antarctic snow accumulation mapped using polarization of 4 3 cm wavelength microwave emission different parts of antarctica receive different amounts of snowfall each year in this paper we map the variations of the mean annual snow accumulation across the ice sheet we also quantify the uncertainty in our estimates more objectively than has been possible for earlier maps the new map is produced using observations from satellites and ground based measurements after a logarithmic transformation these are combined using the geostatistical method of continuous part universal kriging to give an estimate of the snow accumulation within each cell of a rectangular grid covering antarctica we also derive spatial averages over the major drainage systems of the ice sheet along with their confidence intervals we obtain a value of 143 4 kg m 2 a 1 for the average rate of snow accumulation upon the grounded ice sheet of antarctica

a topological system for delineation and codification of the earth s river basins abstract a comprehensive reference system for the earth s river basins is proposed as a support to river basin management global change research and the pursuit of sustainable development a natural system for delineation and codification of basins is presented which is based upon topographic control and the topology of the river network these characteristics make the system well suited for implementation and use with digital elevation models dems and geographic information systems a demonstration of these traits is made with the 30 arcsecond gtopo30 dem for north america the system has additional appeal owing to its economy of digits and the topological information that they carry this is illustrated through presentation of comparisons with usgs hydrologic unit codes and demonstration of the use of code numbers to reveal dependence or independence of water use activities within a basin

hydrological characteristics of the drainage system beneath a surging glacier a rare combination of natural circumstances permits assessment of current theories on water flow beneath glaciers outburst floods from the subglacial lake grimsvotn in iceland took place before during and after surging of skei ararjokull the glacier beneath which the outburst floods drain the observable drainage patterns associated with these floods show the different nature of the basal water conduit system of the glacier during surge and non surge phases during surge conditions basal water is dispersed slowly across the bed in a distributed drainage system but when the glacier is not surging water is transported rapidly through a system of tunnels

modeling subglacial water routing at paakitsoq w greenland 1 the functioning of the greenland ice sheet s subglacial drainage system and its effect on ice dynamics have been inferred largely from hypothetical hydrology dynamics models and from analysis of satellite data and in situ gps measurements despite this there is still uncertainty about how the surface hydrology interacts with the subglacial drainage system and affects basal water pressures and ice flow especially over annual time scales to address this we developed a high spatial 100 m and temporal 1 h resolution distributed physically based subglacial hydrological model and applied it to the paakitsoq region western greenland the model is driven with moulin input hydrographs calculated by a surface routing and lake filling draining model forced ultimately with distributed hourly runoff calculated by a surface mass balance model key outputs from the model are spatially and temporally varying subglacial water pressures and proglacial stream hydrographs early in the melt season short spikes in water pressure lasting less than a day occur as a result of lake drainage events during midsummer there are sustained periods of high water pressure lasting days to weeks even at times when the subglacial system is inferred to be predominantly efficient later in the summer large diurnal fluctuations in water pressure occur with peaks regularly exceeding ice overburden pressure superimposed on a gradually declining trend these phenomena support the results of previous hypothetical modeling efforts and inferences drawn from gps measurements

investigating the stability of subglacial lakes with a full stokes ice sheet model despite the large amount of subglacial lakes present underneath the east antarctic ice sheet and the melt processes involved the hydrology beneath the ice sheet is poorly understood changes in subglacial potential gradients may lead to subglacial lake outbursts discharging excess water through a subglacial drainage system underneath the ice sheet such processes can eventually lead to an increase in ice flow in this paper a full stokes numerical ice sheet model was employed which takes into account the ice flow over subglacial water bodies in hydrostatic equilibrium with the overlying ice sensitivity experiments were carried out for small perturbations in ice flow and basal melt rate as a function of ice thickness general surface slope ice viscosity and lake size in order to investigate their influence on the subglacial potential gradient and the impact on subglacial lake drainage experiments clearly demonstrate that small changes in surface slope are sufficient to start and sustain episodic subglacial drainage events lake drainage can therefore be regarded as a common feature of the subglacial hydrological system and may influence to a large extent the present and future behavior of large ice sheets

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

convection driven melting near the grounding lines of ice shelves and tidewater glaciers subglacial meltwater draining along the bed of fast flowing marine terminating glaciers emerges at the groundingline wheretheiceeithergoesafloattoformaniceshelforterminates inacalvingface theinputof freshwater to the ocean provides a source of buoyancy and drives convective motion alongside the ice ocean interface this process is modeled using the theory of buoyant plumes that has previously been applied to the study of the larger scale circulation beneath ice shelves the plume grows through entrainment of ocean waters and the heat brought into the plume as a result drives melting at the ice ocean interface the equations are nondimensionalized by using scales appropriate for the region where the subglacial drainage rather than the subsequent addition of meltwater supplies the majority of the buoyancy forcing it is found that the melt rate within this region can be approximated reasonably well by a function that is linear in ocean temperature has a cube root dependence on the flux of subglacial meltwater and has a complex dependency on the slope of the ice ocean interface the model is used to investigate variability in melting induced by changes in both ocean temperature and subglacial discharge for a number of realistic examples of ice shelves and tidewater glaciers the results show how warming ocean waters and increasing subglacial drainage both generate increases in melting near the grounding line

