Displays

Sea-level rise is among the greatest risks that arise from anthropogenic global climate change. It is receiving a lot of attention, among others in the IPCC reports, but major questions remain as to the potential contribution from the great continental ice sheets. In recent years, some modelling work has suggested that the ice-component of sea-level rise may be much faster than previously thought, but the rapidity of rise seen in these results depends on inclusion of scientifically debated mechanisms of ice-shelf decay and associated ice-sheet instability. The processes have not been active during historical times, so data are needed from previous warm periods to evaluate whether the suggested rates of sea-level rise are supported by observations or not. Also, we then need to assess which of the ice sheets was most sensitive, and why. The last interglacial (LIG; ~130,000 to ~118,000 years ago, ka) was the last time global sea level rose well above its present level, reaching a highstand of +6 to +9 m or more. Because Greenland Ice Sheet (GrIS) contributions were smaller than that, this implies substantial Antarctic Ice Sheet (AIS) contributions. However, this still leaves the timings, magnitudes, and drivers of GrIS and AIS reductions open to debate. I will discuss recently published sea-level reconstructions for the LIG highstand, which reveal that AIS and GrIS contributions were distinctly asynchronous, and that rates of rise to values above 0 m (present-day sea level) reached up to 3.5 m per century. Such high pre-anthropogenic rates of sea-level rise lend credibility to high rates inferred by ice modelling under certain ice-shelf instability parameterisations, for both the past and future. Climate forcing was distinctly asynchronous between the southern and northern hemispheres as well during the LIG, explaining the asynchronous sea-level contributions from AIS and GrIS. Today, climate forcing is synchronous between the two hemispheres, and also faster and greater than during the LIG. Therefore, LIG rates of sea-level rise should likely be considered minimum estimates for the future.

How to cite: Rohling, E. and Hibbert, F.: Asynchronous Antarctic and Greenland ice-volume contributions to the last interglacial sea-level highstand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1513, https://doi.org/10.5194/egusphere-egu2020-1513, 2019

The East Antarctic Ice Sheet (EAIS) is the single largest potential contributor to future global mean sea level rise, containing a water mass equivalent of 53 m. Recent work has found the overall mass balance of the EAIS to be approximately in equilibrium, albeit with large uncertainties. However, changes in oceanic conditions have the potential to upset this balance. This could happen by both a general warming of the ocean and also by shifts in oceanic conditions allowing warmer water masses to intrude into ice shelf cavities.

We use the Úa numerical ice-flow model, combined with ocean-melt rates parameterized by the PICO box mode, to predict the future contribution to global-mean sea level of the EAIS. Results are shown for the next 100 years under a range of emission scenarios and oceanic conditions on a region by region basis, as well as for the whole of the EAIS. 

How to cite: Jordan, J., Gudmundsson, H., Jenkins, A., Stokes, C., Jamieson, S., and Miles, B.: The contribution of the East Antarctic Ice Sheet to future sea level rise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3099, https://doi.org/10.5194/egusphere-egu2020-3099, 2020

Projections of the Antarctic Ice Sheet (AIS) contribution to future global sea-level rise are highly uncertain, partly due to the potential threat of a collapse of the marine sectors of the AIS. However, whether the inherent instability of such sectors is already underway or is still far away from being triggered remains elusive. One reason for ambiguity in results relies on the uncertainty of basal conditions. Whereas high basal friction can potentially prevent a collapse of the marine zones of the AIS, low basal friction can promote such a process. In addition, future sea-level projections from the AIS are generally run from an equilibrated present-day (PD) state tuned to observational data. However, this procedure neglects the thermal memory of the ice sheet. Furthermore, there is no apparent reason for ruling out that the PD may be subject to a natural drift since the onset of the last deglaciation (~20 kyr BP). Here we study the uncertainty in sea-level projections by investigating the response of the AIS to different RCP scenarios for four different basal-dragging laws. For this purpose we use a three-dimensional ice-sheet-shelf model that is spun up from a deglaciation. Model parameters of all friction laws have been optimized to simulate a realistic PD. In addition, we study the response of the AIS to a sudden CO₂ drop to investigate the potential irreversibility of the ice sheet depending on the RCP scenario and friction law.

