath the pacific we term it the pacific anomaly on the basis of forward travel time and waveform modeling of the observed direct s sdiff scs sks and skks phases sampling a great arc across the anomaly from eastern eurasia to southern south america after correction for the effects of earthquake mislocation and the seismic heterogeneities outside the pacific anomaly seismic observations suggest that the pacific anomaly along the great arc consists of at least two separated portions with a 740 km wide gap between them the western portion of the anomaly is about 1050 km wide extends at least 740 km above the core mantle boundary cmb and exhibits a trapezoidal shape with lateral dimensions increasing slightly with depth the velocity structure of the western portion varies from 3 0 at the top 740 km above the cmb to 3 5 at 100 km above the cmb and an average shear velocity reduction of 5 in the bottom 100 km of the mantle the eastern portion of the anomaly reaches at least 340 km above the cmb beneath the mid pacific with an 1800 km wide base and has a uniform velocity reduction of 3 waveform modeling further suggests a very low velocity layer with a shear velocity reduction of 10 located at the edge of the western portion of the anomaly combining the latest results from others we present a general picture of structural and velocity structures of the pacific anomaly the structural and velocity features suggest that the anomaly represents a cluster of metastable thermo chemical piles

glacier crevasses observations models and mass balance implications we review the findings of approximately 60 years of in situ and remote sensing studies of glacier crevasses as well as the three broad classes of numerical models now employed to simulate crevasse fracture the relatively new insight that mixed mode fracture in local stress equilibrium rather than downstream advection alone can introduce nontrivial curvature to crevasse geometry may merit the reinterpretation of some key historical observation studies in the past three decades there have been tremendous advances in the spatial resolution of satellite imagery as well as fully automated algorithms capable of tracking crevasse displacements between repeat images despite considerable advances in developing fully transient three dimensional ice flow models over the past two decades both the zero stress and linear elastic fracture mechanics crevasse models have remained fundamentally unchanged over this time in the past decade however multidimensional and transient formulations of the continuum damage mechanics approach to simulating ice fracture have emerged the combination of employing damage mechanics to represent slow upstream deterioration of ice strength and fracture mechanics to represent rapid failure at downstream termini holds promise for implementation in large scale ice sheet models finally given the broad interest in the sea level rise implications of recent and future cryospheric change we provide a synthesis of 10 mechanisms by which crevasses can influence glacier mass balance

analysis of a grace global mascon solution for gulf of alaska glaciers we present a high resolution gravity recovery and climate experiment grace mascon solution for gulf of alaska goa glaciers and compare this with in situ glaciological climate and other remote sensing observations our grace solution yields a goa glacier mass balance of 65 11 gt a 1 for the period december 2003 to december 2010 with summer balances driving the interannual variability between october november 2003 and october 2009 we obtain a mass balance of 61 11 gt a 1 from grace which compares well with 65 12 gt a 1 from icesat based on hypsometric extrapolation of glacier elevation changes we find that mean summer june august air temperatures derived from both ground and lower troposphere temperature records were good predictors of grace derived summer mass balances capturing 59 and 72 of the summer balance variability respectively large mass losses during 2009 were likely due to low early melt season surface albedos measured by the moderate resolution imaging spectroradiometer modis and likely associated with the 31 march 2009 eruption of mount redoubt southwestern alaska grace data compared well with in situ measurements at wolverine glacier maritime alaska but poorly with those at gulkana glacier interior alaska we conclude that although goa mass estimates from grace are robust over the entire domain further constraints on subregional and seasonal estimates are necessary to improve fidelity to ground observations