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

three dimensional flow influences on radar layer stratigraphy variations in the depth of radar detectable englacial layers isochrones are commonly used to assess past variability in accumulation rates but little is known about the effect of internal and basal flow variations on isochrone deflections e g bumps troughs in this paper we show how the isochrones are affected by such variation using a three dimensional flow model to investigate changes in the flow mode and in increased basal melting we also investigate how transverse flows with lateral velocity gradients affect the development of isochrones we use the model to visualize how such variations will be seen in radar lines which cross the flow direction we show that in the presence of lateral gradients in the flow field we can produce bumps and troughs when viewed along transects perpendicular to the flow the model results show that the influences of flow convergence melting and changes in flow mode when coupled together affect isochrones over the whole depth of the ice sheet finally changes in the near surface layers cannot be solely attributed to spatial variation in the accumulation rate there can also be a strong signal from changes in the flow mode

a decade of west antarctic subglacial lake interactions from combined icesat and cryosat 2 altimetry we use cryosat 2 interferometric satellite radar altimetry over the mercer and whillans ice streams west antarctica to derive surface elevation changes due to subglacial lake activity at monthly resolution for the period 2010 to 2013 we validate cryosat 2 elevation measurements trends and spatial patterns of change using satellite image differencing and in situ vertical movement from global positioning system gps data two subglacial lake discharge events occur in the same subglacial hydrological catchment within a 9 month period using gps measurements that are spanning the gap between the ice cloud and land elevation satellite and cryosat 2 missions we cross calibrate the two missions to establish the efficacy of cryosat 2 altimetry to measure dynamic changes on the ice sheets

flow switching and water piracy between rutford ice stream and carlson inlet west antarctica rutford ice stream and carlson inlet are neighbouring glaciers in west antarctica rutford ice stream flows at speeds greater than 350m a 1 whereas carlson inlet which has some similar dimensions and supports a similar driving stress flows 10 50 times slower we discuss a range of observations concerning carlson inlet and conclude that there is good indirect evidence that it is a relict ice stream which ceased streaming more than 240 years bp but sufficiently recently that its surface morphology basal water content and basal morphology still retain characteristics produced by streaming an analysis of expected subglacial drainage pathways indicates that carlson inlet is not streaming because it is currently starved of subglacial water which is currently directed beneath rutford ice stream this current state of water piracy by rutford ice stream is however sensitive to minor thickness changes on the ice streams a similar to 120 m 4 thickening of rutford ice stream would divert almost all the subglacial water in the system towards carlson inlet and could reactivate its flow the result highlights the importance of subglacial drainage in controlling ice stream evolution and the requirement for ice sheet models to couple ice flow with subglacial drainage

increased flow speed on a large east antarctic outlet glacier caused by subglacial floods large ice streams and outlet glaciers drain greenland and antarctica an observed acceleration of ice velocity in one of these outlet glaciers byrd glacier east antarctica coincides with a large water discharge from two subglacial lakes allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice

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

drainage networks lakes and water fluxes beneath the antarctic ice sheet antarctica bedmap2 datasets are used to calculate subglacial hydraulic potential and the area depth and volume of hydraulic potential sinks there are over 32 000 contiguous sinks which can be thought of as predicted lakes patterns of subglacial melt are modelled with a balanced ice flux flow model and water fluxes are cumulated along predicted flow pathways to quantify steady state fluxes from the main basin outlets and from known subglacial lakes the total flux from the continent is 21 km3 a 1 byrd glacier has the greatest basin flux of 2 7 km3 a 1 fluxes from subglacial lakes range from 1 10 4 to 1 5 km3 a 1 lake turnover times are calculated from their volumes and fluxes and have median values of 100 a for known active lakes and 500 a for other lakes recurrence intervals of a 0 25 km3 flood range from 2 months to 2000 a median 130 a for known active lakes and from 2 to 2400 a median 360 a for other lakes thus several lakes that have recently been observed to fill and drain may not do so again for many centuries and several lakes that have not so far been observed to fill and drain have the potential to do so even at annual to decadal timescales

