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

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

ocean forcing of the greenland ice sheet calving fronts and patterns of retreat identified by automatic satellite monitoring of eastern outlet glaciers 1 we have developed an automatic method to identify changes in the position of calving glacier margins using daily modis imagery application of the method to 32 ocean terminating glaciers in east greenland produced 26 802 margin positions for a 10 year long period 2000 2009 we report these high resolution data and show that the glaciers exhibit seasonal cycles with magnitudes of advance and retreat proportional to glacier width despite similar seasonality there is a distinct difference between the interannual trends of calving front positions north and south of 69 n all glaciers above this latitude showed very limited or no change when seasonality was excluded while glaciers south of 69 n retreated significantly between 2001 and 2005 2 3 km on average approximately 26 of the retreat of southern glaciers was regained by readvance from 2005 to 2009 to explain the latitudinal boundary of glacier dynamics we review basic climatic factors including summer and winter atmospheric forcing sea ice conditions and ocean temperature we conclude that the southern retreats were strongly influenced by warm oceanic conditions associated with increased transport of subtropical waters to the irminger sea and to fjords and coastal regions south of 69 n northern glaciers remained stable despite significant increase in runoff in this region because fjords at latitudes higher than 69 n are less exposed to subtropical waters the southern retreats illustrate sensitive behavior of calving glaciers and we hypothesize that the calving fronts retreated because they were exposed to rapid ice front melting

sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier 1 large calving events at greenland s largest outlet glaciers are associated with glacial earthquakes and near instantaneous increases in glacier flow speed at some glaciers and ice streams flow is also modulated in a regular way by ocean tidal forcing at the terminus at helheim glacier analysis of geodetic data shows decimeter level periodic position variations in response to tidal forcing however we also observe transient increases of more than 100 in the glacier s responsiveness to such tidal forcing following glacial earthquake calving events the timing and amplitude of the changes correlate strongly with the step like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes the enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice bedrock interface and suggests a new means by which geodetic data may be used to infer glacier properties citation de juan j et al 2010 sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier geophys res lett 37 l12501

multi decadal retreat of greenland s marine terminating glaciers many marine terminating glaciers draining the greenland ice sheet have retreated over the past decade yet the extent and magnitude of retreat relative to past variability is unknown we measure changes in front positions of 210 marine terminating glaciers using landsat imagery spanning nearly four decades and compare decadal scale rates of change with earlier observations we find that 90 of the observed glaciers retreated between 2000 and 2010 approaching 100 in the northwest with rapid retreat observed in all sectors of the ice sheet the current retreat is accelerating and likely began between 1992 and 2000 coincident with the onset of warming following glacier stability and minor advance during a mid century cooling period while it is clear an extensive retreat occurred in the early 20th century a period of increasing air temperatures a comparison of our results with historical observations provides evidence that the current retreat is more widespread the onset of rapid retreat with warming relative to the slow and lagged advance with cooling suggests an asymmetry in the response of marine fronts to external forcing

global detection and location of seismic sources by using surface waves we develop an algorithm for the detection and location of seismic sources using intermediate period 35 150 sec surface waves recorded on a global array of stations continuous vertical seismic waveforms from the global network are collected and a 4 4 global grid of target locations is defined for each target location and each station a surface wave propagation operator is deconvolved from the seismogram to restore any source pulse present the envelope of the seismogram is calculated and cross correlated with a theoretical source pulse shape the resulting waveforms are stacked to improve signal to noise characteristics and the quality strength and timing of the potential detection are determined when a successful event detection is made a finer grid is applied to locate the event with greater precision we apply the algorithm systematically for the period 1993 2003 and catalog the events approximately 2000 events are detected and located for each year and 98 of shallow events in the harvard centroid moment tensor cmt catalog are detected and located by the new algorithm a comparison of 9482 events common to the two catalogs allows the detection strength to be calibrated against the cmt moment magnitude all detected events have estimated moment magnitudes m w 4 5 in each year approximately 100 events not listed in other global seismicity catalogs are detected and located many of these events lie along the ridge transform plate boundaries in the southern hemisphere and may be regular earthquakes that have gone undetected because of poor station coverage other events located in areas where global and regional networks provide good coverage are potentially anomalous and may have escaped detection as a result of their unusual source properties

catastrophic ice shelf break up by an ice shelf fragment capsize mechanism two disintegration events leading to the loss of larsen a and b ice shelves antarctic peninsula in 1995 and 2002 respectively proceeded with extreme rapidity order of several days and reduced an extensive seemingly integrated ice shelf to a jumble of small fragments these events strongly correlate with warming regional climate and accumulation of surface meltwater supporting a hypothesis that meltwater induced propagation of pre existing surface crevasses may have initiated ice shelf fragmentation we address here an additional subsequent mechanism that may sustain and accelerate the ice shelf break up once it begins the proposed mechanism involves the coherent capsize of narrow less than thickness ice shelf fragments by rolling 90 in a direction toward or away from the ice front fragment capsize liberates gravitational potential energy forces open ice shelf rifts and contributes to further fragmentation of the surrounding ice shelf

