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

=== 4.2.2 Observed Changes in the Cryosphere (Snow, Glaciers and Permafrost) ===

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AR5 reported a decrease in snow cover over most of the Northern Hemisphere, decreases in the extent of permafrost and increases in its average temperature, and glacier mass loss in most parts of the world ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). SROCC ( [[#IPCC--2019c|IPCC, 2019c]] ) stated with ''very high'' or ''high confidence'' (a) reduction in seasonal snow cover (snow cover extent decreased by 13.4% per decade for 1967–2018); (b) glacier mass budget of all mountain regions (excluding the Canadian and Russian Arctic, Svalbard, Antarctica, Greenland) was 490 ± 100 kg m –2 yr –1 in 2006–2015; (c) warming of permafrost (e.g., permafrost temperatures increased by 0.39°C in the Arctic for 2007–2017). Tourism and recreation activities have been negatively impacted by declining snow cover, glaciers and permafrost in high mountains ( ''medium confidence'' ).

Recent studies confirmed with ''high confidence'' that snow cover extent continues to decrease across the Northern Hemisphere in all months of the year (see [[#Douville--2021|Douville et al. (2021)]] ; [[#Eyring--2021|Eyring et al. (2021)]] ; [[#Fox-Kemper--2021|Fox-Kemper et al. (2021)]] for more details). From 1922 to 2018, snow cover extent in the Northern Hemisphere peaked in the 1950s to 1970s ( [[#Mudryk--2020|Mudryk et al., 2020]] ) and has consistently reduced since the end of the 20th century ( [[#Hernández-Henríquez--2015|Hernández-Henríquez et al., 2015]] ; [[#Thackeray--2016|Thackeray et al., 2016]] ; [[#Mudryk--2017|Mudryk et al., 2017]] ; [[#Beniston--2018|Beniston et al., 2018]] ; [[#Hammond--2018|Hammond et al., 2018]] ; [[#Thackeray--2019|Thackeray et al., 2019]] ; [[#Mudryk--2020|Mudryk et al., 2020]] ). The consistently negative snow-mass trend of approximately 5 Gt yr −1 in 1981–2018 for all winter-spring months ( [[#Mudryk--2020|Mudryk et al., 2020]] ), including 4.6 Gt yr −1 decrease of snow mass across North America and a negligible trend across Eurasia, has been observed ( [[#Pulliainen--2020|Pulliainen et al., 2020]] ). Negative trends in snow-dominated period duration of 2.0–6.5 weeks per decade was detected from surface and satellite observations during 1971–2014 ( [[#Allchin--2017|Allchin and Déry, 2017]] ), mainly owing to earlier seasonal snowmelt ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). The observed decrease of snow cover metrics (extent, mass, duration) led to changes in runoff seasonality and has impacted water supply infrastructure ( [[#Blöschl--2017|Blöschl et al., 2017]] ; [[#Huss--2017|Huss et al., 2017]] ), particularly in southwestern Russia, western USA and central Asia. In these regions, snowmelt runoff accounts for more than 30% of irrigated water supplies ( [[#Qin--2020|Qin et al., 2020]] ). Negative impacts on hydropower production due to changes in the seasonality of snowmelt have also been documented ( [[#Kopytkovskiy--2015|Kopytkovskiy et al., 2015]] ).

During the last two decades, the global glacier mass loss rate exceeded 0.5-meter water equivalent (m w.e.) per year compared to an average of 0.33 m w.e. yr –1 in 1950–2000. This volume of mass loss is the highest since the start of the entire observation period ( ''very high confidence'' ) ( [[#Zemp--2015|Zemp et al., 2015]] ; [[#Zemp--2019|Zemp et al., 2019]] ; [[#Hugonnet--2021|Hugonnet et al., 2021]] ) (also see Douville et al. 2021; Fox-Kemper et al. 2021; [[#Gulev--2021|Gulev et al. (2021)]] for more details). Regional estimates of glacier mass balance are also mostly negative ( [[#Dussaillant--2019|Dussaillant et al., 2019]] ; [[#Menounos--2019|Menounos et al., 2019]] ; [[#Zemp--2019|Zemp et al., 2019]] ; [[#Douville--2021|Douville et al., 2021]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Hugonnet--2021|Hugonnet et al., 2021]] ), except for West Kunlun, eastern Pamir and northern Karakoram ( [[#Brun--2017|Brun et al., 2017]] ; [[#Lin--2017|Lin et al., 2017]] ; [[#Berthier--2019|Berthier and Brun, 2019]] ). Changes in glacier metrics estimated in post-SROCC publications are summarised in Figure 4.5.

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[[File:e46a1e704a6a9f39cd5b3aa0ddd41c6b IPCC_AR6_WGII_Figure_4_005.png]]

'''Figure 4.5 |''' '''Global and regional estimates of changes in glacier characteristics (elevation, m yr''' '''–1''' '''; mass Gt yr''' '''–1''' ''', mass balance, m.''' '''w.e. yr''' '''–1''' ''') and 95% confidence intervals of the estimates.''' Results are taken from the post-SROCC publications, which are labelled in the chart titles as 1 – ( [[#Hugonnet--2021|Hugonnet et al., 2021]] ); 2 – ( [[#Yang--2020|Yang et al., 2020]] ); 3 – ( [[#Dussaillant--2019|Dussaillant et al., 2019]] ); 4 – ( [[#Davaze--2020|Davaze et al., 2020]] ); 5 – ( [[#Sommer--2020|Sommer et al., 2020]] ); 6 – ( [[#Schuler--2020|Schuler et al., 2020]] ).