morphometry of bedrock meltwater channels on antarctic inner continental shelves implications for channel development and subglacial hydrology abstract expanding multibeam bathymetric data coverage over the last two decades has revealed extensive networks of submarine channels incised into bedrock on the antarctic inner continental shelf the large dimensions and prevalence of the channels implies the presence of an active subglacial hydrological system beneath the past antarctic ice sheet which we can use to learn more about inaccessible subglacial processes here we map and analyse over 2700 bedrock channels situated across 100 000 km2 of continental shelf in the western antarctic peninsula and amundsen sea to produce the first inventory of submarine channels on the antarctic inner continental shelf morphometric analysis reveals highly similar distributions of channel widths depths cross sectional areas and geometric properties with subtle differences between channels in the western antarctic peninsula compared to those in the amundsen sea at 75 3400 m wide 3 280 m deep 160 290 000 m2 in cross sectional area and typically 8 times as wide as they are deep the channels have similar morphologies to tunnel valleys and meltwater channel systems observed from other formerly glaciated landscapes despite differences in substrate geology and glaciological regime we propose that the antarctic bedrock channels formed over multiple glacial cycles through the episodic drainage of at least 59 former subglacial lakes identified on the inner continental shelf

a microbial ecosystem beneath the west antarctic ice sheet liquid water has been known to occur beneath the antarctic ice sheet for more than 40 years but only recently have these subglacial aqueous environments been recognized as microbial ecosystems that may influence biogeochemical transformations on a global scale here we present the first geomicrobiological description of water and surficial sediments obtained from direct sampling of a subglacial antarctic lake subglacial lake whillans slw lies beneath approximately 800 m of ice on the lower portion of the whillans ice stream wis in west antarctica and is part of an extensive and evolving subglacial drainage network the water column of slw contained metabolically active microorganisms and was derived primarily from glacial ice melt with solute sources from lithogenic weathering and a minor seawater component heterotrophic and autotrophic production data together with small subunit ribosomal rna gene sequencing and biogeochemical data indicate that slw is a chemosynthetically driven ecosystem inhabited by a diverse assemblage of bacteria and archaea our results confirm that aquatic environments beneath the antarctic ice sheet support viable microbial ecosystems corroborating previous reports suggesting that they contain globally relevant pools of carbon and microbes that can mobilize elements from the lithosphere and influence southern ocean geochemical and biological systems

high basal melting forming a channel at the grounding line of ross ice shelf antarctica antarctica s ice shelves are thinning at an increasing rate affecting their buttressing ability channels in the ice shelf base unevenly distribute melting and their evolution provides insight into changing subglacial and oceanic conditions here we used phase sensitive radar measurements to estimate basal melt rates in a channel beneath the currently stable ross ice shelf melt rates of 22 2 0 2 m a 1 2500 the overall background rate were observed 1 7 km seaward of mercer whillans ice stream grounding line close to where subglacial water discharge is expected laser altimetry shows a corresponding steadily deepening surface channel two relict channels to the north suggest recent subglacial drainage reorganization beneath whillans ice stream approximately coincident with the shutdown of kamb ice stream this rapid channel formation implies that shifts in subglacial hydrology may impact ice shelf stability

evidence for subglacial water transport in the west antarctic ice sheet through three dimensional satellite radar interferometry 1 radarsat data from the 1997 antarctic mapping mission are used interferometrically to solve for the 3 dimensional surface ice motion in the interior of the west antarctic ice sheet wais an area of 125 km2 in a tributary of the kamb ice stream slumped vertically downwards by up to 50 cm between september 26 and october 18 1997 areas in the bindschadler ice stream also exhibited comparable upward and downward surface displacements as the uplift and subsidence features correspond to sites at which the basal water apparently experiences a hydraulic potential well we suggest transient movement of pockets of subglacial water as the most likely cause for the vertical surface displacements these results and related lidar observations imply that imaging the change in ice surface elevation can help reveal the key role of water in the difficult to observe subglacial environment and its important influence on ice dynamics

rapid discharge connects antarctic subglacial lakes a hidden subglacial world consisting of more than 140 lakes lies beneath the antarctic ice sheet it is widely assumed that these lakes are a static ecosystem cut off from each other and the rest of the world far from it according to a 16 month study of surface elevation changes in the region changes indicating a rapid discharge of water from one lake to another were detected and it is possible that the entire subglacial drainage basin is flushed through periodically by massive water transfers observation of surface elevation changes over 16 months in an area of the east antarctic ice sheet overlying subglacial lakes suggests a rapid discharge of one lake to at least two other lakes involving a water volume of 1 8 km3 in conflict with previous expectations that subglacial lakes have long residence times and slow circulations the existence of many subglacial lakes1 provides clear evidence for the widespread presence of water beneath the east antarctic ice sheet but the hydrology beneath this ice mass is poorly understood2 such knowledge is critical to understanding ice flow basal water transfer to the ice margin glacial landform development and subglacial lake habitats here we present ice sheet surface elevation changes in central east antarctica that we interpret to represent rapid discharge from a subglacial lake our observations indicate that during a period of 16 months 1 8 km3 of water was transferred over 290 km to at least two other subglacial lakes while viscous deformation of the ice roof above may moderate discharge the intrinsic instability of such a system3 suggests that discharge events are a common mode of basal drainage4 if large lakes such as lake vostok or lake concordia1 are pressurizing it is possible that substantial discharges could reach the coast5 6 our observations conflict with expectations that subglacial lakes have long residence times and slow circulations2 7 8 and we suggest that entire subglacial drainage basins may be flushed periodically the rapid transfer of water between lakes would result in large scale solute and microbe relocation and drainage system contamination from in situ exploration is therefore a distinct risk

