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

=== 12.3.4 Snow and Ice ===

<div id="h2-4-siblings" class="h2-siblings"></div>

Cryospheric changes are a focus of ( [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] and were central to the recent IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC; [[#IPCC--2019b|IPCC, 2019b]] ). Here we focus on the ways that scientists use snow and ice CIDs to understand current and future societal impacts and risks.

<div id="12.3.4.1" class="h3-container"></div>

<span id="snow-glacier-and-ice-sheet"></span>
==== 12.3.4.1 Snow, Glacier and Ice Sheet ====

<div id="h3-17-siblings" class="h3-siblings"></div>

A large number of indices have been used in water resource and ecosystem studies to track changes in snow under current and future climate conditions, including measurements of the snow water equivalent at key seasonal dates, the fraction of precipitation falling as snow, the first and last days of snow cover, and cold season temperatures ( [[#Mills--2013|Mills et al., 2013]] ; [[#Pierce--2013|Pierce and Cayan, 2013]] ; [[#Berghuijs--2014|Berghuijs et al., 2014]] ; [[#Klos--2014|Klos et al., 2014]] ; [[#Musselman--2017|Musselman et al., 2017]] ; [[#Rhoades--2018|Rhoades et al., 2018]] ). Impact studies also examine shifts in seasonal streamflow for snow-fed river basins ( [[#Mote--2005|Mote et al., 2005]] ; [[#Pederson--2011|Pederson et al., 2011]] ; [[#Beniston--2014|Beniston and Stoffel, 2014]] ; [[#Coppola--2014b|Coppola et al., 2014b]] , 2018; [[#Fyfe--2017|Fyfe et al., 2017]] ; [[#Islam--2017|Islam et al., 2017]] ; [[#Knouft--2017|Knouft and Ficklin, 2017]] ) as well as the geographic extent of snow cover and the depth of frosts when snow cover’s natural insulation is absent ( [[#Scheurer--2009|Scheurer et al., 2009]] ; [[#Millar--2015|Millar and Stephenson, 2015]] ). Studies examining the impact of snow changes on winter recreation and transportation have used thresholds of about 30 cm snow depth or snow water equivalent &gt;10 cm to determine the length of the season for alpine and cross-country skiing and snowmobiling ( [[#Damm--2017|Damm et al., 2017]] ; [[#Wobus--2017b|Wobus et al., 2017b]] ; [[#Spandre--2019|Spandre et al., 2019]] ; [[#Steiger--2019|Steiger et al., 2019]] ; [[#Abegg--2021|Abegg et al., 2021]] ). Changes in snow quality also affect recreational activities ( [[#Rutty--2017|Rutty et al., 2017]] ), and artificial snowmaking can augment recreational snowpack depending on the number of suitable snowmaking hours (e.g., where wet bulb globe temperature (WBGT) &lt;–2.2°C; [[#Wobus--2017b|Wobus et al., 2017b]] ). Local detail may also be provided by tracking the seasonal rain–snow transition line across space and elevation ( [[#Berghuijs--2014|Berghuijs et al., 2014]] ) ( [[#Pierce--2013|Pierce and Cayan, 2013]] ; [[#Berghuijs--2014|Berghuijs et al., 2014]] ; [[#Klos--2014|Klos et al., 2014]] ; [[#Musselman--2017|Musselman et al., 2017]] ).

Change in ice sheet and glacier spatial extent and surface mass balance is relevant for polar and high mountain ecosystems and downstream assets that rely on glacial water resources (J.R. [[#Lee--2017|]] [[#Lee--2017|Lee et al., 2017]] ; [[#Milner--2017|Milner et al., 2017]] ; [[#Huss--2018|Huss and Hock, 2018]] ; [[#Schaefli--2019|Schaefli et al., 2019]] ). The loss of glaciers reduces the thermal consistency of cold streams suitable for some freshwater species ( [[#Giersch--2017|Giersch et al., 2017]] ), and parks and recreation areas may lose appeal as glaciers and seasonal snow cover retreat ( [[#Gonzalez--2018|Gonzalez et al., 2018]] ; [[#Wang--2019|Wang and Zhou, 2019]] ). Rapid glacial retreat can lead to glacial lakes and outburst floods that endanger downstream communities ( [[#Carrivick--2016|Carrivick and Tweed, 2016]] ; [[#Cook--2016|Cook et al., 2016]] ; [[#Harrison--2018|Harrison et al., 2018]] ).

