Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.5.3.1 Observed Changes of Seasonal Snow Cover ====

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The AR5 ( [[#Vaughan--2013|Vaughan et al., 2013]] ) reported that NH SCE in June ''very likely'' decreased by 11.7 [8.8 to 14.6] % per decade over the 1967–2012 period, exceeding the absolute and relative reductions observed in March and April. The AR5 further reported ''very high confidence'' that NH March and April SCE decreased over the 90 years after 1922. The SROCC only assessed snow cover changes for the Arctic and mountain areas. For the Arctic (north of 60°N), SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) expressed ''high confidence'' in SCE decreases of –3.5 ± 1.9% per decade in May and –13.4 ± 5.4% per decade in June, based on a combination of multiple datasets ( [[#Mudryk--2017|Mudryk et al., 2017]] ). Concerning mountain snow cover, SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) reported with ''high confidence'' that mountain snow cover (both in terms of SCE and maximum SWE) has generally declined since the middle of the 20th century at lower elevations. At higher elevations, SROCC reported ''medium confidence'' in generally insignificant snow cover trends (where these were available). The large-scale assessment provided in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.2|Section 2.3.2.2]] of this Report reports ''very high confidence'' in substantial reductions of NH SCE (particularly in spring) since 1978, and states that there is ''limited evidence'' that this decline extends back to the early 20th century.

Since SROCC, progress has been made in characterizing seasonal NH snow cover changes through the combined analysis of datasets from multiple sources (surface observations, remote sensing, land surface models and reanalysis products). A recent combined dataset ( [[#Mudryk--2020|Mudryk et al., 2020]] ) identified negative NH SCE trends in all months between 1981 and 2018, exceeding –50 × 10 <sup>3</sup> km <sup>2</sup> yr <sup>–1</sup> in November, December, March and May (Figure 9.23a,b). The loss of spring SCE is also reflected in earlier spring snow melt, derived from surface observations ( [[#Bulygina--2011|Bulygina et al., 2011]] ; [[#Brown--2017|Brown et al., 2017]] ), satellite observations ( [[#Wang--2013|Wang et al., 2013]] ; [[#Estilow--2015|Estilow et al., 2015]] ; [[#Anttila--2018|Anttila et al., 2018]] ), and model-based analyses ( [[#Liston--2011|Liston and Hiemstra, 2011]] ). There is considerable inter-dataset and regional variability, but the continental-scale trends of snow-off dates from these datasets are consistently negative ( [[#Brown--2017|Brown et al., 2017]] ; [[#Kouki--2019|Kouki et al., 2019]] ).

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[[File:425f3c95af5739044b1949cc796b6e5b IPCC_AR6_WGI_Figure_9_23.png]]

'''Figure 9.23''' '''|''' '''Observed monthly Northern Hemisphere snow cover (a) trends and (b) anomalies, and snow mass (c) trends and (d) anomalies.''' From the observation-based ensemble discussed in the text ( [[#Mudryk--2020|Mudryk et al., 2020]] ). Trends and anomalies are calculated over the 1981–2018 period. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

Satellite-derived estimates of NH SCE compiled within the National Oceanic and Atmospheric Administration Climate Data Record (NOAA CDR) snow chart extend back to 1967, providing one of the longest environmental data records from spaceborne measurements ( [[#Estilow--2015|Estilow et al., 2015]] ). Continental trends from these coarse resolution estimates (about 200 km) show declining snow cover during the spring period, consistent with surface warming ( [[#Hernández-Henríquez--2015|Hernández-Henríquez et al., 2015]] ; [[#Mudryk--2017|Mudryk et al., 2017]] ). Therefore, as assessed in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.2|Section 2.3.2.2]] , there is ''very high confidence'' that the NH spring SCE has been decreasing since 1978.

Hemispheric reconstructions with simple snow models and in situ observations have extended a pre-satellite record to precede the satellite record and extend back to 1922 ( [[#Brown--2011|Brown and Robinson, 2011]] ), putting the satellite era in historical context. This study, also assessed in AR5, suggests an increase in North American spring (March–April) SCE from 1915 to about 1950, followed by a decrease of the same total magnitude afterwards. In Eurasia, a negative trend in April is visible over the entire 1922–2010 period of record, while in March, a step decrease at about 1985 separates two periods with insignificant trends. Overall, combining March and April, consistency between the continental trends since 1950, and agreement in sign with the NOAA satellite record since 1967, provides ''high confidence'' in Northern Hemisphere spring snow cover decrease since about 1950. Analysis of paleoclimate records ( [[#Pederson--2011|Pederson et al., 2011]] ; [[#Belmecheri--2016|Belmecheri et al., 2016]] ) suggests that recent snowpack reductions in western North America are exceptional on a millennial time scale ( ''medium confidence'' ).