anisotropic shear wave velocity structure of the earth s mantle a global model 1 we combine a new large data set of surface wave phase anomalies long period waveforms and body wave travel times to construct a three dimensional model of the anisotropic shear wave velocity in the earth s mantle our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model simultaneous inversion for velocity and anisotropy as well as discontinuity topographies and implementation of nonlinear crustal corrections for waveforms a comparison of our new three dimensional model s362ani with two other models derived from comparable data sets but using different techniques reveals persistent features 1 strong 200 km thick high velocity anomalies beneath cratons likely representing the continental lithosphere underlain by weaker fast anomalies extending below 250 km which may represent continental roots 2 weak velocity heterogeneity between 250 and 400 km depths 3 fast anomalies extending horizontally up to 2000 3000 km in the mantle transition zone beneath subduction zones 4 lack of strong long wavelength heterogeneity below 650 km suggesting inhibiting character of the upper mantle lower mantle boundary and 5 slow velocity superplumes beneath the pacific and africa the shear wave radial anisotropy is strongest at 120 km depth in particular beneath the central pacific lateral anisotropic variations appreciably improve the fit to data that are predominantly sensitive to the uppermost and lowermost mantle but not to the waveforms that control the transition zone and midmantle depths tradeoffs between lateral variations in velocity and anisotropy are negligible in the uppermost mantle but noticeable at the bottom of the mantle

using upper air conditions to estimate south cascade glacier washington u s a summer balance asimplemodelusesonce dailymeteorologicalvaluesintheu s national centers for environmental predictionand u s national center foratmospheric research ncep ncar re analysis databaseto estimate summerbalanceof south cascade glacier washington u s a each year over1959 99 the rms error 0 30mw e r 2 0 71 is com parable to measurement error the model relates summer balance linearly to temperature t 0 c at 2000m and to snow flux at1650m the altitudes in recent years of the equilib rium line and terminus the snow flux is the product of the humidity and westerly wind component at 850hpa when temperature t 2 c at 1650m temperatures are inter polated linearly betweenthe 850 and 700hpa levels boththe positive 2000mtemperature andthe snow flux are summedfrom 26 aprilto 4 october whenthe summerestimatesare combinedwiththosefromawinterbalancemodelusingthesamedatabase thermserrorin estimating netbalance is 0 40m r 2 0 81 the indicated sensitivities of balance to warm ing of 1 c are 0 51m for summerand 0 24m for winter onthe assumptionthatthetotal 0 75m c 1 sensitivityexists atall altitudes awarmingof only 0 7 c wouldbe sufficientto overcome the1986 98 averagenetbalance 0 5matthe top of theglacier

basal melting beneath a fast flowing temperate tidewater glacier the role of melting at the base of temperate tidewater glaciers is rarely discussed and its potential importance for total glacier mass balance and subglacial dynamics is often overlooked we use columbia glacier alaska usa as an example of a temperate tidewater glacier to estimate the spatial distribution of basal melt due to friction both before and during the glacier s well documented retreat since the early 1980s published data on glacier surface and bed profiles ice flow velocities and surface melt were collated and used as input data for a two dimensional basal melt model we estimate that before the retreat of columbia glacier pre 1980s mean basal melt amounted to 61 mm a 1 increasing to 129 mm a 1 during retreat post 1980s according to our calculations basal melt accounts for 3 and 5 of total glacier melt for the pre retreat and syn retreat i e during retreat glacier profiles respectively these calculations of basal melt are an order of magnitude greater than those typically reported in polar glacier settings basal melting in temperate tidewater settings may be a non negligible process affecting glacier mass balance and subglacial dynamics

south cascade glacier mass balance 1935 2006 a simple model uses daily observations of precipitation and temperature at a nearby weather station to estimate glacier average seasonal mass balance components at south cascade glacier washington usa from 1935 24 years before measurements began at the glacier this is 13 years earlier than measurements that can be derived using the ncep ncar reanalysis database begins 1 january 1948 although the model s error in estimating winter balance and summer balance over 1959 2006 is greater than that of a model using the reanalysis database its error in estimating net balance is comparable the model uses an empirically determined precipita tion ratio between the station and the glacier and a seasonally varying temperature lapse rate de termined from 9 years of measurements at the glacier temperature is used with a degree day for mulation to estimate ablation and to partition precipitation between rain and snow for estimating accumulation both processes are assumed to exist throughout the year and model results are compared seasonally with adjusted observations of winter and summer balances the published mass balance series is adjusted to a constant topography 1970 series in an attempt to remove the influence of changing topography on the glacier s response to climate the reconstructed values prior to 1959 are also with respect to the 1970 glacier topography because precipitation is measured at the weather station rather than being inferred from other meteorological variables it enables us to distinguish more accurately between wet day and dry day conditions including vertical lapse rates of temperature

