the role of the barents sea in the arctic climate system present global warming is amplified in the arctic and accompanied by unprecedented sea ice decline located along the main pathway of atlantic water entering the arctic the barents sea is the site of coupled feedback processes that are important for creating variability in the entire arctic air ice ocean system as warm atlantic water flows through the barents sea it loses heat to the arctic atmosphere warm periods like today are associated with high northward heat transport reduced arctic sea ice cover and high surface air temperatures the cooling of the atlantic inflow creates dense water sinking to great depths in the arctic basins and 60 of the arctic ocean carbon uptake is removed from the carbon saturated surface this way recently anomalously large ocean heat transport has reduced sea ice formation in the barents sea during winter the missing barents sea winter ice makes up a large part of observed winter arctic sea ice loss and in 2050 the barents sea is projected to be largely ice free throughout the year with 4 c summer warming in the formerly ice covered areas the heating of the barents atmosphere plays an important role both in arctic amplification and the arctic heat budget the heating also perturbs the large scale circulation through expansion of the siberian high northward with a possible link to recent continental wintertime cooling large air ice ocean variability is evident in proxy records of past climate conditions suggesting that the barents sea has had an important role in northern hemisphere climate for at least the last 2500 years

climate cod and crops coastal land use in the sw barents sea region during the past 2 5 ka the sw coast of the barents sea is a historical transition zone between the traditional agrarian cultures of western europe and sami hunter gatherers of northern fennoscandia it is today north of the climatic limit of cereal cultivation and the economy is primarily based on fisheries the pollen content of two peat profiles from the outer coastal island soroya c 70 5 n were examined increased anthropogenic influence was seen in the pollen assemblages at 600 100 bc ad 150 550 ad 1300 1550 and ad 1700 2000 as well as a general environmental shift around ad 950 probably towards a wetter climate and more open vegetation the results largely confirm the general held view on agrarian expansion phases in north norway these different phases are discussed from a regional historical economic and environmental perspective during the early iron age the arctic limit for agriculture is extended northward along the coast into the barents sea area probably during periods of more favourable climate from the

antarctic ice sheet melting in the southeast pacific the first oceanographic measurements across a deep channel beneath the calving front of pine island glacier reveal a sub ice circulation driven by basal melting of 10 12 m yr 1 a salt box model described here gives a melt rate similar to that of ice balance and numerical models 5 50 times higher than averages for the george vi and ross ice shelves melting is fueled by relatively warm circumpolar deep water that floods the deep floor of the amundsen and bellingshausen sea continental shelves reaching the deep draft of this floating glacier a revised melt rate for ice shelves in the southeast pacific sector raises circumpolar ice shelf melting to 756 gt yr 1 given prior estimates of surface accumulation and iceberg calving this suggests that the antarctic ice sheet is currently losing mass to the ocean

holocene palaeoclimates from peat stratigraphy macrofossil proxy climate records from three oceanic raised bogs in england and ireland quantified analyses of plant macrofossil remains have been made from three profiles of peat from raised bogs spanning a distance of 425 km from western ireland to northern england the reconstructed vegetation of each profile is related to changing bog surface wetness bsw and since the bogs are ombrotrophic these bsw changes are interpreted in terms of changing climate using age depth models based on a total of 49 radiocarbon dates a number of wetter and drier phases are identified and phase shifts to wetter and or cooler climates are defined prominent coincident changes to wetter conditions are dated in at least two of the profiles to ca 4400 4000 1750 1400 and 1000 cal bp and in all three profiles at 3200 2750 2350 2250 and around 700 cal bp these phases are related to proxy climate changes in other terrestrial data sets from northwest europe and a broad degree of synchroneity is demonstrated

volume loss from antarctic ice shelves is accelerating the floating ice shelves surrounding the antarctic ice sheet restrain the grounded ice sheet flow thinning of an ice shelf reduces this effect leading to an increase in ice discharge to the ocean using 18 years of continuous satellite radar altimeter observations we have computed decadal scale changes in ice shelf thickness around the antarctic continent overall average ice shelf volume change accelerated from negligible loss at 25 64 cubic kilometers per year for 1994 2003 to rapid loss of 310 74 cubic kilometers per year for 2003 2012 west antarctic losses increased by 70 in the past decade and earlier volume gain by east antarctic ice shelves ceased in the amundsen and bellingshausen regions some ice shelves have lost up to 18 of their thickness in less than two decades

