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

==== 12.4.5.4 Snow and Ice ====

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'''Snow:''' Widespread and accelerated declines in snow depth ( [[#Fontrodona%20Bach--2018|Fontrodona Bach et al., 2018]] ) and snow water equivalent ( [[#Marty--2017a|Marty et al., 2017a]] ; see Figure 12.9b) have been observed in Europe. In the Pyrenees a slow snow cover decline has been observed starting from the industrial period with a sharp increase since 1955 ( [[#López-Moreno--2020|López-Moreno et al., 2020]] ). Under the RCP2.6, RCP4.5 and RCP8.5 scenarios the reliability elevation for snowmaking will rise by 200–300 m in the Alps and 400–600 m in the Pyrenees by mid-century. End of century projections of natural snow conditions are highly dependent on the scenario, being stationary for the RCP2.6 and continuously decreasing under RCP8.5 to not have any more natural snow conditions at any of the locations in the French Alps and Pyrenees ( [[#Spandre--2019|Spandre et al., 2019]] ). Similarly Norway and Austria will also see a rising of the natural snow elevation with consequences for the ski season ( [[#Scott--2020|Scott et al., 2020]] ; [[#Steiger--2020|Steiger and Scott, 2020]] ). In the Alps, recent simulations project a reduction in snow water equivalent (SWE) at 1500 m above sea level of 80–90% by 2100 under the A1B scenario and a snow season that would start 2-4 weeks later and end 5-10 weeks earlier than the 1992–2012 average ( [[#Schmucki--2015|Schmucki et al., 2015]] ), which is equivalent to a shift in elevation of about 700 m ( [[#Marty--2017b|Marty et al., 2017b]] ). For elevations above 3000 m above sea level, a decline in SWE of at least 10% is expected by the end of the century even when assuming the largest projected precipitation increase. Similar trends are observed for the Pyrenees and Scandinavia ( [[#López-Moreno--2009|López-Moreno et al., 2009]] ; [[#Räisänen--2012|Räisänen and Eklund, 2012]] ). For the northern French Alps above 1500 m and the Ötztal locations in the Austrian alps SWE has a similar decreasing trend altitudinally dependent for RCP2.6, RCP4.5 and RCP8.5 until mid-century and with significant differentiation among them in the second half of the century up to snow-free conditions under RCP8.5 ( [[#Hanzer--2018|Hanzer et al., 2018]] ; [[#Verfaillie--2018|Verfaillie et al., 2018]] ).

'''Glacier:''' Observations and future projections of European glacier mass changes are assessed in [[IPCC:Wg1:Chapter:Chapter-9#9.5.1%20|Section 9.5.1]] grouped in two main regions: Scandinavia and central Europe regions. It is ''virtually certain'' that glaciers will shrink in the future and there is ''medium confidence'' in the timing and mass change rates ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1|Section 9.5.1]] ). Central Europe is one of the regions where glaciers are projected to lose substantial mass even under low-emissions scenarios ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1.3|Section 9.5.1.3]] ; [[#MedECC--2020|MedECC, 2020]] ). GlacierMIP projections indicate that glaciers in the central Europe region will lose 63 ± 31%, 80 ± 22% and 93 ± 13% of their 2015 mass by the end of the century under RCP2.6, RCP4.5 and RCP8.5 respectively ( [[#Marzeion--2020|Marzeion et al., 2020]] ). For the same scenarios, glaciers in Scandinavia are projected to lose 55 ± 33%, 66 ± 34% and 82 ± 24% of their 2015 mass. The ''virtually certain'' shrink in glaciers is bolstered by RCM simulations from the EURO-CORDEX ensemble, with the Global Glacier Evolution Model (GloGEM) indicating a substantial reduction of glacier ice volumes in the European Alps by 2050 (47–52% with respect to 2017 for RCP2.6, RCP4.5 and RCP8.5). Under RCP2.6, about two-thirds (63 ± 11%) of the present-day (2017) ice volume is projected to be lost by 2100. In contrast, under the strong warming of RCP8.5, glaciers in the European Alps are projected to largely disappear by 2100 (94 ± 4% volume loss compared to 2017; [[#Zekollari--2019|Zekollari et al., 2019]] ).