How to cite: Blasco, J., Tabone, I., Moreno, D., Alvarez-Solas, J., Robinson, A., and Montoya, M.: The uncertainty in Antarctic sea-level rise projections due to ice dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14352, https://doi.org/10.5194/egusphere-egu2020-14352, 2020

Mass loss from the Antarctic Ice Sheet is the main source of uncertainty in projections of future sea-level rise, with important implications for coastal regions worldwide. Enhanced melt beneath ice shelves could destabilise large parts of the ice sheet, and further increase ice loss. Despite advances in our understanding of feedbacks in the ice sheet-ice shelf-ocean system, future projections of ice loss remain poorly constrained in many parts of Antarctica. In particular, there is ongoing debate surrounding the future of the Filchner-Ronne Ice Shelf (FRIS) region. The FRIS has remained relatively unchanged in recent decades, but an increase in air and ocean temperatures in the neighbouring Weddell Sea, could force rapid retreat in the near future. Indeed, previous modelling work has suggested the potential for widespread infiltration of warm water beneath the ice shelf in the second half of the twenty-first century, leading to a drastic increase in basal melting.

Here, we use the ice flow model Úa alongside the ocean box model PICO (Potsdam Ice-shelf Cavity mOdel) to understand the key physical processes and model variability in future projections of sea level rise from the FRIS region. We investigate uncertain model parameters associated with ice dynamics, surface melting and precipitation, ocean temperature forcing, and parameters relating to the strength of basal melt generated by PICO. We optimise the prior distributions of parameters in PICO using observations and a Bayesian approach, leading to improved posterior distributions for use in the following stages of uncertainty quantification. We then run our forward model through the 21^st century for various RCP scenarios and extensive random sampling of uncertain parameters to train an emulator. From this, we present probabilistic projections of potential sea level rise from the FRIS region for different future climate change scenarios, together with a sensitivity analysis to identify the most important parameters that contribute to uncertainty in these projections.

How to cite: Hill, E., Rosier, S., Gudmundsson, H., and Collins, M.: Quantifying uncertainty in future projections of ice loss from the Filchner-Ronne Ice Shelf System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16045, https://doi.org/10.5194/egusphere-egu2020-16045, 2020

The Totten Glacier, located in the Aurora Subglacial Basin of East Antarctica, drains a catchment containing approximately 3.5 m of global sea level rise equivalent ice mass. The This glacier has been losing mass over recent decades, and modelling studies indicate that it is the most vulnerable glacier in East Antarctica to warming oceans and atmosphere over the coming century. Satellite altimetry shows high internal variability in ocean-forced melting of the Totten Ice Shelf; however, the extent to which this variability signal impacts the upstream ice sheet dynamics, and therefore its mass balance, is unknown. Here we use the Ice Sheet System Model (ISSM) combined with a plume and basal melting parameterisation called PICOP to investigate the impact of variability in ocean temperature on the evolution of Totten Glacier. We find that the southernmost portion of the Totten Glacier grounding line - from which the majority of the catchment’s ice is channeled - is stable within only a limited range of background ocean temperatures close to present-day values. In the stable simulations, the magnitude of the ice mass flux depends on the extent to which the ice shelf is pinned on a bed topography rumple located approximately 10 km downstream of its grounding line, but the period of the mass flux is decadal to multi-decadal in each simulation, irrespective of the magnitude of the variability in ocean forcing. We further find that the impact of variability in ocean melt rates decreases as the mean background ocean temperature increases, suggesting that the mean state may have a relatively more important role in the evolution of the Totten Glacier than variability in ocean forcing. Our results have implications for detection and attribution of climate change and internal climate variability in modeling studies, and may inform fieldwork campaigns mapping bed topography in the Aurora Subglacial Basin.

