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

=== 2.2.5 Lake and River Ice ===

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Based on ''limited evidence'' , AR5 reported shorter seasonal ice cover duration during the past decades ( ''low confidence)'' , however, did not specifically address changes in mountain lakes and rivers. Observations of extent, timing, duration and thickness of lake and river ice rely mostly on ''in situ'' measurements (e.g., Sharma et al., 2019) and, increasingly on remote sensing (Duguay et al., 2014 <sup>[[#fn:r218|218]]</sup> ). Lake and river ice studies focusing specifically on mountain regions are rare but observations from lakes in the European Alps, Scandinavia and the Tibetan Plateau show highly variable trends in ice cover duration during the past decades.

For example, Cai et al. (2019) reported shorter ice cover duration for 40 lakes and longer duration for 18 lakes on the Tibetan Plateau during the period 2000–2017. Similarly, using microwave remote sensing, Du et al. (2017) found shorter ice cover duration for 43 out of 71 lakes &gt;50 km 2 including lakes on the Tibetan Plateau during 2002–2015, but only five of these had statistically significant trends ( ''p''   &lt;   0.05), due to large interannual variability. The variable trends in the duration of lake ice cover on the Tibetan Plateau between 2002–2015 corresponded to variable trends in surface water temperatures. Of 52 study lakes in this region, 31 lakes showed a mean warming rate of 0.055 ± 0.033 °C yr -1 , and 21 lakes showed a mean cooling rate of -0.053 ± 0.038 °C yr -1 during 2001–2012 (Zhang et al., 2014 <sup>[[#fn:r219|219]]</sup> ). Kainz et al. (2017) <sup>[[#fn:r220|220]]</sup> reported a significant ( ''p''   &lt;   0.05) increase in the interannual variability in ice cover duration for a subalpine lake in Austria during 1921–2015 in addition to a significant trend in later freeze on, earlier ice break up and shorter ice cover duration. A significant ( ''p'' &lt; 0.05) trend towards shorter ice cover duration was found for another Austrian alpine lake during 1972–2015 (Niedrist et al., 2018 <sup>[[#fn:r209|209]]</sup> ).

Highly variable trends were also found in the timing and magnitude of river ice jams during 1903–2015, as reported by Rokaya et al. (2018) for Canadian rivers, including rivers in the mountains. Most of the variability in river ice trends could be explained by variable water flow, in particular due to flow regulation.

There is ''high confidence'' that air temperature and solar radiation are the most important drivers to explain observed changes of lake ice dynamics (Sharma et al., 2019 <sup>[[#fn:r210|210]]</sup> ). In mountainous regions where the interannual variability in ice cover duration is high, additional drivers become important, for example, morphometry, wind exposure, salinity, and hydrology, in particular hydrological processes driven by glaciers (Kropácek et al., 2013 <sup>[[#fn:r211|211]]</sup> ; Song et al., 2014 <sup>[[#fn:r212|212]]</sup> ; Yao et al., 2016 <sup>[[#fn:r213|213]]</sup> ; Gou et al., 2017 <sup>[[#fn:r214|214]]</sup> ). Despite high spatial and temporal variability in lake and river ice cover dynamics in mountain regions there is ''limited evidence'' ( ''high agreement)'' that further air temperature increases will result in a general trend towards later freezing, earlier break-up, and shorter ice cover duration in the future (Gebre et al., 2014 <sup>[[#fn:r215|215]]</sup> ; Du et al., 2017 <sup>[[#fn:r216|216]]</sup> ).

Overall, there is only ''limited evidence'' on changes in lake and river ice specifically in the mountains, indicating a trend, but not universally, towards shorter lake ice cover duration consistent with increased water temperature.

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