<div id="12.3.4.2" class="h3-container"></div>

<span id="permafrost"></span>
==== 12.3.4.2 Permafrost ====

<div id="h3-18-siblings" class="h3-siblings"></div>

Changes in permafrost temperature, extent and active layer thickness are metrics that track how permafrost thaw below, for example, roads, airstrips, rails and building foundations in high-latitude and mountain regions may destabilize settlements and critical infrastructure ( [[#Pendakur--2016|Pendakur, 2016]] ; [[#Derksen--2018|Derksen et al., 2018]] ; [[#Duvillard--2019|Duvillard et al., 2019]] ; [[#Olsson--2019|Olsson et al., 2019]] ; [[#Streletskiy--2019|Streletskiy et al., 2019]] ). Warmer conditions can also affect ecosystems, built infrastructure and water resources through thawing of especially ice-rich permafrost (≥20% ice content) and by thawing of ice wedges ( [[#Shiklomanov--2017|Shiklomanov et al., 2017]] ; [[#Hjort--2018|Hjort et al., 2018]] ), creation of thermokarst ponds and increased subsurface drainage for polar and high-mountain wetlands ( [[#Walvoord--2016|Walvoord and Kurylyk, 2016]] ; [[#Farquharson--2019|Farquharson et al., 2019]] ) and the release of water pollutants such as mercury ( [[#Burkett--2011|Burkett, 2011]] ; [[#Schaeffer--2012|Schaeffer et al., 2012]] ; [[#Schuster--2018|Schuster et al., 2018]] ).

<div id="12.3.4.3" class="h3-container"></div>

<span id="lake-river-and-sea-ice"></span>
==== 12.3.4.3 Lake, River and Sea Ice ====

<div id="h3-19-siblings" class="h3-siblings"></div>

Reductions in the duration of thick sea, lake and river ice influence ecosystems as well as ice fishing, hunting, dog sledding and snowmobiling, which are recreation activities for some but vital aspects of many traditional indigenous communities ( [[#Durkalec--2015|Durkalec et al., 2015]] ; [[#AMAP--2017|AMAP, 2017]] ; [[#Baztan--2017|Baztan et al., 2017]] ; [[#Arp--2018|Arp et al., 2018]] ; [[#Rokaya--2018|Rokaya et al., 2018]] ; [[#Knoll--2019|Knoll et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Sharma--2019|Sharma et al., 2019]] ). The seasonal extent of thin ice and iceberg density also determines the viability of shipping lanes and seasonal roads ( [[#Valsson--2011|Valsson and Ulfarsson, 2011]] ; [[#Pizzolato--2016|Pizzolato et al., 2016]] ; [[#AMAP--2017|AMAP, 2017]] ; [[#Mullan--2017|Mullan et al., 2017]] ; [[#Sturm--2017|Sturm et al., 2017]] ), oil and gas exploration timing ( [[#Schaeffer--2012|Schaeffer et al., 2012]] ) and the seasonality of phytoplankton blooms ( [[#Oziel--2017|Oziel et al., 2017]] ). Sea ice is a critical aspect of some ecosystems and fisheries ( [[#Massom--2010|Massom and Stammerjohn, 2010]] ; [[#Jenouvrier--2014|Jenouvrier et al., 2014]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ). Various definitions of ‘ice free’ Arctic Ocean conditions can be tailored to represent transportation needs, including thresholds of ice coverage (&lt;5% or &lt;30% or &lt;1 million km <sup>2</sup> ) in September or over a four-month period ( [[#Laliberté--2016|Laliberté et al., 2016]] ; [[#Jahn--2018|Jahn, 2018]] ).

