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

==== 12.4.2.4 Snow and Ice ====

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'''Snow:''' There is no significant interannual trend of total snow cover from 2000 to 2016 over Eurasia (X. [[#Wang--2017|]] [[#Wang--2017|]] [[#Wang--2017|Wang et al., 2017]] a; [[#Sun--2020|Sun et al., 2020]] ). Observations do show significant changes in the seasonal timing of Eurasian snow cover extent (especially for earlier spring snowmelt) since the 1970s, with seasonal changes expected to continue in the future ( ''high confidence'' ) ( [[#Yeo--2017|Yeo et al., 2017]] ; [[#Zhong--2021|Zhong et al., 2021]] ). By 2100, snowline elevations are projected to rise between 400 and 900 m (4.4 to 10.0 m yr <sup>–1</sup> ) in the Indus, Ganges and Brahmaputra basins under the RCP8.5 scenario ( [[#Viste--2015|Viste and Sorteberg, 2015]] ).

'''Glacier:''' Observation and future projection of glacier mass changes in Asia are assessed in [[IPCC:Wg1:Chapter:Chapter-9#9.5.1|Section 9.5.1]] grouped in three main regions: northern Asia, High Mountains of Asia, and Caucuses and Middle East. All regions show continuing decline in glacier mass and area in the coming century ( ''high confidence'' ). Under RCP2.6 the pace of glacier loss slows, but glacier losses increase in RCP8.5 and peak in the mid to late 21st century. GlacierMIP projections indicate that glaciers in the High Mountains of Asia lose 42 ± 25%, 56 ± 24% and 71 ± 21% of their 2015 mass by the end of the century for RCP2.6, RCP4.5 and RCP8.5 scenarios respectively. Under the same scenarios, glaciers in North Asia would lose 57 ± 40%, 72 ± 38% and 85 ± 30% of their mass, and glaciers in the Caucuses and the Middle East would lose 68 ± 32%, 83 ± 19% and 94 ± 13% of their mass (see also [[#Kraaijenbrink--2017|Kraaijenbrink et al., 2017]] ; [[#Rounce--2020|Rounce et al., 2020]] ).

Although enhanced meltwater from snow and glaciers largely offsets hydrological drought-like conditions ( [[#Pritchard--2019|Pritchard, 2019]] ), this effect is unsustainable and may reverse as these cryospheric buffers disappear ( ''medium confidence'' ) ( [[#Gan--2015|Gan et al., 2015]] ; W. [[#Dong--2018|]] [[#Dong--2018|Dong et al., 2018]] ; [[#Huss--2018|Huss and Hock, 2018]] ). In the Himalayas and the TIB region higher temperatures will lead to higher glacier melt rates and significant glacier shrinkage and a summer runoff decrease ( ''medium confidence'' ) ( [[#Sorg--2014|Sorg et al., 2014]] ). Glacier runoff in the Asian high mountains will increase up to mid-century, and after that runoff might decrease due to the loss of glacier storage ( ''medium confidence'' ) ( [[#Lutz--2014|Lutz et al., 2014]] ; [[#Huss--2018|Huss and Hock, 2018]] ; [[#Rounce--2020|Rounce et al., 2020]] ).

Compared with the 1990s, the number of lakes in TIB in the 2010s decreased by 2%, whereas total lake area expanded by 25% (S. [[#Wang--2020|]] [[#Wang--2020|Wang et al., 2020]] ) due to the joint effect of precipitation increase and glacier retreat. Many new lakes are predicted to form as a consequence of continued glacier retreat in the Himalaya-Karakoram region ( [[#Linsbauer--2016|Linsbauer et al., 2016]] ). As many of these lakes will develop at the immediate foot of steep icy peaks with degrading permafrost and decreasing slope stability, the risk of glacier lake outburst floods and floods from landslides into moraine-dammed lakes is increasing in Asian high mountains ( ''high confidence'' ) ( [[#Haeberli--2017|Haeberli et al., 2017]] ; [[#Kapitsa--2017|Kapitsa et al., 2017]] ; [[#Bajracharya--2018|Bajracharya et al., 2018]] ; [[#Narama--2018|Narama et al., 2018]] ; S. [[#Wang--2020|]] [[#Wang--2020|Wang et al., 2020]] ).