an inventory of active subglacial lakes in antarctica detected by icesat 2003 2008 through the detection of surface deformation in response to water movement recent satellite studies have demonstrated the existence of subglacial lakes in antarctica that fill and drain on timescales of months to years these studies however were confined to specific regions of the ice sheet here we present the first comprehensive study of these active lakes for the antarctic ice sheet north of 868 s based on 4 5 years 2003 08 of nasa s ice cloud and land elevation satellite icesat laser altimeter data our analysis has detected 124 lakes that were active during this period and we estimate volume changes for each lake the icesat detected lakes are prevalent in coastal antarctica and are present under most of the largest ice stream catchments lakes sometimes appear to transfer water from one to another but also often exchange water with distributed sources undetectable by icesat suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow thus these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds but also may withdraw water at other times to slow flow

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

submarine melting of the 1985 jakobshavn isbrae floating tongue and the triggering of the current retreat 1 photogrammetric reanalysis of 1985 aerial photos has revealed substantial submarine melting of the floating ice tongue of jakobshavn isbrae west greenland the thickness of the floating tongue determined from hydrostatic equilibrium tapers from 940 m near the grounding zone to 600 m near the terminus feature tracking on orthophotos shows speeds on the july 1985 ice tongue to be nearly constant 18 5 m d 1 indicating negligible dynamic thinning the thinning of the ice tongue is mostly due to submarine melting with average rates of 228 49 m yr 1 0 62 0 13 m d 1 between the summers of 1984 and 1985 the cause of the high melt rate is the circulation of warm seawater thermal forcing of up to 4 2 c beneath the tongue with convection driven by the substantial discharge of subglacial freshwater from the grounding zone we believe that this buoyancy driven convection is responsible for a deep channel incised into the sole of the floating tongue a dramatic thinning retreat and speedup began in 1998 and continues today the timing of the change is coincident with a 1 1 c warming of deep ocean waters entering the fjord after 1997 assuming a linear relationship between thermal forcing and submarine melt rate average melt rates should have increased by 25 57 m yr 1 sufficient to destabilize the ice tongue and initiate the ice thinning and the retreat that followed

changes in west antarctic ice stream velocities observation and analysis 1 we have produced a map of velocity covering much of the siple coast ice streams the map confirms earlier estimates of deceleration on whillans ice stream comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams we have used a theoretical analysis to examine the controls on fast flow this analysis suggests that ice plains very wide ice streams are inherently unstable this instability may be responsible for the current deceleration on the ice plain of whillans ice stream and the shutdown of ice stream c 150 years ago thinning induced reductions in driving stress may also explain some of the observed deceleration particularly in upstream areas the active portions of ice stream c coincide well with the areas where we estimate that melt should be taking place current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain uncertainty remains about the basal conditions on ice stream d while the basal resistance on ice stream e is large enough to ensure basal melting

modeled subglacial water flow routing supports localized intrusive heating as a possible cause of basal melting of mars south polar ice cap the discovery of an 20 km wide area of bright subsurface radar reflections interpreted as liquid water beneath the martian south polar layered deposits spld in data from the mars advanced radar for subsurface and ionosphere sounding marsis instrument and the discovery of two geologically recent potential eskers landforms produced by subglacial melt associated with viscous flow features in martian midlatitudes has suggested recent basal melting of martian ice deposits may be feasible possibly due to locally elevated geothermal heating locations of terrestrial subglacial lakes and major drainage axes have been successfully predicted from subglacial hydraulic potential surfaces calculated from surface topography and ice thickness here we use surface topography from the mars orbiter laser altimeter and spld bed elevations derived from marsis data to calculate the subglacial hydraulic potential surface beneath the spld and determine whether the observed high reflectance area coincides with predicted subglacial lake locations given the sensitivity of terrestrial predictions of lake locations to basal topography we derive over 1 000 perturbed topographies using noise statistics from the marsis data to infer the most likely locations of possible subglacial water bodies and drainage axes our results show that the high reflectance area does not coincide with any substantial predicted lake locations three nearby lake locations are robustly predicted however we interpret this result as suggesting that the high reflectance area assuming the interpretation as liquid is correct is most likely a hydraulically isolated patch of liquid confined by the surrounding cold based ice rather than a topographically constrained subglacial lake