ice front variation and tidewater behavior on helheim and kangerdlugssuaq glaciers greenland we used satellite images to examine the calving behavior of helheim and kangerdlugssuaq glaciers greenland from 2001 to 2006 a period in which they retreated and sped up these data show that many large iceberg calving episodes coincided with teleseismically detected glacial earthquakes suggesting that calving related processes are the source of the seismicity for each of several events for which we have observations the ice front calved back to a large pre existing rift these rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high which agrees well with earlier theoretical predictions in addition to the recent retreat in a period of higher temperatures analysis of several images shows that helheim retreated in the 20th century during a warmer period and then re advanced during a subsequent cooler period this apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water the cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in greenland can advance rapidly greenland s larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance

channelized bottom melting and stability of floating ice shelves 1 the floating ice shelf in front of petermann glacier in northwest greenland experiences massive bottom melting that removes 80 of its ice before calving into the arctic ocean detailed surveys of the ice shelf reveal the presence of 1 2 km wide 200 400 m deep sub ice shelf channels aligned with the flow direction and spaced by 5 km we attribute their formation to the bottom melting of ice from warm ocean waters underneath drilling at the center of one of channel only 8 m above sea level confirms the presence of ice shelf melt water in the channel these deep incisions in ice shelf thickness imply a vulnerability to mechanical break up and climate warming of ice shelves that has not been considered previously

spatial and temporal variations in greenland glacial earthquake activity 1993 2010 1 glacial earthquakes are anomalous earthquakes associated with large ice loss events occurring at marine terminating glaciers primarily in greenland they are detectable teleseismically and a proper understanding of the source mechanism may provide a remote sensing tool to complement glaciological observations of these large outlet glaciers we model teleseismic surface wave waveforms to obtain locations and centroid single force source parameters for 121 glacial earthquakes occurring in greenland during the period 2006 2010 we combine these results with those obtained by previous workers to analyze spatial and temporal trends in glacial earthquake occurrence over the 18 year period from 1993 2010 we also examine earthquake occurrence at six individual glaciers comparing the earthquake record to independently obtained observations of glacier change our findings confirm the inference that glacial earthquake seismogenesis occurs through the capsize of large newly calved icebergs we find a close correspondence between episodes of glacier retreat thinning and acceleration and the timing of glacial earthquakes and document the northward progression of glacial earthquakes on greenland s west coast over the 18 year observing period our results also show that glacial earthquakes occur when the termini of the source glaciers are very close to the glacier grounding line i e when the glaciers are grounded or nearly grounded

centroid single force inversion of seismic waves generated by landslides the centroid moment tensor cmt waveform inversion method of dziewonski et al 1981 is modified to analyze long period seismic waves generated by single force events such as the gigantic landslides associated with the 1980 eruption of mount st helens we refer to the method as centroid single force csf inversion as the result of the inversion we obtain the spatio temporal centroid and three components of a vector we call the csf vector which is the seismic point source representation of a landslide and whose direction is the opposite of the direction of sliding the sealer magnitude of the csf vector measures the overall size of the landslide and is given by mcsf md where m is the mass of the sliding object and d is the sliding distance we apply csf inversion to the long period surface waves generated by the 1980 mount st helens event the 1975 kalapana earthquake and the 1974 mantaro landslide using seismograms from the gdsn ida and hglp networks the csf solution for the st helens event is remarkably consistent with the actual geological observations and proves the efficacy of csf inversion the csf solution for the kalapana event does not fit the overall waveforms better than the cmt solution does however because the cmt solution does not explain the observed love wave radiation pattern it may be necessary to invoke a combination of both types of mechanism for this event although the geometry of the csf solution the direction of the force for the mantaro event is consistent with the actual landslide mcsf is about 5 times smaller than that expected from the mass and travel distance of the landslide estimated by geological observations this discrepancy may suggest either the relatively aseismic nature of this landslide or that the total volume of the slide was overestimated by analogy to single station cmt inversion single station csf inversion also appears to be stable and useful

