Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.9.4 Snow and Ice ====

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'''Snow:''' Atlas.11.1 identified ''likely'' increases in surface mass balance (driven by snowfall) across Antarctica in the 20th century ( ''medium confidence'' ). In the Arctic, overall snow extent and seasonal duration are projected to continue recent declines ( ''high confidence'' ), although mid-winter snowpack increases in some of the coldest and high-elevation locations given higher precipitation totals ( ''medium confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.3|Section 9.5.3]] , Atlas.9 and Atlas.11.2; [[#Bring--2016|Bring et al., 2016]] ; [[#Danco--2016|Danco et al., 2016]] ; [[#AMAP--2017|AMAP, 2017]] ; [[#Meredith--2019|Meredith et al., 2019]] ). Higher temperatures result in a higher percentage of Arctic precipitation falling as rain (particularly in autumn and spring) ( ''high confidence'' ), with most land regions (outside of Greenland and Antarctica) becoming dominated by rainfall (more than 50% of total precipitation) by RCP8.5 2100 ( [[#Bintanja--2017|Bintanja and Andry, 2017]] ; [[#Irannezhad--2017|Irannezhad et al., 2017]] ).

'''Glacier and ice sheet:''' Section 9.5.1 and [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.3|Section 2.3.2.3]] found that glaciers have lost mass in all polar regions since 2000 ( ''high confidence'' ), and [[IPCC:Wg1:Chapter:Chapter-9#9.4|Section 9.4]] assessed ''high confidence'' in Greenland Ice Sheet mass losses since 1980 and Antarctic Ice Sheet losses since 1992 (dominated by West Antarctica, with losses in parts of East Antarctica in the past two decades). New simulations from GlacierMIP ( [[#Marzeion--2020|Marzeion et al., 2020]] ) indicate glaciers in Iceland will lose 31 ± 35%, 41 ± 46% and 53 ± 45% of their mass in 2015 by the end of the century for RCP2.6, RCP4.5 and RCP8.5 scenarios, respectively. [[#Marzeion--2020|Marzeion et al. (2020)]] projected mass losses ( ''high confidence'' ) for those same scenarios in the Greenland Periphery: 22 ± 23%, 29 ± 26%, and 42 ± 28%; Svalbard: 35 ± 34%, 50 ± 36%, and 66 ± 35%; Russian Arctic: 26 ± 26%, 38 ± 28%, and 52 ± 30%; Northern Arctic Canada: 12 ± 13%, 18 ± 12%, and 27 ± 18%; Southern Arctic Canada: 23 ± 27%, 33 ± 29%, and 48 ± 32%; and Antarctic Periphery: 7 ± 12%, 13 ± 10%, and 16 ± 19%. Areas with receding glaciers are also potentially vulnerable to glacial lake outburst floods ( [[#Harrison--2018|Harrison et al., 2018]] ).

'''Permafrost:''' Observations from recent decades (assessed in [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] and [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] ) show increases in permafrost temperature ( ''very high confidence'' ) and active layer thickness ( ''medium confidence'' ) across the Arctic ( [[#AMAP--2017|AMAP, 2017]] ; [[#Derksen--2018|Derksen et al., 2018]] ; [[#Markon--2018|Markon et al., 2018]] ; [[#Biskaborn--2019|Biskaborn et al., 2019]] ; [[#Farquharson--2019|Farquharson et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Romanovsky--2020|Romanovsky et al., 2020]] ). [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] noted that observations of active layer thickness in Antarctica are too limited to assess long-term trends (see also [[#Hrbáček--2018|Hrbáček et al., 2018]] ; [[#Biskaborn--2019|Biskaborn et al., 2019]] ). Future projections indicate continuing increases in permafrost temperature and active layer thickness with loss of permafrost across the Arctic ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] ). [[#Streletskiy--2019|Streletskiy et al. (2019)]] noted that changes to Russian permafrost temperature and active layer thickness are most pronounced in areas where permafrost is continuous (underlying &gt;90% of landmass). CMIP5 analyses by [[#Slater--2013|Slater and Lawrence (2013)]] projected that, by RCP8.5 2100, shallow (&lt;3 m) permafrost would be most probable only in portions of the Canadian Arctic Archipelago and the Russian Arctic coastal and eastern upland regions.

