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

==== 9.5.3.3 Projected Snow Cover Changes ====

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The AR5 ( [[#Collins--2013|Collins et al., 2013]] ) stated that substantial NH spring snow cover reductions at the end of the 21st century were ''very likely'' under strong emissions scenarios, and expressed ''medium confidence'' in the projected geographic patterns of annual maximum SWE changes. Based on studies using downscaled CMIP5 or regional climate model output, either directly or via snowpack models driven by such output, SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) reported ''likely'' SD or mass decreases at lower elevations in many mountain ranges over the 21st century and ''high confidence'' in smaller future changes at higher elevations.

Since AR5, one study ( [[#Brown--2017|Brown et al., 2017]] ), applying a method developed by [[#de%20Elía--2013|de Elía et al. (2013)]] to a CMIP5 sub-ensemble, suggested that over most of the Northern Hemisphere, the projected decrease of SCD will exceed natural variability before this will be the case for annual maximum SWE. The same study reports that, over large parts of Eastern and Western North America and Europe, forced SCD changes are projected to exceed natural variability in the 2020s in spring and autumn, while the signals tend to emerge later in the Arctic regions and particularly late, after 2060, in Eastern Siberia under the RCP8.5 scenario. [[#Thackeray--2016|Thackeray and Fletcher (2016)]] have shown that inter-model spread in projected spring SCE trends could be reduced through improved simulation of spring season warming because of the tight coupling between temperature and SCE linked to the snow-albedo feedback ( [[#Qu--2014|Qu and Hall, 2014]] ; [[#Thackeray--2016|Thackeray and Fletcher, 2016]] ).

Across all emissions scenarios, and with negligible scenario dependence (Figure 9.24b), CMIP6 models consistently (all models and all months) simulate Northern Hemisphere snow cover decrease in response to future GSAT change over the 21st century ( [[#Mudryk--2020|Mudryk et al., 2020]] ). The simulated SCE decrease is close to a linear function of global temperature change for all months (shown in Figure 9.24b for spring, with ''medium confidence'' in an average sensitivity of about –8% per °C of GSAT increase), except when snow cover vanishes. This occurs at about +2°C of GSAT change above the 1995–2014 level (that is, about +3°C above the pre-industrial level) for the months of July and August, and at about +3°C above the 1995–2014 level for June and September. Possible effects of such changes on the hydrological cycle are assessed in [[IPCC:Wg1:Chapter:Chapter-8#8.2.3.1|Section 8.2.3.1]] .

In summary, consistent projections from all generations of global climate models, elementary process understanding and strong covariance between snow cover and temperature on several time scales make it ''virtually certain'' that future Northern Hemisphere snow cover extent and duration will continue to decrease as global climate continues to warm, and process understanding strongly suggests that this also applies to Southern Hemisphere seasonal snow cover ( ''high confidence'' ).

Seasonal snow cover, by definition, has a clear annual cycle with usually complete disappearance in spring and summer and re-formation in autumn or winter. Therefore, there is ''very high confidence'' that the current and projected changes to seasonal snow cover are reversible ( [[#Verfaillie--2018|Verfaillie et al., 2018]] ). In the case of global or regional cooling, abrupt large-scale snow-cover changes, with a transition from seasonal to persistent snow cover due to a strong snow-albedo feedback, are a typical feature of glacial inceptions (e.g., [[#Baum--2003|Baum and Crowley, 2003]] ; [[#Calov--2005|Calov et al., 2005]] ), and these can be irreversible on centennial or longer time scales because of this feedback. In summary, based on physical understanding and the absence of occurrence of such events in climate model projections, abrupt future changes of seasonal snow cover on large scales in the absence of concomitant abrupt atmospheric change as a driver appear ''very unlikely'' in the context of current and projected warming.

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