rapid and synchronous ice dynamic changes in east greenland two major outlet glaciers in east greenland have suddenly begun to accelerate and retreat the speeds of kangerdlugssuaq and helheim remained steady during the 1990s despite progressive and substantial thinning but have abruptly increased within the last two years more than doubling ice flux to the ocean had it been an isolated example the comparable 1998 speed up of jakobshavn isbra in west greenland might have been explained simply by its chance retreat past a pinning point now that two further greenland outlets have exhibited similar behavior a common process seems likely a remarkable correspondence in the inter annual patterns of speed and ice front variation between kangerdlugssuaq and helheim implies a significant sensitivity to regional environmental factors the period of continued warming and thinning appears to have primed these glaciers for a step change in dynamics not included in current models we should expect further greenland outlet glaciers to follow suit copyright 2006 by the american geophysical union

recent changes on greenland outlet glaciers aircraft laser altimeter surveys during the 1990s showed near coastal parts of the greenland ice sheet to be thinning despite slow thickening at higher elevations the ice sheet lost mass to the ocean many outlet glaciers thinned more rapidly than could be explained by increased melting during the recent warmer summers indicating dynamic imbalance between glacier velocity and upstream snow accumulation results from more recent surveys presented here show that thinning rates have increased in most coastal regions for almost half of the surveys these increases might have resulted from increases in summer melting but rapid thinning on others is indicative of dynamic changes that increased with time in particular thinning rates on the three fastest glaciers increased to tens of m a 1 after 2000 and other observations show an approximate doubling in their velocities the deep beds of these glaciers appear to have a strong influence on rates of grounding line retreat and thickness change with periods of glacier acceleration and rapid thinning initiated by flotation and break up of lightly grounded glacier snouts or break up of floating ice tongues near simultaneous thinning of these widely separated glaciers suggests that warming of deeper ocean waters might be a common cause nearby glaciers without deep beds are thinning far more slowly suggesting that basal lubrication as a result of increased surface melting has only a marginal impact on greenland outlet glacier acceleration

laboratory investigations of iceberg capsize dynamics energy dissipation and tsunamigenesis 1 we present laboratory experiments designed to quantify the stability and energy budget of buoyancy driven iceberg capsize box shaped icebergs were constructed out of low density plastic hydrostatically placed in an acrylic water tank containing freshwater of uniform density and allowed or forced if necessary to capsize the maximum kinetic energy translational plus rotational of the icebergs was 15 of the total energy released during capsize and radiated surface wave energy was 1 of the total energy released the remaining energy was directly transferred into the water via hydrodynamic coupling viscous drag and turbulence the dependence of iceberg capsize instability on iceberg aspect ratio implied by the tank experiments was found to closely agree with analytical predictions based on a simple hydrostatic treatment of iceberg capsize this analytical treatment along with the high reynolds numbers for the experiments and considerably higher values for capsizing icebergs in nature indicates that turbulence is an important mechanism of energy dissipation during iceberg capsize and can contribute a potentially important source of mixing in the stratified ocean proximal to marine ice margins

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

investigation of surface melting and dynamic thinning on jakobshavn isbrae greenland jakobshavn isbrae is the most active glacier in greenland with an annual discharge of about 30 km 3 of ice and it is one of the few recently surveyed glaciers to thicken between 1993 and 1998 despite locally warm summers repeated airborne laser altimeter surveys along a 120 km profile in the glacier basin show slow sporadic thickening between 1991 and 1997 suggesting a small positive mass balance but since 1997 there has been sustained thinning of several m a 1 within 20 km of the ice front with lower rates of thinning further inland here we use weather station data from the coast and the ice sheet to estimate the effects on surface elevation of interannual variability in snowfall and surface melt rates and thus to infer the temporal and spatial patterns of dynamic thinning these show the glacier to have been close to balance before 1997 followed by a sudden transition to rapid thinning initially confined to the lower reaches of the glacier below about 500 m elevation but progressively spreading inland until between 1999 and 2001 thinning predominated over the entire surveyed region up to 2000 m elevation if this continues the glacier calving front and probably its grounding line will retreat substantially in the very near future

seafloor morphology and acoustic facies of the george v land shelf to study the seafloor morphology on the george v land shelf east antarctica over 2000km of high frequency 3 5 27 khz echo sounder data were collected between february and march 2000 the shelf can be divided into four acoustic facies a type ia 2 seabed is ice keel turbate found on mid to outer shelf banks on seafloor less than 500m deep b type ib seabed is siliceous mud and diatom ooze drift drape and fill deposits within the george v basin between 750 and 850m depth c type iib seabed is smooth diamicton below 500m depth and occasionally has lowrelief megaflutes or ridge and swale features d type iiic seabed is high relief ridges and canyons from the coast to the deepest part of the george v basin the acoustic facies are explained in terms of glacial and oceanographic influences on the shelf since the last glacial maximum