Recent remote sensing global-scale studies ( [[#Hammond--2018|Hammond et al., 2018]] ; [[#Notarnicola--2020|Notarnicola, 2020]] ) report that, since 2000, snow cover area and/or duration decreased in 78% of global mountain areas ( [[#Notarnicola--2020|Notarnicola, 2020]] ). Due to the shortness of these records and high spatial variability, they only provide ''limited evidence'' in ''medium agreement'' that snow cover area and duration changes over that recent period are more consistently negative at higher (&gt;4000 m) than at lower elevations, and do not alter the ''high confidence'' in longer-term mountain snow cover decrease at lower elevations since the middle of the 20th century that was already reported in SROCC.

As assessed in detail in [[IPCC:Wg1:Chapter:Chapter-3#3.4.2|Section 3.4.2]] , it is ''very likely'' that anthropogenic influence contributed to the observed reductions in Northern Hemisphere spring snow cover since the mid-20th century. The reasons for this assessment are: (i) physical consistency of the observed spring snowpack and surface temperature changes in observations and models; (ii) the strong observed hemispheric and regional spring SCE and SWE trends; and (iii) the general attribution of hemispheric temperature changes to human influence. Consistent between multiple observational products and historical climate model simulations, the observed NH SCE sensitivity to NH land (&gt;30°N) warming ( [[#Mudryk--2017|Mudryk et al., 2017]] ) is approximately –1.9×10 <sup>6</sup> km <sup>2</sup> °C <sup>–1</sup> (95% confidence range of ±0.9×10 <sup>6</sup> km <sup>2</sup> °C <sup>–1</sup> ) throughout the snow season.

Compared to numerous studies on spring SCE changes, less attention has been paid to changes in NH snow cover during the onset period in the autumn, a challenging period to retrieve snow information from optical satellite imagery due to persistent clouds and decreased solar illumination at higher latitudes. Positive trends in October and November SCE in the NOAA CDR ( [[#Hernández-Henríquez--2015|Hernández-Henríquez et al., 2015]] ) are not replicated in other surface, satellite, and model datasets ( [[#Brown--2013|Brown and Derksen, 2013]] ; [[#Peng--2013|Peng et al., 2013]] ; [[#Hori--2017|Hori et al., 2017]] ; [[#Mudryk--2017|Mudryk et al., 2017]] ). The positive trends from the NOAA CDR are also inconsistent with later autumn snow-on dates since 1980 (–0.6 to –1.4 days per decade), based on historical surface observations, model-derived analyses and independent satellite datasets (updated from [[#Derksen--2017|Derksen et al., 2017]] ). The SCE trend sensitivity to surface temperature forcing in the NOAA CDR is anomalous compared to other datasets during October and November ( [[#Mudryk--2017|Mudryk et al., 2017]] ). There is therefore ''medium confidence'' that the NH SCE trend for the 1981–2016 period was also negative during these two months ( [[#Mudryk--2020|Mudryk et al., 2020]] ).

In the low-to-mid latitude (18°S–40°S) South American Andes, a dry-season snow cover decrease of about 12% per decade has been reported for the 1986–2018 period ( [[#Cordero--2019|Cordero et al., 2019]] ), linked to El Niño–Southern Oscillation (ENSO) changes dominant in the northern part, and an additional influence of poleward migration of the westerly wind zone in the southern part of the study area. Further south, long-term warming has been identified as the dominant cause of observed winter snow cover reduction over the 1972–2016 period at about 53°S in Brunswick Peninsula ( [[#Aguirre--2018|Aguirre et al., 2018]] ).

The AR5 ( [[#Hock--2019b|Hock et al., 2019b]] ) reported on SWE and SD in situ observations mostly from mountain areas, the majority of which showed negative trends over their respective observational periods. However, AR5 did not provide an assessment of large-scale snow mass changes across the Northern Hemisphere. The SROCC attributed ''medium confidence'' to reports of negative SWE trends in the Russian Arctic between 1966 and 2014, and stated that seasonal maximum SD trends in the North American Arctic were mostly insignificant and inconsistently positive or negative. It further attributed ''medium confidence'' to gridded products that suggest negative pan-Arctic SWE trends between 1981 and 2016, and ''high confidence'' in a general decline of mountain snow mass at lower elevations, albeit with regional variations.