Regional and global decreasing trends in glacier mass loss are about linear until 1990, after which they accelerated, especially in western Canada, the USA, and the southern Andes ( [[#WGMS--2017|WGMS, 2017]] ). There is a worldwide growth in the number, total area and total volume of glacial lakes by around 50% between 1990 to 2018 due to the global increase in glacier melt rate ( [[#Shugar--2020|Shugar et al., 2020]] ) ( [[#Shugar--2020|Shugar et al., 2020]] ) that can potentially increase risks of glacial lake outburst floods (GLOFs) with significant negative societal impacts ( [[#Ikeda--2016|Ikeda et al., 2016]] ). A drop in glacier runoff has happened in the regions where the glaciers have already passed their peak water stage, for example, in the Canadian Rocky Mountains, European Alps, tropical Andes and North Caucasus ( [[#Bard--2015|Bard et al., 2015]] ; [[#Hock--2019b|Hock et al., 2019b]] ; [[#Rets--2020|Rets et al., 2020]] ). There is ''medium confidence'' that the accelerated melting of glaciers has negatively impacted glacier-supported irrigation systems worldwide ( [[#Buytaert--2017|Buytaert et al., 2017]] ; [[#Nüsser--2017|Nüsser and Schmidt, 2017]] ; [[#Xenarios--2019|Xenarios et al., 2019]] ). Varying impacts on hydropower production ( [[#Schaefli--2019|Schaefli et al., 2019]] ) and tourism industry in some places due to cryospheric changes have also been documented ( [[#Hoy--2016|Hoy et al., 2016]] ; [[#Steiger--2019|Steiger et al., 2019]] ).

Permafrost changes mainly refer to changes in temperature and active layer thickness (ALT) ( [[#Hock--2019b|Hock et al., 2019b]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ). Permafrost temperature near the depth of zero annual temperature amplitude increased globally by 0.29 ± 0.12°C during 2007–2016, by 0.39 ± 0.15°C in the continuous permafrost and by 0.20 ± 0.10°C in the discontinuous permafrost ( [[#Biskaborn--2019|Biskaborn et al., 2019]] ). Thus, permafrost has been warming during the last 3–4 decades ( [[#Romanovsky--2017|Romanovsky et al., 2017]] ) with a rate of 0.4°C–1.4°C per decade throughout the Russian Arctic, 0.1°C–0.8°C per decade in Alaska and Arctic Canada during 2007–2016 ( [[#Biskaborn--2019|Biskaborn et al., 2019]] ) and 0.1°C–0.24°C per decade in the Tibetan plateau ( [[#Wu--2015|Wu et al., 2015]] ). The ALT has also been increasing in the European and Russian Arctic and high-mountain areas of Eurasia since the mid-1990s ( [[#Hock--2019b|Hock et al., 2019b]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Gulev--2021|Gulev et al., 2021]] ). Unfortunately, unlike glaciers and snow, the lack of ''in situ'' observations on permafrost still cannot be compensated for by remote sensing. Still, some methodological progress on this front has been happening recently ( [[#Nitze--2018|Nitze et al., 2018]] ).

There is ''high confidence'' that degradation of the cryospheric components is negatively affecting terrestrial ecosystems, infrastructure and settlements in the high-latitude and high-altitude areas ( [[#Fritz--2017|Fritz et al., 2017]] ; [[#Oliva--2018|Oliva and Fritz, 2018]] ; [[#Streletskiy--2019|Streletskiy et al., 2019]] ). Similarly, communities in the north polar regions and the ecosystems on which they depend for their livelihoods are at risk ( [[#Mustonen--2015|Mustonen, 2015]] ; [[#Pecl--2017|Pecl et al., 2017]] ; [[#Mustonen--2020|Mustonen and Lehtinen, 2020]] ) (Figure 4.6).

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[[File:8c95a164c7dac128be1225e29f014658 IPCC_AR6_WGII_Figure_4_006.png]]

'''Figure 4.6 |''' '''Map of selected observed impacts on cultural water uses of Indigenous Peoples of the cryosphere.''' Map location is approximate; text boxes provide names of the Indigenous Peoples whose cultural water uses have been impacted by climate change; changed climate variable; impact on water; and specific climate impact on cultural water use ( [[#4.3.7|Section 4.3.7]] ).

In summary, the cryosphere is one of the most sensitive indicators of climate change. There is ''high confidence'' that cryospheric components (glaciers, snow, permafrost) are melting or thawing since the end of the 20th and beginning of the 21st century. Widespread cryospheric changes are affecting humans and ecosystems in mid-to-high latitudes and the high-mountain regions ( ''high confidence'' ). These changes are already impacting irrigation, hydropower, water supply, cultural and other services provided by the cryosphere, and populations depending on ice, snow and permafrost.

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