variability of warm deep water inflow in a submarine trough on the amundsen sea shelf the ice shelves in the amundsen sea are thinning rapidly and the main reason for their decline appears to be warm ocean currents circulating below the ice shelves and melting these from below ocean currents transportwarm densewater ontothe shelf channeledby bathymetric troughs leadingto the deep inner basins a hydrographic mooring equipped with an upward looking adcp has been placed in one of these troughs on the central amundsen shelf the two years 2010 11 of mooring data are here used to characterize the inflow of warm deep water to the deep shelf basins during both years the warm layer thickness and temperature peaked in austral fall the along trough velocity is dominated by strong fluctuations that do not vary in the vertical these fluctuations are correlated with the local wind with eastward wind over the shelf and shelf break giving flow toward the ice shelves in addition there is a persistent flow of dense lower circumpolar deep water cdw toward the ice shelves in the bottom layer this bottom intensified flow appears to be driven by buoyancy forces rather than the shelfbreak wind the years of 2010 and 2011 were characterized by a comparatively stationary amundsen sea low and hence there were no strong eastward winds during winter that could drive an upwelling of warm water along the shelf break regardless of this there was a persistent flow of lower cdw in the bottom layer during the two years the average heat transport toward the ice shelves in the trough was estimated from the mooring data to be 0 95 tw

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

the distribution of glacial meltwater in the amundsen sea antarctica revealed by dissolved helium and neon the light noble gases helium he and neon ne dissolved in seawater can be useful tracers of freshwater input from glacial melting because the dissolution of air bubbles trapped in glacial ice results in an approximately tenfold supersaturation using he and ne measurements we determined for the first time the distribution of glacial meltwater gmw within the water columns of the dotson trough dt and in front of the dotson and getz ice shelves dis and gis respectively in the western amundsen sea antarctica in the austral summers of 2011 and 2012 the measured saturation anomalies of he and ne he and ne were in the range of 3 35 and 2 12 respectively indicating a significant presence of gmw throughout the dt the highest values of he 21 were observed at depths of 400 500 m corresponding to the layer between the incoming warm circumpolar deep water and the overlying winter water the high he and ne area extended outside of the shelf break suggesting that gmw is transported more than 300 km offshore the he was substantially higher in front of the dis than the gis and the highest he 31 was observed in the western part of the dis where concentrated outflow from the shelf to the offshore was observed in 2012 the calculated gmw fraction in seawater based on excess he and ne decreased by 30 40 compared with that in 2011 in both ice shelves indicating strong temporal variability in glacial melting

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

freshening of the ross sea during the late 20th century ocean measurements in the ross sea over the past four decades one of the longest records near antarctica reveal marked decreases in shelf water salinity and the surface salinity within the ross gyre these changes have been accompanied by atmospheric warming on ross island ocean warming at depths of 300 meters north of the continental shelf a more negative southern oscillation index and thinning of southeast pacific ice shelves the freshening appears to have resulted from a combination of factors including increased precipitation reduced sea ice production and increased melting of the west antarctic ice sheet

response of the global ocean to greenland and antarctic ice melting 1 we investigate the transient response of the global ocean circulation to enhanced freshwater forcing associated with melting of the greenland and antarctic ice sheets increased freshwater runoff from greenland results in a basin wide response of the north atlantic on timescales of a few years communicated via boundary waves equatorial kelvin waves and westward propagating rossby waves in addition modified air sea interaction plays a fundamental role in setting up the basin scale response of the atlantic circulation in its subpolar and subtropical gyres in particular the modified ocean dynamics and thermodynamics lead to a depression in the central north and south atlantic that would not be expected from linear wave dynamics moreover the heat content increases on basin and global scales in response to anomalous freshwater forcing from greenland suggesting that the ocean s response to enhanced freshwater forcing would be a coupled problem other parts of the world ocean experience a much slower adjustment in response to greenland freshwater forcing communicated via planetary waves but also involving advective diffusive processes especially in the southern ocean over the 50 years considered here most of the sea level increase associated with freshwater input from greenland remains in the atlantic ocean in contrast ice melting around antarctica has a much reduced effect on the global ocean in both cases none of the basins came to a stationary state during the 50 year experiment