antarctic subglacial conditions inferred from a hybrid ice sheet ice stream model abstract subglacial conditions of large polar ice sheets remain poorly understood despite recent advances in satellite observation major uncertainties related to basal conditions such as the temperature field are due to an insufficient knowledge of geothermal heat flow here a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots as well as the distribution of subglacial lakes in this way geothermal heat flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available results of the sensitivity experiments show that 55 of the grounded part of the antarctic ice sheet is at pressure melting point calculated basal melt rates are approximately 65 gt year 1 which is 3 of the total surface accumulation although these sensitivity experiments exhibit small variations in basal melt rates the impact on the ice age of basal layers is quite important in areas that are characterized by relatively low melt rates this may lead to standard deviations up to 50 in a model run over a period of 10 6 years subglacial water flow is concentrated underneath the large outlet glaciers of antarctica the larger subglacial lakes are perched at the head of these subglacial water systems an exception however is the lake system of the recovery ice stream catchment that receives a substantial amount of subglacial melt water from the upstream catchment

connected subglacial lake activity on lower mercer and whillans ice streams west antarctica 2003 2008 we examine patterns of localized surface elevation change in lower mercer and whillans ice streams west antarctica which we interpret as subglacial water movement through a system of lakes and channels we detect and measure the lake activity using repeat track laser altimetry from icesat and image differencing from modis image pairs a hydrostatic potential map for the region shows that the lakes are distributed across three distinct hydrologic regimes our analysis shows that within these regimes some of the subglacial lakes appear to be linked with drainage events in one reservoir causing filling and follow on drainage in adjacent lakes we also observe changes near ice raft a in lower whillans ice stream and interpret them as evidence of subglacial water and other changes at the bed the study provides quantitative information about the properties of this complex subglacial hydrologic system and a relatively unstudied component of ice sheet mass balance subglacial drainage across the grounding line

dynamic distributed drainage implied by the flow evolution of the 1996 1998 adventure trench subglacial lake discharge abstract the transport of subglacial water beneath the east antarctic ice sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities and ultimately global climate periodic outbursts are a critical mechanism in this process recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km 3 traveling some 260 km along the ice bed interface of the adventure subglacial trench between 1996 and 1998 wingham et al 2006 rapid discharge connects antarctic subglacial lakes nature 440 1033 1036 using radar echo sounding data from the adventure subglacial trench region in conjunction with the previously reported satellite observations along with some basic modeling we calculate a mass budget and infer a flow mechanism for the 1996 1998 event the volume released from the source lake exceeded the volume received by the destination lakes by 1 1 km 3 this discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward the downstream release of water from the destination lakes continued until at least 2003 several years after the 1998 cessation of surface subsidence at the source lake by 2003 a total of 1 5 km 3 or nearly 75 of the water released by the source lake had traveled downstream from the destination lakes the temporal evolution of discharge from the outlet can be simulated with the classic ice walled semicircular channel model if and only if the retreat of the source lake shoreline is taken into account further downstream the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable along these reaches mechanical lifting of the ice roof and or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge a distributed system is also consistent with the 3 month delay between water release at the source lake and water arrival at the destination lake observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system ultimately the presence of large subglacial lakes along the flow path of the 1996 1998 adventure subglacial trench flow path delayed the arrival of water to points downstream by approximately 12 months

large subglacial lakes in east antarctica at the onset of fast flowing ice streams water has been found to gather in lakes underneath the antarctic ice sheet and it can move between lakes and drain catastrophically despite this relatively little is known about how subglacial lakes influence ice sheet dynamics now satellite imagery has identified four subglacial lakes similar in total area to lake vostok right at the onset of the recovery glacier ice stream in east antarctica this suggests that subglacial lakes may initiate and maintain rapid ice flow a mechanism that will need to be considered in future ice sheet mass balance assessments satellite imagery is used to identify four subglacial lakes similar in total area to lake vostok right at the onset of the recovery glacier ice stream in east antarctica this suggests that the subglacial lakes can initiate and maintain rapid ice flow a mechanism that will need to be considered in future ice sheet mass balance assessments water plays a crucial role in ice sheet stability and the onset of ice streams subglacial lake water moves between lakes1 and rapidly drains causing catastrophic floods2 the exact mechanisms by which subglacial lakes influence ice sheet dynamics are unknown however and large subglacial lakes3 4 have not been closely associated with rapidly flowing ice streams here we use satellite imagery and ice surface elevations to identify a region of subglacial lakes similar in total area to lake vostok at the onset region of the recovery glacier ice stream in east antarctica and predicted by ice sheet models5 we define four lakes through extensive flat featureless regions of ice surface bounded by upstream troughs and downstream ridges using ice velocities determined using interferometric synthetic aperture radar insar we find the onset of rapid flow moving at 20 to 30 m yr 1 of the tributaries to the recovery glacier ice stream in a 280 km wide segment at the downslope margins of these four subglacial lakes we conclude that the subglacial lakes initiate and maintain rapid ice flow through either active modification of the basal thermal regime of the ice sheet by lake accretion or through scouring bedrock channels in periodic drainage events we suggest that the role of subglacial lakes needs to be considered in ice sheet mass balance assessments