influence of upper air conditions on glaciers in scandinavia a simple model using once daily us national centers for environmental prediction us national center for atmospheric research ncep ncar re analysis upper air values estimates winter balance summer balance and net balance at ten glaciers in norway and two in sweden with 0 37 r 2 0 90 the october may arctic oscillation ao an index of sea level pressure slp in the northern hemisphere correlates with winter balance and net balance with 0 09 r 2 0 82 lower than the model in all but 3 of the 24 cases the october may north atlantic oscillation nao an slp gradient in the north atlantic has lower correlations than the ao for all but 5 of the 24 cases at all ten glaciers with records beginning before 1987 net balance became more positive after 1988 owing mainly to increased winter balance with summer balance changing little although winter temperatures increased they were still well below freezing so the rain snow division of the precipitation was only slightly affected increase in winter balance was due to increased precipitation caused by a change in atmospheric circulation resulting in more frequent westerly flow concurrent with the warming at 850 hpa westerly flow is 2 5 c warmer than easterly flow westerly flow warmed by 0 3 c easterly by 0 7 c both the ao and nao with which winter balance is positively correlated at all 12 glaciers were more positive after 1988

rapid erosion of soft sediments by tidewater glacier advance taku glacier alaska usa 2 0 0 1 m a 1 1940 2005 at l 3 km h 350 m and 1 5 0 2 m a 1 1952 2005 at l 1 5 km h 250 m detailed mapping over a 4 km 2 area of the terminus revealed a deeply incised channel in line with a major outlet stream glaciofluvial processes must play the dominant role in the subglacial erosion and removal of these unlithified sediments citation motyka r j m truffer e m kuriger and a k bucki 2006 rapid erosion of soft sediments by tidewater glacier advance taku glacier alaska usa geophys res lett 33 l24504 doi 10 1029 2006gl028467

using surface velocities to calculate ice thickness and bed topography a case study at columbia glacier alaska usa information about glacier volume and ice thickness distribution is essential for many glaciological applications but direct measurements of ice thickness can be difficult and costly we present a new method that calculates ice thickness via an estimate of ice flux we solve the familiar continuity equation between adjacent flowlines which decreases the computational time required compared to a solution on the whole grid we test the method on columbia glacier a large tidewater glacier in alaska usa and compare calculated and measured ice thicknesses with favorable results this shows the potential of this method for estimating ice thickness distribution of glaciers for which only surface data are available we find that both the mean thickness and volume of columbia glacier were approximately halved over the period 1957 2007 from 281m to 143m and from 294km 3 to 134km 3 respectively using bedrock slope and considering how waves of thickness change propagate through the glacier we conduct a brief analysis of the instability of columbia glacier which leads us to conclude that the rapid portion of the retreat may be nearing an end

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

static mass balance sensitivity of arctic glaciers and ice caps using a degree day approach the purpose of this study is to enhance our understanding of the response of glaciers to climate change global sea level is affected by changes in glacier ice volume and melt water from glaciers is a principal water source in many regions this study applies glacier mass balance modelling with varying complexity and spatial resolution ranging from individual glaciers to regional and global assessments of glacier mass losses glaciers located in maritime environments generally show considerably higher mass balance sensitivities than those in continental settings on average an assumed increase in annual precipitation of 10 tends to offset the effect of an annual temperature change of 1 k by roughly 20 two case studies at storglaciaren sweden and hofsjokull iceland involve model results of future mass balance change and glacier melt induced changes in runoff applying a temperature and precipitation scenario for iceland in 2050 results in increased total runoff from hofsjokull by roughly one third and results emphasize the role of the firn layer in delaying water flow through glaciers yielding a redistribution of discharge within the year based on a global gridded data set of glacierized area the sea level equivalent from all mountain glaciers and ice caps outside the ice sheets in greenland and antarctica during 1961 2004 caused by changes in temperature and precipitation is estimated to be 0 58 0 34 mm a 1 the mountain glaciers and ice caps around antarctica alone contribute almost 40 of the global estimate and hence their contribution is considerably larger than previously assumed