How to cite: McCormack, F., Morlighem, M., Gwyther, D., Roberts, J., and Pelle, T.: The impact of internal variability in ocean-induced melting on Totten Glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4196, https://doi.org/10.5194/egusphere-egu2020-4196, 2020

The distribution of relict moraines in the Transantarctic Mountains affords geologic constraint of past ice-marginal positions of the East Antarctic Ice Sheet (EAIS). We describe the directly dated glacial-geologic record from Roberts Massif, an ice-free area in the central Transantarctic Mountains, to provide a comprehensive record of ice sheet change at this site since the Miocene and to capture ice sheet response to warmer-than-present climate conditions. The record is constrained by cosmogenic ³He, ¹⁰Be, ²¹Ne, and ²⁶Al surface-exposure ages from > 160 dolerite and sandstone erratics on well-preserved moraines and drift units. Our data set indicates that a cold-based EAIS was present, and similar to its current configuration, for long periods over the last ~14.5 Myr, including the mid-Miocene, Late Pliocene, and early-to-mid Pleistocene, with moraine ages increasing with distance from and elevation above the modern ice margin. We also report extremely low erosion rates over the duration of our record, reflecting long-term polar desert conditions at Roberts Massif. The age-elevation distribution of moraines at Roberts Massif is consistent with a persistent EAIS extent during glacial maxima, accompanied by slow, isostatic uplift of the massif due to subglacial erosion. Although our data are not a direct measure of ice volume, the Roberts Massif glacial record indicates that the EAIS was present and of similar extent to today during periods when global temperature was believed to be warmer and/or atmospheric CO₂ concentrations were likely higher than today.

How to cite: Bromley, G., Balter, A., Balco, G., and Jackson, M.: A 14.5 million-year geologic record of East Antarctic Ice Sheet fluctuations in the central Transantarctic Mountains, constrained with multiple cosmogenic nuclides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19999, https://doi.org/10.5194/egusphere-egu2020-19999, 2020

The future response of the Antarctic ice sheet to rising temperatures remains highly uncertain. A useful period for assessing the sensitivity of Antarctica to warming is the Last Interglacial (LIG; 129-116 kyr), which experienced warmer polar temperatures and higher global mean sea level (GMSL +6 to 9 m) relative to present day. LIG sea level cannot be fully explained by Greenland Ice Sheet melt (~2 m), ocean thermal expansion and melting mountain glaciers (~1 m), suggesting substantial Antarctic mass loss was initiated by warming of Southern Ocean waters, resulting from a weakening Atlantic Meridional Overturning Circulation in response to North Atlantic surface freshening.  Here we report a blue-ice record of ice-sheet and environmental change from the Weddell Sea Embayment at the periphery of the marine-based West Antarctic Ice Sheet (WAIS) which is underlain by major methane hydrate reserves. Constrained by a widespread volcanic horizon and supported by ancient microbial DNA analyses, we provide the first evidence for substantial mass loss across the Weddell Sea Embayment during the Last Interglacial, most likely driven by ocean warming and associated with destabilization of sub-glacial hydrates. Ice-sheet modelling supports this interpretation and suggests that millennial-scale warming of the Southern Ocean could have triggered a multi-meter rise in global sea levels. Our data indicate that Antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice-climate feedbacks that further amplify warming.

How to cite: Turney, C., Fogwill, C., and Golledge, N. and the The AntarcticScience.com Team: Early Last Interglacial ocean warming drove substantial ice mass loss from Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12771, https://doi.org/10.5194/egusphere-egu2020-12771, 2020

Containing ~52 m sea level rise equivalent ice mass (SLRe), the East Antarctic Ice Sheet (EAIS) is a major component of the global sea level budget; yet, uncertainty remains in how this ice sheet will respond to enhanced atmospheric and oceanic thermal forcing through the turn of the century. To address this uncertainty, we model the most dynamic catchments of EAIS out to 2100 using the Ice Sheet System Model. We employ three basal melt rate parameterizations to resolve ice-ocean interactions and force our model with anomalies in both surface mass balance and ocean thermal forcing from both CMIP5 and CMIP6 model output. We find that this sector of EAIS gains approximately 10 mm SLRe by 2100 under high emission scenarios (RCP8.5 and SSP585), and loses mass under low emission scenarios (RCP2.6). All basins within the domain either gain mass or are in near mass balance through the 86-year experimental period, except the Aurora Subglacial Basin. The primary region of mass loss in this basin is located within 50 km upstream of Totten Glacier’s grounding line, which loses up to 6 mm SLRe by 2100. Glacial discharge from Totten is modulated by buttress supplied by a 10 km ice plain, located along the southern-most end of Totten’s grounding line. This ice plain is sensitive to brief changes in ocean temperature and once ungrounded, glacial discharge from Totten accelerates by up to 70% of it present day configuration. In all, we present plausible bounds on the contribution of a large sector of EAIS to global sea level rise out to the end of the century and target Totten as the most vulnerable glacier in this region. In doing so, we reduce uncertainty in century-scale global sea level projections and help steer scientific focus to the most dynamic regions of EAIS.

