(d)
Fig. 15: (a) March and (b) September sea ice extent in the Arctic
(Million km2). (c) and (d) same as (a) and (b) but for
the Antarctic region. The purple line indicates the observed sea ice
extent from the sea ice portal meereisportal.de (Grosfeld et.al 2016).
For comparison, observational National Snow and Ice Data Center (NSIDC)
(Fetterer et al. 2017) and ERA5 reanalysis (Copernicus Climate Change
Service (C3S), 2017; Hersbach et al., 2020) sea-ice extent are shown in
inlays along with the ones from sea ice portal. The observation
uncertainty is small and does not affect the conclusions.
Like for the sea-ice extent, the decline of Arctic sea-ice thickness is
also evident from the historical simulation during the freezing season,
most pronounced from around mid 20th century till recent years (Fig.
16). Simulated sea ice thickness in the Antarctic shows a weaker decline
than that in the Arctic. We compare the simulated ensemble mean
thickness in the Arctic with recent satellite thickness data from
CS2SMOS (Ricker et al., 2017), which is constructed by merging CryoSat-2
and SMOS thickness together using the optimal interpolation method. Sea
ice thickness in the historical simulation falls well into the observed
range from 2010 to 2013. Basin-scale observations for sea ice thickness
in the Antarctic are rather limited. A more detailed evaluation against
observations for Antarctic ice thickness is therefore currently not
possible.