small fluctuations in the density and thickness of a dry firn column it is of practical importance to have a description on time scales of 1 100 years of the relationship between the mass imbalance of an ice sheet and its rate of change of thickness in this paper a linearized treatment off the relationship is described closed form expressions are derived that relate the time variant density in an isothermal firn layer to the fluctuations in accumulation rate and density that occur at the surface these expressions are used to provide a spectral description of the contribution of surface accumulation and surface density fluctuations to the rate of change of thickness of an ice sheet using these the contribution of firn densification to the variability of ice sheet thickness is examined as a function of the time interval over which the ice sheet is observed this contribution is illustrated for sites in antarctica and greenland it is concluded that it is important to give greater attention than hitherto to the spatial scale of accumulation fluctuations if satellite observations of ice sheet elevation change are to be used to estimate ice sheet imbalance over short time intervals

fast recession of a west antarctic glacier satellite radar interferometry observations of pine island glacier west antarctica reveal that the glacier hinge line position retreated 1 2 0 3 kilometers per year between 1992 and 1996 which in turn implies that the ice thinned by 3 5 0 9 meters per year the fast recession of pine island glacier predicted to be a possible trigger for the disintegration of the west antarctic ice sheet is attributed to enhanced basal melting of the glacier floating tongue by warm ocean waters

spatial and temporal evolution of pine island glacier thinning 1995 2006 we use ers 2 and envisat satellite radar altimetry to examine spatial and temporal changes in the rate of thinning of the pine island glacier west antarctica during the period 1995 to 2006 we show that the pattern of thinning has both accelerated and spread inland to encompass tributaries flowing into the central trunk of the glacier within the 5 400 km 2 central trunk the average rate of volume loss quadrupled from 2 6 0 3 km 3 yr 1 in 1995 to 10 1 0 3 km 3 yr 1 in 2006 the region of lightly grounded ice at the glacier terminus is extending upstream and the changes inland are consistent with the effects of a prolonged disturbance to the ice flow such as the effects of ocean driven melting if the acceleration continues at its present rate the main trunk of pig will be afloat within some 100 years six times sooner than anticipated citation wingham d j d w wallis and a shepherd 2009 spatial and temporal evolution of pine island glacier thinning 1995 2006 geophys res lett 36 l17501 doi 10 1029 2009gl039126

warm ocean is eroding west antarctic ice sheet 1 satellite radar measurements show that ice shelves in pine island bay have thinned by up to 5 5 m yr 1 over the past decade the pattern of shelf thinning mirrors that of their grounded tributaries the pine island thwaites and smith glaciers and ocean currents on average 0 5 c warmer than freezing appear to be the source the synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini and the drawdown of grounded ice shows that antarctica is more sensitive to changing climates than was previously considered

modelling circumpolar deep water intrusions on the amundsen sea continental shelf antarctica results are presented from an isopycnic coordinate model of ocean circulation in the amundsen sea focusing on the delivery of circumpolar deep water cdw to the inner continental shelf around pine island bay the warmest waters to reach this region are channeled through a submarine trough accessed via bathymetric irregularities along the shelf break temporal variability in the influx of cdw is related to regional wind forcing easterly winds over the shelf edge change to westerlies when the amundsen sea low migrates west and south in winter spring this drives seasonal on shelf flow while inter annual changes in the wind forcing lead to inflow variability on a decadal timescale a modelled period of warming following low cdw influx in the late 1980 s and early 1990 s coincides with a period of observed thinning and acceleration of pine island glacier

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

shallow conduit system at kilauea volcano hawaii revealed by seismic signals associated with degassing bursts 1 eruptive activity at the summit of kilauea volcano hawaii beginning in march 2008 and continuing to the present time is characterized by episodic explosive bursts of gas and ash from a vent within halemaumau pit crater these bursts are accompanied by seismic signals that are well recorded by a broadband network deployed in the summit caldera we investigate in detail the dimensions and oscillation modes of the source of a representative burst in the 1 10 s band an extended source is realized by a set of point sources distributed on a grid surrounding the source centroid where the centroid position and source geometry are fixed from previous modeling of very long period vlp data in the 10 50 s band the source time histories of all point sources are obtained simultaneously through waveform inversion carried out in the frequency domain short scale noisy fluctuations of the source time histories between adjacent sources are suppressed with a smoothing constraint whose strength is determined through a minimization of the akaike bayesian information criterion abic waveform inversions carried out for homogeneous and heterogeneous velocity structures both image a dominant source component in the form of an east trending dike with dimensions of 2 9 2 9 km the dike extends 2 km west and 0 9 km east of the vlp centroid and spans the depth range 0 2 3 1 km the source model for a homogeneous velocity structure suggests the dike is hinged at the source centroid where it bends from a strike e 27 n with northern dip of 85 west of the centroid to a strike e 7 n with northern dip of 80 east of the centroid the oscillating behavior of the dike is dominated by simple harmonic modes with frequencies 0 2 hz and 0 5 hz representing the fundamental mode 11 and first degenerate mode 12 21 of the dike although not strongly supported by data in the 1 10 s band a north striking dike segment is required for enhanced compatibility with the model elaborated in the 10 50 s band this dike provides connectivity between the east trending dike and the new vent within halemaumau pit crater waveform inversions with a dual dike model suggest dimensions of 0 7 0 7 km to 2 6 2 6 km for this segment further elaboration of the complex dike system under halemaumau does not appear to be feasible with presently available data

