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

===== 3.4.1.1.3 Drivers =====

Despite uncertainties due to sparse observations (Cowtan and Way, 2014 <sup>[[#fn:r1371|1371]]</sup> ), surface temperature has increased across Arctic land areas in recent decades (Hawkins and Sutton, 2012 <sup>[[#fn:r1372|1372]]</sup> ; Fyfe et al., 2013 <sup>[[#fn:r1373|1373]]</sup> ), driving reductions in Arctic snow extent and duration ( ''high confidence'' ) ''.'' Changes in Arctic snow extent can be directly related to extratropical temperature increases (Brutel-Vuilmet et al., 2013 <sup>[[#fn:r1374|1374]]</sup> ; Thackeray et al., 2016 <sup>[[#fn:r1375|1375]]</sup> ; Mudryk et al., 2017 <sup>[[#fn:r1376|1376]]</sup> ). Based on multiple historical datasets, there is a consistent temperature sensitivity for Arctic snow extent, with approximately 800,000 km 2 of snow cover lost per degrees Celsius warming in spring (Brown and Derksen, 2013 <sup>[[#fn:r1377|1377]]</sup> ; Brown et al., 2017), and 700,000–800,000 km 2 lost in autumn (Derksen and Brown, 2012 <sup>[[#fn:r1379|1379]]</sup> ; Brown and Derksen, 2013 <sup>[[#fn:r1380|1380]]</sup> ) ( ''high confidence'' ).

There is ''high confidence'' that darkening of snow through the deposition of black carbon and other light absorbing particles enhances snow melt (Bullard et al., 2016 <sup>[[#fn:r1381|1381]]</sup> ; Skiles et al., 2018 <sup>[[#fn:r1382|1382]]</sup> ; Boy et al., 2019 <sup>[[#fn:r1383|1383]]</sup> ). The global direct radiative forcing for black carbon in seasonal snow and over sea ice is estimated to be 0.04 W m –2 , but the effective forcing can be up to threefold greater at regional scales due to the enhanced albedo feedback triggered by the initial darkening (Bond et al., 2013). Lawrence et al. (2011) <sup>[[#fn:r1393|1393]]</sup> estimate the present-day radiative effect of black carbon and dust in land-based snow to be 0.083 W m –2 , only marginally greater than the simulated 1850 effect (0.075 W m –2 ) due to offsetting effects from increased black carbon emissions and reductions in dust darkening ( ''medium confidence'' ). Kylling et al. (2018) <sup>[[#fn:r1394|1394]]</sup> estimate a surface radiative effect of 0.292 W m –2 caused by dust deposition (largely transported from Asia) to Arctic snow, approximately half of the black carbon central scenario estimate of Flanner et al. (2007) <sup>[[#fn:r1395|1395]]</sup> . The forcing from brown carbon deposited in snow (associated with both combustion and secondary organic carbon) is estimated to be 0.09−0.25 W m –2 , with the range due to assumptions of particle absorptivity (Lin et al., 2014 <sup>[[#fn:r1396|1396]]</sup> ) ( ''low confidence'' ).

Precipitation remains a sparse and highly uncertain measurement over Arctic land areas: ''in situ'' datasets remain uncertain (Yang, 2014 <sup>[[#fn:r1397|1397]]</sup> ) and are largely regional (Kononova, 2012 <sup>[[#fn:r1398|1398]]</sup> ; Vincent et al., 2015 <sup>[[#fn:r1399|1399]]</sup> ). Atmospheric reanalyses show increases in Arctic precipitation in recent decades (Lique et al., 2016 <sup>[[#fn:r1400|1400]]</sup> ; Vihma et al., 2016 <sup>[[#fn:r1401|1401]]</sup> ), but there remains ''low confidence'' in reanalysis-based closure of the Arctic freshwater budget due to a wide spread between available reanalysis derived precipitation estimates (Lindsay et al., 2014 <sup>[[#fn:r1484|1484]]</sup> ). Despite improved process understanding, estimates of sublimation loss during blowing snow events remain a key uncertainty in the mass budget of the Arctic snowpack (Sturm and Stuefer, 2013 <sup>[[#fn:r1485|1485]]</sup> ).

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