upper air model of summer balance on mount rainier usa in 2003 07 summer balance was measured at altitudes between 1700 and 3382 m a s l on two glaciers on mount rainier washington state usa 46 85 n 121 72 w 4400 m a s l south facing nisqually glacier and east northeast facing emmons glacier upper air temperatures at the nearest gridpoint in the ncep ncar reanalysis database are used in a distributed over altitude positive degree day pdd model for each glacier the model used the same coefficients at all altitudes for all years the rms model error was 0 65 r 2 0 87 and 0 78 m a 1 w e r 2 0 93 for nisqually and emmons glaciers respectively although pdd work generally uses different coefficients for snow and ice surfaces and the duration of exposure of those surfaces varies with altitude error in this single coefficient model is nearly uncorrelated with altitude values of coefficients obtained are within the range of those found in other pdd work the degree day coefficient however differs markedly between the two glaciers and is shown to be controlled by the difference between them in vertical gradient of measured summer balance it is smaller for nisqually glacier where solar radiation is a stronger contributor to melt and larger for emmons glacier where it is a weaker contributor over 1948 2007 when the model calibrated over 2003 07 was applied to the upper air temperatures estimated summer balance was 0 4 m a 1 less negative over 1962 83 than before and 0 6 m a 1 less negative than after corresponding roughly with changes of the northeast pacific sea surface temperatures

fast glacier flow ice streams surging and tidewater glaciers fast flowing glaciers are unusual but important jakobshavns glacier in western greenland illustrates this point figure 1 considered to be the world s fastest glacier its maximum observed flow rate measured in midsummer near the floating terminus is 8360 m yr 1 lingle et al 1981 except for a tidal influence near the terminus the flow is believed to be fairly steady bindschadler 1984

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

contribution of alaskan glaciers to sea level rise derived from satellite imagery over the last 50 years retreating glaciers and ice caps gic contributed 0 5 mm yr to sea level rises slr and one third is believed to originate from ice masses bordering the gulf of alaska however these estimates of ice wastage in alaska are based on methods that measure a limited number of glaciers and extrapolate the results to estimate ice loss for the many thousands of others how these methods capture the complex pattern of decadal elevation changes at the scale of individual glacier and mountain range is unclear here combining a comprehensive glacier inventory with elevation changes derived from sequential digital elevation models dems we found that between 1962 and 2006 alaskan glaciers lost 41 9 8 6 km 3 yr water equivalent w e and contributed 0 12 0 02 mm yr to slr our ice loss is 34 lower than previous estimates reasons for our lower values include the higher spatial resolution of our glacier inventory and the reduction of ice thinning under debris and at the glacier margins which were not resolved in earlier work estimates of mass loss from gic in other mountain regions could be subject to similar revisions

extent of the low velocity region in the lowermost mantle beneath the western pacific detected by the vietnamese broadband seismograph array 1 we present evidence showing the extent of the low velocity region in the lowermost mantle beneath the western pacific we analyzed s ss scs s and sscs ss travel times observed by the vietnamese broadband seismograph array deployed as part of the ocean hemisphere project the abrupt changes in scs s and sscs ss travel times suggest that the western geographical boundary of the low velocity region is located around 140 e and is sharp more than 4 velocity contrast within 200 km the dependency of s and ss travel time anomalies of epicentral distances suggests that the strong low velocity region is confined to within 400 km from the cmb core mantle boundary the existence of lateral heterogeneities with a 100 km scale inside the low velocity region is also suggested by the abrupt changes in s and scs waveforms

quantifying the effects of climate and surface change on glacier mass balance when a mass balance is computed using an outdated map that computation does not reveal the actual mass change but older maps often must be used for practical reasons we present a method by which with a few additional measurements each year a mass balance computed with an outdated map can be transformed into an actual mass change this is done by taking into account the influence of changes in areal extent and changes in the surface elevation of the glacier since the map was made this method is applied to south cascade glacier washington u s a as an example the computed cumulative mass balance from 1970 to 1997 would have been 16 too negative if the 1970 map had not been updated while the actual volume change of a glacier is relevant to hydrological studies the change that would have occurred on a constant or static surface is more relevant to certain glacier dynamics problems and most climate problems we term this the reference surface balance and propose that such a balance which deliberately omits the influence of changes in area and surface elevation is better correlated to climatic variations than the conventional one which incorporates those influences

