Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.9.2 Wet and Dry ====

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'''Mean precipitation:''' Atlas.11.2 indicated ''medium confidence'' in observed increases in Arctic precipitation, with the largest rises in the cold season. Antarctic precipitation showed no significant overall trend since the 1970s, with a positive trend over the 20th century (Sections 9.4.2.1 and Atlas.11.1). Increases in Arctic and Antarctic precipitation during the 21st century are ''very'' ''likely'' , with projected percentage increases that are much higher than most subpolar regions of the world (Figure Atlas.29).

'''Floods and heavy precipitation:''' Observations and model projections indicate ''high confidence'' in increasing Arctic river runoff in response to increasing total precipitation ( [[#Box--2019|Box et al., 2019]] ; [[#Durocher--2019|Durocher et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ) with a shift towards earlier meltwater flooding ( [[#AMAP--2017|AMAP, 2017]] ). Higher Arctic precipitable water totals are also connected with observed increases in heavy precipitation and convective activity ( ''high confidence'' ) ( [[#Ye--2015|Ye et al., 2015]] ; [[#Kharin--2018|Kharin et al., 2018]] ; [[#Chernokulsky--2019|Chernokulsky et al., 2019]] ). Higher flood magnitudes are also driven by future increases in rain-on-snow event days, amounts, and runoff, which are more significant in the Arctic than in mid-latitudes (where seasonal snow cover is often further reduced; [[#AMAP--2017|AMAP, 2017]] ; [[#Jeong--2018b|Jeong and Sushama, 2018b]] ).

'''Landslide and snow avalanche:''' There is a growing number of studies on mass movements in polar regions. Although there is ''low confidence'' in widespread observational trends for landslides or snow avalanches, a rise in the number of future landslides is supported by strong links to increases in heavy precipitation, glacier retreat, and thawing of ice-rich permafrost that can lead to retrogressive thaw slumps in Arctic regions ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] ; [[#Kokelj--2015|Kokelj et al., 2015]] ; [[#Derksen--2018|Derksen et al., 2018]] ; [[#Lewkowicz--2019|Lewkowicz and Way, 2019]] ; [[#Patton--2019|Patton et al., 2019]] ; [[#Ward%20Jones--2019|Ward Jones et al., 2019]] ).

'''Aridity and drought:''' Recent decades have seen a general decrease in Arctic aridity, with projections indicating a continuing trend towards reduced aridity ( ''high confidence'' ) as increased moisture transport leads to higher precipitation, humidity and streamflow ( [[#Meredith--2019|Meredith et al., 2019]] ) and a corresponding decrease in dry days ( [[#Khlebnikova--2019a|Khlebnikova et al., 2019a]] ). There is ''low confidence'' overall of recent or projected drought changes in polar regions ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ) even as increasing evidence shows that drainage from permafrost thaw, higher potential evapotranspiration, and changing seasonal patterns of melt have caused lake reduction and soil moisture deficits in several areas that match with projections of future drought increase despite overall precipitation increases ( [[#Andresen--2015|Andresen and Lougheed, 2015]] ; [[#Bring--2016|Bring et al., 2016]] ; [[#Spinoni--2018a|Spinoni et al., 2018a]] ; [[#Feng--2019|Feng et al., 2019]] ; [[#Finger%20Higgens--2019|Finger Higgens et al., 2019]] ).

'''Fire weather:''' Fire season lengthened from 1979 to 2015 over Arctic portions of North America ( [[#Jain--2017|Jain et al., 2017]] ), corresponding also to a 1975–2015 increase in lightning-ignited fires in Arctic North-Western North America ( [[#Girardin--2013|Girardin et al., 2013]] ; [[#Veraverbeke--2017|Veraverbeke et al., 2017]] ). [[#Abatzoglou--2019|Abatzoglou et al. (2019)]] climate model simulations project significant fire weather index increases in boreal forests of Arctic Europe, Arctic Russia and Arctic North-Eastern North America ( ''medium confidence'' ). Trends towards more frequent fires in tundra regions are expected to continue, driven in particular by increasing potential evapotranspiration and changes in vegetation ( ''high confidence'' ) ( [[#Hu--2015|Hu et al., 2015]] ; [[#AMAP--2017|AMAP, 2017]] ; [[#Young--2017|Young et al., 2017]] ).

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