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

==== 4.4.2.1 Arctic Sea Ice ====

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The AR5 assessed that for RCP8.5, Arctic sea ice coverage in September will drop below 1 million km <sup>2</sup> , or become practically ice-free, at some point between 2040 and 2060 ( [[#Collins--2013|Collins et al., 2013]] ). Since AR5, there has been substantial progress in understanding the response of Arctic sea ice to near-term changes in external forcing. In particular, it is ''very likely'' that different trajectories of the near-term evolution of anthropogenic forcing cause distinctly different likelihood ranges for very low sea ice coverage to occur over the next two decades ( [[#Notz--2018|Notz and Stroeve, 2018]] ). For example, there is an ''unlikely'' drop of September Arctic sea ice coverage to below 1 million km <sup>2</sup> before 2040 for RCP 2.6, and a ''likely'' drop of September Arctic sea ice coverage to below 1 million km <sup>2</sup> before 2040 for RCP 8.5 ( ''medium confidence'' given the single study). The much higher likelihood of a practically sea ice free Arctic Ocean during summer before 2040 in RCP8.5 compared to RCP2.6 is consistent with related studies assessed in SROCC that find a substantially increased likelihood of an ice-free Arctic Ocean for 2.0°C compared to 1.5°C mean global warming relative to pre-industrial levels ( [[#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]] ).

Based on results from CMIP6 models, we conclude that Arctic SIA will decrease in September in the near term (Figure 4.15, ''high confidence'' ). In the case of 10-year trends ending in the near term, 79% of the simulations considered across all the core SSPs project decreasing Arctic sea ice area in September. Due to less of an influence from internal variability, this number rises to 98% in the case of 30-year trends. A more detailed assessment of near-term Arctic sea ice changes can be found in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.3.1). A detailed assessment of Antarctic sea ice changes is in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.3.2).

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[[File:cc363822ef266bdf1b51f3f8cad32393 IPCC_AR6_WGI_Figure_4_15.png]]

'''Figure 4.15 |''' '''CMIP6 linear trends in September Arctic sea-ice area for 10-year, 20-year, and 30-year periods ending in 2021–2040 following five SSPs.''' Plotted are the 5–95% ranges across the ensembles of simulations. The numbers at the top of the plot are the number of model simulations in each SSP ensemble. The numbers near the bottom of the plot indicate the percentage of simulations across all the SSPs with decreasing sea-ice area. Results are from concentration-driven simulations. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

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