Projecting Antarctica's contribution to future sea level rise from basal ice shelf melt using linear response functions of 16 ice sheet models (LARMIP-2)

Abstract

Abstract. The sea level contribution of the Antarctic ice sheet constitutes a large uncertainty in future sea level projections. Here we apply a linear response theory approach to 16 state-of-the-art ice sheet models to estimate the Antarctic ice sheet contribution from basal ice shelf melting within the 21st century. The purpose of this computation is to estimate the uncertainty of Antarctica's future contribution to global sea level rise that arises from large uncertainty in the oceanic forcing and the associated ice shelf melting. Ice shelf melting is considered to be a major if not the largest perturbation of the ice sheet's flow into the ocean. However, by computing only the sea level contribution in response to ice shelf melting, our study is neglecting a number of processes such as surface-mass-balance-related contributions. In assuming linear response theory, we are able to capture complex temporal responses of the ice sheets, but we neglect any self-dampening or self-amplifying processes. This is particularly relevant in situations in which an instability is dominating the ice loss. The results obtained here are thus relevant, in particular wherever the ice loss is dominated by the forcing as opposed to an internal instability, for example in strong ocean warming scenarios. In order to allow for comparison the methodology was chosen to be exactly the same as in an earlier study (Levermann et al., 2014) but with 16 instead of 5 ice sheet models. We include uncertainty in the atmospheric warming response to carbon emissions (full range of CMIP5 climate model sensitivities), uncertainty in the oceanic transport to the Southern Ocean (obtained from the time-delayed and scaled oceanic subsurface warming in CMIP5 models in relation to the global mean surface warming), and the observed range of responses of basal ice shelf melting to oceanic warming outside the ice shelf cavity. This uncertainty in basal ice shelf melting is then convoluted with the linear response functions of each of the 16 ice sheet models to obtain the ice flow response to the individual global warming path. The model median for the observational period from 1992 to 2017 of the ice loss due to basal ice shelf melting is 10.2 mm, with a likely range between 5.2 and 21.3 mm. For the same period the Antarctic ice sheet lost mass equivalent to 7.4 mm of global sea level rise, with a standard deviation of 3.7 mm (Shepherd et al., 2018) including all processes, especially surface-mass-balance changes. For the unabated warming path, Representative Concentration Pathway 8.5 (RCP8.5), we obtain a median contribution of the Antarctic ice sheet to global mean sea level rise from basal ice shelf melting within the 21st century of 17 cm, with a likely range (66th percentile around the mean) between 9 and 36 cm and a very likely range (90th percentile around the mean) between 6 and 58 cm. For the RCP2.6 warming path, which will keep the global mean temperature below 2 ∘C of global warming and is thus consistent with the Paris Climate Agreement, the procedure yields a median of 13 cm of global mean sea level contribution. The likely range for the RCP2.6 scenario is between 7 and 24 cm, and the very likely range is between 4 and 37 cm. The structural uncertainties in the method do not allow for an interpretation of any higher uncertainty percentiles. We provide projections for the five Antarctic regions and for each model and each scenario separately. The rate of sea level contribution is highest under the RCP8.5 scenario. The maximum within the 21st century of the median value is 4 cm per decade, with a likely range between 2 and 9 cm per decade and a very likely range between 1 and 14 cm per decade.

Long‐term projections of sea‐level rise from ice sheets

“…Of the models that have carried out whole‐continent scenario‐based predictive simulations, many have employed simplifications to allow tractable experiments, such as excluding an evolving surface mass balance scheme (Ritz et al, ) or using simplified (time series) environmental forcings instead of spatially distributed climatologies (N. R. Golledge et al, ). These approximations aside, there is some agreement between scenario‐based ice sheet models that, under an unmitigated emissions scenario, Antarctica will contribute up to 40 cm to sea level by 2100 (N. R. Golledge et al, , ); Ritz et al, ; Schlegel et al, ), which is consistent with AR5 and previous modeling approaches that made projections using WAIS‐only models (Cornford et al, ) or that employed different methods for forecasting future mass loss, such as the “linear response function” approach (Levermann et al, , ).…”

Ocean-forced evolution of the Amundsen Sea catchment, West Antarctica, by 2100

“…Cornford et al (2015) found a 1.5 to 4.0 cm SLE in response to the A1B scenario from CMIP3, which is consistent with our findings, despite the A1B scenario being of a lower-magnitude forcing than RCP8.5. Furthermore, a 16-member ice sheet model inter- comparison study projecting the response to an RCP8.5 scenario by Levermann et al (2020) gave a 90 % likelihood upper-bound SLE contribution of approximately 9 cm relative to the year 2000, with a median of 2 cm. Whilst the uncertainty range in their investigation is derived from the differences between the ice sheet models, and thus their resolutions and model physics, the study does not account for uncertainty associated with individual model configuration, which would result in a greater uncertainty range in SLE projections.…”

The role of history and strength of the oceanic forcing in sea level projections from Antarctica with the Parallel Ice Sheet Model

“…Observations show that the Antarctic Ice Sheet is currently not in equilibrium and that its contribution to global sea level rise is increasing (Shepherd et al, 2018). Its future contribution is the largest uncertainty in sea level projections (Oppenheimer, 2020) with its evolution driven by snowfall increases (e.g., Ligtenberg et al, 2013;Frieler et al, 2015) that are counteracted by increased ocean forcing (e.g., Hellmer et al, 2012;Naughten et al, 2018) and potentially instabilities such as the marine ice sheet instability (Weertman, 1974;Schoof, 2007) and the marine ice cliff instability . In recent years, sea level projections of the Antarctic Ice Sheet were conducted with individual ice sheet models (e.g., and extended by comprehensive community efforts such as the Ice Published by Copernicus Publications on behalf of the European Geosciences Union.…”