'''Permafrost:''' In Europe, permafrost is found in high mountains and in Scandinavia, as well as in Arctic Islands (e.g., Iceland, Novaya Zemlia or Svalbard). In recent decades permafrost has been lost ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] ) and accelerated warming at high altitudes and latitudes has favoured an increase of permafrost temperatures of the order of 0.2 ± 0.1°C between 2007 and 2016 ( [[#Romanovsky--2018|Romanovsky et al., 2018]] ; [[#Noetzli--2019|Noetzli et al., 2019]] ). Over the 21st century, permafrost is ''very'' ''likely'' to undergo increasing thaw and degradation under all scenarios ( [[#Hock--2019|Hock et al., 2019]] ) and it is ''virtually certain'' that permafrost extent and volume will decrease with increase of global warming ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] ).

Permafrost thawing is projected to affect the frequency and magnitude of high-mountain mass wasting processes ( [[#Stoffel--2012|Stoffel and Huggel, 2012]] ). The temporal frequency of periglacial debris flows in the Alps is ''unlikely'' to change significantly by the mid-21st century but is ''likely'' to decrease during the second part of the century under the A1B scenario, especially in summer ( [[#Stoffel--2011|Stoffel et al., 2011]] , 2014). There is ''medium confidence'' that most of the Northern Europe periglacial processes will disappear by the end of the century, even in the RCP2.6 scenario ( [[#Aalto--2017|Aalto et al., 2017]] ). The magnitude of debris flow events might increase ( [[#Lugon--2010|Lugon and Stoffel, 2010]] ) and the debris-flow season may last longer under the A1B scenario ( [[#Stoffel--2018|Stoffel and Corona, 2018]] ) ''.'' Quantitative data for the European Alps is highly site dependent ( [[#Haeberli--2013|Haeberli, 2013]] ).

'''Heavy snowfall, ice storms and hail:''' There is ''low confidence'' that climate change will affect ice and snow-related episodic hazards ( ''limited evidence'' ). The change in snowpack in the Alps is expected to lead to a possible reduction in overall avalanche activity by end of the century ( ''low confidence'' ), except possibly in winter and at high altitudes ( [[#Castebrunet--2014|Castebrunet et al., 2014]] ).

For ice storms, or freezing rainstorms, there is also ''limited evidence'' due to a limited number of studies. Heavy snowfalls have decreased in frequency in the past decades and this is expected to continue in the future climate ( ''low confidence'' ) ( [[#Beniston--2018|Beniston et al., 2018]] ). Freezing rain is projected to increase in western, central and southern Europe by the end of the century under RCP4.5 and RCP8.5 ( ''low confidence'' ) ( [[#Kämäräinen--2018|Kämäräinen et al., 2018]] ). Rain-on-snow events, are decreasing in northern regions ( [[#Pall--2019|Pall et al., 2019]] ) and by 48% on average in southern Scandinavia ( [[#Poschlod--2020|Poschlod et al., 2020]] ) due to decreases in snowfall.

'''In summary, future snow cover extent and seasonal duration will reduce''' ( high confidence ''') and it is''' virtually certain '''that glaciers will continue to shrink.''' '''A reduction of glacier ice volume is projected in the European Alps and Scandinavia''' ( high confidence '''). There is''' high confidence '''that permafrost will undergo increasing thaw and degradation over the 21st century.''' '''Most of the Northern Europe periglacial will disappear by the end of the century even for a lower emissions scenario''' ( medium confidence ''') and the debris-flow season may last longer in a warming climate''' ( medium confidence ''').'''

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