How to cite: Pelle, T., Morlighem, M., and S. McCormack, F.: Aurora Basin, the weak underbelly of East Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2456, https://doi.org/10.5194/egusphere-egu2020-2456, 2020

The fast ice in the McMurdo Sound plays an important role in the coastal ecological systems and climate changes, but its seasonal and interannual variations are poorly understood. In this study, the fast ice phenology and extent variation are investigated using Sentinel-1 Synthetic Aperture Radar (SAR) images from 2017 to 2019, and the factors controlling the fast ice development are explored. The results showed that the fast ice edge presented obvious seasonal change. In 2017/2018 and 2018/2019 years it arrived at northernmost during May – July, and keeps north until the end of December or January, and then moves south, arriving at most south on February or March. However, there are some difference between these two years. The date the fast ice edge arrived at northernmost in 2018 was about two months later than in 2017, but the ending time at the northern edge was about one month earlier (31 Dec 2018 vs 30 Jan 2018). The time when it retreated to the southernmost in 2019 was about one month before that in 2017 or 2018. It seems the longer the edge stays in the northernmost, the later it retreats to the southernmost, and it may not completely disappear; the shorter the edge stays in the northernmost, the earlier it retreats to the southernmost, and it may completely disappear. The dominant factor controlling the beginning and end dates are air temperature. This statement still needs to be confirmed when more data will be processed and analyzed in near future.

How to cite: Dai, L.:  Seasonal variability of fast ice edge in the McMurdo Sound between 2017 and 2019 based on Sentinel-1 SAR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2857, https://doi.org/10.5194/egusphere-egu2020-2857, 2020

One of the largest uncertainties in projecting future global mean sea level (GSML) rise in response to anthropogenic global warming originates from the Antarctic ice sheet (AIS) contribution. Previous studies suggested that a potential AIS collapse due to the Marine Ice Sheet Instability (MISI) and Marine Ice Cliff Instability (MICI) may contribute up to 1m GMSL rise by the year 2100. However, these estimates were based on uncoupled ice sheet models that do not capture interactions between the AIS and the ocean and atmosphere. Here, we explore future GMSL projections using a three-dimensional coupled climate-ice sheet model (LOVECLIP) that simulates ice sheet dynamics in both hemispheres. The model was forced by increasing CO₂ concentrations following the Shared Socioeconomic Pathway (SSP) 1-1.9, 2-4.5 and 5-8.5 scenarios. Over the next 80 years, the corresponding GMSL contribution from AIS amounts to about 2cm, 8cm and 11cm, respectively. Additional sensitivity experiments show that AIS meltwater flux in response to the SSP 5-8.5 CO₂ concentrations causes subsurface Southern Ocean warming which leads to an additional 20% AIS melting and a reduction in Southern Hemispheric future warming.

How to cite: Park, J.-Y., Schloesser, F., Timmermann, A., Choudhury, D., Lee, J.-Y., Nellikkattil, A. B., and Pollard, D.: Estimating Antarctic Ice Sheet Contributions to Future Sea Level Rise Using a Coupled Climate-Ice Sheet Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6572, https://doi.org/10.5194/egusphere-egu2020-6572, 2020

Obliquity pacing of Antarctic glaciations during the Quaternary

The frequency of Antarctic glaciations during the Quaternary are not well understood. Benthic oxygen isotope records provide evidence for eccentricity paced global ice volume changes since c. 800 000 years and the ice core records (such as EPICA) also appear to have 100 000 year cycles over the last 800 000 years. However, the benthic oxygen isotope records are a global average – not an Antarctic record. Quaternary, sedimentary records proximal to the ice margin (such as the ANDRILL AND-1B record) are needed to understand better the recent glacial history of Antarctica. 

Here we present results from the 6.21 m long, NBP03-01A-20PCA sedimentary record which was recovered from the outer continental margin of the Ross Embayment.

Sediments comprise mud with numerous clasts and paleomagnetic analyses revealed magnetic reversals at 4.21 m, 5.74 m, and 5.85 m depth. These reversals are correlated with C1n-C1r.1r-C1r.1n-C1r.2r geomagnetic reversals which have corresponding ages of 773 ka, 990 ka, and 1070 ka. 

