Subglacial lakes have long been considered hydraulically isolated water bodies underneath ice sheets. This view changed radically with the advent of repeat-pass satellite altimetry and the discovery of multiple lake discharges and water infill, associated with water transfer over distances of more than 200 km. The presence of subglacial lakes also influences ice dynamics, leading to glacier acceleration. Furthermore, subglacial melting under the Antarctic ice sheet is more widespread than previously thought, and subglacial melt rates may explain the availability for water storage in subglacial lakes and water transport. Modelling of subglacial water discharge in subglacial lakes essentially follows hydraulics of subglacial channels on a hard bed, where ice sheet surface slope is a major control on triggering subglacial lake discharge. Recent evidence also points to the development of channels in deformable sediment in West Antarctica, with significant water exchanges between till and ice. Most active lakes drain over short time scales and respond rapidly to upstream variations. Several Antarctic subglacial lakes exhibit complex interactions with the ice sheet due to water circulation. Subglacial lakes can therefore—from a modelling point of view—be seen as confined small oceans underneath an imbedded ice shelf.

The West Antarctic Ice Sheet (WAIS) is considered the major contributor to global sea level rise in the Last Interglacial (LIG) and potentially in the future. Exposed fossil reef terraces suggest sea levels in excess of 7m in the last warm era, of which probably not much more than 2 m are considered to originate from melting of the Greenland Ice Sheet. We simulate the evolution of the Antarctic Ice Sheet during the LIG with a 3-D thermomechanical ice sheet model forced by an atmosphere-ocean general circulation model (AOGCM). Our results show that high LIG sea levels cannot be reproduced with the atmosphere-ocean forcing delivered by current AOGCMs. However, when taking reconstructed Southern Ocean temperature anomalies of several degrees, sensitivity studies indicate a Southern Ocean temperature anomaly threshold for total WAIS collapse of 2-3 degrees C, accounting for a sea level rise of 3-4m during the LIG. Potential future Antarctic Ice Sheet dynamics range from a moderate retreat to a complete collapse, depending on rate and amplitude of warming.

ABSTRACTLakes beneath the Antarctic Ice Sheet are known to decrease traction at the ice base and therefore can have a great impact on ice dynamics. However, the total extent of Antarctic subglacial lakes is still unknown. We address this issue by combining modeling and remote-sensing strategies to predict potential lake locations using the general hydraulic potential equation. We are able to reproduce the majority of known lakes, as well as predict the existence of many new and so far undetected potential lakes. To validate our predictions, we analyzed ice-penetrating radar profiles from radio-echo sounding flights acquired over 1994–2013 in Dronning Maud Land, East Antarctica, and this led to the identification of 31 new subglacial lakes. Based on these findings, we estimate the total number of Antarctic subglacial lakes to be ~1300, a factor of three higher than the total number of lakes discovered to date. We estimate that only ~30% of all Antarctic subglacial lakes and ~65% of the total estimated lake-covered area have been discovered, and that lakes account for 0.6% of the Antarctic ice/bed interface.

This manuscript presents results of a simple model which uses data constraining ice sheet geometry in the region draining the West Antarctic ice sheet into Ross Ice Shelf to estimate subglacial water routing and subglacial drainage basins. The authors assume that the drainage area is a proxy of subglacial water flux by assuming that the basal melting rate is spatially uniform in the study area, in spite of prior modeling results inconsistent with such an assumption (Beem et al., 2010; Joughin et al., 2003). Finally, they use the modern rates of ice surface elevation changes to estimate evolution of subglacial water routing and subglacial drainage basins 200 yeears into the future.

A general ocean circulation model is coupled with a 3D-thermodynamical ice-sheet/shelf model to simulate the response of the Filchner-Ronne Ice Shelf (FRIS, Antarctica) and coastal parts of its catchment basin to a postulated inflow of Warm Deep Water into the ice-shelf cavity on a 1000-yr timescale. Prescribed ocean warming (based on climate projections) enters the ice-shelf cavity in the up to 1500 m deep Filchner Trough and penetrates deep into the sub-ice cavity. Increasing basal melt rates induce geometry changes of the cavity, which in turn have an impact on the ocean circulation and therefore the modelled melt rates. Highest melt rates of about 20 m yr(-1) follow the (up to 180 km) retreating grounding line. Basal mass loss reaches about 250 km(3) yr(-1), doubling the present-day value. The most vulnerable areas below the FRIS are the Bailey Ice Stream and the area between the Institute and Moeller Ice Streams, where the increased melting accounts for about 80 km of the modelled grounding line retreat on the backward sloping bedrock. The potential additional contribution to the eustatic sea level rise due to the grounded-ice loss, simulated in an ensemble approach against a transient control experiment, is about 0.05 mm yr(-1) during the first 500 yr and about 0.17 mm yr(-1) thereafter. (C) 2015 Elsevier B.V. All rights reserved.