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

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

climatic change and contemporaneous land use phases north and south of the alps 2300 bc to 800 ad fluctuations in the 14c curve and subsequent gaps of archaeological findings at 800 650 and 400 100 bc in western and central europe may indicate major climate driven land abandonment phases to address this hypothesis radiocarbon dated sediments from four lakes in switzerland were studied palynologically pollen analysis indicates contemporaneous phases of forest clearances and of intensified land use at 1450 1250 bc 650 450 bc 50 bc 100 ad and around 700 ad these land use expansions coincided with periods of warm climate as recorded by the alpine dendroclimatic and greenland oxygen isotope records our results suggest that harvest yields would have increased synchronously over wide areas of central and southern europe during periods of warm and dry climate combined interpretation of palaeoecological and archaeological findings suggests that higher food production led to increased human populations positive long term trends in pollen values of cerealia and plantago lanceolata indicate that technical innovations during the bronze and iron age e g metal ploughs scythes hay production fertilising methods gradually increased agricultural productivity the successful adoption of yield increasing advances cannot be explained by climatic determinism alone combined with archaeological evidence our results suggest that despite considerable cycles of spatial and demographic reorganisation repeated land abandonments and expansions as well as large scale migrations and population decreases human societies were able to shift to lower subsistence levels without dramatic ruptures in material culture however our data imply that human societies were not able to compensate rapidly for harvest failures when climate deteriorated agriculture in marginal areas was abandoned and spontaneous reforestations took place on abandoned land south and north of the alps only when the climate changed again to drier and warmer conditions did a new wide spread phase of forest clearances and field extensions occur allowing the reoccupation of previously abandoned areas spatial distribution of cereal cultivation and growth requirements of cerealia species suggest that increases in precipitation were far more decisive in driving crop failures over central and southern europe than temperature decreases

impact of local winter cooling on the melt of pine island glacier antarctica the rapid thinning of the ice shelves in the amundsen sea is generally attributed to basal melt driven by warm water originating from the continental slope we examine the hypothesis that processes taking place on the continental shelf contribute significantly to the interannual variability of the ocean heat content and ice shelf melt rates a numerical model is used to simulate the circulation of ocean heat and the melt of the ice shelves over the period 2006 2013 the fine model grid grid spacing 1 5 km explicitly resolves the coastal polynyas and mesoscale processes the ocean heat content of the eastern continental shelf exhibits recurrent decreases around september with a magnitude that varies from year to year the heat loss is primarily caused by surface heat fluxes along the eastern shore in areas of low ice concentration polynyas the cold winter water intrudes underneath the ice shelves and reduces the basal melt rates ocean temperatures upstream i e at the shelf break are largely constant over the year and cannot account for the cold events the cooling is particularly marked in 2012 and its effect on the ocean heat content remains visible over the following years the study suggests that ocean atmosphere interactions in coastal polynyas contribute to the interannual variability of the melt of pine island glacier

new ice core evidence for a volcanic cause of the a d 536 dust veil in new zealand human cryptosporidiosis demonstrates spring and autumn peaks of incidence with the spring peak being three times greater in magnitude than the autumn peak the imbalance between the two peaks is notable and may be associated with the high livestock density in new zealand in the summer and autumn the cryptosporidiosis rate was positively associated with temperatures in the current and previous month highlighting the importance of outdoor recreation to transmission no associations between spring incidence and weather were found providing little support for the importance of drinking water pathways imported travel cases do not appear to be an important factor in the aetiology of cryptosporidiosis in new zealand

rapid sea level rise along the antarctic margins in response to increased glacial discharge melting of the antarctic ice sheet and ice shelves reduces the salinity of the coastal seas satellite and model data suggest that the freshwater discharge has also caused coastal antarctic sea level to rise by about 2 mm yr 1 more than the regional mean

large multidecadal salinity trends near the pacific antarctic continental margin abstract ocean temperature and salinity measurements on and near the antarctic continental shelf in the southwest pacific sector are evaluated for evidence of temporal change shelf water in the southwest ross sea has declined in salinity by 0 03 decade 1 from 1958 to 2008 while its temperatures have increased in proportion to the influence of salinity on the sea surface freezing point modified deep water intrusions that reach the central ross ice shelf have freshened at a similar rate and cooled by 0 5 c since the late 1970s salinity has decreased by 0 08 decade 1 in the westward coastal and slope front currents consistent with increased melting of continental ice upstream in the amundsen sea overturning of those near surface waters during winter sea ice formation and mixing across the slope front is sufficient to account for the 5 decade shelf water salinity change a strong correlation between the freshening and change in the southern annular mode index suggests a link with the large scale atmosp