rapid thinning of parts of the southern greenland ice sheet aircraft laser altimeter surveys over southern greenland in 1993 and 1998 show three areas of thickening by more than 10 centimeters per year in the southern part of the region and large areas of thinning particularly in the east above 2000 meters elevation the ice sheet is in balance but thinning predominates at lower elevations with rates exceeding 1 meter per year on east coast outlet glaciers these high thinning rates occur at different latitudes and at elevations up to 1500 meters which suggests that they are caused by increased rates of creep thinning rather than by excessive melting taken as a whole the surveyed region is in negative balance

interpretation of earthquake epicenter and cmt centroid locations in terms of rupture length and direction abstract a comparison is made between hypocenter and centroid estimates of shallow earthquakes to investigate whether the difference between these two quantities contains useful information on fault rupture length and direction an initial comparison of international seismological centre isc and harvard centroid moment tensor cmt catalog locations suggests that for earthquakes below mw 6 5 the difference in location is dominated by location errors whilst larger earthquakes appear to show the effect of increasing spatial rupture however for this data set combined location errors greater than 25 km are observed for a majority of earthquakes imposing a limit on interpretation we use these constraints to investigate this approach using a consistent earth model in the re analysis of eight previously well studied events new hypocenter estimates based on teleseismic p wave arrivals from the isc bulletin and a 3 d velocity model of the mantle s p12 wm13 are compared with new centroid locations obtained using the same 3 d model and waveform data from international deployment of accelerometers ida global digital seismic network gdsn and re calibrated high gain long period hglp stations the distance between our new hypocenter and centroid locations is shown to provide reliable information on both the rupture length and direction the rupture lengths and azimuths generally agree with the most recent previous results from other methods and provide an absolute estimate of the location of the rupture

spread of ice mass loss into northwest greenland observed by grace and gps 1 greenland s main outlet glaciers have more than doubled their contribution to global sea level rise over the last decade recent work has shown that greenland s mass loss is still increasing here we show that the ice loss which has been well documented over southern portions of greenland is now spreading up along the northwest coast with this acceleration likely starting in late 2005 we support this with two lines of evidence one is based on measurements fromthegravityrecoveryandclimateexperiment grace satellite gravity mission launched in march 2002 the other comes from continuous global positioning system gps measurements from three long term sites on bedrock adjacent to the ice sheet the grace results provide a direct measure of mass loss averaged over scales of a few hundred km the gps data are used to monitor crustal uplift caused by ice mass loss close to the sites the grace results can be used to predict crustal uplift which can be compared with the gps data in addition to showing that the northwest ice sheet margin is now losing mass the uplift results from both the gps measurements and the grace predictions show rapid acceleration in southeast greenland in late 2003 followed by a moderate deceleration in 2006 because that latter deceleration is weak southeast greenland still appears to be losing ice mass at a much higher rate than it was prior to fall 2003 in a more general sense the analysis described here demonstrates that gps uplift measurements can be used in combination with grace mass estimates to provide a better understanding of ongoing greenland mass loss an analysis approach that will become increasingly useful as long time spans of data accumulate from the 51 permanent gps stations recently deployed around the edge of the ice sheet as part of the greenland gps network gnet citation khan s a j wahr m bevis i velicogna and e kendrick 2010 spread of ice mass loss into northwest greenland observed by grace and gps geophys res lett 37 l06501 doi 10 1029 2010gl042460

possible mechanisms for glacial earthquakes the large glacial earthquakes reported on by ekstrom et al 2003 2006 and tsai and ekstrom 2007 have previously been evaluated in terms of their seismic characteristics in this paper we attempt to take constraints such as known glacial ice properties outlet glacier size calving style and meltwater variability to construct a self consistent physical model of the glacial earthquake process since many glaciological parameters are poorly constrained we parameterize a number of important processes and estimate a wide range of possible values for some properties the range of model outputs is thus fairly large but it is still difficult to match observational constraints under most conditions we find that only a small class of models is able to satisfy the major observational constraints these models are characterized by 1 lost basal resistance coupled to viscoelastic deformation with extensive internal crevassing or with low effective elastic modulus and possibly low effective viscosity or 2 by nonequilibrium calving such as having large icebergs capsize into the glacier front although observational constraints cannot definitively rule out any of the proposed classes of mechanisms the calving model has much stronger support fortunately the various models make different predictions regarding observables that can potentially be measured in the near future