Der Westantarktische Eisschild hat aufgrund seiner unter dem Meeresspiegel gegrundeten Basis einen besonderen Einfluss auf den globalen Meereisspiegelanstieg. Anderungen in der Ozeantemperatur oder im Zustrom warmer Wassermassen auf den Kontinentalschelf und in die Schelfeiskavernen hinein fuhren zu einem vermehrten Ausdunnen der Schelfeisgebiete, die eine ruckstauende Wirkung auf den Abfluss von Eismassen aus dem Inland ausuben. Fur viele Regionen der Westantarktis kommt hinzu, dass sich der Meeresboden landeinwarts neigt. Dies stellt eine instabile Position fur die Grundungszone der Schelfeise dar und einmal in Bewegung gebracht, fuhrt es zu einem fortschreitenden Ruckzug und einem vermehrten Eisabfluss. Simulationen mit eisdynamischen Modellen unter der Annahme einer zukunftigen Klimaerwarmung prognostizieren einen verstarkten Beitrag insbesondere der Westantarktis zum globalen Meeresspiegel. Fur die nachsten hundert bis tausend Jahre kann dieser bis zu einem halben Meter betragen. Es ist sogar mit einem teilweisen Kollaps des Westantarktischen Eisschild zu rechnen, der den globalen Meeresspiegel um mehrere Meter ansteigen lassen wurde. The West Antarctic Ice Sheet has a particular influence on global sea-level rise owing to its base being below sea level. Changes in ocean temperature or in the inflow of warm water masses onto the continental shelf and into the ice-shelf cavities lead to increased thinning of the ice shelves which exert a buttressing effect on the outflow of ice from the ice sheet. For widespread regions of West Antarctica, in addition the seabed slopes downward inland. This causes possible instabilities of the grounding-line position of the ice shelves and once set in motion, might cause an irreversible retreat of the grounding line and an increased ice loss. Simulations with thermomechanical ice-sheet models, based on future climate-change scenarios, predict an increasing contribution in particular of West Antarctica to global sea-level rise. For the next few hundred to a thousand years, this contribution can be up to half a meter. Even a partial collapse of the West Antarctic Ice Sheet is possible, which would raise the global sea level by several meters.

We use surface air temperature to evaluate the decadal forecast skill of the fully coupled Max Planck Institut Earth System Model (MPI‐ESM) initialized using only surface wind stress applied to the ocean component of the model (Modini: Model initialization by partially coupled spin‐up). Our analysis shows that the greenhouse gas forcing alone results in a significant forecast skill on the 2–5 and 6–9 year range even for uninitialized hindcasts. For the first forecast year, the forecast skill of Modini is generally comparable with previous initialization procedures applied to MPI‐ESM. But only Modini is able to generate a significant skill (correlation) in the tropical Pacific for a 2–5 year (and to a lesser extent for a 6–9 year) hindcast. Modini is also better able to capture the observed hiatus in global warming in hindcast mode than the other methods. Finally, we present forecasts for 2015 and the average of years 2016–2019 and 2020–2024, predicting an end to the hiatus.

Components of the climate system, such as ice sheets and marine sediments serve as invaluable archives, which can be tapped into, to reconstruct paleoclimate conditions. The relative abundance of hydrogen and oxygen isotopes in ice cores is a proxy for past local temperature evolution. However the translation of these proxies into temperature is not straightforward. Complex interdependencies in the climate system can hide or override the local climate signal at which the ice core was drilled. Using 3D ice sheet modelling in concert with passive tracer advection one can simulate the isotopic distribution in ice sheets and compare them to ice core data. Combining this method in a coupled climate model environment, containing atmosphere and ocean components, one can theoretically simulate the isotopic cycle from the source to the actual ice record. Such an approach would greatly support the interpretation of proxy data whilst constraining the output of 3D ice sheet models (ISMs). We present the implementation of passive tracer advection in our 3D ISM RIMBAY (Thoma et al. in Geosci Model Dev 1:1–21, 2014, Goeller et al. in Cryosphere 7:1095–1106, 2014) and asses the potential of the method to reproduce chronologies of the polar ice sheets.

