Editing IPCC:AR6/WGI/Chapter-3 (section)

==== 3.4.3.1 Glaciers ====

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Glaciers are defined as perennial surface land ice masses independent of the Antarctic and Greenland Ice Sheets (Sections 9.5 and 2.3.2.3). The AR5 assessed that anthropogenic influence had ''likely'' contributed to the retreat of glaciers observed since the 1960s ( [[#Bindoff--2013|Bindoff et al., 2013]] ), based on a high level of scientific understanding and robust estimates of observed mass loss, internal variability, and glacier response to climatic drivers. The SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) concluded that atmospheric warming was ''very likely'' the primary driver of glacier recession.

Simulations of glacier mass changes under climate change rely on glacier models driven by climate model output, often in collaborative research efforts such as GlacierMIP ( [[#Hock--2019a|Hock et al., 2019a]] ; [[#Marzeion--2020|Marzeion et al., 2020]] ). The GlacierMIP project is a systematic coordinated modelling effort designed to further understanding of glacier loss using global models. While the low resolution and remaining biases of climate model-derived boundary forcing data is a limitation, the release of the Randolph Glacier Inventory ( [[#Pfeffer--2014|Pfeffer et al., 2014]] ; [[#RGI%20Consortium--2017|RGI Consortium, 2017]] ) has supported more sophisticated, systematic and comprehensive modelling of glaciers worldwide ( [[#Hock--2019a|Hock et al., 2019a]] ).

A regional study considering 85 Northern Hemisphere glacier systems concluded that there is a discernible human influence on glacier mass balance, with glacier model simulations driven by CMIP5 historical and greenhouse gas-only simulations showing a glacier mass loss, whereas those driven by natural-only forced simulations showed a net glacier growth ( [[#Hirabayashi--2016|Hirabayashi et al., 2016]] ). In addition, a study of the role of climate change in glacier retreat using a simple mass-balance model for 37 glaciers worldwide, concluded that observed length changes would not have occurred without anthropogenic climate change, with observed length variations exceeding those associated with internal variability by several standard deviations in many cases ( [[#Roe--2017|Roe et al., 2017]] ). [[#Roe--2021|Roe et al. (2021)]] used the same model to estimate that at least 85% of cumulative glacier mass loss since 1850 is attributable to anthropogenic influence. While [[#Marzeion--2014|Marzeion et al. (2014)]] found that anthropogenic influence contributed only 25 ± 35% of glacier mass loss for the period 1851–2010, their naturally-forced simulations exhibited a substantial negative mass balance, which [[#Roe--2021|Roe et al. (2021)]] argued is unrealistic. Moreover, [[#Marzeion--2014|Marzeion et al. (2014)]] estimated that anthropogenic influence contributed 69 ± 24% of glacier mass loss for the period 1991 to 2010, consistent with a progressively increasing fraction of mass loss attributable to anthropogenic influence found by [[#Roe--2021|Roe et al. (2021)]] .

In summary, considering together the SROCC assessment that atmospheric warming was ''very likely'' the primary driver of glacier recession, the results of Roe et al. (2017, 2021) and our assessment of the dominant role of anthropogenic influence in driving atmospheric warming ( [[#3.3.1|Section 3.3.1]] ), we conclude that human influence is ''very likely'' the main driver of the near-universal retreat of glaciers globally since the 1990s.

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