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

==== 2.3.1.1 Changes in River Runoff ====

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AR5 reported increased winter flows and a shift in timing towards earlier spring snowmelt runoff peaks during previous decades ( ''robust evidence, high agreement'' ). In glacier-fed river basins, it was projected that melt water yields from glaciers will increase for decades in many regions but then decline ( ''very high confidence'' ). These findings have been further supported and refined by a wealth of new studies since AR5.

Recent studies indicate considerable changes in the seasonality of runoff in snow and glacier dominated river basins ( ''very high confidence'' ; Table SM2.9). Several studies have reported an increase in average winter runoff over the past decades, for example in Western Canada (Moyer et al., 2016 <sup>[[#fn:r238|238]]</sup> ), the European Alps (Bocchiola, 2014 <sup>[[#fn:r239|239]]</sup> ; Bard et al., 2015 <sup>[[#fn:r240|240]]</sup> ) and Norway (Fleming and Dahlke, 2014 <sup>[[#fn:r241|241]]</sup> ), due to more precipitation falling as rain under warmer conditions. Summer runoff has been observed to decrease in basins, for example in Western Canada (Brahney et al., 2017 <sup>[[#fn:r242|242]]</sup> ) and the European Alps (Bocchiola, 2014 <sup>[[#fn:r243|243]]</sup> ), but to increase in several basins in High Mountain Asia (Mukhopadhyay and Khan, 2014 <sup>[[#fn:r244|244]]</sup> ; Duethmann et al., 2015 <sup>[[#fn:r245|245]]</sup> ; Reggiani and Rientjes, 2015 <sup>[[#fn:r246|246]]</sup> ; Engelhardt et al., 2017 <sup>[[#fn:r247|247]]</sup> ). Both increases, for example, in Alaska (Beamer et al., 2016 <sup>[[#fn:r248|248]]</sup> ) and the Tien Shan (Wang et al., 2015 <sup>[[#fn:r249|249]]</sup> ; Chen et al., 2016 <sup>[[#fn:r250|250]]</sup> ), and decreases, for example, in Western Canada (Brahney et al., 2017 <sup>[[#fn:r251|251]]</sup> ) have also been found for average annual runoff. In Western Austria, Kormann et al. (2015) detected an increase in annual flow at high elevations and a decrease at low elevations between 1980–2010.

These contrasting trends for summer and annual runoff often result from spatially variable changes in the contribution of glacier and snow melt. As glaciers shrink, annual glacier runoff typically first increases, until a turning point, often called ‘peak water’ is reached, upon which runoff declines (FAQ 2.1). There is ''robust evidence'' and ''high agreement'' that peak water in glacier-fed rivers has already passed with annual runoff declining especially in mountain regions with predominantly smaller glaciers, for example, in the tropical Andes (Frans et al., 2015 <sup>[[#fn:r252|252]]</sup> ; Polk et al., 2017 <sup>[[#fn:r253|253]]</sup> ), Western Canada (Fleming and Dahlke, 2014 <sup>[[#fn:r254|254]]</sup> ; Brahney et al., 2017 <sup>[[#fn:r255|255]]</sup> ) and the Swiss Alps (Huss and Fischer, 2016 <sup>[[#fn:r256|256]]</sup> ). A global modelling study (Huss and Hock, 2018 <sup>[[#fn:r257|257]]</sup> ) suggests that peak water has been reached before 2019 for 82–95% of the glacier area in the tropical Andes, 40–49% in Western Canada and USA, and 55–67% in Central Europe (including European Alps and Pyrenees) and the Caucasus (Figure 2.6).

Projections indicate a continued increase in winter runoff in many snow and/or glacier-fed rivers over the 21st century ( ''high confidence'' ) regardless of the climate scenario, for example, in North America (Schnorbus et al., 2014 <sup>[[#fn:r258|258]]</sup> ; Sultana and Choi, 2018 <sup>[[#fn:r259|259]]</sup> ), the European Alps (Addor et al., 2014 <sup>[[#fn:r260|260]]</sup> ; Bosshard et al., 2014 <sup>[[#fn:r261|261]]</sup> ), Scotland (Capell et al., 2014 <sup>[[#fn:r262|262]]</sup> ) and High Mountain Asia (Kriegel et al., 2013 <sup>[[#fn:r263|263]]</sup> ) due to increased winter snowmelt and more precipitation falling as rain in addition to increases in precipitation in some basins (Table SM2.9). There is ''robust evidence'' ( ''high agreement'' ) that summer runoff will decline over the 21st century in many basins for all emission scenarios, for example, in Western Canada and USA (Shrestha et al., 2017 <sup>[[#fn:r264|264]]</sup> ), the European Alps (Jenicek et al., 2018 <sup>[[#fn:r265|265]]</sup> ), High Mountain Asia (Prasch et al., 2013 <sup>[[#fn:r266|266]]</sup> ; Engelhardt et al., 2017 <sup>[[#fn:r267|267]]</sup> ) and the tropical Andes (Baraer et al., 2012 <sup>[[#fn:r268|268]]</sup> ), due to less snowfall and decreases in glacier melt after peak water. A global-scale projection suggests that decline in glacier runoff by 2100 (RCP8.5) may reduce basin runoff by 10% or more in at least one month of the melt season in several large river basins, especially in High Mountain Asia during dry seasons, despite glacier cover of less than a few percent (Huss and Hock, 2018 <sup>[[#fn:r269|269]]</sup> ).