Time series analysis of continuous Anhysteretic Remanent Magnetisation (ARM) data, which are controlled primarily by the concentration of magnetic minerals, revealed strong obliquity paced cycles between c. 800 ka and 350 ka. The presence of obliquity cycles prompted us to carry out core scanning XRF and grain size analyses. The archive half was scanned in a itrax XRF core scanner at the Marine and Geology Repository at Oregon State University and high density grain size analyses were conducted at the University of Otago. 

We identified obliquity paced cycles in the titanium elemental data over the same period which we suggest represent variations in the terrigenous material in the core. Weaker obliquity cycles are also present in the >2mm grain size fraction which we suggest is controlled by the proximity of the ice shelf front. 

We suggest that the presence of obliquity paced cycles in our data series indicate that the Ross Ice Shelf calving line advance and retreat cycles were paced with obliquity until at least 350 ka and that the mid-Pleistocene transition occurred later in the Southern Hemisphere than in the North. 

How to cite: Ohneiser, C., Beltran, C., Hulbe, C., Moy, C., Riesselman, C., Worthington, R., and Condon, D.: Obliquity pacing of Antarctic glaciations during the Quaternary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11191, https://doi.org/10.5194/egusphere-egu2020-11191, 2020

Over the last 5 Million years, outstanding warm interglacial periods (i.e. ‘super-interglacials’) occurred under low atmospheric CO₂ levels that may feature extensive Antarctica ice sheet collapse. Here, we focus on the extreme super-interglacial known as Marine Isotope Stage 31 (MIS31) that took place 1.072 million years ago and is the subject of intense debate.

Our Southern Ocean organic biomarker based paleotemperature reconstructions show that the surface ocean was warmer by ~5 °C than today between 50 °S and the Antarctic ice margin. We used these ocean temperature records to constrain the climate and ice sheet simulations to explore the impact of ocean warming on the Antarctic ice sheets. Our results show that low amplitude short term oceanic modifications drove the collapse of the West Antarctic Ice Sheet (WAIS) and deflation of sectors of the East Antarctic Ice Sheet (EAIS) resulting in sustained sea-level rise of centimeters to decimeters per decade.

We suggest the WAIS retreated because of anomalously high Southern Hemisphere insolation combined with the intrusion of Circumpolar Deep Water onto the continental shelf under poleward-intensified winds leading to a shorter sea ice season and ocean warming at the continental margin. Under this scenario, the extreme warming we observe likely reflects the extensively modified oceanic and hydrological circulation patterns following ice sheet collapse. Our work highlights the sensitivity of the Antarctic ice sheets to relatively minor oceanic and/or atmospheric perturbations that could be at play in the near future.

How to cite: Beltran, C., Golledge, N. R., Ohneiser, C., Kowalewski, D. E., Sicre, M.-A., Hageman, K. J., Smith, R. O., Wilson, G. S., and Mainié, F.: Rapid Antarctic ice sheet retreat under low atmospheric CO2, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12028, https://doi.org/10.5194/egusphere-egu2020-12028, 2020

The response of the marine sectors of the East Antarctic Ice Sheet to future global warming represents a major source of uncertainty in sea level projections. If greenhouse gas emissions continue unbridled, ice loss in these areas may contribute up to several meters to long-term global sea level rise. In East Antarctica, thinning of the ice cover of the George V and Sabrina Coast is currently taking place, and its destabilization in past warm climate periods has been implied. The extent of such past interglacial retreat episodes cannot yet be quantitatively derived from paleo proxy records alone. Ice sheet modelling constrained by paleo observations is therefore critical to assess the stability of the East Antarctic Ice Sheet during warmer climates. We propose that a runaway retreat during the Last Interglacial of the George V Coast grounding line into the Wilkes Subglacial Basin would either leave a clear imprint on the water isotope composition in the neighbouring Talos Dome ice-core record or prohibit the preservation of an ice core record from the Last Interglacial alltogether. We test this hypothesis using a dynamic ice sheet model and infer that the marine Wilkes Basin ice sheet remained stable throughout the Last Interglacial (130,000-120,000 years ago). Our analysis provides the first constraint on Last Interglacial East Antarctic grounding line stability by benchmarking ice sheet model simulations with ice core records. Our findings also imply that ambitious mitigation efforts keeping global temperature rise in check could safeguard this region from irreversible ice loss in the long term.

How to cite: Sutter, J., Eisen, O., Werner, M., Grosfeld, K., Kleiner, T., and Fischer, H.: Limited Retreat of the Wilkes Basin Ice Sheet during the Last Interglacial., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13504, https://doi.org/10.5194/egusphere-egu2020-13504, 2020

Over recent decades, the Antarctic Ice Sheet has witnessed large spatial variations at its surface through the surface mass balance (SMB). Since the complex Antarctic topography, working at high resolution is crucial to represent accurately the dynamics of SMB. While ice cores provide a mean to infer the SMB over centuries, the view is very spatially constrained. Global Climate models estimate the spatial distribution of SMB over centuries, but with a too coarse resolution with regards to the large variations due to local orographic effects. We have therefore explored a methodology to statistically downscale the SMB components from the climate model historical simulations (1850-present day). An analogue method is set up over a period of 30 years with the ERA-Interim reanalysis (1979-2010 AD) and associated with SMB components from the Regional Atmospheric Climate Model (RACMO) at 5 km spatial resolution over Dronning Maud in East Antarctica. The same method is then applied to the period from 1850 to present days using an ensemble of 10 simulations from the CESM2 model. This method enables to derive a spatial distribution of SMB. In addition, the changes in precipitation delivery mechanisms can be unveiled.

How to cite: Ghilain, N., Vannitsem, S., Dalaiden, Q., and Goosse, H.: Reconstructing the distribution of surface mass balance over East Antarctica (DML) from 1850 to present day, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13959, https://doi.org/10.5194/egusphere-egu2020-13959, 2020

Simulations of the glacial-interglacial history of the Antarctic Ice Sheet provide insights into dynamic threshold behavior and estimates of the ice sheet's contributions to global sea-level changes, for the past, present and future. However, boundary conditions are weakly constrained, in particular at the interface of the ice-sheet and the bedrock. We use the Parallel Ice Sheet Model (PISM) to investigate the dynamic effects of different choices of input data and of various parameterizations on the sea-level relevant ice volume. We evaluate the model's transient sensitivity to corresponding parameter choices and to different boundary conditions over the last two glacial cycles and provide estimates of involved uncertainties. We also present isolated and combined effects of climate and sea-level forcing on glacial time scales. 

How to cite: Albrecht, T., Winkelmann, R., and Levermann, A.: PISM paleo simulations of the Antarctic Ice Sheet over the last two glacial cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15081, https://doi.org/10.5194/egusphere-egu2020-15081, 2020

We present Surface Mass Balance (SMB) and surface melt rates projections in West Antarctica for the end of the 21^st century using the MAR regional atmosphere and firn model (Gallée 1994; Agosta et al. 2019) forced by a CMIP5-rcp85 multi-model-mean seasonal anomaly added to the ERA-Interim 6-hourly reanalysis.

First of all, we assess the validity of our projection method, following a perfect-model approach, with MAR constrained by the ACCESS-1.3 present-day and future climates. Changes in large-scale variables are well captured by our anomaly-based projection method, and errors on surface melting and SMB projections are typically 10%.

Based on the CMIP5-rcp85 multi-model mean, SMB over the grounded ice sheet in the Amundsen sector is projected to increase by 35% over the 21^st century. This corresponds to a SMB sensitivity to near-surface warming of 8.3%.°C^-1. Increased humidity, resulting from both higher water vapour saturation in warmer conditions and decreased sea-ice concentrations, are shown to favour increased SMB in the future scenario.

Ice-shelf surface melt rates at the end of the 21^st century are projected to become 6 to 15 times larger than presently, depending on the ice shelf under consideration. This is due to enhanced downward longwave radiative fluxes related to increased humidity, and to an albedo feedback leading to more absorption of shortwave radiation. Interestingly, only three ice shelves produce runoff (Abbot, Cosgrove and Pine Island) in the future climate. For the other ice shelves (Thwaites, Crosson, Dotson, Getz), the future melt-to-snowfall ratio remains too low to produce firn air depletion and subsequent runoff.