oceanic heat transport onto the amundsen sea shelf through a submarine glacial trough glaciers which drain the west antarctic ice sheet wais into the amundsen sea are accelerating and thinning rapidly these observations have been attributed to the regional oceanography whereby heat contained within circumpolar deep water cdw drives the basal melting of floating glaciers on the basis of new data we calculate that 2 8 terra watts 1012 of oceanic heat flow onto the continental shelf and toward the glaciers via a submarine glacial trough this is enough to account for most of the basal melting in the entire region suggesting the ocean is supplying an excess of heat toward the antarctic continent

ocean circulation and sea ice thinning induced by melting ice shelves in the amundsen sea a 1 12 ocean model configuration of the amundsen sea sector is developed to better understand the circulation induced by ice shelf melt and the impacts on the surrounding ocean and sea ice eighteen sensitivity experiments to drag and heat exchange coefficients at the ice shelf ocean interface are performed the total melt rate simulated in each cavity is function of the thermal stanton number and for a given thermal stanton number melt is slightly higher for lower values of the drag coefficient sub ice shelf melt induces a thermohaline circulation that pumps warm circumpolar deep water into the cavity the related volume flux into a cavity is 100 500 times stronger than the melt volume flux itself ice shelf melt also induces a coastal barotropic current that contributes 45 12 of the total simulated coastal transport due to the presence of warm circumpolar deep waters the melt induced inflow typically brings 4 20 times more heat into the cavities than the latent heat required for melt for currently observed melt rates approximately 6 31 of the heat that enters a cavity with melting potential is actually used to melt ice shelves for increasing sub ice shelf melt rates the transport in the cavity becomes stronger and more heat is pumped from the deep layers to the upper part of the cavity then advected toward the ocean surface in front of the ice shelf therefore more ice shelf melt induces less sea ice volume near the ice sheet margins

oceanographic controls on the variability of ice shelf basal melting and circulation of glacial meltwater in the amundsen sea embayment antarctica ice shelves in the amundsen sea embayment have thinned accelerating the seaward flow of ice sheets upstream over recent decades this imbalance is caused by an increase in the ocean driven melting of the ice shelves observations and models show that the ocean heat content reaching the ice shelves is sensitive to the depth of thermocline which separates the cool fresh surface waters from warm salty waters yet the processes controlling the variability of thermocline depth remain poorly constrained here we quantify the oceanic conditions and ocean driven melting of cosgrove pine island glacier pig thwaites crosson and dotson ice shelves in the amundsen sea embayment from 1991 to 2014 using a general circulation model ice shelf melting is coupled to variability in the wind field and the sea ice motions over the continental shelf break and associated onshore advection of warm waters in deep troughs the layer of warm salty waters at the calving front of pig and thwaites is thicker in austral spring june to october than in austral summer december to march whereas the seasonal cycle at the calving front of dotson is reversed furthermore the ocean driven melting in pig is enhanced by an asymmetric response to changes in ocean heat transport anomalies at the continental shelf break melting responds more rapidly to increases in ocean heat transport than to decreases this asymmetry is caused by the inland deepening of bathymetry and the glacial meltwater circulation around the ice shelf

global contraction of antarctic bottom water between the 1980s and 2000s abstracta statistically significant reduction in antarctic bottom water aabw volume is quantified between the 1980s and 2000s within the southern ocean and along the bottom most southern branches of the meridional overturning circulation moc aabw has warmed globally during that time contributing roughly 10 of the recent total ocean heat uptake this warming implies a global scale contraction of aabw rates of change in aabw related circulation are estimated in most of the world s deep ocean basins by finding average rates of volume loss or gain below cold deep potential temperature surfaces using all available repeated hydrographic sections the southern ocean is losing water below 0 c at a rate of 8 2 2 6 106 m3 s 1 this bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near surface recovery apparently through a southward surge of circumpolar deep water from the north to the north smaller losses of bottom waters are see

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

holocene vegetation and climate dynamics of the boreal alpine ecotone of northwestern fennoscandia climate change with respect to summer temperature throughout the holocene is inferred from oscillations in the local pinus sylvestris alnus incana and betula pubescens forest lines as recorded by fossil pollen and plant macrofossils in lake sediments at four altitudinal levels mt skrubben 848 m a s l in dividalen was deglaciated down to below 280 m a s l during 10 800 10 300 cal yr bp betula pubescens established 10100 cal yr bp at 280 m a s l and expanded up to near the summit during the next 700 years birch woodland prevailed on the mountain plateau until 3300 cal yr bp local pinus sylvestris stands are recorded up to 400 m a s l at 8450 cal yr bp and 548 m a s l about 8160 cal yr bp alnus incana expanded from 400 to nearly 790 m a s l during the period 7900 7600 cal yr bp the maximum forest distribution lasted until ca 6000 cal yr bp marked climatic deteriorations caused lowering of the forest lines around 4600 and 3000 cal yr bp reconstruction of the summer temperature indicated mean july temperatures at 400 m a s l of 1 5 3 c above the present during the period of maximum forest expansion whereas 3 c above the present temperature at 548 m a s l this is in accordance with other regional temperature reconstructions from northern europe copyright 2008 john wiley sons ltd

