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

==== 12.4.6.4 Snow and Ice ====

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'''Snow:''' The seasonal extent of snow cover has reduced over North America in recent decades ( ''robust evidence'' , ''high agreement'' ) (see also Sections 2.3.2.2 and 9.5.3, and Figure Atlas.25). The average snow-cover extent in North America decreased at a rate of about 8500 km <sup>2</sup> yr <sup>–1</sup> over the 1972–2015 period, reducing the average snow cover season by two weeks, primarily due to earlier spring melt ( [[#US%20EPA--2016|US EPA, 2016]] ). Observations indicate earlier spring snowpack melting ( [[#Dudley--2017|Dudley et al., 2017]] ) and a reduction in end-of-season snowpack metrics important to water resources over the Rocky Mountains (particularly since 1980) and Pacific Northwest ( [[#Pederson--2013|Pederson et al., 2013]] ; [[#Kormos--2016|Kormos et al., 2016]] ; [[#Kunkel--2016|Kunkel et al., 2016]] ; [[#Fyfe--2017|Fyfe et al., 2017]] ; [[#Mote--2018|Mote et al., 2018]] ). In situ measurements in Canada show more heterogenous trends in snow amount and density ( [[#Brown--2019|Brown et al., 2019]] ).

Climate change is expected to reduce the total snow amount and the length of the snow cover season over most of North America, with a corresponding decrease in the proportion of total precipitation falling as snow and a reduction in end-of-season snowpack ( ''high confidence'' ) (see Atlas.9.5). Changes include a reduction in the number of days with snowfall in across all of North America, with the exception of northern Canada ( [[#Danco--2016|Danco et al., 2016]] ; [[#McCrary--2019|McCrary and Mearns, 2019]] ), a delay of about a week in first snowfall in the western USA by 2050 under RCP8.5 ( [[#Pierce--2013|Pierce and Cayan, 2013]] ), and more prominent reductions in Canadian snow cover in the October –December period ( [[#Mudryk--2018|Mudryk et al., 2018]] ). Reduced total snowpack and earlier snowmelt lower dry season streamflow ( [[#Kormos--2016|Kormos et al., 2016]] ; [[#Rhoades--2018|Rhoades et al., 2018]] ). Figure 12.10b shows a reduction in days suitable for skiing (SWE &gt; 10 cm; [[#Wobus--2017b|Wobus et al., 2017b]] ) across the USA and southern Canada, although some portions of northern central Canada see an increase.

'''Glacier:''' Section 9.5.1 assessed that glaciers in Alaska, western Canada and the western USA are expected to continue to lose mass and areal extent ( ''high confidence'' ). Compared to their 2015 state, glaciers in the western Canada and the USA region will lose 62 ± 30%, 75 ± 29% and 85 ± 23%, of their mass by the end of the century for RCP2.6, RCP4.5 and RCP8.5 scenarios, respectively ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Meanwhile glaciers in Alaska will lose 26 ± 21%, 31 ± 24% and 44 ± 27%, of their 2015 mass under the same scenarios. The overall loss of glacial mass can act as a meltwater supply for freshwater resources, although this is expected to peak in the middle of the century and then fade as glaciers disappear ( [[#Fyfe--2017|Fyfe et al., 2017]] ; [[#Derksen--2018|Derksen et al., 2018]] ). Continued shrinkage of glaciers is projected to create further glacial lakes ( ''medium confidence'' ) similar to those that have led to outburst floods in Alaska and Canada ( [[#Carrivick--2016|Carrivick and Tweed, 2016]] ; [[#Harrison--2018|Harrison et al., 2018]] ).

'''Permafrost:''' Warmer ground temperatures are expected to extend the geographical extent and depth of permafrost thaw across northern North America ( ''very'' ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] ). Observations across Canada show that permafrost temperature is increasing and the active layer is getting thicker ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] ; [[#Derksen--2018|Derksen et al., 2018]] ; [[#Biskaborn--2019|Biskaborn et al., 2019]] ; [[#Romanovsky--2020|Romanovsky et al., 2020]] ). [[#Slater--2013|Slater and Lawrence (2013)]] note that the RCP8.5 end-of-century period in North America only has shallow permafrost as the most probable condition in the Canadian Archipelago. [[#Melvin--2017|Melvin et al. (2017)]] noted the loss of shallow permafrost in five RCP8.5 CMIP5 models across a wide swathe of southern Alaska by 2050, along with increases of active layer thickness. There is ''high confidence'' in continued reductions in mountain near-surface permafrost area with high spatial variability given local snow and temperature changes ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] ; [[#Peng--2018|Peng et al., 2018]] ; [[#Hock--2019|Hock et al., 2019]] ).