a historical approach to continental flood basalt volcanism insights into pre volcanic rifting sedimentation and early alkaline magmatism abstract continental flood basalts are widely thought to be produced from mantle plume heads however plume theories do not observe any role for lithospheric rifting before flood basalt events and consider irrelevant the fact that most if not all continental flood basalts have erupted through deep rifts containing thick sedimentary sequences at best plume theories invoke selective capture of such deep rifts or lithospheric thinspots by rising mantle plume heads however the fact that cfbs of the world erupted through deep ancient rift zones and alternative dynamical considerations of flood basalt genesis directly lead towards a new historical approach to flood basalt emplacement this approach takes cognizance of the basic unity of geological history and processes satisfactorily explains pre volcanic rifting sedimentation mantle metasomatism and early pre tholeiite enriched alkaline magmatism for tens of millions of years both incubating and impacting plume heads ought to lead to pre volcanic lithospheric doming which is usually not observed in flood basalts continental or oceanic flood basalt events instead seem to be derived by convective partial melting during sudden lithospheric pull apart splitting along pre existing lithospheric discontinuities such as deep rifts or fracture zones

investigations of an ice plain in the mouth of pine island glacier antarctica we present newly acquired airborne radar data showing ice thickness and surface elevation for pine island glacier antarctica these data when combined with earlier measurements suggest the presence of a lightly grounded area immediately above the grounding line of pine island glacier we identify this region as a ice plain it lies close to the centre line of the glacier has an elevation above buoyancy of 28 km the upstream edge of the ice plain is defined by a coupling line the configuration of the ice plain implies that nearby thinning of the ice stream would result in substantial grounding line retreat we suggest that the grounding line retreat of pine island glacier observed between 1992 and 1996 probably commenced sometime after 1981

new boundary conditions for the west antarctic ice sheet subglacial topography beneath pine island glacier 1 predictions about future changes in the amundsen sea sector of the west antarctic ice sheet wais have been hampered by poorly known subglacial topography extensive airborne survey has allowed us to derive improved subglacial topography for the pine island glacier basin the trunk of this glacier lies in a narrow 250 km long 500 m deep sub glacial trough suggesting a long lived and constrained ice stream two tributaries lie in similar troughs others lie in less defined shallower troughs the lower basin of the glacier is surrounded by bedrock which after deglaciation and isostatic rebound could rise above sea level this feature would impede ice sheet collapse initiated near the grounding line of this glacier and prevent its progress into the deepest portions of wais the inland slope of the bed beneath the trunk of the glacier however confirms potential instability of the lower basin containing sufficient ice to raise global sea by 24 cm

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

timing of recent accelerations of pine island glacier antarctica 1 we have used interferometric synthetic aperture radar insar data and sequential landsat imagery to identify and temporally constrain two acceleration events on pine island glacier pig these two events are separated by a period of at least seven years 1987 1994 the change in discharge between two flux gates indicates that the majority of the increase in discharge associated with the second acceleration originates well inland 80 km from the grounding line an analysis indicates that changes in driving stress consistent with observed thinning rates are sufficient in magnitude to explain much of the acceleration

antarctic elevation change from 1992 to 1996 satellite radar altimeter measurements show that the average elevation of the antarctic ice sheet interior fell by 0 9 0 5 centimeters per year from 1992 to 1996 if the variability of snowfall observed in antarctic ice cores is allowed for the mass imbalance of the interior this century is only 0 06 0 08 of the mean mass accumulation rate

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

mass balance of the antarctic ice sheet the antarctic contribution to sea level rise has long been uncertain while regional variability in ice dynamics has been revealed a picture of mass changes throughout the continental ice sheet is lacking here we use satellite radar altimetry to measure the elevation change of 72 of the grounded ice sheet during the period 1992 2003 depending on the density of the snow giving rise to the observed elevation fluctuations the ice sheet mass trend falls in the range 5 85gt yr 1 we find that data from climate model reanalyses are not able to characterise the contemporary snowfall fluctuation with useful accuracy and our best estimate of the overall mass trend growth of 27 29 gt yr 1 is based on an assessment of the expected snowfall variability mass gains from accumulating snow particularly on the antarctic peninsula and within east antarctica exceed the ice dynamic mass loss from west antarctica the result exacerbates the difficulty of explaining twentieth century sea level rise

inland thinning of pine island glacier west antarctica the pine island glacier pig transports 69 cubic kilometers of ice each year from 10 of the west antarctic ice sheet wais it is possible that a retreat of the pig may accelerate ice discharge from the wais interior satellite altimetry and interferometry show that the grounded pig thinned by up to 1 6 meters per year between 1992 and 1999 affecting 150 kilometers of the inland glacier the thinning cannot be explained by short term variability in accumulation and must result from glacier dynamics

