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

===== 2.3.2.1.3 Floods =====

Glacier-related floods, including floods from lake outbursts (GLOFs), are documented for most glacierised mountain ranges and are among the most far-reaching glacier hazards. Past events affected areas tens to hundreds of kilometres downstream (Carrivick and Tweed, 2016 <sup>[[#fn:r532|532]]</sup> ). Retreating glaciers produced lakes at their fronts in many high mountain regions in recent decades (Frey et al., 2010 <sup>[[#fn:r533|533]]</sup> ; Gardelle et al., 2011 <sup>[[#fn:r534|534]]</sup> ; Loriaux and Casassa, 2013 <sup>[[#fn:r535|535]]</sup> ). Lake systems in High Mountain Asia also often developed on the surface of downwasting, low-slope glaciers where they coalesced from temporally variable supraglacial lakes (Benn et al., 2012 <sup>[[#fn:r536|536]]</sup> ; Narama et al., 2017 <sup>[[#fn:r537|537]]</sup> ). Corroborating SREX and AR5 findings, there is ''high confidence'' that current global glacier shrinkage caused new lakes to form and existing lakes to grow in most regions, for instance in South America, High mountain Asia and Europe (Loriaux and Casassa, 2013 <sup>[[#fn:r538|538]]</sup> ; Paul and Mölg, 2014 <sup>[[#fn:r539|539]]</sup> ; Zhang et al., 2015 <sup>[[#fn:r540|540]]</sup> ; Buckel et al., 2018 <sup>[[#fn:r541|541]]</sup> ). Exceptions occurred and are expected to occur in the future for few lakes where evaporation, runoff and reduced melt water influx in total led to a negative water balance (Sun et al., 2018a <sup>[[#fn:r542|542]]</sup> ). Also, advancing glaciers temporarily dammed rivers, lake sections, or fjords (Stearns et al., 2015 <sup>[[#fn:r543|543]]</sup> ), for instance through surging (Round et al., 2017 <sup>[[#fn:r544|544]]</sup> ), causing particularly large floods once the ice dams breached. Outbursts from water bodies in and under glaciers are able to cause floods similar to those from surface lakes but little is known about the processes involved and any trends under climate change. In some cases, the glacier thermal regime played a role so that climate driven changes in thermal regime are expected to alter the hazard potential, depending on local conditions (Gilbert et al., 2012 <sup>[[#fn:r545|545]]</sup> ). Another source of large water bodies under glaciers and subsequent floods has been subglacial volcanic activity (Section 2.3.2.1.4). There is also ''high confidence'' that the number and area of glacier lakes will continue to increase in most regions in the coming decades, and new lakes will develop closer to steep and potentially unstable mountain walls where lake outbursts can be more easily triggered by the impact of landslides (Frey et al., 2010 <sup>[[#fn:r546|546]]</sup> ; ICIMOD, 2011 <sup>[[#fn:r547|547]]</sup> ; Allen et al., 2016a <sup>[[#fn:r548|548]]</sup> ; Linsbauer et al., 2016 <sup>[[#fn:r549|549]]</sup> ; Colonia et al., 2017 <sup>[[#fn:r550|550]]</sup> ; Haeberli et al., 2017 <sup>[[#fn:r551|551]]</sup> ).

In contrast to the number and size of glacier lakes, trends in the number of glacier-related floods are not well known for recent decades (Carrivick and Tweed, 2016 <sup>[[#fn:r552|552]]</sup> ; Harrison et al., 2018 <sup>[[#fn:r553|553]]</sup> ), although a number of periods of increased and decreased flood activity have been documented for individual glaciers in North America and Greenland, spanning decades (Geertsema and Clague, 2005 <sup>[[#fn:r554|554]]</sup> ; Russell et al., 2011 <sup>[[#fn:r555|555]]</sup> ). A decrease in moraine-dammed glacier lake outburst floods in recent decades suggests a response of lake outburst activity being delayed by some decades with respect to glacier retreat (Harrison et al., 2018 <sup>[[#fn:r556|556]]</sup> ) but inventories might significantly underestimate the number of events (Veh et al., 2018 <sup>[[#fn:r557|557]]</sup> ). For the Himalaya, Veh et al. (2019) found no increase in the number of glacier lake outburst floods since the late 1980s. The degradation of permafrost and the melting of ice buried in lake dams have been shown to lower dam stability and contribute to outburst floods in many high mountain regions (Fujita et al., 2013 <sup>[[#fn:r558|558]]</sup> ; Erokhin et al., 2017 <sup>[[#fn:r559|559]]</sup> ; Narama et al., 2017 <sup>[[#fn:r560|560]]</sup> ).

Floods originating from the combination of rapidly melting snow and intense rainfall, referred to as rain-on-snow events, are some of the most damaging floods in mountain areas (Pomeroy et al., 2016 <sup>[[#fn:r561|561]]</sup> ; Il Jeong and Sushama, 2018). The hydrological response of a catchment to a rain-on-snow event depends on the characteristics of the precipitation event, but also on turbulent fluxes driven by wind and humidity, which typically provide most of the melting energy during such events (Pomeroy et al., 2016 <sup>[[#fn:r562|562]]</sup> ), and the state of the snowpack, in particular the liquid water content (Würzer et al., 2016 <sup>[[#fn:r563|563]]</sup> ). An increase in the occurrence of rain-on-snow events in high-elevation zones, and a decrease at the lowest elevations have been reported (Western USA, 1949–2003, McCabe et al. (2007); Oregon, 1986–2010, Surfleet and Tullos (2013) <sup>[[#fn:r564|564]]</sup> ; Switzerland, 1972–2016, Moran-Tejéda et al. (2016), central Europe, 1950–2010, Freudiger et al. (2014) <sup>[[#fn:r565|565]]</sup> . These trends are consistent with studies carried out at the scale of the Northern Hemisphere (Putkonen and Roe, 2003 <sup>[[#fn:r566|566]]</sup> ; Ye et al., 2008 <sup>[[#fn:r567|567]]</sup> ; Cohen et al., 2015 <sup>[[#fn:r568|568]]</sup> ). There are no studies found on this topic in Africa and South America. In summary, evidence since AR5 suggests that rain-on-snow events have increased over the last decades at high elevations, particularly during transition periods from autumn to winter and winter to spring ( ''medium confidence'' ). The occurrence of rain-on-snow events has decreased over the last decade in low-elevation or low-latitude areas due to a decreasing duration of the snowpack, except for the coldest months of the year ( ''medium confidence'' ).

Il Jeong and Sushama (2018) projected an increase in rain-on-snow events in winter and a decrease in spring, for the period 2041–2070 (RCP4.5 and RCP8.5) in North America, corroborated by Musselman et al. (2018). Their frequency in the Swiss Alps is projected to increase at elevations higher than 2,000 m a.s.l. (SRES A1B, 2025, 2055, and 2085) ( Beniston and Stoffel, 2016 <sup>[[#fn:r569|569]]</sup> ) . This study showed that the number of rain-on-snow events may increase by 50%, with a regional temperature increase of 2°C to 4°C, and decrease with a temperature rise exceeding 4°C. In Alaska, an overall increase of rain-on-snow events is projected, however with a projected decline in the southwestern/southern region (Bieniek et al., 2018 <sup>[[#fn:r570|570]]</sup> ). In summary, evidence since AR5 suggests that the frequency of rain-on-snow events is projected to increase and occur earlier in spring and later in autumn at higher elevation and to decrease at lower elevation ( ''high confidence'' ).

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