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

meltwater flux and runoff modeling in the ablation area of jakobshavn isbrae west greenland the temporal variability in surface snow and glacier melt flux and runoff were investigated for the ablation area of lakobshavn isbrae west greenland high resolution meteorological observations both on and outside the greenland ice sheet gris were used as model input realistic descriptions of snow accumulation snow and glacier ice melt and runoff are essential to understand trends in ice sheet surface properties and processes snowmodel a physically based spatially distributed meteorological and snow evolution modeling system was used to simulate the temporal variability of lakobshavn isbrre accumulation and ablation processes for 2000 01 2006 07 winter snow depth observations and modis satellite derived summer melt observations were used for model validation of accumulation and ablation simulations agreed well with observed values simulated annual surface melt varied from as low as 3 83 x 10 sup 9 m sup 3 2001 02 to as high as 8 64 x 10 sup 9 m sup 3 2004 05 modeled surface melt occurred at elevations reaching 1 870 m a s l for 2004 05 while the equilibrium line altitude ela fluctuated from 990 to 1 210 m a s l during the simulation period the snowmodel meltwater retention and refreezing routines considerably reduce the amount of meltwater available as ice sheet runoff without these routines the lakobshavn surface runoff would be overestimated by anmore average of 80 from september october through may june no runoff events were simulated the modeled interannual runoff variability varied from 1 81 x 10 sup 9 m sup 3 2001 02 to 5 21 x 10 sup 9 m sup 3 2004 05 yielding a cumulative runoff at the jakobshavn glacier terminus of approx 2 25 m w eq to approx 4 5 m w eq respectively the average modeled lakobshavn runoff of approx 3 4 km sup 3 y sup 1 was merged with previous estimates of jakobshavn ice discharge to quantify the freshwater flux to illulissat icefiord for both runoff and ice discharge the average trends are similar indicating increasing insignificant influx of freshwater to the illulissat icefiord for the period 2000 01 2006 07 this study suggests that surface runoff forms a minor part of the overall jakobshavn freshwater flux to the fiord around 7 approx 3 4 km sup 3 y sup 1 of the average annual freshwater flux of approx 51 0 km sup 3 y sup 1 originates from the surface runoff less

the triggering of sub glacial lake drainage during the rapid glacier drawdown crane glacier antarctic peninsula ice surface altimetry from icesat 1 and nasa aircraft altimeter overflights spanning 2002 09 indicate that a region of lower crane glacier antarctic peninsula shows an unusual temporal pattern of elevation loss a period of very rapid drawdown 91 m a 1 between september 2004 and september 2005 bounded by periods of large but more moderate rates 23 m a 1 until september 2004 12 m a 1 after september 2005 the region of increased drawdown is 4 5 km 2 2 km based on satellite aster and spot 5 stereo image digital elevation model dem differencing spanning the event in a later differential dem the anomalous drawdown feature is not seen bathymetry in crane glacier fjord reveals a series of flat lying formerly subglacial deeps interpreted as lake sediment basins we conclude that the elevation change feature resulted from drainage of a small deep subglacial lake we infer that the drainage event was induced by hydraulic forcing of subglacial water past a downstream obstruction however only a fraction of crane glacier s increase in flow speed that occurred near the time of lake drainage derived from image feature tracking appears to be directly attributable to the event instead retreat of the ice front off a subglacial ridge 6 km downstream of the lake is likely the dominant cause of renewed fast flow and more negative mass balance in the subsequent 4 years

three dimensional mapping by cryosat 2 of subglacial lake volume changes we analyze data acquired by the cryosat 2 interferometric radar altimeter and demonstrate its novel capability to track topographic features on the antarctic ice sheet we map the perimeter and depth of a 260 km2 surface depression above an antarctic subglacial lake sgl and in combination with ice cloud and land elevation satellite laser altimetry chart decadal changes in sgl volume during 2007 2008 between 4 9 and 6 4 km3 of water drained from the sgl and peak discharge exceeded 160 m3 s 1 the flood was twice as large as any previously recorded and equivalent to 10 of the meltwater generated annually beneath the ice sheet the ice surface has since uplifted at a rate of 5 6 2 8 m yr 1 our study demonstrates the ability of cryosat 2 to provide detailed maps of ice sheet topography its potential to accurately measure sgl drainage events and the contribution it can make to understanding water flow beneath antarctica key points assessment of novel cryosat 2 interferometric altimetry over land ice demonstration of interferometric capability to track off nadir topography mapping of largest antarctic subglacial lake drainage event observed to date

