Ocean bathymetry exerts a strong control on ice sheet-ocean interactions within Antarctic ice-shelf cavities, where it can limit the access of warm, dense water at depth to the underside of floating ice shelves. However, ocean bathymetry is challenging to measure within or close to ice-shelf cavities. It remains unclear how uncertainty in existing bathymetry datasets affect simulated sub-ice shelf melt rates. Here we infer linear sensitivities of ice shelf melt rates to bathymetric shape with grid-scale detail by means of the adjoint of an ocean general circulation model. Both idealised and realistic-geometry experiments of sub-ice shelf cavities in West Antarctica reveal that bathymetry has a strong impact on melt in localised regions such as topographic obstacles to flow. Moreover, response of melt to bathymetric perturbation is found to be non-monotonic, with deepening leading to either increased or decreased melt depending on location. Our computational approach provides a comprehensive way of identifying regions where refined knowledge of bathymetry is most impactful, and also where bathymetric errors have relatively little effect on modelled ice sheet-ocean interactions.

Projections of the sea level contribution from the Greenland and Antarctic ice sheets rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous CMIP5 effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.

The projected contribution to sea-level rise from the Greenland Ice Sheet currently has a large spread in literature, ranging from about 14 to 255 mm by the year 2100. Part of this spread is due to uncertainty in mass loss from ocean-terminating outlet glaciers in response to terminus retreat. Here, we use a diffusive-kinematic wave formulation of glacier thinning to show that steep bed features can limit thinning from diffusing inland from a glacier’s terminus. This simplified model allows us to rank 141 of Greenland’s outlet glaciers based on their potential to allow thinning to diffuse far inland and, thus, contribute to sea-level rise over the next century. We then target two glaciers: Kakivfaat Sermiat (KAK) in West Greenland and Kangerlussuaq Gletscher (KLG) in East Greenland. Both glaciers have a high potential to contribute to sea-level rise but with contrasting bed geometries; KAK has relatively low ice flux but its geometry can allow thinning to diffuse far inland while KLG has high ice flux but a geometry that will limit thinning to 30 km inland of its terminus. We simulate mass loss from each glacier, in response to prescribed terminus retreat, using a higher-order numerical model, and find very different response times of mass loss from the two glaciers over the next century. KLG reaches a new steady state by 2100, while the slow inland diffusion of thinning causes KAK to continue its response into the next century and beyond. As a result, KAK contributes nearly twice the volume of ice to sea-level rise of KLG by year 2200, suggesting that low-flux glaciers that can allow thinning to spread far into the ice sheet interior may contribute much to sea-level rise as high-flux glaciers that limit thinning to their lowest reaches. By identifying the glaciers around the ice sheet with the highest potential to contribute to sea-level rise, we hope to help focus future higher-order numerical modeling studies working toward narrowing the range in sea-level rise projections.

Numerical, process-based simulations of tidewater glacier evolution are necessary to project future sea-level change under various climate scenarios. Previous work has shown that nonlinearities in tidewater glacier and ice stream dynamics can lead to biases in simulated ice mass change in the presence of noisy forcings. Ice sheet modeling projections that will be used in the upcoming IPCC Assessment Report 6 (AR6) utilize atmospheric and oceanic forcings at annual temporal resolution, omitting any higher frequency forcings. Here, we quantify the effect of seasonal (<1 year) tidewater glacier terminus oscillations on decadal-scale (30 year) mass change. We use an idealized geometry to mimic realistic tidewater glacier geometries, and investigate the impact of the magnitude of seasonal oscillations, bed slope at the glacier terminus, and basal friction law. We find that omitting seasonal terminus motion results in biased mass change projections, with up to an 18% overestimate of mass loss when seasonality is neglected. The bias is most sensitive to the magnitude of the seasonal terminus oscillations and exhibits very little sensitivity to choice of friction law. Our results show that including seasonality is required to eliminate a potential bias in ice sheet mass change projections. In order to achieve this, seasonality in atmospheric and oceanic forcings must be adequately represented and observations of seasonal terminus positions and tidewater glacier thickness changes must be acquired to evaluate numerical models.