2020, Sun, Sainan, Pattyn, Frank, Simon, Erika G., Albrecht, Torsten, Cornford, Stephen, Calov, Reinhard, Dumas, Christophe, Gillet-Chaulet, Fabien, Goelzer, Goelzer, Golledge, Nicholas R., Greve, Ralf, Hoffman, Matthew J., Humbert, Angelika, Kazmierczak, Elise, Kleiner, Thomas, Leguy, Gunter R., Lipscomb, William H., Martin, Daniel, Morlighem, Mathieu, Nowicki, Sophie, Pollard, David, Price, Stephen, Quiquet, Aurélien, Seroussi, Hélène, Schlemm, Tanja, Sutter, Johannes, van de Wal, Roderik S.W., Winkelmann, Ricarda, Zhang, Tong

Antarctica's ice shelves modulate the grounded ice flow, and weakening of ice shelves due to climate forcing will decrease their 'buttressing' effect, causing a response in the grounded ice. While the processes governing ice-shelf weakening are complex, uncertainties in the response of the grounded ice sheet are also difficult to assess. The Antarctic BUttressing Model Intercomparison Project (ABUMIP) compares ice-sheet model responses to decrease in buttressing by investigating the 'end-member' scenario of total and sustained loss of ice shelves. Although unrealistic, this scenario enables gauging the sensitivity of an ensemble of 15 ice-sheet models to a total loss of buttressing, hence exhibiting the full potential of marine ice-sheet instability. All models predict that this scenario leads to multi-metre (1-12 m) sea-level rise over 500 years from present day. West Antarctic ice sheet collapse alone leads to a 1.91-5.08 m sea-level rise due to the marine ice-sheet instability. Mass loss rates are a strong function of the sliding/friction law, with plastic laws cause a further destabilization of the Aurora and Wilkes Subglacial Basins, East Antarctica. Improvements to marine ice-sheet models have greatly reduced variability between modelled ice-sheet responses to extreme ice-shelf loss, e.g. compared to the SeaRISE assessments. Copyright © The Author(s) 2020.

2019, Seroussi, Hélène, Nowicki, Sophie, Simon, Erika, Abe-Ouchi, Ayako, Albrecht, Torsten, Brondex, Julien, Cornford, Stephen, Dumas, Christophe, Gillet-Chaulet, Fabien, Goelzer, Heiko, Golledge, Nicholas R., Gregory, Jonathan M., Greve, Ralf, Hoffman, Matthew J., Humbert, Angelika, Huybrechts, Philippe, Kleiner, Thomas, Larour, Eric, Leguy, Gunter, Lipscomb, William H., Lowry, Daniel, Mengel, Matthias, Morlighem, Mathieu, Pattyn, Frank, Payne, Anthony J., Pollard, David, Price, Stephen F., Quiquet, Aurélien, Reerink, Thomas J., Reese, Ronja, Rodehacke, Christian B., Schlegel, Nicole-Jeanne, Shepherd, Andrew, Sun, Sainan, Sutter, Johannes, Van Breedam, Jonas, van de Wal, Roderik S. W., Winkelmann, Ricarda, Zhang, Tong

Ice sheet numerical modeling is an important tool to estimate the dynamic contribution of the Antarctic ice sheet to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMIP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic ice sheets. Following initMIP-Greenland, initMIP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMIP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the surface mass balance anomaly but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, and the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue.

2018, Goelzer, Heiko, Nowicki, Sophie, Edwards, Tamsin, Beckley, Matthew, Abe-Ouchi, Ayako, Aschwanden, Andy, Calov, Reinhard, Gagliardini, Olivier, Gillet-Chaulet, Fabien, Golledge, Nicholas R., Gregory, Jonathan, Greve, Ralf, Humbert, Angelika, Huybrechts, Philippe, Kennedy, Joseph H., Larour, Eric, Lipscomb, William H., Le clec'h, Sébastien, Lee, Victoria, Morlighem, Mathieu, Pattyn, Frank, Payne, Antony J., Rodehacke, Christian, Martin Rückamp, Martin, Saito, Fuyuki, Schlegel, Nicole, Seroussi, Helene, Shepherd, Andrew, Sun, Sainan, van de Wal, Roderik, Ziemen, Florian A.

Earlier large-scale Greenland ice sheet sea-level projections (e.g. those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. The goal of this initMIP-Greenland intercomparison exercise is to compare, evaluate, and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within the Coupled Model Intercomparison Project Phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of (1) the initial present-day state of the ice sheet and (2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly); they should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions, and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises.