Editing IPCC:AR6/SROCC/Chapter-3 (section)

==== 3.4.1.3 Freshwater Systems ====

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There is increasing awareness of the influence of a changing climate on freshwater systems across the Arctic, and associated impacts on hydrological, biogeophysical and ecological processes (Prowse et al., 2015 <sup>[[#fn:r1456|1456]]</sup> ; Walvoord and Kurylyk, 2016 <sup>[[#fn:r1457|1457]]</sup> ), and northern populations (Takakura, 2018 <sup>[[#fn:r1458|1458]]</sup> ) (Section 3.4.3.3.1). Assessing these impacts requires consideration of complex interconnected processes, many of which are incompletely observed. The increasing imprint of human development, such as flow regulation on major northerly flowing rivers adds complexity to the determination of climate-driven changes.

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===== 3.4.1.3.1 Freshwater ice =====

Long-term ''in situ'' river ice records indicate that the duration of ice cover in Russian Arctic rivers decreased by 7–20 days between 1955 and 2012 (Shiklomanov and Lammers, 2014 <sup>[[#fn:r1459|1459]]</sup> ) ( ''high confidence'' ). This is consistent with historical reductions in Arctic river ice cover derived from models (Park et al., 2015) and regional analysis of satellite data (Cooley and Pavelsky, 2016 <sup>[[#fn:r1461|1461]]</sup> ).

Analysis of satellite imagery between 2000 and 2013 identified a significant trend of earlier spring ice break-up across all regions of the Arctic (Šmejkalová et al., 2016 <sup>[[#fn:r1462|1462]]</sup> ); independent satellite data showed approximately 80% of Arctic lakes experienced declines in ice cover duration during 2002–2015, due to both a later freeze-up and earlier break-up (Du et al., 2017 <sup>[[#fn:r1463|1463]]</sup> ) ( ''high confidence'' ). There are indications that lake ice across Alaska has thinned in recent decades (Alexeev et al., 2016 <sup>[[#fn:r1464|1464]]</sup> ), but ice thickness trends are not available at the pan-Arctic scale. Analysis of satellite data over northern Alaska show that approximately one-third of bedfast lakes (the entire water volume freezes by the end of winter) experienced a regime change to floating ice over the 1992–2011 period (Surdu et al., 2014 <sup>[[#fn:r1465|1465]]</sup> ; Arp et al., 2015 <sup>[[#fn:r1466|1466]]</sup> ). This can result in degradation of underlying permafrost (Arp et al., 2016 <sup>[[#fn:r1467|1467]]</sup> ; Bartsch et al., 2017 <sup>[[#fn:r1468|1468]]</sup> ). Lakes of the central and eastern Canadian High Arctic are transitioning from a perennial to seasonal ice regime (Surdu et al., 2016 <sup>[[#fn:r1469|1469]]</sup> ).

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===== 3.4.1.3.2 Runoff and surface water =====

A general trend of increasing discharge has been observed for large Siberian (Troy et al., 2012 <sup>[[#fn:r1470|1470]]</sup> ; Walvoord and Kurylyk, 2016 <sup>[[#fn:r1471|1471]]</sup> ) and Canadian (Ge et al., 2013 <sup>[[#fn:r1472|1472]]</sup> ; Déry et al., 2016 <sup>[[#fn:r1473|1473]]</sup> ) rivers that drain to the Arctic Ocean ( ''medium confidence'' ). Between 1976 and 2017, trends are 3.3 ± 1.6% for Eurasian rivers and 2.0 ± 1.8% for North American rivers (Holmes et al., 2018 <sup>[[#fn:r1474|1474]]</sup> ) (Figure 3.10). Extreme regional runoff events have also been identified (Stuefer et al., 2017 <sup>[[#fn:r1475|1475]]</sup> ). An observed increase in baseflow in the North American (Walvoord and Striegl, 2007 <sup>[[#fn:r1476|1476]]</sup> ; St. Jacques and Sauchyn, 2009) and Eurasian Arctic (Smith et al., 2007 <sup>[[#fn:r1477|1477]]</sup> ; Duan et al., 2017 <sup>[[#fn:r1478|1478]]</sup> ) over the last several decades is attributable to permafrost thaw and concomitant enhancement in groundwater discharge. The timing of spring season peak flow is generally earlier (Ge et al., 2013 <sup>[[#fn:r1479|1479]]</sup> ; Holmes et al., 2015 <sup>[[#fn:r1480|1480]]</sup> ). There is consistent evidence of decreasing summer season discharge for the Yenisei, Lena, and Ob watersheds in Siberia (Ye et al., 2003 <sup>[[#fn:r1481|1481]]</sup> ; Yang et al., 2004a <sup>[[#fn:r1482|1482]]</sup> ; Yang et al., 2004b <sup>[[#fn:r1483|1483]]</sup> ) and the majority of northern Canadian rivers (Déry et al., 2016 <sup>[[#fn:r1484|1484]]</sup> ). Long-term records indicate water temperature increases (Webb et al., 2008 <sup>[[#fn:r1485|1485]]</sup> ; Yang and Peterson, 2017 <sup>[[#fn:r1486|1486]]</sup> ); attribution to rising air temperatures is complicated by the influence of reservoir regulation over Siberian regions (Liu et al., 2005 <sup>[[#fn:r1487|1487]]</sup> ; Lammers et al., 2007 <sup>[[#fn:r1488|1488]]</sup> ). Increases in discharge and water temperature in the spring season represent notable freshwater and heat fluxes to the Arctic Ocean (Yang et al., 2014 <sup>[[#fn:r1489|1489]]</sup> ).