a three dimensional calving model numerical experiments on johnsons glacier livingston island antarctica calving from tidewater glaciers and ice shelves accounts for around half the mass loss from both polar ice sheets yet the process is not well represented in prognostic models of ice dynamics benn and others proposed a calving criterion appropriate for both grounded and floating glacier tongues or ice shelves based on the penetration depth of transverse crevasses near the calving front computed using nye s formula the criterion is readily incorporated into glacier and ice sheet models but has not been fully validated with observations we apply a three dimensional extension of benn and others criterion incorporated into a full stokes model of glacier dynamics to estimate the current position of the calving front of johnsons glacier antarctica we find that two improvements to the original model are necessary to accurately reproduce the observed calving front 1 computation of the tensile deviatoric stress opening the crevasse using the full stress solution and 2 consideration of such a tensile stress as a function of depth our modelling results also suggest that johnsons glacier has a polythermal structure rather than the temperate structure suggested by earlier studies

rapid changes in ice discharge from greenland outlet glaciers using satellite derived surface elevation and velocity data we found major short term variations in recent ice discharge and mass loss at two of greenland s largest outlet glaciers their combined rate of mass loss doubled in less than a year in 2004 and then decreased in 2006 to near the previous rates likely as a result of fast re equilibration of calving front geometry after retreat total mass loss is a fraction of concurrent gravity derived estimates pointing to an alternative source of loss and the need for high resolution observations of outlet dynamics and glacier geometry for sea level rise predictions

alaska tidewater glacier terminus positions 1948 2012 a significant portion of the world s glacier ice drains through tidewater outlets though much remains unknown about the response to recent climate change of tidewater glaciers we present a 64 year record of length change for 50 alaska tidewater glaciers we use u s geological survey topographic maps to provide a base length for glaciers before 1970 using all available cloud free landsat images we manually digitize calving front outlines for each glacier between 1972 and 2012 resulting in a total of more than 10 000 outlines tidewater glacier lengths vary seasonally focusing on the 36 glaciers terminating in tidewater throughout the study period we find a mean standard deviation seasonal variation of 60 85 m a 1 we use these oscillations to determine the significance of interannual changes in glacier length all 36 glaciers underwent at least one period 1 year of significant advance or retreat 28 glaciers underwent at least one period of both significant advance and retreat over the entire period 1948 2012 24 of these glaciers retreated a total uncertainty of 107 95 0 29 km 11 advanced a total of 7 71 0 20 and one chenega glacier did not change significantly retreats and advances are highly variable in time several glaciers underwent rapid short term retreats of a few years duration these retreats occurred after large changes in summer sea surface temperature anomalies further study is needed to determine what triggered these retreats no coherent regional behavior signal is apparent in the length record although two subregions show a coherence similar to recent observations in greenland

coexisting shear and compressional wave seismic velocity discontinuities beneath the central pacific stacking of broadband shear waves s and short period compressional waves p provides compelling evidence for the existence of a localized discontinuous increase in both s and p wave velocity 230 km above the core mantle boundary cmb beneath the central pacific modelling indicates a 1 7 increase in s velocity v s and a 0 75 increase in p velocity vp similar features have been detected intermittently on a global basis but seldom with s and p structure being resolved in the same location previous evidence for anisotropy near the cmb beneath the central pacific suggests that the discontinuity may be related to anisotropic fabrics induced by flow in the thermal boundary layer above the core