Projected changes in annual runoff in glacier dominated basins are complex including increases and decreases over the 21st century for all scenarios depending on the time period and the timing of peak water ( ''high confidence'' ) (Figure 2.6). Local and regional-scale projections in High Mountain Asia, the European Alps, and Western Canada and USA suggest that peak water will generally be reached before or around the middle of the century. These finding are consistent with results from global-scale modelling of glacier runoff (Bliss et al., 2014 <sup>[[#fn:r270|270]]</sup> ; Huss and Hock, 2018 <sup>[[#fn:r271|271]]</sup> ) indicating generally earlier peak water in regions with little ice cover and smaller glaciers (e.g., Low Latitudes, European Alps and Pyrenees, and the Caucasus) and later peak water in regions with extensive ice cover and large glaciers (e.g., Alaska, Southern Andes). In some regions (e.g., Iceland) peak water from most glacier area is projected to occur earlier for RCP2.6 than RCP8.5, caused by decreasing glacier runoff as glaciers find a new equilibrium. In contrast melt-driven glacier runoff continues to rise for the higher emission scenario. There is ''very high confidence'' that spring peak runoff in many snow-dominated basins around the world will occur earlier in the year, up to several weeks, by the end of the century caused by earlier snowmelt (e.g., Coppola et al., 2014; Bard et al., 2015 <sup>[[#fn:r272|272]]</sup> ; Yucel et al., 2015 <sup>[[#fn:r273|273]]</sup> ; Islam et al., 2017 <sup>[[#fn:r274|274]]</sup> ; Sultana and Choi, 2018 <sup>[[#fn:r275|275]]</sup> ).

In addition to changes in ice and snow melt, changes in other variables such as precipitation and evapotranspiration due to atmospheric warming or vegetation change affect runoff amounts and timing (e.g., Bocchiola, 2014; Lutz et al., 2016 <sup>[[#fn:r276|276]]</sup> ). Changes in melt water from ice and snow often dominates the runoff response to climate change at higher elevations, while changes in precipitation and evapotranspiration become increasingly important at lower elevations (Kormann et al., 2015 <sup>[[#fn:r277|277]]</sup> ). Permafrost thaw may affect runoff by releasing water from ground ice (Jones et al., 2018 <sup>[[#fn:r278|278]]</sup> ) and indirectly by changing hydrological pathways or ground water recharge as permafrost degrades (Lamontagne-Hallé et al., 2018 <sup>[[#fn:r279|279]]</sup> ). The relative importance of runoff from thawing permafrost compared to runoff from melting glaciers is expected to be greatest in arid areas where permafrost tends to be more abundant (Gruber et al., 2017 <sup>[[#fn:r280|280]]</sup> ). Because glaciers react more rapidly to climate change than permafrost, runoff in some mountain landscapes may become increasingly affected by permafrost thaw in the future (Jones et al., 2018 <sup>[[#fn:r281|281]]</sup> ).

In summary, there is ''very high confidence'' that glacier and snow cover decline have affected and will continue to change the amounts and seasonality of river runoff in many snow-dominated and/or glacier-fed river basins. The average winter runoff is expected to increase ( ''high confidence'' ), and spring peak maxima will occur earlier ( ''very'' ''high confidence'' ). Although observed and projected trends in annual runoff vary substantially among regions and can even be opposite in sign, there is ''high confidence'' that average annual runoff from glaciers will have reached a peak, with declining runoff thereafter, at the latest by the end of the 21st century in all regions regardless emission scenario. The projected changes in runoff are expected to affect downstream water management, related hazards and ecosystems (Section 2.3.2, 2.3.3).

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'''Figure 2.6'''

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'''Figure 2.6 | Timing of peak water from glaciers in different regions (Figure 2.1) under two emission scenarios for Representative Concentration Pathways RCP2.6 and RCP8.5. Peak water refers to the year when annual runoff from the initially glacier-covered area will start to decrease due to glacier shrinkage after a period of melt induced increase. The […]'''
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Figure 2.6 | Timing of peak water from glaciers in different regions (Figure 2.1) under two emission scenarios for Representative Concentration Pathways RCP2.6 and RCP8.5. Peak water refers to the year when annual runoff from the initially glacier-covered area will start to decrease due to glacier shrinkage after a period of melt induced increase. The bars are based on Huss and Hock (2018) who used a global glacier model to compute the runoff of all individual glaciers in a region until year 2100 based on 14 General Circulation Models (GCMs). Depicted is the area of all glaciers that fall into the same 10-year peak water interval expressed as a percentage of each region’s total glacier area, i.e., all bars for the same RCP sum up to 100% glacier area. Shadings of the bars distinguish different glacier sizes indicating a tendency for peak water to occur later for larger glaciers. Circles/diamonds mark timing of peak water from individual case studies based on observations or modelling (Table SM2.10). Circles refer to results from individual glaciers regardless of size or a collection of glaciers covering 150 km2 glacier coverage. Case studies based on observations or scenarios other than RCP2.6 and RCP8.5 are shown in both the left and right set of panels.

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