Editing IPCC:AR6/SROCC/Chapter-2 (section)

===== 2.3.2.1.4 Combined hazards and cascading events =====

The largest mountain disasters in terms of reach, damage and lives lost that involve ice, snow and permafrost occurred through a combination or chain of processes. New evidence since SREX and AR5 have these findings (Anacona et al., 2015a <sup>[[#fn:r571|571]]</sup> ; Evans and Delaney, 2015) <sup>[[#fn:r572|572]]</sup> . Some process chains occur frequently, while others are rare, specific to local circumstances and difficult to anticipate. Glacier lake outbursts were in many mountain regions and over recent decades documented to have been triggered by impact waves from snow-, ice- or rock-avalanches, landslides, iceberg calving events, or by temporary blockage of surface or subsurface drainage channels (Benn et al., 2012 <sup>[[#fn:r573|573]]</sup> ; Narama et al., 2017 <sup>[[#fn:r574|574]]</sup> ). Rock-slope instability and catastrophic failure along fjords caused tsunamis (Hermanns et al., 2014 <sup>[[#fn:r575|575]]</sup> ; Roberts et al., 2014 <sup>[[#fn:r576|576]]</sup> ). For instance, a landslide generated wave in 2015 at Taan Fjord, Alaska, ran up 193 m on the opposite slope and then travelled more than 20 km down the fjord (Higman et al., 2018 <sup>[[#fn:r577|577]]</sup> ). Earthquakes have been a starting point for different types of cascading events, for instance by causing snow-, ice- or rock-avalanches, and landslides (van der Woerd et al., 2004 <sup>[[#fn:r578|578]]</sup> ; Podolskiy et al., 2010 <sup>[[#fn:r579|579]]</sup> ; Cook and Butz, 2013 <sup>[[#fn:r580|580]]</sup> ; Sæmundsson et al., 2018 <sup>[[#fn:r581|581]]</sup> ). Glaciers and their moraines, including morainic lake dams, seem however, not particularly prone to earthquake triggered failure (Kargel et al., 2016 <sup>[[#fn:r582|582]]</sup> ).

Landslides and rock avalanches in glacier environments were often documented to entrain snow and ice that fluidise, and incorporate additional loose glacial sediments or water bodies, thereby multiplying their mobility, volume and reach (Schneider et al., 2011 <sup>[[#fn:r583|583]]</sup> ; Evans and Delaney, 2015 <sup>[[#fn:r584|584]]</sup> ). Rock avalanches onto glaciers triggered glacier advances in recent decades, for instance in North America, New Zealand and Europe, mainly through reducing surface melt (Deline, 2009 <sup>[[#fn:r585|585]]</sup> ; Reznichenko et al., 2011 <sup>[[#fn:r586|586]]</sup> ; Menounos et al., 2013 <sup>[[#fn:r587|587]]</sup> ). In glacier covered frozen rock walls, particularly complex thermal, mechanical, hydraulic and hydrologic interactions between steep glaciers, frozen rock and its ice content, and unfrozen rock sections lead to combined rock/ice instabilities that are difficult to observe and anticipate (Harris et al., 2009 <sup>[[#fn:r588|588]]</sup> ; Fischer et al., 2013 <sup>[[#fn:r589|589]]</sup> ; Ravanel et al., 2017 <sup>[[#fn:r590|590]]</sup> ). There is ''limited evidence'' of observed direct event chains to project future trends. However, from the observed and projected degradation of permafrost, shrinkage of glaciers and increase in glacier lakes it is reasonable to assume that event chains involving these could increase in frequency or magnitude, and that accordingly hazard zones could expand.

Volcanoes covered by snow and ice often produce substantial melt water during eruptions. This typically results in floods and/or lahars (mixtures of melt water and volcanic debris) which can be exceptionally violent and cause large-scale loss of life and destruction to infrastructure (Barr et al., 2018 <sup>[[#fn:r591|591]]</sup> ). The most devastating example from recent history occurred in 1985, when the medium-sized eruption of Nevado del Ruiz volcano, Colombia, produced lahars that killed more than 23,000 people some 70 km downstream (Pierson et al., 1990 <sup>[[#fn:r592|592]]</sup> ). Hazards associated with ice and snow-clad volcanoes have been reported mostly from the Cordilleras of the Americas, but also from the Aleutian arc (USA), Mexico, Kamchatka (Russia), Japan, New Zealand and Iceland (Seynova et al., 2017 <sup>[[#fn:r593|593]]</sup> ). In particular, under Icelandic glaciers, volcanic activity and eruptions melted large amounts of ice and caused especially large floods if water accumulated underneath the glacier (Björnsson, 2003 <sup>[[#fn:r594|594]]</sup> ; Seneviratne et al., 2012 <sup>[[#fn:r595|595]]</sup> ). There is ''medium confidence'' that the overall hazard related to floods and lahars from ice- and snow-clad volcanoes will gradually diminish over years-to-decades as glaciers and seasonal snow cover continue to decrease under climate change (Aguilera et al., 2004 <sup>[[#fn:r596|596]]</sup> ; Barr et al., 2018 <sup>[[#fn:r597|597]]</sup> ). On the other hand, shrinkage of glaciers may uncover steep slopes of unconsolidated volcanic sediments, thus decreasing in the future the resistance of these volcano flanks to heavy rain fall and increasing the hazard from related debris flows (Vallance, 2005 <sup>[[#fn:r598|598]]</sup> ). In summary, future changes in snow and ice are expected to modify the impacts of volcanic activity of snow- and ice-clad volcanoes ( ''high confidence)'' although in complex and locally variable ways and at a variety of time scales (Barr et al., 2018 <sup>[[#fn:r599|599]]</sup> ; Swindles et al., 2018 <sup>[[#fn:r600|600]]</sup> ).

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