Large-scale fully coupled Earth system models (ESMs) are usually applied in climate projections like the IPCC (Intergovernmental Panel on Climate Change) reports. In these models internal variability is often within the correct order of magnitude compared with the observed climate, but due to internal variability and arbitrary initial conditions they are not able to reproduce the observed timing of climate events or shifts as for instance observed in the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), or the Atlantic Meridional Overturning Circulation (AMOC). Additional information about the real climate history is necessary to constrain ESMs; not only to emulate the past climate, but also to introduce a potential forecast skill into these models through a proper initialisation. We attempt to do this by extending the fully coupled climate model Max Planck Institute Earth System Model (MPI-ESM) using a partial coupling technique (Modini-MPI-ESM). This method is implemented by adding reanalysis wind-field anomalies to the MPI-ESM's inherent climatological wind field when computing the surface wind stress that is used to drive the ocean and sea ice model. Using anomalies instead of the full wind field reduces potential model drifts, because of different mean climate states of the unconstrained MPI-ESM and the partially coupled Modini-MPI-ESM, that could arise if total observed wind stress was used. We apply two different reanalysis wind products (National Centers for Environmental Prediction, Climate Forecast System Reanalysis (NCEPcsfr) and ERA-Interim reanalysis (ERAI)) and analyse the skill of Modini-MPI-ESM with respect to several observed oceanic, atmospheric, and sea ice indices. We demonstrate that Modini-MPI-ESM has a significant skill over the time period 1980–2013 in reproducing historical climate fluctuations, indicating the potential of the method for initialising seasonal to decadal forecasts. Additionally, our comparison of the results achieved with the two reanalysis wind products NCEPcsfr and ERAI indicates that in general applying NCEPcsfr results in a better reconstruction of climate variability since 1980.

Interactions between Antarctic ice shelf regions and the ocean and their contribution to oceanic water mass formation: Ice shelves represent the main areas of ice discharge from the Antarctic Ice Sheet into the Southern Ocean. At the transition between ice and ocean, ice shelves affect both, the mass balance and dynamics of the ice sheet and the water mass formation in the Southern Ocean by ice shelf melting and calving at the shelf front, respectively. Under constant boundary conditions (mass flux from the ice sheet, ice accumulation and melting) a balance between mass gain and mass loss exists, and the ice shelves, except for recurring calving events, establish an almost invariant shape. In addition, ice shelves exert a buttressing force on the ice sheet and, therefore, are a regulating factor of ice discharge. Under changing climatic conditions, manifested for example in a changing oceanic circulation and temperature, this equilibrium state might be disturbed. In a model study with a coupled ocean - ice shelf - ice sheet model, the influence of changing climate boundary conditions on the mass loss in the Atlantic sector of the Antarctic Ice Sheet, the water mass formation in the Weddell Sea, and the sea level is investigated. Starting from scenario runs of the Intergovernmental Panel on Climate Change (IPCC) Assessment Report IV (AR4), which describe the climate development to the year 2100, the dynamic coupling of ocean circulation and ice sheet dynamics predict a significant mass loss for the Filchner-Ronne Ice Shelf and in particular a retreat of its southern grounding line. This mass loss is equivalent to a doubling of today’s observed losses of glaciers and ice caps and sets the previously assumed stability of the East Antarctic Ice Sheet to climate change into question.

We present first results from a coupled model setup, consisting of the state-of-the-art ice sheet model RIMBAY (Revised Ice Model Based on frAnk pattYn), and the community earth system model COSMOS. We show that special care has to be provided in order to ensure physical distributions of the forcings as well as numeric stability of the involved models. We demonstrate that a suitable statistical downscaling is crucial for ice sheet stability, especially for southern Greenland where surface temperatures are close to the melting point. The downscaling of net snow accumulation is based on an empirical relationship between surface slope and rainfall. The simulated ice sheet does not show dramatic loss of ice volume for pre-industrial conditions and is comparable with present-day ice orography. A sensitivity study with high CO2 level is used to demonstrate the effects of dynamic ice sheets onto climate compared to the standard setup with prescribed ice sheets.

The mass export of the West Antarctic Ice Sheet (WAIS) is dominated by fast flowing ice streams which transport ice from the interior of the ice sheet towards its coast lines with velocities of several hundred meters per year. Understanding their dynamics is considered as a key to estimate the contributions of the WAIS to global sea level rise. This study focuses on the Ross Ice Streams (RIS) at the Siple Coast where observations reveal a high variability of ice stream pathways and velocities in the past. A widely spread and meters thick basal layer of unconsolidated sediments beneath the ice sheet creates the precondition for high basal sliding rates by sediment deformation. However, the exact locations of the RIS are determined by the pathways of basal melt water flow. We compute the subglacial water flow paths for the present-day ice sheet geometry with a balance flux approach and find high correlations between areas of enhanced subglacial water flow and the locations of the RIS. Moreover, the ice flow velocities of the particular ice streams are found to be correlated with the sizes of the water catchment areas draining underneath. For projections we apply surface elevation change rates observed by ICESat and CryoSat-2 to the present-day ice sheet geometry for 200 years and thus estimate the evolution of basal water pathways and catchment areas at the Siple Coast. The results of the simulations using the elevation change rates derived by the particular satellite campaigns show a high consistency. According to them, a major hydraulic tributary of the Kamb and Whillans Ice Stream (KIS and WIS) will be redirected underneath the Bindschadler Ice Stream (BIS) within the next 200 years. The water catchment area feeding underneath the BIS is estimated to grow by about 50% while the lower part of the stagnated KIS becomes increasingly separated from the upper hydraulic tributaries of the Siple Coast. This might be a continuation of the subglacial hydraulic processes which caused the past stagnation of the KIS and could also explain the observed deceleration of the WIS. Furthermore, this might also lead to a future increase of the ice velocities within the BIS and an increased ice drainage of the corresponding hinterland.