a dislocation model for the earthquake cycle at convergent plate boundaries summary a kinematic model for the earthquake cycle at convergent plate boundaries has been constructed on the basis of dislocation theory we model the lithosphere asthenosphere system by a stratified semi infinite medium under gravity consisting of an elastic surface layer an intervening layer with maxwell viscoelasticity and an elastic substratum the steady motion of plate convergence is naturally represented by uniform slip at a constant rate on the upper boundary of the descending oceanic plate the occurrence of interplate earthquakes is regarded as a perturbation of the steady state plate motion and represented by a periodic sequence of step slips on a finite seismic zone of the plate interface based on dislocation theory we can show that the steady slip on the interface deeper than the lithosphereasthenosphere boundary does not contribute to surface deformation in the steady state accordingly the surface deformation associated with the earthquake cycle is given by the superposition of viscoelastic responses to the steady slip on the interface shallower than the lithosphere asthenosphere boundary the steady backslip on the seismic zone and the periodic sequence of seismic slips we have computed cyclic patterns of vertical displacements at the free surface for two representative cases taking account of gravity effects the patterns of vertical displacements differ notably in the latter stage of the cycle depending on whether or not the seismic zone extends through the entire thickness of the lithosphere after the completion of one earthquake cycle our model yields a certain amount of permanent deformation resulting from the steady plate convergence the pattern of the permanent deformation that is characterized by steep uplift on the continental side sharp subsidence at the plate boundary and gentle uplift on the oceanic side is irrespective of the cyclic process of stress accumulation and release repeated on the seismic zone the earthquake cycle model developed here provides a possible explanation for the formation of earthquake origin marine terraces

step wise changes in glacier flow speed coincide with calving and glacial earthquakes at helheim glacier greenland this is the publisher s version also available electronically from http onlinelibrary wiley com

jakobshavn glacier west greenland 30 years of spaceborne observations early 1960 s reconnaissance satellite images are compared to more recent image and map data in an inter annual and seasonal study of west central greenland margin fluctuations from 1962 to 1992 ice sheet margins to the north and south of jakobshavn glacier retreated despite a decline in average summer temperatures the retreat may be reversing along the southern flank of the ice stream where regional mass balance estimates are positive from 1950 to fluctuated 2 5 km around its annual mean position the total calving flux during the summer is more than six times that during winter we identified that summer melting and the break up of sea ice and icebergs in the fjord are important in controlling the rate of iceberg production if correct calving rates may be expected to increase should climate become warmer in the near future

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

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

seventeen antarctic seismic events detected by global surface waves and a possible link to calving events from satellite images 1 we detect 17 seismic events in antarctica from 1997 to 2009 by applying a surface wave detector to global seismic data we locate these events using a waveform cross correlation method and find that most occurred near the coast of antarctica and are clustered in three regions four events are on the ronne ice shelf close to the location of a 1998 calving event five events are near the vanderford glacier and eight events are near the ninnis glacier the observed rayleigh and love waves for these events have similar amplitudes and a two lobed radiation pattern matching the expected amplitude behavior of a single force source model using such a model we obtain best fitting horizontal force directions for the 14 events that have relatively better signal to noise ratios analysis of coastline changes from modis images before and after the detected events show that two events on vanderford glacier and one event near ninnis glacier are likely associated with calving events moreover the inferred force directions for the seismic events appear consistent with local ice flow directions both satellite observations and modeling results strongly suggest a link between seismic events and calving processes in the two regions however the force directions on the ronne ice shelf are aligned with observed rift propagation directions suggesting that these events may arise from rifting processes

mass balance of the greenland ice sheet at dye 3 the mass balance of the greenland ice sheet a t dye 3 is estimated on the basis of 0 bserva tions of ice thickness accumulation rate surface velocities and surface strain rates the calculations indicate a rate of increase of surface elevation of 3 cm year with 95 confidence limits of 3 cm year and 9 cm year previous estimates of the mass balance of the greenland ice sheet by the same method reported large imbalances these are most probably due to lack of precise data and the use of quantities measured at the surface as representative of depth averaged quantities the most reliable observations indicate that the interior regions of the greenland ice sheet are at present thickening at a rate of a few centimetres per year a contributing cause for this may be the slow thinning of a bottom layer of relatively soft wisconsin ice resume bilan de masse de l lndlandsis groenlandais a dye 3 le bilan de masse est estime it partir des mesures d epaisseur taux d accumulation vitesses et vitesses de deformation superficielles le calcul don ne un epalss lssement de 3 cm an 1 avec un intervalle de confiance au seuil de 95 allant de 3 cm an 1 it 9 cm an i les estimations anterieures du bilan de l lndlandsis obtenues par la meme methode sont examinees on en deduit que les valeurs de bilan tres eloignees de l equilibre sont certainement dues au manque de donnees preclses et ou a i utilisation de donnees superficielles comme valeurs moyennes sur une

greenland ice sheet increased coastal thinning repeated laser altimeter surveys and modelled snowfall summer melt show average ice loss from greenland between 1997 and 2003 was 80 12 km3 yr 1 compared to about 60 km3 yr 1 for 1993 4 1998 9 half of the increase was from higher summer melting with the rest caused by velocities of some glaciers exceeding those needed to balance upstream snow accumulation velocities of one large glacier almost doubled between 1997 and 2003 resulting in net loss from its drainage basin by about 20 km3 of ice between 2002 and 2003 copyright 2004 by the american geophysical union