sand mass accumulation rate as a proxy for wind regimes in the sw barents sea during the past 3 ka aeolian sand mass accumulation rates were determined in a coastal mire complex on the north norwegian island soroya this measure of aeolian activity is used as a proxy for past storm intensity in the sw barents sea region at least until the seventeenth century when human activities began to obscure the natural signal aeolian erosion started to increase in the sixth century and consistently high values are recorded from the eleventh century onwards periods of relatively increased aeolian activity can be seen 1200 800 bc 550 150 bc ad 500 850 ad 1000 1400 and ad 1700 2000 increased aeolian activity here indicates a higher frequency of east tracking storm systems at 70 n potential causes for such conditions are increased advection of atlantic water and intensification and or northward displacement of the main winter storm tracks normally associated with moist westerlies and positive nao conditions other noteworthy climatic events are 1 a short but severe wet spell is recorded in the late tenth

tropical forcing of circumpolar deep water inflow and outlet glacier thinning in the amundsen sea embayment west antarctica outlet glaciers draining the antarctic ice sheet into the amundsen sea embayment ase have accelerated in recent decades most likely as a result of increased melting of their ice shelf termini by warm circumpolar deep water cdw an ocean model forced with climate reanalysis data shows that beginning in the early 1990s an increase in westerly wind stress near the continental shelf edge drove an increase in cdw inflow onto the shelf the change in local wind stress occurred predominantly in fall and early winter associated with anomalous high sea level pressure slp to the north of the ase and an increase in sea surface temperature sst in the central tropical pacific the slp change is associated with geopotential height anomalies in the middle and upper troposphere characteristic of a stationary rossby wave response to tropical sst forcing rather than with changes in the zonally symmetric circulation tropical pacific warming similar to that of the 1990s occurred in the 1940s and thus is a candidate for initiating the current period of ase glacier retreat

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

the effect of basal channels on oceanic ice shelf melting 1 the presence of ice shelf basal channels has been noted in a number of antarctic and greenland ice shelves but their impact on basal melting is not fully understood here we use the massachusetts institute of technology general circulation model to investigate the effect of ice shelf basal channels on oceanic melt rate for an idealized ice shelf resembling the floating tongue of petermann glacier in greenland the introduction of basal channels prevents the formation of a single geostrophically balanced boundary current instead the flow is diverted up the right hand coriolis favored side of each channel with a return flow in the opposite direction on the left hand side as the prescribed number of basal channels is increased the mean basal melt rate decreases in agreement with previous studies for a small number of relatively wide channels the subice flow is found to be a largely geostrophic horizontal circulation the reduction in melt rate is then caused by an increase in the relative contribution of weakly melting channel crests and keels for a larger number of relatively narrow channels the subice flow changes to a vertical overturning circulation this change in circulation results in a weaker sensitivity of melt rates to channel size the transition between the two regimes is governed by the rossby radius of deformation our results explain why basal channels play an important role in regulating basal melting increasing the stability of ice shelves

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

impacts of warm water on antarctic ice shelf stability through basal channel formation the antarctic ice sheet is buttressed by floating ice shelves remote sensing data show extensive basal channels particularly in west antarctica which grow quickly in response to warm water intrusion antarctica s ice shelves provide resistance to the flow of grounded ice towards the ocean if this resistance is decreased as a result of ice shelf thinning or disintegration1 acceleration of grounded ice can occur increasing rates of sea level rise loss of ice shelf mass is accelerating especially in west antarctica where warm seawater is reaching ocean cavities beneath ice shelves2 here we use satellite imagery airborne ice penetrating radar and satellite laser altimetry spanning the period from 2002 to 2014 to map extensive basal channels in the ice shelves surrounding antarctica the highest density of basal channels is found in west antarctic ice shelves within the channels warm water flows northwards eroding the ice shelf base and driving channel evolution on annual to decadal timescales our observations show that basal channels are associated with the development of new zones of crevassing suggesting that these channels may cause ice fracture we conclude that basal channels can form and grow quickly as a result of warm ocean water intrusion and that they can structurally weaken ice shelves potentially leading to rapid ice shelf loss in some areas