The ice flow at the margins of the Antarctic Ice Sheet (AIS) is moderated by large ice shelves. Their buttressing effect substantially controls the mass balance of the AIS and thus its contribution to sea level rise. Recent results of ocean circulation models indicate that warm circumpolar water of the Southern Ocean may override the continental slope front and boost basal ice shelf melting. In particular, simulations demonstrate the redirection of a warm coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf (FRIS) within the next decades. The increase of water temperature in the sub-shelf cavity is estimated to dramatically raise the basal shelf melting. Coupled simulations with a finite elements ocean model and a three-dimensional thermomechanical ice flow model reveal that the consequent thinning of the FRIS would lead to an extensive grounding line retreat associated with a vast mass loss of the AIS. In this subsequent study, we aim for an enhanced understanding of the complex feedbacks between ocean circulation and ice dynamics of the grounded AIS. Therefor, we focus on the ice streams which are draining into the FRIS and dominating the mass transport from grounded to floating ice. For a better representation of these fast-flowing ice streams we expand the above ice flow model by the incorporation of local processes at the ice base. There, sediment deformation and lubrication by subglacial hydrology locally allow high basal sliding rates and thus create the precondition for the development of ice streams. Based on satellite-observed ice surface velocity patterns we identify such areas with low basal drag and parametrize the ice flow model accordingly. As a result, the modeled ice flow patterns will depict velocity and locations of observed ice streams in the catchment of the FRIS more realistically. We present first results of this advanced ice-flow modeling approach, anticipating an even larger response of the AIS on increased sub-shelf melting rates in future coupled simulations.

Glaciers and ice caps exhibit currently the largest cryospheric contributions to sea level rise. Modelling the dynamics and mass balance of the major ice sheets is therefore an important issue to investigate the current state and the future response of the cryosphere in response to changing environmental conditions, namely global warming. This requires a powerful, easy-to-use, versatile multi-approximation ice dynamics model. Based on the well-known and established ice sheet model of Pattyn (2003) we develop the modular multi-approximation thermomechanic ice model RIMBAY, in which we improve the original version in several aspects like a shallow ice–shallow shelf coupler and a full 3D-grounding-line migration scheme based on Schoof's (2007) heuristic analytical approach. We summarise the full Stokes equations and several approximations implemented within this model and we describe the different numerical discretisations. The results are cross-validated against previous publications dealing with ice modelling, and some additional artificial set-ups demonstrate the robustness of the different solvers and their internal coupling. RIMBAY is designed for an easy adaption to new scientific issues. Hence, we demonstrate in very different set-ups the applicability and functionality of RIMBAY in Earth system science in general and ice modelling in particular.

We apply a coupled thermomechanical ice sheet—self-gravitating viscoelastic solid Earth model (SGVEM), allowing for the dynamic exchange of ice thickness and bedrock deformation, in order to investigate the effect of viscoelastic deformation on ice dynamics and vice versa. In a synthetic glaciation scenario, we investigate the interaction between the ice sheet and the solid Earth deformation, the glacial-isostatic adjustment (GIA), accounting for an atmospheric forcing depending on the ice sheet surface altitude. We compare the results from the coupled model to runs with the common elastic lithosphere/relaxing asthenosphere (ELRA) model, where the lithosphere is represented by a thin plate and the mantle relaxes with one characteristic relaxation time, as well as to a rigid Earth without any deformation. We find that the deformational behaviour of the SGVEM on ice dynamics (i.e. stored ice volume, ice thickness and velocity field) is comparable to the ELRA for an optimal choice of the parameters in steady state, but exhibits differences in the transient behaviour. Beyond the ice sheet, in the region of peripheral forebulge, the differences in the transient surface deformation between ELRA and SGVEM are substantial, demonstrating the inadequacy of the ELRA model for interpreting constraints on GIA in the periphery of the ice sheet, such as sea-level indicators and GPS uplift rates.

Predictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (<500m), or subgrid interpolation of the grounding line. The latter still requires nominal grid sizes of <5 km. For larger grid spacings, appropriate parameterizations for ice flux may be imposed at the grounding line, but the short-time transient behaviour is then incorrect and different from models that do not incorporate grounding-line parameterizations. The numerical error associated with predicting grounding-line motion can be reduced significantly below the errors associated with parameter ignorance and uncertainties in future scenarios.