A large proportion of low-lying Arctic land areas are covered by lakes because permafrost limits surface water drainage and supports ponding even across areas with high moisture deficits (Grosse et al., 2013 <sup>[[#fn:r1490|1490]]</sup> ). While thaw in continuous permafrost is linked to intensified thermokarst activity and subsequent ponding (resulting in lake/wetland expansion), observations of change in surface water coverage across the Arctic are regionally variable (Nitze et al., 2017 <sup>[[#fn:r1491|1491]]</sup> ; Ulrich et al., 2017 <sup>[[#fn:r1492|1492]]</sup> ; Pastick et al., 2019 <sup>[[#fn:r1493|1493]]</sup> ). In landscapes with degrading ice-wedge polygons, subsidence can reduce inundation, increase runoff, and decrease surface water (Liljedahl et al., 2016 <sup>[[#fn:r1494|1494]]</sup> ; Perreault et al., 2017 <sup>[[#fn:r1495|1495]]</sup> ). In discontinuous permafrost, thaw opens up pathways of subsurface flow, improving the connection among inland water systems which supports the drainage of lakes and overall reduction in surface water cover (Jepsen et al., 2013 <sup>[[#fn:r1496|1496]]</sup> ). Enhanced subsurface connectivity from thaw in discontinuous permafrost serves tempers short-term lake fluctuations (Rey et al., 2019 <sup>[[#fn:r1497|1497]]</sup> ).

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===== 3.4.1.3.3 Drivers =====

There is ''high confidence'' that environmental drivers of Arctic surface water change are diverse and depend on local and regional factors such as permafrost properties and geomorphology (Nitze et al., 2018 <sup>[[#fn:r1498|1498]]</sup> ). Thermokarst lake expansion has been observed in the continuous permafrost of northern Siberia (Smith et al., 2005 <sup>[[#fn:r1499|1499]]</sup> ; Polishchuk et al., 2015 <sup>[[#fn:r1500|1500]]</sup> ) and Alaska (Jones et al., 2011 <sup>[[#fn:r1501|1501]]</sup> ); surface water area reduction has been observed in discontinuous permafrost of central and southern Siberia (Smith et al., 2005 <sup>[[#fn:r1502|1502]]</sup> ; Sharonov et al., 2012 <sup>[[#fn:r1503|1503]]</sup> ), western Canada (Labrecque et al., 2009 <sup>[[#fn:r1504|1504]]</sup> ; Carroll et al., 2011 <sup>[[#fn:r1505|1505]]</sup> ; Lantz and Turner, 2015 <sup>[[#fn:r1506|1506]]</sup> ) and interior Alaska (Chen et al., 2012 <sup>[[#fn:r1507|1507]]</sup> ; Rover et al., 2012 <sup>[[#fn:r1508|1508]]</sup> ). Increased evaporation from warmer/longer summers, decreased recharge due to reductions in snow melt volume, and dynamic processes such as ice-jam flooding (Chen et al., 2012 <sup>[[#fn:r1509|1509]]</sup> ; Bouchard et al., 2013 <sup>[[#fn:r1510|1510]]</sup> ; Jepsen et al., 2015 <sup>[[#fn:r1511|1511]]</sup> ) are important considerations for understanding observed surface water area change across the Arctic.

Satellite and model-derived estimates of evapotranspiration show increases across the Arctic (Rawlins et al., 2010 <sup>[[#fn:r1512|1512]]</sup> ; Liu et al., 2014 <sup>[[#fn:r1513|1513]]</sup> ; Liu et al., 2015b <sup>[[#fn:r1514|1514]]</sup> ; Fujiwara et al., 2016 <sup>[[#fn:r1515|1515]]</sup> ; Suzuki et al., 2018 <sup>[[#fn:r1516|1516]]</sup> ) ( ''medium confidence'' ). Increases in the seasonal active layer thickness impact temporary water storage and thus runoff regimes in drainage basins. Formation of taliks underneath lakes and rivers may result in reconnection of surface with sub-permafrost ground water aquifers with varying hydrological consequences depending on local geological and hydraulic settings (Wellman et al., 2013 <sup>[[#fn:r1517|1517]]</sup> ).

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