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

==== 2.3.2.2 Terrestrial Snow Cover ====

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The AR5 concluded that snow cover extent (SCE) had decreased in the NH, especially in spring ( ''very high confidence'' ). For 1967–2012, the largest change was in June and March–April SCE ''very likely'' declined. No trends were identified for the SH due to limited records and large variability. The SROCC concluded with ''high confidence'' that Arctic June SCE declined between 1967 and 2018 and in nearly all mountain regions, snow cover declined in recent decades.

Analysis of the combined in situ observations ( [[#Brown--2002|Brown, 2002]] ) and the multi-observation product (Mudryk et al. 2020) indicates that since 1922, April SCE in the NH has declined by 0.29 million km <sup>2</sup> per decade, with significant interannual variability (Figure 2.22) and regional differences (Section 9.5.3.1). The limited pre-satellite era data does not allow for a similar assessment for the entire spring-summer period. Assessment of SCE trends in the NH since 1978 indicates that for the October to February period there is substantial uncertainty in trends with the sign dependent on the observational product. Analysis using the NOAA Climate Data Record shows an increase in October to February SCE ( [[#Hernández-Henríquez--2015|Hernández-Henríquez et al., 2015]] ; [[#Kunkel--2016|Kunkel et al., 2016]] ) while analyses based on satellite borne optical sensors ( [[#Hori--2017|Hori et al., 2017]] ) or multi-observation products ( [[#Mudryk--2020|Mudryk et al., 2020]] ) show a negative trend for all seasons (Section 9.5.3.1 and Figure 9.23). The greatest declines in SCE have occurred during boreal spring and summer, although the estimated magnitude is dataset dependent ( [[#Rupp--2013|Rupp et al., 2013]] ; [[#Estilow--2015|Estilow et al., 2015]] ; [[#Bokhorst--2016|Bokhorst et al., 2016]] ; [[#Thackeray--2016|Thackeray et al., 2016]] ; [[#Connolly--2019|Connolly et al., 2019]] ).

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[[File:ce87e8bb066a3b89434858a8b908ff2d IPCC_AR6_WGI_Figure_2_22.png]]

'''Figure 2.22''' '''|''' '''April snow cover extent (SCE) for the Northern Hemisphere (1922–2018).''' Shading shows ''very likely'' range. The trend over the entire 1922–2018 period (black line) is –0.29 (± 0.07) million km <sup>2</sup> per decade. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

There has been a commensurate decrease in the snow-cover duration and persistence, particularly in higher latitudes due to earlier spring melt and, in some cases, later autumn onset of snow cover ( [[#Chen--2015|Chen et al., 2015]] ; [[#Derksen--2015|Derksen et al., 2015]] ; [[#Hori--2017|Hori et al., 2017]] ; [[#Hammond--2018|Hammond et al., 2018]] ). Arctic snow-cover duration has decreased by 2–4 days per decade since the 1970s ( [[#Brown--2017|Brown et al., 2017]] ). Significant decreases in snow-cover duration have been documented over western Eurasia since 1978 ( [[#Hori--2017|Hori et al., 2017]] ).

For the NH, maximum snow depth has generally decreased since the 1960s, with more robust trends for North America and greater uncertainty for Eurasia ( [[#Kunkel--2016|Kunkel et al., 2016]] ). Several satellite-based passive microwave and other products indicate general declines in pre-melt snow water equivalent since 1981 although there is regional and inter-dataset variability ( [[#Brown--2017|Brown et al., 2017]] ; [[#Jeong--2017|Jeong et al., 2017]] ; [[#Marty--2017|Marty et al., 2017]] ; [[#Mortimer--2020|Mortimer et al., 2020]] ; [[#Mudryk--2020|Mudryk et al., 2020]] ; [[#Pulliainen--2020|Pulliainen et al., 2020]] , Section 9.5.3).

In summary, substantial reductions in spring snow cover extent have occurred in the NH since 1978 ( ''very high confidence'' ) with ''limited evidence'' that this decline extends back to the early 20th century. Since 1981 there has been a general decline in NH spring snow water equivalent ( ''high confidence'' ).

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