global seismicity of 2003 centroid moment tensor solutions for 1087 earthquakes abstract centroid moment tensor cmt solutions are presented for 1087 earthquakes that occurred during 2003 the solutions are obtained using the method of dziewonski et al dziewonski a m chou t a woodward j h 1981 determination of earthquake source parameters from waveform data for studies of global and regional seismicity j geophys res 86 2825 2852 and applying corrections for aspherical earth structure as represented by the whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward dziewonski a m woodward r l 1992 acoustic imaging at the planetary scale in emert h harjes h p eds acoustical imaging vol 19 plenum press new york pp 785 797 the model of inelastic attenuation of durek and ekstrom durek j j ekstrom g 1996 a radial model of inelasticity consistent with long period surface wave attenuation bull seism soc am 86 144 158 is used to predict the decay of the waveforms the focal mechanisms of the largest or otherwise significant earthquakes of 2003 are reviewed

grace observes small scale mass loss in greenland using satellite gravity data between february 2003 and january 2008 we examine changes in greenland s mass distribution on a regional scale during this period greenland lost mass at a mean rate of 179 25 gt yr equivalent to a global mean sea level change of 0 5 0 1 mm yr rates increase over time suggesting an acceleration of the mass loss driven by mass loss during summer the largest mass losses occurred along the southeastern and northwestern coast in the summers of 2005 and 2007 when the ice sheet lost 279 gt and 328 gt of ice respectively within 2 months in 2007 a strong mass loss is observed during summer at elevations above 2000 m for the first time since the start of the observations

analysis of glacial earthquakes in 2003 ekstrom et al reported on the detection of a new class of earthquakes that occur in glaciated regions with the vast majority being in greenland the events have a characteristic radiation pattern and lack the high frequency content typical of tectonic earthquakes it was proposed that the events correspond to large and sudden sliding motion of glaciers here we present an analysis of all 184 such events detected in greenland between 1993 and 2005 fitting the teleseismic long period surface waves to a landslide model of the source we obtain improved locations timing force amplitudes and force directions after relocation the events cluster into seven regions all of which correspond to regions of very high ice flow and most of which are named outlet glaciers these regions are daugaard jensen glacier kangerdlugssuaq glacier helheim glacier the southeast greenland glaciers the northwest greenland glaciers rinks isbrae and jakobshavn isbrae event amplitudes range from 0 1 to 2 0 10 14 kg m force directions are consistent with sliding in the direction of glacial flow over a period of about 50 s each region has a different temporal distribution of events all glaciers are more productive in the summer but have their peak activity in different months over the study period kangerdlugssuaq has had a constant number of events each year whereas jakobshavn had most events in 1998 1999 and the number of events in helheim and the northwest greenland glaciers has increased substantially between 1993 and 2005 the size distribution of events in kangerdlugssuaq is peaked above the detection threshold suggesting that glacial earthquakes have a characteristic size

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

jakobshavn isbrae west greenland flow velocities and tidal interaction of the front area from 2004 field observations 1 during the summer of 2004 the front area of the jakobshavn isbrae was monitored using a geodetic photogrammetric survey with temporarily coincident precise observations of local ocean tides in the disko bay close to ilulissat the geodetic and photogrammetric observations were conducted at the southern margin of the glacier front the largest observed horizontal flow velocities are in the central part of the front with values up to 45 m d this is a factor of 2 greater than the average velocities at the front area observed in the last century our new observations confirm previous estimates of an acceleration of glacier flow during the last decade the photogrammetric survey provided flow trajectories for 4000 surface points with a time resolution of 30 min these flow trajectories were used to compare the vertical motion of the glacier with the observed tides the existence of a free floating glacier tongue in 2004 was confirmed by these data however it occupied only a small belt of at most a few 100 m width in the central part of the glacier front horizontal motion did not appear to depend on the tidal phase unlike some of the fast moving ice streams of west antarctica