oceanographic observations at the shelf break of the amundsen sea antarctica the part of the west antarctic ice sheet that drains into the amundsen sea is currently thinning at such a rate that it contributes nearly 10 of the observed rise in global mean sea level acceleration of the outlet glaciers appears to be caused by thinning at their downstream ends where the ice goes afloat indicating that the changes are probably being forced from the ocean observations made since the mid 1990s on the amundsen sea continental shelf have revealed that the deep troughs carved by previous glacial advances are flooded by almost unmodified circumpolar deep water cdw with temperatures around 3 4 c above the freezing point and that this water mass drives rapid melting of the floating ice here we report observations of water properties and currents made in the region where one of those troughs reaches the continental shelf edge we estimate the absolute circulation within the trough from a combination of detided acoustic doppler current profiler data and geostrophic shear derived from conductivity temperature depth sections the shelf edge region is characterised by a landward deepening of the pycnocline separating cdw from the overlying colder and fresher surface waters this feature the so called antarctic slope front asf is almost circumpolar in extent and is typically a full depth feature the pycnocline intersecting the seabed over the upper continental slope however the asf is weaker in the amundsen sea where it is rarely a full depth feature geostrophic shear associated with the asf leads to a weakening of the associated westward current with depth and an eastward undercurrent of varying strength has been reported at other locations at the time of our amundsen sea observations the westward surface flow was weak giving rise to a strong eastward undercurrent flowing along the continental shelf edge and upper slope at the upstream western side of the trough the undercurrent turns south driving a net on shelf flow of cdw in the western part of the trough and leaving a weakened shelf edge flow that re establishes itself on the downstream eastern side of the trough an analogous feature was captured albeit crudely in an earlier coarse resolution model of the circulation on the amundsen sea shelf and variability in its strength associated with variability in the surface wind stress was the main cause of variations in the heat content of the waters on the inner continental shelf our observations thus lend support to the earlier hypothesis that changes in atmospheric forcing over the continental shelf edge could be the ultimate driver of changes in the west antarctic ice sheet outlet glaciers that drain into the amundsen sea

the international bathymetric chart of the southern ocean ibcso version 1 0 a new bathymetric compilation covering circum antarctic waters the international bathymetric chart of the southern ocean ibcso version 1 0 is a new digital bathymetric model dbm portraying the seafloor of the circum antarctic waters south of 60 degrees s

oceanic controls on the mass balance of wilkins ice shelf antarctica 0 8 m a 1 driven by a mean basal melt rate of wb 1 3 0 4 m a 1 interannual variability was large associated with changes in both surface mass accumulation and wb basal melt rate declined significantly around 2000 from 1 8 0 4 m a 1 for 1992 2000 to 0 75 0 55 m a 1 for 2001 2008 the latter value corresponding to approximately steady state ice shelf mass observations of ocean temperature t obtained during 2007 2009 by instrumented seals reveal a cold deep halo of winter water ww t 1 6 c surrounding wis the base of the ww in the halo is 170 m approximately the mean ice draft for wis we hypothesize that the transition in wb in 2000 was caused by a small perturbation 10 20 m in the relative depths of the ice base and the bottom of the ww layer in the halo we conclude that basal melting of thin ice shelves like wis is very sensitive to upper ocean and coastal processes that act on shorter time and space scales than those affecting basal melting of thicker west antarctic ice shelves such as george vi and pine island glacier

modeling ice shelf cavities in a z coordinate ocean general circulation model 1 processes at the ice shelf ocean interface and in particular in ice shelf cavities around antarctica have an observable effect on the solutions of basin scale to global coupled ice ocean models despite this these processes are not routinely represented in global ocean and climate models it is shown that a new ice shelf cavity model for z coordinate models can reproduce results from an intercomparison project of earlier approaches with vertical or isopycnic coordinates as a proof of concept ice shelves are incorporated in a 100 year global integration of a z coordinate model in this simulation glacial meltwater can be traced as far as north as 15 s the observed effects of processes in the ice shelf cavities agree with previous results from a coordinate model notably the increase in sea ice thickness however melt rates are overestimated probably because the parameterization of basal melting does not suit the low resolution of this configuration