'''Lake, river and sea ice:''' Anthropogenic warming reduces the seasonal extent of lake and river ice over many North American freshwater systems, with ice-free winter conditions pushing further north with rising temperatures ( ''high confidence'' ). Observations in Central and Eastern North America show reduced average seasonal lake-ice cover duration ( [[#Benson--2012|Benson et al., 2012]] ; [[#Mason--2016|Mason et al., 2016]] ; [[#US%20EPA--2016|US EPA, 2016]] ). Satellite observations show declines in lake ice ( [[#Du--2017|Du et al., 2017]] ) and loss of more than 20% of winter river-ice length in much of Alaska (2008–2018 compared to 1984–1994; [[#Yang--2020a|Yang et al., 2020a]] ). Spring lake and river ice in Canada is projected to break up 10–25 days earlier while autumn freeze-up occurs 5–15 days later by mid-century, with larger declines in lake-ice season closer to the coasts ( [[#Dibike--2012|Dibike et al., 2012]] ) and for rivers in the Rocky Mountains and north-eastern USA ( [[#Yang--2020a|Yang et al., 2020a]] ), although global models have difficulty with frozen freshwater system dynamics ( [[#Derksen--2018|Derksen et al., 2018]] ). Substantial ice loss is projected over the Laurentian Great Lakes ( [[#Hewer--2019|Hewer and Gough, 2019]] ; [[#Matsumoto--2019|Matsumoto et al., 2019]] ). The southern extent of lakes experiencing intermittent winter ice cover moves northward with rising temperature, pushing nearly out of the continental USA at low elevations under a 4.5°C GWL ( [[#Sharma--2019|Sharma et al., 2019]] ). Higher spring flows and the potential for winter thaws are also projected to heighten the threat of ice jams ( [[#Rokaya--2018|Rokaya et al., 2018]] ; [[#Bonsal--2019|Bonsal et al., 2019]] ) while reducing the seasonal viability of ice roads and recreational use ( [[#Pendakur--2016|Pendakur, 2016]] ; [[#Mullan--2017|Mullan et al., 2017]] ; [[#Knoll--2019|Knoll et al., 2019]] ).

Seasonal sea ice coverage along the majority of Canadian and Alaskan coastlines is declining ( ''robust evidence'' , ''high agreement'' ) and there is ''high confidence'' that sea ice loss continues under climate change, as further assessed in [[#12.4.9|Section 12.4.9]] .

'''Heavy snowfall:''' There is ''low agreement'' ( ''limited evidence'' ) for observed changes in heavy snowfall in North America. [[#Kluver--2015|Kluver and Leathers (2015)]] noted a 1930–2008 frequency increase for all snow intensities in the northern Great Plains but declines in heavier snow events in the Pacific Northwest and declines in the south-eastern USA. [[#Changnon--2018|Changnon (2018)]] found that most extreme 30-day high-snowfall periods in the 1900–2016 record over the eastern USA occurred in the 1959–1987 period, which lies between the 1930s Dust Bowl and recent warming. There is ''low agreement'' and ''medium evidence'' for broad projected changes to heavy snowfall over North America given increased heavy precipitation and warmer winter temperatures. Several recent regional studies have projected that low-intensity events decrease more rapidly than heavy snowfall events, resulting in an increase in the snowfall proportion from heavy snowfall events even as the number of such events decreases ( [[#O’Gorman--2014|O’Gorman, 2014]] ; [[#Lute--2015|Lute et al., 2015]] ; [[#Zarzycki--2016|Zarzycki, 2016]] ; [[#Janoski--2018|Janoski et al., 2018]] ; [[#Ashley--2020|Ashley et al., 2020]] ).

'''Ice storm:''' There is ''limited evidence'' in the literature of unique changes in ice storms observed or projected over North America. [[#Groisman--2016|Groisman et al. (2016)]] examined 40 years of observations and found weak decreases in freezing rain events over the south-eastern USA in the most recent decade. [[#Ning--2015|Ning and Bradley (2015)]] project that the average snow–rain transition line, which is associated with mixed precipitation, moves 2° latitude northward over Eastern North America by the end of the 21st century under RCP4.5 (4° under RCP8.5; see also [[#Klima--2015|Klima and Morgan, 2015]] ).

'''Hail:''' There is ''limited evidence'' and ''low agreement'' for observed changes in the frequency or intensity of North American hail storms. J.T. [[#Allen--2015|]] [[#Allen--2015|Allen et al. (2015)]] and [[#Allen--2018|Allen (2018)]] found that temporal inconsistencies in the US and Canadian hail records made long-term climate analysis difficult, although B.H. [[#Tang--2019|]] [[#Tang--2019|Tang et al. (2019)]] identified an increasing frequency of environmental conditions conducive for large hail (diameter ≥ 5 cm) over the central and eastern USA. There is ''limited evidence'' and ''medium agreement'' in projections of increased hail damage potential over North America. Some regional and convective-permitting model projections indicate a longer hail season with fewer events and larger hail sizes that result in higher hail damage potential ( [[#Brimelow--2017|Brimelow et al., 2017]] ; [[#Trapp--2019|Trapp et al., 2019]] ).

'''Snow avalanche:''' There is ''limited evidence'' of directional changes in snow avalanches over North America. [[#Mock--2000|Mock and Birkeland (2000)]] identified a 1969–1995 decrease in snow avalanches over the western United States, although they note the heavy influence of natural variability. A similar decline was observed over western Canada ( [[#Bellaire--2016|Bellaire et al., 2016]] ; [[#Sinickas--2016|Sinickas et al., 2016]] ), but clear trends are difficult to discern given sparse observations and shifts in avalanche management. We concur with the SROCC assessment of ''medium confidence'' and ''high agreement'' that snow avalanche hazards generally decrease at low elevations given lower snowpack, even as high elevations are increasingly susceptible to wet-snow avalanches ( [[#Hock--2019|Hock et al., 2019]] ; see also [[#Lazar--2008|Lazar and Williams, 2008]] ).

'''Observations and projections agree that snow and ice CIDs over North America are characterized by reduction in glaciers and the seasonality of snow and ice formation, loss of shallow permafrost, and shifts in the rain/snow transition line that alters the seasonal and geographic range of snow and ice conditions in the coming decades''' ( very high confidence ''').'''

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