northern hemisphere geography of ice covered rivers although river ice is a major component of the cryosphere and is particularly important to many river processes including extreme events its full geographical coverage has never been documented recognizing that the freeze up and break up of river ice is closely linked to the timing of 0 c air temperatures this study analyses the spatial extent of river networks relative to the location of three 0 c isotherm periods these were defined to represent a suite of ice affected conditions that would be experienced for 6 3 or 0 5 month periods the briefest interval possibly leading only to a very thin and transient ice cover or simply border frazil ice formation four different gis databases were used to represent the river networks the percentages of the total northern hemisphere land mass average river network influenced by cold temperatures conducive to ice formation were 52 45 and 25 56 47 and 28 respectively the related southern position of the isotherms ranged from 33 n 35 n and 50 n in central north america to a nearly consistent 27 n for eurasia reflecting the influence of the high elevation central plateau region also identified are the lengths of major rivers that fall within the three 0 c isotherm boundaries included are some of the world s largest rivers including the lena mackenzie ob yellow yukon and yenisey rivers although their percentage of ice affected coverage varied for the three isotherm periods from a consistent 100 for the lena and yukon rivers to as little as 23 at the 6 month interval for the yellow river copyright 2009 her majesty the queen in right of canada published by john wiley sons ltd

time dependence of the spatial pattern of accumulation rate in east antarctica deduced from isochronic radar layers using a 3 d numerical ice flow model 1 in east antarctica surface mass balance data can only be obtained from the sparsely distributed ice cores when considering time periods greater than a few decades observations of internal layers measured by airborne ice penetrating radar in principle permit extrapolation of mass balance information from these ice cores we use radar survey lines gathered in the 1970s and a three dimensional numerical model to investigate the feasibility of such extrapolation seeking to match the calculations of englacial layer geometry with observations first we justify the use of a three dimensional model by showing that simple vertical flow models cannot explain all the observations and that horizontal advection is a significant glacial process then we examine processes that affect calculations of layer geometry finding that spatial accumulation rate patterns are extremely important while geothermal heat flux and flow mode sliding or internal deformation are of substantially less importance where the layer is from the last glacial maximum 17 5 ka we find a very good match between the spatial pattern of accumulation rates inferred from this layer and the modern spatial pattern of accumulation rates when considering deeper layers from beyond the current interglacial we find that a different spatial accumulation rate pattern must have existed in addition to the known change in accumulation rate from ice cores the glacial spatial accumulation rate pattern would have had proportionally greater accumulation at the south pole than now compared with the vostok and dome c ice cores

a subglacial water flow model for west antarctica the current generation of continental scale ice sheet models cannot successfully reproduce the complex ice water sediment interactions of the west antarctic ice sheet wais in a physically meaningful manner the potential of a thin film based subglacial water flow model for incorporation into a continental scale coupled ice water flow model of the wais is evaluated in this paper the subglacial water flow model is applied to the ross sea sector of the wais in both a steady state and time dependent form to derive the equilibrium water depth for the present day configuration the potential for coupling the model to an ice flow model is then demonstrated using a variable sliding parameter that is a function of the subglacial water depth a coupled ice water flow model using the parameterization tested in this paper could have the potential for reproducing the surface elevation velocity and thermal regime of the wais successfully these requirements are crucial in modelling the evolution of the wais and must be addressed before reliable continental scale predictive models can be utilized

numerical analysis of rapid water transfer beneath antarctica we use a simple energy conservation model and a model based on rothlisberger s theory for steady state water flow in a subglacial conduit to model water movement between lakes in the adventure subglacial trench region of east antarctica during a 1996 98 jokulhlaup using available field evidence to constrain the models suggests that water flow would likely be accommodated in a tunnel with a cross sectional area of 36 m 2 and a value for k the reciprocal of manning s roughness parameter larger than the 12 5 m 1 3 s 1 previously calculated we also use nye s theory for time dependent conduit water flow to model the temporal evolution of conduit discharge cross sectional area water pressure and lake draining and filling during the flood we initially assume one source and one sink lake we perform sensitivity tests on the input parameter set matching modeled source and sink lake depth changes with measured surface elevation data using a simple function for vertical ice deformation in which surface deformation scales linearly to the lake depth change we find the scaling factor is of the order 4 10 3 of the ice thickness the most likely value of k lies in the range 55 68 m 1 3 s 1 and the ratio of source to sink lake radii is approximately 1 1 4 finally we experiment using nye s theory to model water movement between one source and three sink lakes the model fails to produce the observed patterns of water movement as indicated by the surface deformation data

hydrological controls on patterns of surface internal and basal motion during three spring events haut glacier d arolla switzerland three early melt season high velocity events or spring events occurred on haut glacier d arolla switzerland during the melt seasons of 1998 and 1999 the events involve enhanced glacier velocity during periods of rapidly increasing bulk discharge in the proglacial stream and high subglacial water pressures however differences in spatial patterns of surface velocity internal ice deformation rates the spatial extent of high subglacial water pressures and in rates of subglacial sediment deformation suggest different hydrological and mechanical controls the data from two of the events suggest widespread ice bed decoupling particularly along a subglacial drainage axis creating the highest rates of basal motion and plug flow in the overlying ice the other event showed evidence of less extensive ice bed decoupling and sliding along the drainage axis with more mechanical support for ice overburden transferred to areas adjacent to decoupled areas we suggest that 1 plug flow may be a common feature on glaciers experiencing locally induced reductions in basal drag 2 under certain circumstances enhanced surface motion may be due in part to non locally forced enhanced bed deformation and 3 subglacial sediment deformation is confined to a depth of the order of centimetres to decimetres