surface mass balance thinning and iceberg production columbia glacier alaska 1948 2007 a mass balance model using upper air meteorological data for input was calibrated with surface mass balance measured mainly during 1977 78 at 67 sites on columbia glacier alaska between 135 and 2645 m a s l root mean square error model vs measured is 1 0 m w e a 1 w ithr 2 0 88 a remarkable result of the analysis was that both precipitation and the factor in the positive degree day model used to estimate surface ablation were constant with altitude the model was applied to reconstruct glacier wide components of surface mass balance over 1948 2007 surface ablation 4k m 3 ice eq a 1 ice equivalent has changed little throughout the period from 1948 until about 1981 when drastic retreat began the surface mass balance was positive but changes in glacier geometry were small so the positive balance was offset by calving 0 9 km 3 ice eq a 1 during retreat volume loss of the glacier accounted for 92 of the iceberg production calving increased to 4 3 km 3 ice eq a 1 from 1982 to 1995 and after that until 2007 to 8 0 km 3 ice eq a 1 which was about twice the loss by surface ablation whereas prior to retreat it was only about a quarter as much calving is calculated as the difference between glacier wide surface mass balance and geodetically determined volume change

iceberg calving during transition from grounded to floating ice columbia glacier alaska 1 the terminus of columbia glacier alaska unexpectedly became ungrounded in 2007 during its prolonged retreat visual observations showed that calving changed from a steady release of low volume bergs to episodic flow perpendicular rifting propagation and release of very large icebergs a style reminiscent of calving from ice shelves here we compare passive seismic and photographic observations through this transition to examine changes in calving mechanical changes accompany the visible changes in calving style post flotation generation of seismic energy during calving is substantially reduced we propose this is partly due to changes in source processes

estimating south cascade glacier washington u s a mass balance from a distant radiosonde and comparison with blue glacier a simple flux model using twice daily measurements of wind humid ity and temperature from standard upper air levels in a distant radiosonde estimated winter balance of south cascade glacier over 1959 98 with error 0 24 m w e correla tion between net and winter balance is strong the model estimates net balance with error 0 53 m w e over the past 40 years average net balance of south cascade glacier has been strongly negative 0 46 m w e and it has been shrinking steadily in com parison 200 km wsw at blue glacier the average balance has been less negative 0 13 m w e that glacier has undergone little change over the 40 years balance histories of the two glaciers are positively correlated r 0 54 and south cascade has been more out of balance than blue presumably because it is still adjusting to climate change since the little ice age recent warming and drying has made the net balance of both glaciers strongly negative since 1976 0 84 m w e at south cascade 0 56 m w e at blue if south cascade glacier were in balance with the 1986 1998 climate it would be about 1 4 of its present area if conditions persist retreat of both glaciers is inevitable

a strong lateral shear velocity gradient and anisotropy heterogeneity in the lowermost mantle beneath the southern pacific 1 velocity heterogeneity and seismic anisotropy in the d region beneath the southern pacific ocean is investigated using broadband shear waves from southwest pacific and south american earthquakes recorded by permanent and temporary stations in antarctica australia south america and on islands in the south pacific shear velocity perturbations vs are inferred from the differential travel times between hand picked horizontally polarized sh direct or diffracted s arrivals and vertically polarized sv sks arrivals derived patterns in vs roughly agree with global shear velocity models but reveal a stronger shear velocity lateral gradient north of approximately 52 s where vs transitions from approximately 0 5 to 1 0 relative to radially averaged reference models over less than 600 km along the base of d waveform analyses provide an even stronger constraint on the transitional region where there is a change in waveform behavior occurring over length scales less than 300 km differential travel times between skks and sks calculated by cross correlating sks with hilbert transformed skks support large vs amplitudes and help to constrain the location of the transitional region anisotropy in d ks is inferred from handpicked differential travel times between the sv and sh components of direct or diffracted s and display a slight spatial trend where the sh component precedes the sv in the north and east of the study area but arrives after sv in the center and southwest of it however there is high ks variability in the center of the study region there is no apparent correlation between vs and ks abrupt changes in the character of velocity and anisotropy in this region of d may be related to chemical heterogeneity at the boundary of the pacific superswell as well as small scale convection in the deep mantle

