Fig. 14: (a) Annual mean 2 m temperature and (b) precipitation response according to the SSP585 scenario 2071-2100 compared to the historical period 1985-2014. Dotted (hatched) areas represent areas where simulated changes are larger than (smaller than) 2 standard deviations (1 standard deviation) of the internal variability based on yearly means of the 500-year control simulation.

5.2.2 Sea ice extent

The simulated changes in sea-ice extent are shown in Fig. 15 for the Arctic (a, b) and the Antarctic (c, d) during March and September according to piControl, historical and tier1 scenario experiments (i.e. ssp126, ssp245, ssp370 and ssp585), along with observations of the last decades.
The strongest decline trend in sea-ice extent can be seen in the Arctic, during September (Fig. 15b). Starting between 2025 and 2030 there are isolated years with virtually sea ice free Arctic summers (1 Million km2 sea ice extent or less) independent of climate change mitigation efforts (see also Notz et al., 2020). Starting from around 2050, except for SSP126, there are subsequent summers of a virtually ice-free Arctic ocean. The observed September sea-ice extent according to AWI’s Sea Ice Portal (Grosfeld et al., 2016; derived from the University Bremen AMSR-ASI product, see Spreen et al. 2008) for 1979 to 2019 is shown (in purple) on top of AWI-CM outputs, confirming that AWI-CM sea ice extent agrees well with observations both in terms of the average and in terms of the rate of sea ice decline. However, the September Arctic sea ice concentration is underestimated in AWI-CM simulations of the last 30 years as shown in section 4.5. This needs to be taken in consideration when interpreting the projections of the future Arctic sea ice cover. According to the multi-model CMIP5 ensembles of September sea-ice extent, Arctic sea ice was melting even faster than predictions, even though observations remained within the first standard deviation of the models due to high internal variability of the participating models (Stroeve et al., 2015). In comparison to CMIP5, AWI-CM shows stronger sensitivity to the forcings. Unlike multi-model CMIP5 ensembles, ice-free Septembers will be expected not only for SSP585 (corresponding to RCP 8.5 in CMIP5), but also for SSP245 (corresponding to RCP 4.5 in CMIP5) and SSP370 (new pathway). IPCC AR5 reported September sea-ice extent reduction in 2081-2100 with respect to the average of the last 20 years of historical experiments (1986-2005) to be 43% for RCP2.6 and 94% for RCP8.5 (IPCC, 2013, p. 92). According to our simulations, the September Arctic sea-ice extent declines by the end of this century (2081—2100) with respect to the last 20 years of historical experiments (1995—2014) according to AWI-CM SSP126 and SSP585 are 64% and 99.99%, respectively. The inter-ensemble variability for both historical and scenario (ssp370) experiments is small. This means that the results are robust against internal variability.
Likewise, Arctic sea-ice extent during March shows a continuously negative trend for historical and scenario experiments (Fig. 15a). This negative trend seems to be independent of the scenario until the mid-21st century which implies that the impact of mitigation efforts might not be seen before that in terms of Arctic winter sea ice. However afterwards, sea-ice extent stabilizes at around 14 million km² for SSP126 and SSP245. As detailed in Fig. 15a, scenarios incorporating higher radiative forcings (SSP370 and SSP585) predict accelerating decline of sea-ice extent. According to the high-end scenario of SSP585, by 2100, Arctic March sea-ice extent will be half of its value at the beginning of the century.
IPCC AR5 (IPCC, 2014: Climate Change 2014: Synthesis Report p.48) reported low confidence in near-term projections of Antarctic sea ice extent. This was due to the mismatch between CMIP5 models (strong simulated decline) and observations (no decline) along with very limited understanding of the origin of this mismatch. According to IPCC AR5, it is suspected that this phenomenon is likely due to regional variability within the Antarctic (IPCC, 2013, p.303). A study of individual CMIP5 models also suggested that although these models cannot replicate the observed Antarctic sea-ice extent trend, the observation still remains within the natural variability of better performing models (Turner et al., 2015). Furthermore, Bintanja (2013) showed that this sea-ice expansion could indeed be due to Antarctic sea ice shelf melting, which is not represented in CMIP5 models.
Similar to CMIP5 models, AWI-CM predicts declining Antarctic sea-ice extent for both September and March over recent decades (Figs. 15c and d) - which is in contrast to observations - and furthermore till the end of the century. In addition, the simulated difference between late winter and late summer Antarctic sea ice extent is more pronounced than in observations. Overall, interannual variability for Antarctic sea-ice extent is larger than for the Arctic, which agrees with the findings regarding CMIP5 models by Turner et al. (2015). Similarly to the Arctic sea ice, mitigation efforts only start to have a noticeable impact from around 2050.
(a)