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

==== 3.4.2.1 Seasonal Snow ====

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Historical simulations from CMIP5 models tend to underestimate observed reductions in spring snow cover extent due to uncertainty in the parameterisation of snow processes (Essery, 2013 <sup>[[#fn:r1518|1518]]</sup> ; Thackeray et al., 2014 <sup>[[#fn:r1519|1519]]</sup> ), challenges in simulating snow-albedo feedback (Qu and Hall, 2014 <sup>[[#fn:r1520|1520]]</sup> ; Fletcher et al., 2015 <sup>[[#fn:r1521|1521]]</sup> ; Li et al., 2016b <sup>[[#fn:r1522|1522]]</sup> ), unrealistic temperature sensitivity (Brutel-Vuilmet et al., 2013 <sup>[[#fn:r1523|1523]]</sup> ; Mudryk et al., 2017 <sup>[[#fn:r1524|1524]]</sup> ), and biases in climatological spring snow cover (Thackeray et al., 2016 <sup>[[#fn:r1525|1525]]</sup> ). The role of precipitation biases is not well understood (Thackeray et al., 2016 <sup>[[#fn:r1526|1526]]</sup> ).

Reductions in Arctic snow cover duration are projected by the CMIP5 multi-model ensemble due to later snow onset in the autumn and earlier snow melt in spring (Brown et al., 2017 <sup>[[#fn:r1527|1527]]</sup> ) driven by increased surface temperature over essentially all Arctic land areas (Hartmann et al., 2013). There is ''high confidence'' that projected snow cover declines are proportional to the amount of future warming in each model realisation (Thackeray et al., 2016 <sup>[[#fn:r1528|1528]]</sup> ; Mudryk et al., 2017 <sup>[[#fn:r1529|1529]]</sup> ). Projections to mid-century are primarily dependent on natural variability and model dependent uncertainties rather than the choice of forcing scenario (Hodson et al., 2013 <sup>[[#fn:r1530|1530]]</sup> ). By end of century, however, differences between scenarios emerge. Under RCP2.6 and RCP4.5, Arctic snow cover duration stabilises at 5–10% reduction (compared to a 1986–2005 reference period); under RCP8.5, snow cover duration declines reach –15 to –25% (Brown et al., 2017 <sup>[[#fn:r1531|1531]]</sup> ) (Figure 3.10) ( ''high confidence'' ).

Positive Arctic snow water equivalent changes emerge across the eastern Eurasian Arctic by mid-century for both RCP4.5 and RCP8.5 (Brown et al., 2017 <sup>[[#fn:r1532|1532]]</sup> ) ( ''medium confidence'' ). Projected snow water equivalent increases across the North American Arctic are only modest, emerge later in the century, and only under RCP8.5 (Brown et al., 2017 <sup>[[#fn:r1533|1533]]</sup> ). These projected increases are due to enhanced snowfall (Krasting et al., 2013 <sup>[[#fn:r1534|1534]]</sup> ) from a more moisture-rich Arctic atmosphere coupled with winter season temperatures that remain sufficiently low for precipitation to fall as snow. There is ''low confidence'' in changes to snow properties such as density and stratigraphy (relevant for understanding the impacts of changes to Arctic snow on ecosystems) which are not resolved directly by climate model simulations, but require detailed snow physics models.

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