Editing IPCC:AR6/WGI/Chapter-Atlas (section)

==== Atlas.11.1.4 Assessment and Synthesis of Projections ====

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This section provides an assessment of projections in temperature, precipitation and SMB. See [[IPCC:Wg1:Chapter:Chapter-9#9.4.2|Section 9.4.2]] for projected changes in the ice-sheet total mass balance and relevant processes, and see [[IPCC:Wg1:Chapter:Chapter-4#4.3.1|Section 4.3.1]] (Table 4.2) and [[IPCC:Wg1:Chapter:Chapter-4#4.5|Section 4.5.1]] for Antarctic temperature projections relative to other regions.

The Antarctic region is ''very likely'' to experience a significant increase in annual mean temperature and precipitation by the end of this century under all emissions scenarios used in CMIP5 and CMIP6 (Figure Atlas.29; [[#Bracegirdle--2015|Bracegirdle et al., 2015]] , 2020b; [[#Frieler--2015|Frieler et al., 2015]] ; [[#Lenaerts--2016|Lenaerts et al., 2016]] ; [[#Previdi--2016|Previdi and Polvani, 2016]] ; [[#Palerme--2017|Palerme et al., 2017]] ). Ensemble means (and 10th–90th percentile ranges) of end-of-century (2081–2100) projected Antarctic surface air temperature change from 35 CMIP6 models and relative to 1995–2014 are 1.2°C (0.5°C–2.0°C) for the SSP1-2.6 emissions scenarios, 2.3°C (1.3°C–3.4°C) for SSP2-4.5, 3.5°C (2°C–5°C) for SSP3-7.0, and 4.4°C (2.8°C–6.4°C) for SSP5-8.5 (Interactive Atlas). Both temperature and precipitation projections are characterized by a relatively large multi-model range (Figure Atlas.29 and the Interactive Atlas). A strong regional variability is present with the projected changes over coastal Antarctica not scaling linearly with global forcing. While continental mean temperatures are linearly related to global mean temperatures in CMIP6 models, the relative increase in coastal temperatures are higher for low-emissions scenarios due to stronger relative Southern Ocean warming and relatively stronger effects of ozone recovery ( [[#Bracegirdle--2020b|Bracegirdle et al., 2020b]] ). A higher multi-model average increase in temperature is projected by CMIP6 models compared to CMIP5, with a 1.3°C higher mean Antarctic near-surface temperature at the end of the 21st century ( [[#Kittel--2021|Kittel et al., 2021]] ). While similar median temperature changes are projected for WAN and EAN, the former shows larger spread and higher projected temperature range in both CMIP5 and CMIP6 models and for all scenarios (Figure Atlas.29). CORDEX-Antarctica simulations show a mean and range in the future temperature changes similar to the subset of CMIP5 models used to drive them for the RCP8.5 scenario and 1.5°C, 2°C and 3°C GWLs (Figure Atlas.29).

There is ''high confidence'' that projected future surface air temperature increase over Antarctica will be accompanied by precipitation increase (Figure Atlas.29). CMIP6 models show a similar or larger but more constrained increase in precipitation (more models agreeing with larger precipitation increase) for the same GWLs compared to CMIP5. For example, over WAN during JJA for 3°C GWL, CMIP6 and CMIP5 models project a median 15% increase in precipitation with a 10th–90th percentile range of 7–25% in CMIP6 models and of 3–24% in CMIP5. Average precipitation changes relative to 1995–2014 over WAN and EAN are largely similar; they show projected increases for SSP2-4.5 (SSP5-8.5) of around 5% (5%) for 2021–2040, 7% (10%) for 2041–2060, and 12% (25%) for 2081–2100 with smaller increases projected for SSP1-2.6 emissions, reaching around 5% in 2081–2100. Regionally, the largest relative precipitation increase is projected (under all scenarios) for the eastern part of WAN, the western AP, large parts of the EAN plateau and over coastal EAN within 0°E–90°E longitudinal sector (Interactive Atlas). The largest increase in absolute precipitation amount is projected along the coastal regions, with the largest increase over coastal WAN and the western AP, and is projected to be largely driven by the increase in maximum five-day precipitation (Interactive Atlas), which is in line with the dominant contribution of extreme snowfall events to the total annual precipitation in the present Antarctic climate ( [[#Boening--2012|Boening et al., 2012]] ; [[#Gorodetskaya--2014|Gorodetskaya et al., 2014]] ; [[#Turner--2020|Turner et al., 2020]] ). Under all emissions scenarios, the coastal precipitation increase corresponds to the snowfall increase, except for the northern and central part of the western AP, where snowfall is projected to decrease and rainfall to increase (similarly to the tendency towards increased precipitation, decreased snowfall and increase in rainfall over the Southern Ocean; Interactive Atlas).