<div id="12.3.4.4" class="h3-container"></div>

<span id="heavy-snowfall-and-ice-storm"></span>
==== 12.3.4.4 Heavy Snowfall and Ice Storm ====

<div id="h3-20-siblings" class="h3-siblings"></div>

Heavy snowfall is a substantial concern for cities, settlements and key transportation and energy infrastructure ( [[#Ward--2013|Ward, 2013]] ; [[#Palko--2017|Palko, 2017]] ; [[#Janoski--2018|Janoski et al., 2018]] ; [[#Collins--2019|Collins et al., 2019]] ). Heavy snowfall can interfere with transportation ( [[#Herring--2018|Herring et al., 2018]] ) and cause a loss of both work and school days depending on local snow removal infrastructure. Freezing rain and ice storms can be treacherous for road and air travel ( [[#Tamerius--2016|Tamerius et al., 2016]] ), and can knock down power and telecommunication lines if ice accumulation is high ( [[#Degelia--2016|Degelia et al., 2016]] ). Rain-on-snow events can create a solid barrier that hinders wildlife and livestock grazing that is important to indigenous communities ( [[#Forbes--2016|Forbes et al., 2016]] ). Shifts in the frequency, seasonal timing and regions susceptible to ice storms alter risks for agriculture and infrastructure ( [[#Lambert--2011|Lambert and Hansen, 2011]] ; [[#Klima--2015|Klima and Morgan, 2015]] ; [[#Ning--2015|Ning and Bradley, 2015]] ; [[#Groisman--2016|Groisman et al., 2016]] ).

<div id="12.3.4.5" class="h3-container"></div>

<span id="hail"></span>
==== 12.3.4.5 Hail ====

<div id="h3-21-siblings" class="h3-siblings"></div>

Information on the changing frequency and size distribution of hail can help stakeholders build resilience for agriculture, vehicles, transportation infrastructure and buildings, solar panels and wild species that see critical damage at particular hail size thresholds ( [[#Dessens--2007|Dessens et al., 2007]] ; [[#Webb--2009|Webb et al., 2009]] ; [[#Patt--2013|Patt et al., 2013]] ; [[#Fiss--2019|Fiss et al., 2019]] ). Most climate models do not directly resolve hail and therefore studies often examine proxies associated with severe mesoscale storms ( [[#Tippett--2015|Tippett et al., 2015]] ; [[#Prein--2018|Prein and Holland, 2018]] ), although some regional studies now utilize hail-resolving models ( [[#Mahoney--2012|Mahoney et al., 2012]] ; [[#Brimelow--2017|Brimelow et al., 2017]] ).

<div id="12.3.4.6" class="h3-container"></div>

<span id="snow-avalanche"></span>
==== 12.3.4.6 Snow Avalanche ====

<div id="h3-22-siblings" class="h3-siblings"></div>

Information about the changing frequency and seasonal timing of snow avalanches is important to assess threats to transportation routes, infrastructure, recreational skiing and people living in alpine communities ( [[#Lazar--2008|Lazar and Williams, 2008]] ; [[#Mock--2017|Mock et al., 2017]] ; [[#Ballesteros-Cánovas--2018|Ballesteros-Cánovas et al., 2018]] ; [[#Hock--2019|Hock et al., 2019]] ). Like landslides and other mass movements, snow avalanches are not directly resolved by climate models and are thus tracked using proxy climate information describing snow avalanche susceptibility, particularly the snow water equivalent, and triggering mechanisms such as warm spells, high winds, rain-on-snow and heavy precipitation ( [[#Hock--2019|Hock et al., 2019]] ). The quality of snow also provides insight into avalanche hazards ( [[#Mock--2017|Mock et al., 2017]] ), with the seasonal altitude of wet snowpack (&gt;0.5% liquid water by volume) particularly important in determining characteristics of potential avalanches ( [[#Castebrunet--2014|Castebrunet et al., 2014]] ).

<div id="12.3.5" class="h2-container"></div>

<span id="coastal"></span>