Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.3.2.1 Arctic Sea Ice ====

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The AR5 assessed from CMIP5 simulations that there will be year-round reductions of Arctic sea ice coverage by the end of this century ( [[#Collins--2013|Collins et al., 2013]] ). These range from 43% under RCP2.6 and 94% under RCP8.5 in September, and from 8% under RCP2.6 and 34% under RCP8.5 in March ( ''medium confidence'' ). Based on a five-member selection of CMIP5 models, AR5 further assessed that for RCP8.5, Arctic sea ice coverage in September will drop below 1 million km <sup>2</sup> and be practically ice free at some point between 2040 and 2060. The SROCC further assessed that the probability of an ice-free Arctic in September for stabilized global warming of 1.5°C and 2.0°C is approximately 1% and 10–35%, respectively ( [[#IPCC--2019|IPCC, 2019]] ).

With regards to the model selection in AR5, model evaluation studies have since identified shortcomings of the CMIP5 models to match the observed distribution of sea ice thickness in the Arctic ( [[#Stroeve--2014|Stroeve et al., 2014]] ; [[#Shu--2015|Shu et al., 2015]] ) and the observed evolution of albedo on seasonal scales ( [[#Koenigk--2014|Koenigk et al., 2014]] ). It was also found that many models’ deviation from observed sea ice cover climatology cannot be explained by internal variability, whereas the models’ deviation from observed sea ice cover trend (over the satellite period) can often be explained by internal variability ( [[#Olonscheck--2017|Olonscheck and Notz, 2017]] ). This hinders a selection of models according to their simulated trends, which additionally has been shown to only have a weak effect on the magnitude of simulated future trends (Stroeve and [[#Notz--2015|Notz, 2015]] ).

Based on results from the CMIP6 models, we conclude that on average the Arctic will become practically ice-free in September by the end of the 21st century under SSP2-4.5, SSP3-7.0, and SSP5-8.5 ( ''high confidence'' ) (Figure 4.2c and Table 4.4). Also, in the CMIP6 models, Arctic SIA in March decreases in the future, but to a much lesser degree, in percentage terms, than in September ( ''high confidence'' ) (Table 4.4). A more detailed assessment of projected Arctic and also Antarctic sea ice change can be obtained in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.3.1).

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'''Table 4.4''' '''|''' '''CMIP6 Arctic sea ice area for selected months, time periods, and across five SSPs.''' Displayed are the multi-model averages across the individual models and, in parentheses, the 5 '''–''' 95% ranges. The number of models used in these calculations are shown in Figure 4.2c.

{| class="wikitable"
|-
| colspan="2"| Month and Time Period

| '''SSP1-1.9 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

| '''SSP1-2.6 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

| '''SSP2-4.5 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

| '''SSP3-7.0 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

| '''SSP5-8.5 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

|-
| rowspan="3"| '''September'''

| 2021–2040

| 2.6 (1.1, 6.5)

| 2.7 (0.6, 6.4)

| 2.8 (0.7, 6.4)

| 3.1 (1.1, 6.4)

| 2.5 (0.4, 5.8)

|-
| 2041–2060

| 2.2 (0.3, 6.5)

| 2.0 (0.2, 6.1)

| 1.7 (0.1, 5.6)

| 1.7 (0.1, 5.7)

| 1.2 (0.0, 5.2)

|-
| 2081–2100

| 2.4 (0.2, 6.2)

| 1.7 (0.0, 6.0)

| 0.8 (0.0, 4.6)

| 0.5 (0.0, 3.3)

| 0.3 (0.0, 2.2)

|-
| rowspan="3"| '''March'''

| 2021–2040

| 14.0 (11.4, 18.7)

| 14.9 (11.9, 25.8)

| 14.9 (11.9, 23.5)

| 15.0 (11.7, 27.3)

| 14.9 (11.9, 24.7)

|-
| 2041–2060

| 13.8 (10.9, 18.3)

| 14.5 (10.9, 25.7)

| 14.3 (11.1, 23.3)

| 14.2 (10.5, 27.1)

| 13.9 (10.2, 24.5)

|-
| 2081–2100

| 13.7 (10.9, 18.5)

| 14.2 (10.6, 25.7)

| 13.1 (9.5, 22.2)

| 11.8 (5.4, 25.5)

| 9.7 (3.1, 21.6)

|}

Studies focusing on the relationship of sea ice extent and changes in external drivers have consistently found a much-reduced likelihood of a practically ice-free Arctic Ocean during summer for global warming of 1.5°C than for 2.0°C ( [[#Screen--2017|Screen and Williamson, 2017]] ; [[#Jahn--2018|Jahn, 2018]] ; [[#Niederdrenk--2018|Niederdrenk and Notz, 2018]] ; [[#Notz--2018|Notz and Stroeve, 2018]] ; [[#Sigmond--2018|Sigmond et al., 2018]] ; [[#Olson--2019|Olson et al., 2019]] ). This is shown here in a large initial-condition ensemble of observationally constrained model simulations where GSAT are stabilized at 1.5°C, 2.0°C and 3.0°C warming relative to 1850–1900 in the RCP8.5 scenario (Figure 4.5). Temperature stabilization is achieved by switching off all the anthropogenic emissions around the time that GSAT first reaches the stabilization thresholds. Simulations have been observationally constrained to correct for a model bias in simulated historical September sea ice extent. In these simulations, Arctic sea ice coverage in September is simulated, on average, to drop below 1 million km <sup>2</sup> around 2040, consistent with the AR5 set of assessed models ( [[#Sigmond--2018|Sigmond et al., 2018]] ). The individual model simulations, for which there are twenty for each stabilized temperature level, show that the probability of the Arctic becoming practically ice free at the end of the 21st century is significantly higher for 2°C warming than for 1.5°C warming above 1850–1900 levels ( ''high confidence'' ).

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[[File:47131a59275ac70c2dcc3d9dfc87e61a IPCC_AR6_WGI_Figure_4_5.png]]

'''Figure 4.5''' '''|''' '''Arctic sea ice extent in September in a large initial-condition ensemble of observationally-constrained simulations of an Earth system model (CanESM2).''' The black and red curves are averages over twenty simulations following historical forcings to 2015 and RCP8.5 extensions to 2100. The other curves are averages of over 20 simulations each after global surface air temperature has been stabilized at the indicated degree of global mean warming relative to 1850–1900. The bars to the right are the minimum to maximum ranges over 2081–2100 ( [[#Sigmond--2018|Sigmond et al., 2018]] ). The horizontal dashed line indicates a practically sea ice-free Arctic. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

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