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

complex network of channels beneath an antarctic ice shelf ice shelves play an important role in stabilizing the interior grounded ice of the large ice sheets the thinning of major ice shelves observed in recent years possibly in connection to warmer ocean waters coming into contact with the ice shelf base has focused attention on the ice ocean interface here we reveal a complex network of sub ice shelf channels under the fimbul ice shelf antarctica mapped using ground penetrating radar over a 100 km2 grid the channels are 300 500 m wide and 50 m high among the narrowest of any reported observing narrow channels beneath an ice shelf that is mainly surrounded by cold ocean waters with temperatures close to the surface freezing point shows that channelized basal melting is not restricted to rapidly melting ice shelves indicating that spatial melt patterns around antarctica are likely to vary on scales that are not yet incorporated in ice ocean models

vertical motions in the uranian atmosphere an analysis of radio observations abstract we present a new 6 cm radio map of uranus that shows latitudinal features probably caused by vertical motions in the atmosphere a comparison with older data and voyager infrared measurements suggests that large scale vertical motions have not changed significantly in the last 8 years and that these motions are deep seated and extend from the 0 1 to the 45 bar level a height of 250 km

evidence from ice shelves for channelized meltwater flow beneath the antarctic ice sheet subglacial meltwater channels beneath the antarctic ice sheet have been reported but the nature and distribution of these meltwater pathways are unclear remote sensing observations reveal persistent channelized features beneath the filchner ronne ice shelf in west antarctica suggesting widespread channelized flow driven by melting

the supply of subglacial meltwater to the grounding line of the siple coast west antarctica recent satellite studies have shown that active subglacial lakes exist under the antarctic ice streams and persist almost to their grounding lines when the lowest lying lakes flood the water crosses the grounding line and enters the sub ice shelf cavity modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice shelf base we examine the spatial and temporal variability in subglacial water supply to the grounding lines of the siple coast ice streams by combining estimates for lake volume change derived from ice cloud and land elevation satellite icesat data with a model for subglacial water transport our results suggest that subglacial outflow tends to concentrate towards six embayments in the siple coast grounding line although mean grounding line outflow is 60m3 s 1 for the entire siple coast maximum local grounding line outflow may temporarily exceed 300 m3 s 1 during the synchronized flooding of multiple lakes in a hydrologic basin variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the ross ice shelf cavity the temporal variability in grounding line outflow results in a net reduction in long term average melt rate but temporary peak melting rates may exceed the long term average by a factor of three

high sensitivity of subglacial hydrological pathways in antarctica to small ice sheet changes 1 the flow of water beneath the antarctic ice sheet has recently been shown to involve drainage from and exchange between subglacial lakes with flow routing over distances of 100km as subglacial water is known to be important to ice sheet dynamics changes to the flow routing of basal water may yield an ice sheet response here we show that the directions of subglacial hydrological flowpaths are highly sensitive to the ice sheet surface topography at certain sites in antarctica surface elevation changes of between 5 and 15 m are found to be sufficient to produce significant changes to the subglacial water flowpaths which affect the potential drainage routes of large subglacial lakes we identify the reasons for this high sensitivity and recognise several locations in east antarctica where changes to the drainage regime could have important effects on large scale basal water distribution

seasonal evolution of subglacial drainage and acceleration in a greenland outlet glacier an acceleration of ice mass loss has been observed near the margin of the greenland ice sheet partly as a result of faster ice motion observations by gps receivers reveal high seasonal variability in ice motion with summer motion up to 220 higher than winter background levels the greenland ice sheet contains enough water to raise sea levels by 7 m however its present mass balance and future contribution to sea level rise is poorly understood1 accelerated mass loss has been observed near the ice sheet margin partly as a result of faster ice motion2 3 4 surface melt waters can reach the base of the ice sheet and enhance basal ice motion5 6 however the response of ice motion to seasonal variations in meltwater supply is poorly constrained both in space and time here we present ice motion data obtained with global positioning system receivers located along a 35 km transect at the western margin of the greenland ice sheet throughout a summer melt season our measurements reveal substantial increases in ice velocity during summer up to 220 above winter background values these speed up events migrate up the glacier over the course of the summer the relationship between melt and ice motion varies both at each site throughout the melt season and between sites we suggest that these patterns can be explained by the seasonal evolution of the subglacial drainage system similar to hydraulic forcing mechanisms for ice dynamics that have been observed at smaller glaciers

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes