Editing IPCC:AR6/WGI/Chapter-8 (section)

===== 8.3.1.7.2 Seasonal snow cover =====

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The AR5 assessed that Northern Hemisphere (NH) snow cover extent (SCE) has decreased since the late 1960s, especially in spring ( ''very high confidence'' ). This is confirmed by recent studies ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.2|Section 2.3.2.2]] ; [[#Kunkel--2016|Kunkel et al., 2016]] ). AR6 assesses that NH spring snow cover has been decreasing since 1978 ( ''very high confidence'' ) and that this trend extends back to 1950 ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.3|Section 9.5.3]] ). Human-caused global warming is the dominant driver of this observed decline ( [[IPCC:Wg1:Chapter:Chapter-3#3.4.2|Section 3.4.2]] ; [[#Estilow--2015|Estilow et al., 2015]] ). Model simulations suggest that surface temperature responses at hemispheric/regional scales explain between 40% and 85% of the SCE trend variability ( [[#Mudryk--2017|Mudryk et al., 2017]] ). A decreasing trend in snowfall has also been detected in the NH (Figure 8.1; [[#Rupp--2013|Rupp et al., 2013]] ). Snowfall as a proportion of precipitation has decreased significantly in recent years ( [[#Berghuijs--2014|Berghuijs et al., 2014]] ). However, a late-20th-century increase in snowfall in West Antarctica observed in ice cores has been linked to a combination of factors including the anthropogenically forced deepening of the Amundsen Sea Low ( [[#Thomas--2015|Thomas et al., 2015]] , 2017).

Observations show a rapid recent decrease of spring SCE in NH, mostly in Eurasia and North America, closely linked to temperature change, for example, March to April SCE is decreasing at 3.4% ± 1.1 % per decade (1979–2005; [[#Brown--2011|Brown and Robinson, 2011]] ; [[#Hernández-Henríquez--2015|Hernández-Henríquez et al., 2015]] ). An overall increasing annual trend of the NH SCE since the late 1980s has been observed, in contrast to decreasing trends over 1960s to 1980s that are dominated by the autumn and winter seasons ( [[#Barry--2020|Barry and Gan, 2020]] ). Such recent positive trends in snow cover extent are however at odds with other surface and satellite datasets and with the negative trends simulated by most CMIP5 and CMIP6 models ( [[#Mudryk--2017|Mudryk et al., 2017]] , 2020). [[#Hernández-Henríquez--2015|Hernández-Henríquez et al. (2015)]] also detected positive trends in October to November SCE in in the NOAA SCE Climate Data Record (NOAA-CDR), which are not replicated in other datasets ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.3|Section 9.5.3]] ). [[#Wu--2018|Wu et al. (2018)]] found slower snowmelt rates over the NH in 1980–2017, with higher ablation rates in locations with deep snow water equivalent (SWE), but due to the reduction of SWE in deep snowpacks, moderate/high ablation rates showed decreasing trends. [[#Santolaria-Otín--2020|Santolaria-Otín and Zolina (2020)]] reported weak but significant decline in SCE in autumn over northern Eurasia and North America during 1979 – 2005, and similarly for spring, except for northern Siberia which showed higher spring SCE. [[#Kapnick--2012|Kapnick and Hall (2012)]] detected significant loss of spring mountain snowpack in western USA in 1950 – 2008. For Canada, extensive decreasing snow depths, SCE and duration were detected since mid-1970s, especially in western Canada during winter and spring (DeBeer et al. , 2016). [[#Berghuijs--2014|Berghuijs et al. (2014)]] show that across the continental USA, catchments with more snowfall than rainfall generally have higher mean streamflow, which will probably decrease with smaller fractions of precipitation falling as snow because of climate warming.

In summary, a decline in the spring NH snow cover extent, snow depth and duration has been observed since the late 1960s and has been attributed to human influence ( ''high confidence'' ). Depending on the region and season, there is ''low-to-medium confidence'' in the main drivers of snow cover changes, although various regions exhibit a shortening of the snow cover season which is consistent with global warming. A more detailed assessment of observed changes in seasonal snow cover is provided in [[IPCC:Wg1:Chapter:Chapter-9#9.5.3|Section 9.5.3]] .

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