From 2000 to 2100, the grounded Antarctic SMB is projected to mitigate sea level rise for RCP4.5 (RCP8.5) by the following sea level equivalents (SLEs), 0.03 ± 0.02 m (0.08 ± 0.04 m SLE) from 30 CMIP5 models and for SSP2-4.5 (SSP5-8.5) by 0.03 ± 0.03 m SLE (0.07 ± 0.04 m SLE) from 24 CMIP6 models ( [[#Gorte--2020|Gorte et al., 2020]] ). Subsets or downscaling of CMIP AOGCMs lead to 21st-century cumulative projections in the range of 0.05 ± 0.03 m SLE for CMIP5 RCP8.5 and 0.08 ± 0.04 m SLE for CMIP6 SSP5-8.5 ( [[#Gorte--2020|Gorte et al., 2020]] ; [[#Nowicki--2020|Nowicki et al., 2020]] ; [[#Seroussi--2020|Seroussi et al., 2020]] ; [[#Kittel--2021|Kittel et al., 2021]] ). Use of model subsets reduces spread leading to either lower or higher climate sensitivity in the Antarctic depending on the selection method. For example, models selected by [[#Gorte--2020|Gorte et al. (2020)]] based on SMB ice-core reconstruction from [[#Medley--2019|Medley and Thomas (2019)]] tend to underestimate strongly winter sea ice area ( [[#Agosta--2015|Agosta et al., 2015]] ; [[#Roach--2020|Roach et al., 2020]] ) and show reduced 21st-century increase in Antarctic SMB compared to the full ensembles ( [[#Agosta--2015|Agosta et al., 2015]] ; [[#Bracegirdle--2015|Bracegirdle et al., 2015]] ). A different subset of models is used for ISMIP6 ( [[IPCC:Wg1:Chapter:Chapter-9#9.4.2.3|Section 9.4.2.3]] ) which gives a lower increase in Antarctic SMB than the full ensemble for CMIP5 but a larger increase for CMIP6.

Polar-CORDEX RCMs show higher variability in precipitation projections compared to CMIP5 models with a similar spatial pattern of the areas with precipitation increase over continental Antarctica but with higher local magnitude, and also showing a larger increase over the Weddell Sea ice shelves (Interactive Atlas). CMIP5 and CMIP6 models, bias adjusted based on regional climate model simulations, showed that the projected warming is expected to result in increased surface melting over the Antarctic ice shelves, with meltwater runoff under RCP8.5 and SSP5-8.5 becoming larger than precipitation over ice shelves over the period 2045–2050, surpassing intensities that were linked with the collapse of Larsen A and B ice shelves ( [[#Trusel--2015|Trusel et al., 2015]] ; [[#Kittel--2021|Kittel et al., 2021]] ). Given the existing uncertainty in the present precipitation and SMB simulations and the significant range in the projected precipitation increase under various emissions scenarios in CMIP5, CMIP6 and CORDEX models, there is ''medium confidence'' that the future Antarctic SMB will have a negative contribution to sea level during the 21st century under all emissions scenarios (see [[IPCC:Wg1:Chapter:Chapter-9#9.4.2.3|Section 9.4.2.3]] for assessment of the drivers of future Antarctic ice-sheet change and [[IPCC:Wg1:Chapter:Chapter-9#9.4.2.6|Section 9.4.2.6]] for longer time scales).

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