rapid response of helheim glacier in greenland to climate variability over the past century during the early 2000s the greenland ice sheet experienced the largest ice mass loss of the instrumental record 1 largely as a result of the acceleration thinning and retreat of large outlet glaciers in west and southeast greenland 2 5 the quasi simultaneous change in the glaciers suggests a common climate forcing increasing air 6 and ocean 7 8 temperatures have been indicated as potential triggers here we present a record of calving activity of helheim glacier east greenland that extends back to about ad 1890 based on an analysis of sedimentary deposits from sermilik fjord where helheim glacier terminates specifically we use the annual deposition of sand grains as a proxy for iceberg discharge our record reveals large fluctuations in calving rates but the present high rate was reproduced only in the 1930s a comparison with climate indices indicates that high calving activity coincides with a relatively strong influence of atlantic water and a lower influence of polar water on the shelf off greenland as well as with warm summers and the negative phase of the north atlantic oscillation our analysis provides evidence that helheim glacier responds to short term fluctuations of large scale oceanic and atmospheric conditions on timescales of 3 10 years the forcings behind the rapid increase in mass loss from the greenland ice sheet in the early 2000s ref 1 are still debated it is unclear whether the mass loss will continue in the near future and ifso atwhatrate theseuncertaintiesareaconsequenceofour limited understanding of mechanisms regulating ice sheet variability and the response of fast flowing outlet glaciers to climate variability in southeast greenland helheim glacier one of the regions largestglaciers thinned acceleratedandretreatedduringtheperiod 20032005

impact of hydrodynamics on seismic signals generated by iceberg collisions full glacier thickness icebergs are frequently observed to capsize as they calve into the ocean as they capsize they may collide with the glaciers termini previous studies have hypothesized that such collisions are the source of teleseismic glacial earthquakes we use laboratory scale experiments force balance modeling and theoretical arguments to show that 1 the contact forces during these collisions are strongly influenced by hydrodynamic forces and 2 the associated glacial earthquake magnitudes expressed as twice integrated force histories are related to the energy released by the capsizing icebergs plus a hydrodynamic term that is composed of drag forces and hydrodynamic pressure our experiments and first order modeling efforts suggest that due to hydrodynamic forces both contact force and glacial earthquake magnitudes may not be directly proportional to the energy released by the capsizing icebergs as might be expected most importantly however our results highlight the need to better understand the hydrodynamics of iceberg capsize prior to being able to accurately interpret seismic signals generated by iceberg collisions

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

analysis of low frequency seismic signals generated during a multiple iceberg calving event at jakobshavn isbrae greenland 1 we investigated seismic signals generated during a large scale multiple iceberg calving event that occurred at jakobshavn isbrae greenland on 21 august 2009 the event was recorded by a high rate time lapse camera and five broadband seismic stations located within a few hundred kilometers of the terminus during the event two full glacier thickness icebergs calved from the grounded or nearly grounded terminus and immediately capsized the second iceberg to calve was two to three times smaller than the first the individual calving and capsize events were well correlated with the radiation of low frequency seismic signals 0 1 hz dominated by love and rayleigh waves in agreement with regional records from previously published glacial earthquakes these low frequency seismic signals had maximum power and or signal to noise ratios in the 0 05 0 1 hz band similarly full waveform inversions indicate that these signals were also generated by horizontal single forces acting at the glacier terminus the signals therefore appear to be local manifestations of glacial earthquakes although the magnitudes of the signals twice time integrated force histories were considerably smaller than previously reported glacial earthquakes we thus speculate that such earthquakes may be a common if not pervasive feature of all full glacier thickness calving events from grounded termini finally a key result from our study is that waveform inversions performed on low frequency calving generated seismic signals may have only limited ability to quantitatively estimate mass losses from calving in particular the choice of source time function has little impact on the inversion but dramatically changes the earthquake magnitude accordingly in our analysis it is unclear whether the smaller or larger of the two calving icebergs generated a larger seismic signal

changes in ice front position on greenland s outlet glaciers from 1992 to 2007 1 previous studies in greenland show that retreat of tidewater glaciers may be linked to recent increases in ice loss raising greenland s contribution to sea level rise we examined ice front changes of 203 tidewater glaciers land terminating glaciers and glaciers terminating with ice shelves to understand greenland glacier behavior over three periods 1992 2000 2000 2006 and 2006 2007 we observed synchronous ice sheet wide increases in tidewater retreat during 2000 2006 relative to 1992 2000 coinciding with a 1 1 c increase in mean summer temperature at coastal weather stations rates of retreat for the southeast and east slowed during 2006 2007 when temperatures were slightly cooler than the 2000 2006 average our work suggests that regional greenland tidewater retreat responds strongly to climate change with higher temperatures corresponding to increasing retreat and helps confirm a link between ice thickness velocity and ice front position