Since AR5, the number of global or hemispheric-scale gridded SWE products has substantially increased. A validation and intercomparison ( [[#Mortimer--2020|Mortimer et al., 2020]] ) of datasets – derived from: (i) reanalysis-based products; (ii) a combined surface observation – passive microwave remote sensing product; and (iii) stand-alone passive microwave products – has led to better understanding of the strengths and limitations of each. These gridded products consistently identify negative trends in maximum pre-melt SWE across the 1981–2016 period over Eurasia and North America (Figure 9.23c,d; [[#Mudryk--2020|Mudryk et al., 2020]] ). To further constrain SWE uncertainty, [[#Pulliainen--2020|Pulliainen et al. (2020)]] implemented a bias correction based on snow course observations which yielded a current best estimate for the average 1980–2018 March SWE over NH non-alpine land north of 40°N of 2938 [ ''likely'' range 2846–3062] Gt. Using this method, the bias-corrected GlobSnow v3.0 dataset suggests a 4.6 Gt yr <sup>–1</sup> decrease of March SWE over this 39-year period across North America, and a negligible trend across Eurasia. These SWE trends are consistent with the continental SCE trends over this period, as assessed above, but strong regional and temporal variability only allows ''medium confidence'' in the signs and magnitudes of these trends. However, there is ''high confidence'' in a general decline of NH spring SWE since 1981 ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.2|Section 2.3.2.2]] ). In the longer term (see also [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.2|Section 2.3.2.2]] ), annual maximum SD trends from site measurements confirm mostly negative trends in North America ( [[#Kunkel--2016|Kunkel et al., 2016]] ) between 1960–1961 and 2014–2015, and strong spatial variability in Eurasia ( [[#Zhong--2018|Zhong et al., 2018]] ) between 1966 and 2012, with spatial patterns bearing some resemblance to the shorter satellite-based trends reported by [[#Pulliainen--2020|Pulliainen et al. (2020)]] . However, over this longer period, the Eurasian measurements ( [[#Zhong--2018|Zhong et al., 2018]] ) exhibit, on average, a positive trend. On the Qinghai-Tibet Plateau, site measurements between 1961 and 2010 ( [[#Xu--2017|Xu et al., 2017]] ) suggest a shift from an initial increase of spring SD until about 1980 to a decreasing trend afterwards.

Concerning the assessment of SWE trends in mountainous regions, SROCC noted a need for observations spanning several decades because of very strong temporal variability. Moreover, determining SWE trends in mountain regions is challenging because the coarse resolution (typically 25 to 50 km) of gridded SWE products is inadequate in areas of mountainous terrain ( [[#Snauffer--2016|Snauffer et al., 2016]] ). Based on a compilation of a large number of studies of SWE trends in mountain regions, SROCC noted strong regional variations, but a general consistency in greater reductions in SWE at lower elevations associated with shifts from solid to liquid precipitation. A recent synthesis of snow observations in the European Alps ( [[#Matiu--2021|Matiu et al., 2021]] ) shows a 1971–2019 seasonal (November to May) SD trend of –8.4% per decade, along with negative maximum SD and seasonal snow cover duration trends. The trends are stronger and more significant during transitional seasons and at transitional (from no snow to snow) altitudes, and exhibit strong regional variations, consistent with earlier reports for the Swiss and Austrian Alps ( [[#Schöner--2019|Schöner et al., 2019]] ) and the Pyrenees (López‐Moreno et al., 2020).

In summary, since AR5, intercomparison, dataset blending of gridded products, and bias correction using snow course measurements contributed to an improved estimate of the average 1980–2018 March SWE over NH non-alpine land north of 40°N of 2938 [ ''likely'' range 2846–3062] Gt, with ''medium confidence'' in the magnitudes of continental-scale trends over that period. However, there is ''high confidence'' in a general decline of NH spring SWE since 1981 ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.2|Section 2.3.2.2]] ). In mountain areas, in situ observations tend to suggest that annual maximum SWE reductions are generally stronger at elevation bands where shifts from solid to liquid precipitation affected the snow mass.

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