How to cite: Jourdain, N., Donat-Magnin, M., Kittel, C., Agosta, C., Amory, C., Gallée, H., Krinner, G., and Chekki, M.: Surface mass balance and melting projections over the Amundsen coastal region, West Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16488, https://doi.org/10.5194/egusphere-egu2020-16488, 2020

The surface mass balance (SMB) of the Antarctic ice sheet is often considered as a negative contributor to the sea level rise as present snowfall accumulation largely compensates for ablation through wind erosion, sublimation and runoff. The latter is even almost negligible since current Antarctic surface melting is limited to relatively scarce events over generally peripheral areas and refreezes almost entirely into the snowpack. However, melting can significantly affect the stability of ice shelves through hydrofracturing, potentially leading to their disintegration, acceleration of grounded ice and increased sea level rise. Although a large increase in snowfall is expected in a warmer climate, more numerous and stronger melting events could conversely lead to a larger risk of ice shelf collapse. In this study, we provide an estimation of the SMB of the Antarctic ice sheet for the end of the 21st century by forcing the state-of-the-art regional climate model MAR with three different global climate models. We chose the models (from both the Coupled Model Intercomparison Project Phase 5 and 6 - CMIP5 and CMIP6) providing the best metrics for representing the current Antarctic climate. While the increase in snowfall largely compensates snow ablation through runoff in CMIP5-forced projections, CMIP6-forced simulations reveal that runoff cannot be neglected in the future as it accounts for a maximum of 50% of snowfall and becomes the main ablation component over the ice sheet. Furthermore, we identify a tipping point (ie., a warming of 4°C) at which the Antarctic SMB starts to decrease as a result of enhanced runoff particularly over ice shelves. Our results highlight the importance of taking into account meltwater production and runoff and indicate that previous model studies neglecting these processes yield overestimated SMB estimates, ultimately leading to underestimated Antarctic contribution to sea level rise. Finally, melt rates over each ice shelf are higher than those that led to the collapse of the Larsen A and B ice shelves, suggesting a high probability of ice shelf collapses all over peripheral Antarctica by 2100.

How to cite: Kittel, C., Amory, C., Agosta, C., Jourdain, N., Hofer, S., Delhasse, A., and Fettweis, X.: Decreasing Antarctic surface mass balance due to runoff-dominated ablation by 2100, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17355, https://doi.org/10.5194/egusphere-egu2020-17355, 2020

Over glacial-interglacial cycles, the evolution of an ice sheet is influenced by Glacial isostatic adjustment (GIA) via two negative feedback loops. Firstly, vertical bedrock deformation due to a changing ice load alters ice-sheet surface elevation. For example, an increasing ice load leads to a lower bedrock elevation that lowers ice-sheet surface elevation. This will increase surface melting of the ice sheet, following an increase of atmospheric temperature at lower elevations. Secondly, bedrock deformation will change the height of the grounding line of the ice sheet. For example, a lowering bedrock height following ice-sheet advance increases the melt due to ocean water that in turn leads to a retreat of the grounding line and a slow-down of ice-sheet advance.     
               GIA is mainly determined by the viscosity of the interior of the solid Earth which is radially and laterally varying. Underneath the Antarctic ice sheet, there are relatively low viscosities in West Antarctica and higher viscosities in East Antarctica, in turn affecting the response time of the above mentioned feedbacks. However, most ice-dynamical models do not consider the lateral variations of the viscosity in the GIA feedback loops when simulating the evolution of the Antarctic ice sheet. The method developed by Gomez et al. (2018) includes the feedback between GIA and ice-sheet evolution and alternates between simulations of the two models where each simulation covers the full time period. We presents a different method to couple ANICE, a 3-D ice-sheet model, to a 3-D GIA finite element model. In this method the model computations alternates between the ice-sheet and GIA model until convergence of the result occurs at each timestep. We simulate the evolution of the Antarctic ice sheet from 120 000 years ago to the present. The results of the coupled simulation will be discussed and compared to results of the uncoupled ice-sheet model (using an ELRA GIA model) and the method developed by Gomez et al. (2018).