accelerated sea level rise from west antarctica recent aircraft and satellite laser altimeter surveys of the amundsen sea sector of west antarctica show that local glaciers are discharging about 250 cubic kilometers of ice per year to the ocean almost 60 more than is accumulated within their catchment basins this discharge is sufficient to raise sea level by more than 0 2 millimeters per year glacier thinning rates near the coast during 2002 2003 are much larger than those observed during the 1990s most of these glaciers flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates providing exit routes for ice from further inland if ice sheet collapse is under way

inland thinning of the amundsen sea sector west antarctica 1 together with the pine island glacier pig the thwaites tg and smith sg glaciers are the principal drainage systems of the amundsen sea as sector of western antarctica here we use satellite radar altimetry and interferometry to show that a rapid thinning of ice has occurred within the fastest flowing sections of all as outlet glaciers the pattern of thinning extends to distances greater than 150 km inland between 1991 and 2001 the tg and sg thinned by more than 25 and 45 m at their grounding lines and a total of 154 16 km 3 of ice or 0 43 mm of eustatic sea level rise was lost from the as sector glaciers to the ocean we show that the thickness changes may have caused the pig tg and sg to retreat inland by over 8 4 and 7 km respectively in line with independent estimates of grounding line migration

sensitivity of pine island glacier west antarctica to changes in ice shelf and basal conditions a model study we use a finite element model of coupled ice stream ice shelf flow to study the sensitivity of pine island glacier west antarctica to changes in ice shelf and basal conditions by tuning a softening coefficient of the ice along the glacier margins and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream we are able to match model velocity to that observed with interferometric synthetic aperture radar insar we use the model to investigate the effect of small perturbations on ice flow we find that a 5 5 13 reduction in our initial ice shelf area increases the glacier velocity by 3 5 10 at the grounding line the removal of the entire ice shelf increases the grounding line velocity by 70 the changes in velocity associated with ice shelf reduction are felt several tens of km inland alternatively a 5 reduction in basal shear stress increases the glacier velocity by 13 at the grounding line by contrast softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity hence both the ice shelf buttressing and the basal shear stress contribute significant resistance to the flow of pine island glacier

changes in west antarctic ice stream dynamics observed with alos palsar data the advanced land observation system alos phased array synthetic aperture radar palsar is an l band frequency 1 27 ghz radar capable of continental scale interferometric observations of ice sheet motion here we show that palsar data yield excellent measurements of ice motion compared to c band 5 6 ghz radar data because of greater temporal coherence over snow and firn we compare palsar velocities from year 2006 in pine island bay west antarctica with those spanning years 1974 to 2007 between 1996 and 2007 pine island glacier sped up 42 and ungrounded over most of its ice plain smith glacier accelerated 83 and ungrounded as well their largest speed up are recorded in 2007 thwaites glacier is not accelerating but widening with time and its eastern ice shelf doubled its speed total ice discharge from these glaciers increased 30 in 12 yr and the net mass loss increased 170 from 39 15 gt yr to 105 27 gt yr longer term velocity changes suggest only a moderate loss in the 1970s as the glaciers unground into the deeper smoother beds inland the mass loss from this region will grow considerably larger in years to come copyright 2008 by the american geophysical union

reassessment of the potential sea level rise from a collapse of the west antarctic ice sheet theory has suggested that the west antarctic ice sheet may be inherently unstable recent observations lend weight to this hypothesis we reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution we obtain a value for the global eustatic sea level rise contribution of about 3 3 meters with important regional variations the maximum increase is concentrated along the pacific and atlantic seaboard of the united states where the value is about 25 greater than the global mean even for the case of a partial collapse

mass changes of the greenland and antarctic ice sheets and shelves and contributions to sea level rise 1992 2002 changes in ice mass are estimated from elevation changes derived from 10 5 years greenland and 9 years antarctica of satellite radar altimetry data from the european remote sensing satellites ers 1 and 2 for the first time the dh dt values are adjusted for changes in surface elevation resulting from temperature driven variations in the rate of firn compaction the greenland ice sheet is thinning at the margins 42 2g t a 1 below the equilibrium line altitude ela and growing inland 53 2g t a 1 above the ela with a small overall mass gain 11 3g t a 1 0 03 mm a 1 sle sea level equivalent the ice sheet in west antarctica wa is losing mass 47 4g t a 1 and the ice sheet in east antarctica ea shows a small mass gain 16 11 gt a 1 for a combined net change of 31 12 gt a 1 0 08 mm a 1 sle the contribution of the three ice sheets to sea level is 0 05 0 03 mm a 1 t he antarctic ice shelves show corresponding mass changes of 95 11 gt a 1 in wa and 142 10 gt a 1 in ea thinning at the margins of the greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate the marked thinnings in the pine island and thwaites glacier basins of wa and the totten glacier basin in ea are probably ice dynamic responses to long term climate change and perhaps past removal of their adjacent ice shelves the ice growth in the southern antarctic peninsula and parts of ea may be due to increasing precipitation during the last century