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

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

continued evolution of jakobshavn isbrae following its rapid speedup 1 several new data sets reveal that thinning and speedup of jakobshavn isbrae continue following its recent rapid increase in speed as its floating ice tongue disintegrated the present speedup rate of 5 a 1 over much of the fast moving region appears to be a diffusive response to the initial much larger speedup near the front there is strong seasonality in speed over much of the fast flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short typically 6 km floating ice tongue this modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003 analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving major calving only resumes in late winter when much of this ice clears from the fjord the collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea ice concentration in disko bay this decline may have extended the length of the calving season for several consecutive years leading to the ice tongue s collapse

ocean regulation hypothesis for glacier dynamics in southeast greenland and implications for ice sheet mass changes synchronous acceleration and thinning of southeast se greenland glaciers during the early 2000s was the main contributor that resulted in the doubling of annual discharge from the ice sheet we show that this acceleration was followed by a synchronized and widespread slowdown of the same glaciers in many cases associated with a decrease in thinning rates and we propose that ice sheet ocean interactions are the first order regional control on these recent mass changes sea surface temperature and mooring data show that the preceding dynamic thinning coincides with a brief decline in the cold east greenland coastal current egcc and east greenland current we suggest this decline was partly induced by a reduction in ice sheet runoff which allowed warm water from the irminger current to reach the se greenland coast a restrengthening of the cold waters coincides with the glaciers subsequent slowdown we argue that this warming and subsequent cooling of the coastal waters was the cause of the glaciers dynamic changes we further suggest that the restrengthening of the egcc resulted in part from cold water input by increased glacier calving during the speedup and increased ice sheet runoff we hypothesize that the main mechanism for ice sheet mass loss in se greenland is highly sensitive to ocean conditions and is likely subject to negative feedback mechanisms copyright 2010 by the american geophysical union

seasonality and increasing frequency of greenland glacial earthquakes some glaciers and ice streams periodically lurch forward with sufficient force to generate emissions of elastic waves that are recorded on seismometers worldwide such glacial earthquakes on greenland show a strong seasonality as well as a doubling of their rate of occurrence over the past 5 years these temporal patterns suggest a link to the hydrological cycle and are indicative of a dynamic glacial response to changing climate conditions

seasonal variation in velocity before retreat of jakobshavn isbrae greenland 1 using repeat pass satellite feature tracking the dynamic behaviour of jakobshavn isbrae which drains around 7 of the greenland ice sheet is investigated in improved detail through its recent period of thinning acceleration and retreat the departure in 1995 from the normal seasonal invariance in velocity near the grounding line suggests a possible change in dynamics well before the subsequent thinning phase the initiation of the acceleration and retreat is pinpointed to spring 1998 and may have been prompted by the same mechanism that gave rise to the spring 1995 flow increase

substantial thinning of a major east greenland outlet glacier aircraft laser altimeter surveys in 1993 and 1998 over kangerdlugssuaq glacier in east greenland reveal thinning over the 5 year interim of several meters for all surveyed areas within 70 km of the seaward ice front rising to 50 meters in the final 5 km such rapid thinning is best explained by increased discharge velocities and associated creep thinning most probably caused by enhanced lubrication of the glacier bed the calving ice front over the past decade has occupied approximately the same location as in 1966 velocity estimates for 1995 96 are about the same as those for 1966 and 1988 but significantly less than for 1999 suggesting that major thinning began after 1995

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

glacial rumblings from jakobshavn ice stream greenland the steep increase in greenland s glacial earthquake activity detected by the global seismographic network since the late 1990s suggests that a close inspection of these events might provide clues to the nature and origin of such seismic activity here we discuss the detection of large unexpected seismic events of extraordinarily long duration 10 40 min occurring about once every 2 days and localized in the ice stream that feeds the earth s fastest moving glacier jakobshavn isbrae from the east these glacial rumblings represent an ice mass wasting process that is greater and more frequent than glacial earthquakes have suggested probably triggered by calving the rumblings are all very similar regardless of duration and all end with a sharp earthquake like event in which the largest seismic amplitude is in the rumbling and that might signal the collapse of large ice masses upstream by calculating the total amount of seismic energy released as rumblings we estimate that the maximum seasonal amount of ice moved seismogenically down the ice stream is up to 12 km 3 or 30 of the average annual iceberg discharge in jakobshavn

glacial earthquakes in greenland and antarctica glacial earthquakes are a new class of seismic events first discovered as signals in long period seismograms recorded on the global seismographic network most of these events occur along the coasts of greenland where they are spatially related to large outlet glaciers glacial earthquakes show a strong seasonality with most events occurring during the late summer the rateofglacial earthquakeoccurrenceincreasedbetween2000and2005 with a stabilization of earthquake frequency at 2003 2004 levels in 2006 2008 recent observations establish a strong temporal correlation between the distinct seismic signals of glacial earthquakes and large ice loss events in which icebergs of cubic kilometer scale collapse against the calving face linking the seismogenic process to the force exerted by these icebergs on the glacier andtheunderlyingsolidearth asuddenchangeinglacierspeedresultsfrom these glacial earthquake calving events seasonal and interannual variations in glacier terminus position account for general characteristics of the temporal variation in earthquake occurrence glacial earthquakes in antarctica are less well studied but they exhibit several characteristics similar to glacial earthquakes in greenland