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)
Authors: Levermann Anders , Winkelmann Ricarda , Albrecht Torsten , Goelzer Heiko , Golledge Nicholas R. , Greve Ralf , Huybrechts Philippe , Jordan Jim , Leguy Gunter , Martin Daniel , Morlighem Mathieu , Pattyn Frank , Pollard David , Quiquet Aurelien , Rodehacke Christian , Seroussi Helene , Sutter Johannes , Zhang Tong , Van Breedam Jonas , Calov Reinhard , DeConto Robert , Dumas Christophe , Garbe Julius , Gudmundsson G. Hilmar , Hoffman Matthew J. , Humbert Angelika , Kleiner Thomas , Lipscomb William H. , Meinshausen Malte , Ng Esmond , Nowicki Sophie M. J. , Perego Mauro , Price Stephen F. , Saito Fuyuki , Schlegel Nicole-Jeanne , Sun Sainan , van de Wal Roderik S. W. 
Published: February 2020

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.