How to cite: van der Wal, W., van Calcar, C., de Boer, B., and Blank, B.: Feedback between ice dynamics and bedrock deformation with 3D viscosity in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19278, https://doi.org/10.5194/egusphere-egu2020-19278, 2020

The Antarctic ice sheet is a deeply uncertain component of future sea level under anthropogenic climate change. To shed light on the ice sheets response to warmer climates in the past and its’ response to future warming, periods in Earth’s geological record can serve as instructive modelling targets. The mid-Pliocene warm period (3.3 – 3.0 Ma) is characterised by global mean surface temperatures ~2.7-4^oC above pre-industrial, atmospheric CO₂ concentrations of ~400ppm and eustatic sea level rise on the order of ~10-30m above modern. The mid-Pliocene sea level record is subject to large uncertainties. The upper end of this record implies a significant contribution from Antarctica and possible collapse of regions of the ice sheet, driven by marine ice sheet instabilities.

We present a suite of BISICLES ice sheet model simulations, forced with a subset of Pliocene Modelling Intercomparison Project (PlioMIP phase 1) coupled atmosphere-ocean climate models, that represent the Pliocene Antarctic ice sheet. This ensemble captures a range of possible ice sheet model responses to a warm Pliocene-like climate under different parameter choices, sampled in a Latin hypercube design. Modelled Antarctic sea level contribution is compared to reconstructions of Pliocene sea level, to explore the extent to which available data with large uncertainties can constrain the model parameter values.

Our aim with this work is to provide insights on Antarctic contribution to sea level in the warm mid-Pliocene. We seek to characterise the role of ice-ocean, ice-atmosphere and ice-bedrock parameter uncertainty in BISICLES on the ice sheet sea level contribution range, and whether cliff instability processes are necessary in reproduce high Pliocene sea levels in this ice sheet model.

How to cite: O'Neill, J., Edwards, T., Gregoire, L., Gandy, N., Dolan, A., Wernecke, A., Cornford, S., de Boer, B., Kelman, I., and van de Flierdt, T.: Modelling the Antarctic Ice Sheet in the warm Mid-Pliocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19348, https://doi.org/10.5194/egusphere-egu2020-19348, 2020

About 20% of all snow accumulation in Antarctica occurs on the ice shelfs and ice rises, locations within the ice shelf where the ice is locally grounded on topography. These ice rises largely control the spatial surface mass balance (SMB) distribution by inducing snowfall variability due to orographic uplift and by inducing wind erosion due altering the wind conditions. Moreover these ice rises buttress the ice flow and represent an ideal drilling locations for ice cores.

In this study we assess the connection between snowfall variability and wind erosion to provide a better understanding of how ice rises impact SMB variability, how well this is captured in the regional atmospheric climate model RACMO, and the implications of this SMB variability for ice rises as an ice core drilling side. By combining ground penetrating radar profiles from two ice rises in Dronning Maud Land with ice core dating we reconstruct spatial and temporal SMB variations across both ice rises from 1982 to 2017. Subsequently, the observed SMB is compared with output from RACMO, SnowModel to quantify the contribution of the different processes that control the spatial SMB variability across the ice rises. Finally, the observed SMB is compared with Sentinel-1 backscatter data to extrapolate spatial SMB trends over larger areas.

Our results show snowfall-driven differences of up to ~ 0.24 m w.e./yr between the windward and the leeward side of both ice rises as well as a local erosion driven minimum at the peak of the ice rises. RACMO captures the snowfall-driven differences, but overestimates their magnitude, whereas the erosion on the peak can be reproduced by SnowModel with RACMO forcing. Observed temporal variability of the average SMBs calculated for 4 time intervals in the 1982-2017 range are low at the peak of the easternmost ice rise (~ 0.03 m w.e./yr), while being three times higher (~ 0.1 m w.e./yr) on the windward side of the ice rise. This implicates that at the peak of the ice rise, higher snowfall, driven by regional processes, such as orographic uplift, is balanced out by local erosion.  Comparison of the observed SMB gradients with Sentinel-1 data finally shows the potential of SAR satellite observations to represent spatial variability in SMB across ice shelves and ice rises.

How to cite: Kausch, T., Lhermitte, S., Lenaerts, J. T. M., Wever, N., Inoue, M., Pattyn, F., Sun, S., Wauthy, S., Tison, J.-L., and van de Berg, W. J.: Impact of coastal East Antarctic ice rises on surface mass balance: insights from observations and modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20370, https://doi.org/10.5194/egusphere-egu2020-20370, 2020