'''Sea ice:''' Consistent with SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ), [[IPCC:Wg1:Chapter:Chapter-9#9.3.1|Section 9.3.1]] and [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.1|Section 2.3.2.1.1]] assess ''very high confidence'' that Arctic sea ice thickness, extent, and average age have significantly decreased over the past four decades, with largest declines in September (when sea ice is at an annual minimum). Declines in landfast ice are most rapid in the Laptev Sea ( [[#Selyuzhenok--2015|Selyuzhenok et al., 2015]] ), with warming also breaking perennial landfast ice blocking ocean channels (‘ice plugs’) in the Canadian Archipelago ( [[#Pope--2017|Pope et al., 2017]] ), and landfast ice declining in the cold season by 7% per decade across the Arctic (1976–2007; [[#Yu--2014|Yu et al., 2014]] ). Observed trends and projections suggest that perennial sea ice is being replaced by thin, seasonal ice, although multi-year ice will persist above the Canadian Archipelago and drift into sea transportation lanes ( [[#Howell--2016|Howell et al., 2016]] ; [[#Derksen--2018|Derksen et al., 2018]] ). Trends from 1979 to 2013 show slightly earlier spring melt for Arctic sea ice, but substantially delayed autumn freeze-up and a melt season lengthened by more than 3 days per decade off northern Alaska and Canada with the exception of portions of the Bering Sea ( [[#Parkinson--2014|Parkinson, 2014]] ; [[#Stroeve--2014|Stroeve et al., 2014]] ). [[IPCC:Wg1:Chapter:Chapter-9#9.3.2|Section 9.3.2]] assessed ''low confidence'' in long-term trends in sea ice extent or thickness near Antarctica.

Future declines in Arctic sea ice are ''virtually certain'' , although there is ''low confidence'' in declines of Antarctic sea ice given dynamical processes in the Southern Ocean and the recovery of stratospheric ozone ( [[IPCC:Wg1:Chapter:Chapter-9#9.3|Section 9.3]] ; [[#Meredith--2019|Meredith et al., 2019]] ). Projections of an ‘ice-free’ Arctic vary, depending on definitions representing transportation needs, however [[#Laliberté--2016|Laliberté et al. (2016)]] noted that the median of 42 CMIP5 models projected &lt;5% sea ice for the month of September by 2050, with equivalent conditions for the entirety of the August–October period by 2090. [[IPCC:Wg1:Chapter:Chapter-9#9.3.1|Section 9.3.1]] assessed ''high confidence'' that practically ice-free conditions (&lt;1 million km <sup>2</sup> in the September mean) would ''likely'' first appear before 2050 even under strong mitigation scenarios ( [[#Sigmond--2018|Sigmond et al., 2018]] ; [[#Stroeve--2018|Stroeve and Notz, 2018]] ; Notz and SIMIP Community, 2020).

'''Lake and river ice:''' There is ''high confidence'' in observations of significant declines in seasonal ice cover thickness and duration over most Arctic lakes, with many lakes projected to lose more than one month of ice cover by mid-century ( ''medium confidence'' ) ( [[#Meredith--2019|Meredith et al., 2019]] ; [[#Sharma--2019|Sharma et al., 2019]] ). Some lakes that previously froze to the bottom (‘bedfast’) now maintain liquid bottom water year round, and others shift from perennial to seasonal ice cover ( [[#Surdu--2016|Surdu et al., 2016]] ; [[#Engram--2018|Engram et al., 2018]] ). [[#Yang--2020a|Yang et al. (2020a)]] identified a decline in Arctic cold-season river ice extent in satellite observations (particularly in Alaska) and projected reductions in average Northern Hemisphere seasonal river ice duration of 6.10 ± 0.08 days per degree global surface air temperature.

'''Heavy snowfall and ice storm:''' There is ''limited evidence'' of changes in heavy snowfall due to competing influences of shortened snowfall seasonality with more intense (and larger overall) precipitation in the Arctic. Episodic heavy snowfall trends in Antarctica are difficult to separate from large interannual variability ( ''limited evidence'' ) (Gorodetskaya et al., 2014, Turner et al., 2020). ''Limited evidence'' also hinders clear signals in ice storms, although warming shifts the freezing line (around which ice storms occur) poleward and upslope ( [[#Bintanja--2017|Bintanja and Andry, 2017]] ). [[#Groisman--2016|Groisman et al. (2016)]] used 40 years of observations to identify an increase of freezing rain events in Norway, North America, and eastern and western Russia. Increases in winter rainfall have led to more frequent development of difficult wildlife and livestock grazing conditions as basal ice conditions coat the ground below snowpack ( [[#Peeters--2019|Peeters et al., 2019]] ).

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'''Table 12.11''' '''|''' '''Summary of confidence in direction of projected change in climatic impact-drivers in the polar regions, representing their aggregate characteristic changes for mid-century for scenarios RCP4.5, SSP2-4.5, SRES A1B, or above within each AR6 region (defined in Chapter 1), approximately corresponding (for CIDs that are independent of sea level rise) to global warming levels between 2°C and 2.4°C (see [[#12.4|Section 12.4]] for more details of the assessment method).''' The table also includes the assessment of observed or projected time-of-emergence of the CID change signal from the natural interannual variability if found with at least ''medium confidence'' in [[#12.5.2|Section 12.5.2]] . Note that the Arctic portions of the NEU, NEN and NWN differ from the full AR6 regions assessed in the Europe and North America sections above (see also Figure 1.18c).

[[File:35c186ec3522c779e133883cf3c3179a IPCC_AR6_WGI_Chapter12_Table_12_11_1.jpg]]

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