'''Permafrost''' : Permafrost is thawing in Asia ( ''high confidence'' ). Temperatures in the cold continuous permafrost of north-eastern East Siberia rose from the 1980s up to 2017, and the active layer thicknesses in Siberia and Russian Far East generally increased from late 1990s to 2017 ( [[#Romanovsky--2018|Romanovsky et al., 2018]] ). The change in mean annual ground temperature for northern Siberia is about +0.1 to +0.3°C per decade since 2000 ( [[#Romanovsky--2018|Romanovsky et al., 2018]] ). Ground temperature in the permafrost regions of TIB (taking 40% of TIB currently) increased (0.02–0.26°C per decade for different boreholes) during 1980 to 2018, and the active layer thickened at a rate of 19.5 cm per decade (L. [[#Zhao--2020|]] [[#Zhao--2020|Zhao et al., 2020]] ). There is ''high confidence'' that permafrost in Asian high mountains will continue to thaw and the active layer thickness will increase ( [[#Bolch--2019|Bolch et al., 2019]] ). The permafrost area is projected to decline by 13.4–27.7% and 60–90% in TIB (L. [[#Zhao--2020|]] [[#Zhao--2020|Zhao et al., 2020]] ) and 32% ± 11% and 76% ± 12% in Russia ( [[#Guo--2016|Guo and Wang, 2016]] ) by the end of the 21st century under the RCP2.6 and RCP8.5 scenarios respectively ( ''high confidence'' ).

'''Lake and river ice:''' Lake ice cover duration got shorter in many lakes in TIB ( [[#Yao--2016|Yao et al., 2016]] ; [[#Cai--2019|Cai et al., 2019]] ; [[#Guo--2020|Guo et al., 2020]] ) and some other areas such as north-west China ( [[#Cai--2020|Cai et al., 2020]] ) and north-east China ( [[#Yang--2019|Yang et al., 2019]] ) in the last two decades ( ''high confidence'' ). River ice cover extent decreased in TIB as well (H. [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ; [[#Yang--2020a|Yang et al., 2020a]] ). Climate warming also leads to a significant reduction in the period with ice phenomena and the decrement of ice regime hazard in Russian lowland rivers ( [[#Agafonova--2017|Agafonova et al., 2017]] ), and the Inner Mongolia reach of the Yellow River in northern China ( [[#Wan--2020|Wan et al., 2020]] ) ( ''high confidence'' ). Lake ice and river ice in Asia are expected to decline with projected increases in surface air temperature towards the end of this century ( ''high confidence'' ) ( [[#Guo--2020|Guo et al., 2020]] ; [[#Yang--2020a|Yang et al., 2020a]] ).

'''Heavy snowfall and ice storm:''' Observed trends in heavy snowfall and ice storms are uncertain. Annual maximum snow depth decreased for the period between 1962 and 2016 on the western side of both eastern and western Japan, at rates of 12.3% and 14.6% per decade respectively ( [[#MOE--2018|MOE et al., 2018]] ). Observational results generally show a decrease in the frequency and an increase in the mean intensity of snowfalls in most Chinese regions ( ''medium confidence'' ) ( [[#Zhou--2018|]] [[#Zhou--2018|B. Zhou et al., 2018]] ). Because of the decrease in the snow frequency, the occurrence of large-scale snow disasters in TIB decreased ( ''low confidence'' ) ( [[#Qiu--2018|Qiu et al., 2018]] ; S. [[#Wang--2019|]] [[#Wang--2019|]] [[#Wang--2019|]] [[#Wang--2019|Wang et al., 2019]] ). Large parts of northern high-latitude continents (including Siberia and RFE) have experienced cold snaps and heavy snowfalls in the past few winters, and the reduction of Arctic sea ice would increase the chance of heavy snowfall events in those regions in the coming decades ( ''medium confidence'' ) ( [[#Song--2017|Song and Liu, 2017]] ). Heavy snowfall is projected to occur more frequently in Japan’s Northern Alps, the inland areas of Honshu Island and Hokkaido Island ( [[#Kawase--2016|Kawase et al., 2016]] , 2020; [[#MOE--2018|MOE et al., 2018]] ), and the heavy wet snowfall can be enhanced over the mountainous regions in central Japan and northern part of Japan ( [[#Ohba--2020|Ohba and Sugimoto, 2020]] ) ( ''medium confidence'' ).

'''Hail:''' The hailstorm in the Asian region shows a decreasing trend in several regions ( ''low confidence'' , ''limited evidence'' ). In China severe weather days including thunderstorms, hail and/or damaging wind have decreased by 50% from 1961 to 2010 (M. [[#Li--2016|]] [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ; [[#Zhang--2017|Zhang et al., 2017]] ), and the hail size decreased since 1980 ( [[#Ni--2017|Ni et al., 2017]] ). A rate of decrease of 0.214 hail days per decade has also been reported for Mongolia between 1984–2013, where the annual number of hail days averaged is 0.74 ( [[#Lkhamjav--2017|Lkhamjav et al., 2017]] ).

'''Snow avalanche:''' There is as yet ''limited evidence'' for the evolution of avalanches in Asia. Tree-ring-based snow avalanche reconstructions in the Indian Himalayas show an increase in avalanche occurrence and runout distances in recent decades ( [[#Ballesteros-Cánovas--2018|Ballesteros-Cánovas et al., 2018]] ).

'''In summary, snowpack and glaciers are projected to continue decreasing and permafrost to continue thawing in Asia''' ( high confidence '''). There is''' medium confidence '''of increasing heavy snowfall in some regions, but''' limited evidence '''on future changes in hail and snow avalanches.'''

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