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

evolving force balance at columbia glacier alaska during its rapid retreat 1 changes in driving and resistive stresses play an essential role in governing the buoyancy forces that are important controls on the speed and irreversibility of tidewater glacier retreats we describe changes in geometry velocity and strain rate and present a top down force balance analysis performed over the lower reach of columbia glacier our analysis uses new measurements and estimates of basal topography and photogrammetric surface velocity measurements made between 1977 and 2001 while assuming depth independent strain sensitivity tests show that the method is robust and insensitive to small changes in the calculation parameters spatial distributions of ice speed show little correspondence with driving stress instead spatial patterns of ice speed exhibit a nonlinear correspondence with basal drag primary resistance to flow comes from basal drag but lateral drag becomes increasingly more important throughout the retreat which may account for observed increases in speed maximum basal drag is always located in a prominent constriction located 12 km upstream from the preretreat terminus once the terminus retreated into deep water off the terminal moraine marking the modern maximum extent the upstream location of this maximum basal drag helped to promote thinning and decrease effective pressure in the lower region by limiting replenishing ice flow from upstream an increase in both ice velocity and calving resulted initiating what appears to be an irreversible retreat

rapid wastage of alaska glaciers and their contribution to rising sea level we have used airborne laser altimetry to estimate volume changes of 67 glaciers in alaska from the mid 1950s to the mid 1990s the average rate of thickness change of these glaciers was 0 52 m year extrapolation to all glaciers in alaska yields an estimated total annual volume change of 52 15 km 3 year water equivalent equivalent to a rise in sea level sle of 0 14 0 04 mm year repeat measurements of 28 glaciers from the mid 1990s to 2000 2001 suggest an increased average rate of thinning 1 8 m year this leads to an extrapolated annual volume loss from alaska glaciers equal to 96 35 km 3 year or 0 27 0 10 mm year sle during the past decade these recent losses are nearly double the estimated annual loss from the entire greenland ice sheet during the same time period and are much higher than previously published loss estimates for alaska glaciers they form the largest glaciological contribution to rising sea level yet measured

evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath africa we present data which indicate that the broad low shear velocity anomaly beneath southern africa is stronger and more extensive than previously thought recordings of earthquakes in the southwestern atlantic ocean at an array of broadband seismic stations in eastern africa show anomalously large propagation time delays of the shear phases s scs and sks which vary rapidly with epicentral distance by forward modeling we estimate that the low velocity anomaly extends from the core mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3 lower than in standard models such as prem strong velocity contrasts exist at its margins 2 over about 300 km these seismic characteristic are consistent with recent numerical simulations of lower mantle mega plume formation

fast tidewater glaciers some iceberg calving outlet glaciers flow continuously at speeds normally associated with surging glaciers arid exhibit dramatic instability scenarios related to those suggested for marine ice sheets no temperate tidewater glaciers are known to have floating termini but many polar and subpolar tidewater glaciers do the fast flow of temperate calving glaciers is almost entirely due to basal sliding and appears to be a function of the effective pressure on the bed which may approach zero and the longitudinal back stress on the terminus the terminus boundary condition the calving relation is imperfectly known yet is vital to the dynamics of these glaciers calving relations for grounded tidewater glaciers have been suggested on empirical grounds but have not been rigorously tested the calving relations for floating termini are virtually unknown this together with the imperfect understanding of basal sliding inhibits confidence in our understanding of the stability of these glaciers columbia glacier alaska is an instructive example because observations have been made on the major changes in its geometry calving rate and dynamics that have occurred in less than 10 years the calving flux has increased more rapidly than the glacier flux causing thinning and retreat as a result the ice velocity has increased markedly the short term velocity changes relate to changes in back pressure ice recession tidal changes and the flux of water injected to the bed these results have relevance to the mechanisms of basal sliding glacier surges and the stability of marine ice streams

modeling the response of nioghalvfjerdsfjorden and zachariae isstrom glaciers greenland to ocean forcing over the next century recent studies have shown that the northeast greenland ice sheet region negis has been undergoing significant acceleration and dynamic thinning since 2010 and these changes are closely related to regional atmospheric and oceanic warming here we model the response of nioghalvfjerdsfjorden 79north and zachariae isstrom zi to ocean forcing to investigate their evolution over the coming decades our model suggests that 79north will retreat slowly over the next century whereas zi will lose its floating ice tongue completely and retreat rapidly for 70 years after 70 years zi will stabilize 30 km upstream of its current position on a topographic ridge frontal melt rates need to reach 6 m day in the summer to dislodge the glacier from this ridge zi will then continue a fast and unstoppable retreat contributing more than 16 2 mm to global sea level rise by 2100

