Article révisé par les pairs
Résumé : Abstract. The sea level contribution of the Antarctic ice sheetconstitutes a large uncertainty in future sea level projections. Here weapply a linear response theory approach to 16 state-of-the-art ice sheetmodels to estimate the Antarctic ice sheet contribution from basal ice shelfmelting within the 21st century. The purpose of this computation is toestimate the uncertainty of Antarctica's future contribution to global sealevel rise that arises from large uncertainty in the oceanic forcing and theassociated ice shelf melting. Ice shelf melting is considered to be a majorif not the largest perturbation of the ice sheet's flow into the ocean.However, by computing only the sea level contribution in response to iceshelf melting, our study is neglecting a number of processes such assurface-mass-balance-related contributions. In assuming linear responsetheory, we are able to capture complex temporal responses of the ice sheets,but we neglect any self-dampening or self-amplifying processes. This isparticularly relevant in situations in which an instability is dominating theice loss. The results obtained here are thus relevant, in particular wherever theice loss is dominated by the forcing as opposed to an internal instability,for example in strong ocean warming scenarios. In order to allow forcomparison the methodology was chosen to be exactly the same as in anearlier study (Levermannet al., 2014) but with 16 instead of 5 ice sheet models. We includeuncertainty in the atmospheric warming response to carbon emissions (fullrange of CMIP5 climate model sensitivities), uncertainty in the oceanictransport to the Southern Ocean (obtained from the time-delayed and scaledoceanic subsurface warming in CMIP5 models in relation to the global meansurface warming), and the observed range of responses of basal ice shelfmelting to oceanic warming outside the ice shelf cavity. This uncertainty inbasal ice shelf melting is then convoluted with the linear responsefunctions of each of the 16 ice sheet models to obtain the ice flow responseto the individual global warming path. The model median for theobservational period from 1992 to 2017 of the ice loss due to basal iceshelf melting is 10.2 mm, with a likely range between 5.2 and 21.3 mm. Forthe same period the Antarctic ice sheet lost mass equivalent to 7.4 mm ofglobal 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 toglobal mean sea level rise from basal ice shelf melting within the 21stcentury of 17 cm, with a likely range (66th percentile around the mean) between9 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 theglobal mean temperature below 2 ∘C of global warming and is thusconsistent with the Paris Climate Agreement, the procedure yields a median of13 cm of global mean sea level contribution. The likely range for theRCP2.6 scenario is between 7 and 24 cm, and the very likely range isbetween 4 and 37 cm. The structural uncertainties in the method do notallow for an interpretation of any higher uncertainty percentiles. We provideprojections for the five Antarctic regions and for each model and eachscenario separately. The rate of sea level contribution is highest underthe RCP8.5 scenario. The maximum within the 21st century of the medianvalue is 4 cm per decade, with a likely range between 2 and 9 cm per decadeand a very likely range between 1 and 14 cm per decade.

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