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

getz ice shelf melting response to changes in ocean forcing the large and complex getz ice shelf extends along nearly half of the west antarctic coastline in the amundsen sea and is exposed to a more variable ocean environment than most other pacific sector ice shelves ocean temperature salinity and dissolved oxygen profiles acquired near its sub ice cavity openings are used here to estimate seawater transports and meltwater fractions more complete coverage during 2000 and 2007 brackets most of the variability observed from 1994 to 2011 and yearlong records near one ice front support the use of summer profiles to determine annual basal melt rates we find area average rates of 1 1 and 4 1 m yr higher in 2007 when a larger volume of warmer deep water occupied the adjacent continental shelf and the ocean circulation was stronger results are consistent with changes in thermocline depths relative to ice shelf draft and mass transports onto the adjacent continental shelf we also calculate steady state and actual melting of 2 5 and 4 6 m yr in 2007 2008 from satellite measurements of ice flux modeled accumulation and thinning from 2003 to 2008 this implies a positive mass balance in 2000 but negative in 2007 when the getz was producing more meltwater than any of the larger slower melting or smaller faster melting ice shelves 2013 american geophysical union all rights reserved

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

quantitative multiproxy assessment of long term patterns of holocene environmental change from a small lake near abisko northern sweden quantitative reconstructions are made of holocene changes in climatic and environmental conditions from analyses of pollen chironomids and diatoms in identical stratigraphic levels of a sediment c

stronger ocean circulation and increased melting under pine island glacier ice shelf the ice shelf buttressing antarctica s pine island glacier has been melting rapidly observations taken between 1994 and 2009 show that meltwater production has increased by about 50 since 1994 as a result of a stronger circulation below the ice shelf

climatic changes cause synchronous population dynamics and adaptive strategies among coastal hunter gatherers in holocene northern europe this dataset contains 735 radiocarbon date from coastal sites in arctic norway the dates are used for palaeodemographic modeling based on summed probability distribution methodology

modeled trends in antarctic sea ice thickness abstractunlike the rapid sea ice losses reported in the arctic satellite observations show an overall increase in antarctic sea ice concentration over recent decades however observations of decadal trends in antarctic ice thickness and hence ice volume do not currently exist in this study a model of the southern ocean and its sea ice forced by atmospheric reanalyses is used to assess 1992 2010 trends in ice thickness and volume the model successfully reproduces observations of mean ice concentration thickness and drift and decadal trends in ice concentration and drift imparting some confidence in the hindcasted trends in ice thickness the model suggests that overall antarctic sea ice volume has increased by approximately 30 km3 yr 1 0 4 yr 1 as an equal result of areal expansion 20 103 km2 yr 1 or 0 2 yr 1 and thickening 1 5 mm yr 1 or 0 2 yr 1 this ice volume increase is an order of magnitude smaller than the arctic decrease and about half the size of the increased freshwater s

calving fluxes and basal melt rates of antarctic ice shelves iceberg calving has been assumed to be the dominant cause of mass loss for the antarctic ice sheet with previous estimates of the calving flux exceeding 2 000 gigatonnes per year1 2 more recently the importance of melting by the ocean has been demonstrated close to the grounding line and near the calving front3 4 5 so far however no study has reliably quantified the calving flux and the basal mass balance the balance between accretion and ablation at the ice shelf base for the whole of antarctica the distribution of fresh water in the southern ocean and its partitioning between the liquid and solid phases is therefore poorly constrained here we estimate the mass balance components for all ice shelves in antarctica using satellite measurements of calving flux and grounding line flux modelled ice shelf snow accumulation rates6 and a regional scaling that accounts for unsurveyed areas we obtain a total calving flux of 1 321 144 gigatonnes per year and a total basal mass balance of 1 454 174 gigatonnes per year this means that about half of the ice sheet surface mass gain is lost through oceanic erosion before reaching the ice front and the calving flux is about 34 per cent less than previous estimates derived from iceberg tracking1 2 7 in addition the fraction of mass loss due to basal processes varies from about 10 to 90 per cent between ice shelves we find a significant positive correlation between basal mass loss and surface elevation change for ice shelves experiencing surface lowering8 and enhanced discharge9 we suggest that basal mass loss is a valuable metric for predicting future ice shelf vulnerability to oceanic forcing

improved representation of east antarctic surface mass balance in a regional atmospheric climate model this study evaluates the impact of a recent upgrade in the physics package of the regional atmospheric climate model racmo2 on the simulated surface mass balance smb of the antarctic ice sheet the modelled smb increases in particular over the grounded ice sheet of east antarctica 44 gt a 1 with a small change in west antarctica this mainly results from an increase in precipitation which is explained by changes in the cloud microphysics including a new parameter ization for ice cloud supersaturation and changes in large scale circulation patterns which alter topographically forced precipitation the spatial changes in smb are evaluated using 3234 in situ smb observations and ice balance velocities and the temporal variability using grace satellite retrievals the in situ observations and balance velocities show a clear improvement of the spatial representation of the smb in the interior of east antarctica which has become considerably wetter no improvements are seen for west antarctica and the coastal regions a comparison of model smb temporal variability with grace satellite retrievals shows no significant change in performance

