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

=== 4.4.2 Projected Changes in the Cryosphere (Snow, Glaciers and Permafrost) ===

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AR5 noted that global glacier mass loss is ''very likely'' to increase further during the 21st century ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). According to the SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ), it is ''very likely'' that glaciers will continue to lose mass throughout the 21st century: from 18% (by 2100, relative to 2015) for RCP2.6 to 36% for RCP8.5. AR5 ( [[#Collins--2013)|Collins et al., 2013)]] and SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) reported with ''high confidence'' that permafrost would continue to thaw in the 21st century, but the projections are uncertain. Constraining warming to 1.5°C would prevent the thawing of a permafrost area of 1.5 to 2.5 million km 2 compared to thawing under 2°C ( ''medium confidence'' ) ( [[#IPCC--2018b|IPCC, 2018b]] ). AR5 ( [[#Collins--2013)|Collins et al., 2013)]] and SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) concluded that Northern Hemisphere snow extent and mass would likely reduce by the end of the 21st century, both in plain and mountain regions. AR6 assessed with ''medium confidence'' that under RCP2.6 and RCP8.5 from 2015 to 2100, glaciers are expected to lose 18% and 36% of their early 21st-century mass, respectively (AR6 WGI, ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )).

Global glacier mass loss since 2015 and 2100 was projected to be 18 ± 13% by 2100 with 0.9 – 2.3°C global warming and 36 ± 20% with 3.2 – 5.4°C global warming ( [[#Marzeion--2020|Marzeion et al., 2020]] ), which corresponds with previous findings ( [[#Radić--2014|Radić et al., 2014]] ; [[#Hock--2019a|Hock et al., 2019a]] ; [[#Shannon--2019|Shannon et al., 2019]] ). The regional glacier loss rate projections are unevenly distributed worldwide and considerably vary between scenarios ( [[#Huss--2018|Huss and Hock, 2018]] ; [[#Hock--2019a|Hock et al., 2019a]] ). In most regions, ‘peak water’ has already been reached or is expected to be reached before mid-century (with an earlier ‘peak water’ for RCP2.6 scenario compared with RCP8.5) ( [[#Huss--2018|Huss and Hock, 2018]] ; [[#Pritchard--2019|Pritchard, 2019]] ; [[#Marzeion--2020|Marzeion et al., 2020]] ; [[#Rounce--2020|Rounce et al., 2020]] ). The influence of the expected subsequent decrease in glacier runoff by the end of the 21st century will be more pronounced during droughts and dry seasons ( [[#Farinotti--2016|Farinotti et al., 2016]] ; [[#Huss--2016|Huss and Fischer, 2016]] ; [[#Hanzer--2018|Hanzer et al., 2018]] ; [[#Brunner--2019|Brunner et al., 2019]] ).

Such changes in runoff could potentially lead to water shortages for over 200 million people in the high mountains of Asia ( [[#Pritchard--2019|Pritchard, 2019]] ; [[#Shahgedanova--2020|Shahgedanova et al., 2020]] ). There is ''medium confidence'' that under a 4°C warming scenario, 40% of current irrigated demand in sub-basins relying primarily on snowmelt runoff would need to be supplemented from other water sources ( [[#Qin--2020|Qin et al., 2020]] ). Basins where such alternate sources are not available will face agricultural water scarcity ( [[#4.5.1|Section 4.5.1]] ). Globally, 1.5 billion people are projected to critically depend on runoff from the mountains by the mid-21 st century under the RCP6.0 scenario ( [[#Viviroli--2020|Viviroli et al., 2020]] ). Furthermore, there is ''medium confidence'' that projected changes in snow and glacier melt runoff will affect water inputs to hydropower, leading to a decline in hydroelectricity production in mountain basins, for example, in India ( [[#Ali--2018|Ali et al., 2018]] ), Switzerland ( [[#Schaefli--2019|Schaefli et al., 2019]] ) and the USA ( [[#Lee--2016|Lee et al., 2016]] ) ( [[#4.5.2|Section 4.5.2]] ) (IPCC AR6 WGI, 2021) (Sections 9.5.1.3 and 8.4.1.7.1).

Projections of snow cover metrics [IPCC AR6 WGI, 2021 ( [[IPCC:Wg2:Chapter:Chapter-9#9.5.3|Section 9.5.3.3]] )] suggest a further decrease in snow water equivalent (SWE) and snow cover extent (SCE), though the inter-model spread is considerable ( [[#Lute--2015|Lute et al., 2015]] ; [[#Thackeray--2016|Thackeray et al., 2016]] ; [[#Kong--2017|Kong and Wang, 2017]] ; [[#Henderson--2018|Henderson et al., 2018]] ) ( ''high confidence'' ). The projected CMIP6 SCE and SWE changes share the broad features of the CMIP5 projections: SCE is expected to decrease in the Northern Hemisphere by approximately 20%, relative to the 1995–2014 mean value, around 2060 and stabilise afterwards under the RCP2.6 scenario, while the RCP8.5 scenario leads to snow cover losses up to 60% by 2100 ( [[#Mudryk--2020|Mudryk et al., 2020]] ). Regionally, the SWE loss will probably lead to more frequent snow droughts; for example, the frequency of consecutive snow droughts is projected to increase to 80–100% of years at 4 ° C warming in western Canada ( [[#Shrestha--2021|Shrestha et al., 2021]] ) and 42% of years under the RCP8.5 scenario in the western USA ( [[#Marshall--2019|Marshall et al., 2019]] ) by 2100. Thus, by the mid- to late 21st century, for more than 2/3 of snow-dominated areas in the western USA, the ability to predict seasonal droughts and prepare robust water management plans will decline ( [[#Livneh--2020|Livneh and Badger, 2020]] ) ( [[#4.4.5|Section 4.4.5]] ).

There is a ''high agreement'' between the CMIP6 projections and the previous findings that permafrost will undergo increasing thaw and degradation during the 21st century worldwide ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ) . The CMIP6 models project that the annual mean frozen volume in the top 2 m of the soil could decrease by 10–40% for every degree increase of global temperature ( [[#Burke--2020|Burke et al., 2020]] ; [[#Yokohata--2020b|Yokohata et al., 2020b]] ). The CMIP5-based equilibrium sensitivity of permafrost extent to stabilised global mean warming is established to be about 4.0 × 10 6 km 2 °C –1 ( [[#Chadburn--2017|Chadburn et al., 2017]] ). The southern boundary of the permafrost is projected the move to the north: 1°–3.5° northward (relative to 1986–2005) at the level of 1.5°C temperature rise ( [[#Kong--2017|Kong and Wang, 2017]] ).

The observational knowledge gaps ( [[#4.2.2|Section 4.2.2]] ) impede efforts to calibrate and evaluate models that simulate the past and future evolution of the cryosphere and its social impacts.

In summary, in most basins fed by glaciers, runoff is projected to increase initially in the 21st century and then decline ( ''medium confidence'' ). Projections suggest a further decrease in seasonal snow cover extent and mass in mid to high latitudes and high mountains ( ''high confidence'' ), though the projection spread is considerable. Permafrost will continue to thaw throughout the 21st century ( ''high confidence'' ). There is ''medium confidence'' that future changes in cryospheric components will negatively affect irrigated agriculture and hydropower production in regions dependent on snowmelt runoff.

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