a new probe of ulvz s wave velocity structure array stacking of scs waveforms 1 s wave velocity reductions in thin layers above the core mantle boundary cmb are resolved by modeling precursors to the tangential components of core reflected scs phases unlike most phases used to study ultra low velocity zone ulvz structure scs provides direct sensitivity to shear velocity structure vs recordings of deep earthquakes in the tonga fiji and south american subduction zones at broadband seismic networks in western north america are used to map small scale ulvz variability a two layer low vs structure exists under the central pacific an abrupt 0 8 2 0 decrease 60 86 km above the cmb is underlain by a 24 30 km thick ulvz layer having reductions of 3 3 7 4 with respect to prem no corresponding velocity reductions are found under the cocos plate the structure above the ulvz may be associated with reverse transformation of post perovskite to perovskite in a steep thermal gradient in a boundary layer above the cmb

an sh hybrid method and shear velocity structures in the lowermost mantle beneath the central pacific and south atlantic oceans 1 an sh hybrid method is developed for calculating synthetic seismograms involving two dimensional localized heterogeneous structures the hybrid method is a combination of analytic and numerical methods with the numerical method finite difference applied in the heterogeneous region only and analytical methods applied outside the region generalized ray theory solutions from a seismic source are used to initiate the finite difference calculation and seismic responses at the earth s surface are obtained from the finite difference output by applying the kirchhoff theory we apply the hybrid method and study sh wave propagation near the bottom of the mantle the shear velocity structures and the interaction of sh waves with these velocity structures at the base of the mantle beneath the central pacific and south atlantic oceans are of particular importance the observed sh waves sampling these two regions of the core mantle boundary show very different characteristics across the epicentral distance range of 83 108 the sh waves sampling the core mantle boundary beneath the central pacific show a linearly increasing delay of 4 s from 98 to 108 and discernible scs phases up to an epicentral distance of 102 the sh waves propagating through the base of the mantle beneath the south atlantic ocean on the other hand exhibit a linearly increasing delay of 10 s from 98 to 108 discernible multiple scs phases with a same slowness as the direct sh waves up to 108 and rapid variations of waveforms across small epicentral distances synthetic tests indicate that while the observations sampling the central pacific can be explained by a negative shear velocity gradient of 3 relative to the preliminary reference earth model at the bottom 300 km of the mantle those sampling the south atlantic ocean require a 300 km thick bottom boundary layer with a larger negative velocity gradient up to 10 and steeply dipping edges the negative velocity gradient at the base of mantle beneath the south atlantic ocean can be best explained by partial melt driven by a compositional change produced in the early earth s history and a vertical thermal gradient within the layer while that beneath the central pacific may reasonably be attributed to pure thermal effects within a thermal boundary layer

glaciers dominate eustatic sea level rise in the 21st century ice loss to the sea currently accounts for virtually all of the sea level rise that is not attributable to ocean warming and about 60 of the ice loss is from glaciers and ice caps rather than from the two ice sheets the contribution of these smaller glaciers has accelerated over the past decade in part due to marked thinning and retreat of marine terminating glaciers associated with a dynamic instability that is generally not considered in mass balance and climate modeling this acceleration of glacier melt may cause 0 1 to 0 25 meter of additional sea level rise by 2100

sensitivity of mass balance of five swiss glaciers to temperature changes assessed by tuning a degree day model a degree day model is used to assess the sensitivity of the mass balance of five swiss glaciers to temperature changes the model uses temperature data extrapolated from nearby climate stations and is tuned by varying precipitation to make the model fit the observed distribution of mass balance with altitude once the model is tuned the effect of temperature change is simulated by recalculating the mass balance with the same parameters as before but with a temperature increase of 1 c throughout the year the largest mass balance changes involving increased ablation of 1 m w e a 1 c 1 occur at the snout with a progressively smaller increase with altitude the area averaged sensitivities for the five glaciers are 0 7 to 0 9 m w e a 1 c 1 if annual precipitation also increased by 20 it would partly offset the effect of the 1 c higher temperatures but could not compensate for it