variability of circumpolar deep water transport onto the amundsen sea continental shelf through a shelf break trough 1 the presence of warm circumpolar deep water cdw intrusions on the amundsen continental shelf has been linked to recent thinning of the outlet glaciers draining the west antarctic ice sheet into the amundsen sea inflow of the cdw onto the shelf is thought to occur within a series of troughs that intersect the continental shelf break we use observations between 1994 and 2011 and a numerical model to investigate the variability of cdw transport in a trough intersecting the shelf break at 113 w the location of the main cdw inflow into the trough varies between its eastern flank and center while the western part of the trough is filled by a recirculation that commonly entrains cooler water originating further south on the shelf thermocline depth decreases between the early and late 2000s with an indication that the depth of the 1994 thermocline was similar to the later years mooring results show that the cdw layer cools and thins in summer and thickens and warms in winter in addition to a deeper thermocline in summer we observe a stronger presence of lower cdw in the bottom of the trough heat flux onto the shelf is controlled by current velocities rather than cdw temperature and the majority of the heat is carried onto the shelf by background flow rather than episodic events

sedimentary records of holocene palaeo mudflow events in tianshui basin of the western loess plateau china abstract during our field investigations a holocene aeolian loess soil profile containing three units of palaeo mudflow deposits is identified in tianshui basin of the western loess plateau china through field observation and the analysis of particle size characteristics three palaeo mudflow deposit layers are differentiated from the interbedding aeolian loess and soil and they record three episodes of palaeo mudflow events based on stratigraphic correlation osl dates and archeological constraints the three episodes of palaeo mudflow events are estimated to occur in 7000 6000 5500 4800 and 4200 4000 yr bp respectively comparison of the ages of the three paleo mudflow events with the paleoclimatic proxy data from different regions indicates that these three episodes of palaeo mudflow events were most likely associated with the climatic instabilities and prehistorical human activities the results of our study as well as the results of other studies may provide further insights into the holocene climate change and also into the prehistoric human activities in environmentally sensitive areas

ice shelf basal channels in a coupled ice ocean model a numerical model for an interacting ice shelf and ocean is presented in which the iceshelf base exhibits a channelized morphology similar to that observed beneath petermann gletscher s greenland floating ice shelf channels are initiated by irregularities in the ice along the grounding line and then enlarged by ocean melting to a first approximation spatially variable basal melting seaward of the grounding line acts as a steel rule die or a stencil imparting a channelized form to the ice base as it passes by ocean circulation in the region of high melt is inertial in the along channel direction and geostrophically balanced in the transverse direction melt rates depend on the wavelength of imposed variations in ice thickness where it enters the shelf with shorter wavelengths reducing overall melting petermann gletscher s narrow basal channels may therefore act to preserve the ice shelf against excessive melting overall melting in the model increases for a warming of the subsurface water the same sensitivity holds for very slight cooling but for cooling of a few tenths of a degree a reorganization of the spatial pattern of melting leads surprisingly to catastrophic thinning of the ice shelf 12 km from the grounding line subglacial discharge of fresh water along the grounding line increases overall melting the eventual steady state depends on when discharge is initiated in the transient history of the ice showing that multiple steady states of the coupled system exist in general

the response of the southern ocean and antarctic sea ice to freshwater from ice shelves in an earth system model abstractthe possibility that recent antarctic sea ice expansion resulted from an increase in freshwater reaching the southern ocean is investigated here the freshwater flux from ice sheet and ice shelf mass imbalance is largely missing in models that participated in phase 5 of the coupled model intercomparison project cmip5 however on average precipitation minus evaporation p e reaching the southern ocean has increased in cmip5 models to a present value that is about greater than preindustrial times and 5 22 times larger than estimates of the mass imbalance of antarctic ice sheets and shelves 119 544 two sets of experiments were conducted from 1980 to 2013 in cesm1 cam5 one of the cmip5 models artificially distributing freshwater either at the ocean surface to mimic iceberg melt or at the ice shelf fronts at depth an anomalous reduction in vertical advection of heat into the surface mixed layer resulted in sea surface cooling at high southern latitudes and an associated increase in sea i