(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.