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

===== 9.4.2.3.1 Surface mass balance =====

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The AR5 projected a negative contribution from Antarctic surface mass balance (SMB) changes to sea level over the 21st century (i.e., mitigating sea level rise), due to increased snowfall associated with warmer air temperatures. Sensitivity of SMB to Antarctic surface air temperature change varied from 3.7 to 7% °C <sup>–1</sup> , and the sea level projections assumed a sensitivity of 5.1 ± 1.5% °C <sup>–1</sup> from CMIP3 era models ( [[#Gregory--2006|Gregory and Huybrechts, 2006]] ) to estimate SMB changes from Antarctic temperatures in the CMIP5 ensemble. Since the AR5, analyses of CMIP5 and CMIP6 models have found Antarctic temperature sensitivity for accumulation (precipitation minus sublimation) of 3.5 to 8.7% °C <sup>–1</sup> ( [[#Frieler--2015|Frieler et al., 2015]] ), for SMB of 6.0 to 9.9% °C <sup>–1</sup> ( [[#Previdi--2016|Previdi and Polvani, 2016]] ) and for precipitation of around 4 to 9% °C <sup>–1</sup> (±1 standard deviation ranges; [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ). An accumulation sensitivity estimate derived from ice core data lies in the middle of the range, around 6% °C <sup>–1</sup> ( [[#Frieler--2015|Frieler et al., 2015]] ). These are consistent, within uncertainties, with each other and AR5, under the approximation that SMB is dominated by snowfall.

The AR5 found that the median and ''likely'' sea level contributions due to SMB from 1986–2005 to 2100 were –0.05 (–0.09 to –0.02) m under RCP8.5 and –0.02 (–0.05 to 0.00) m under RCP2.6. The SROCC did not present a separate SMB contribution, instead showing total Antarctic projections derived from ice-sheet models ( [[#9.4.2.5|Section 9.4.2.5]] ). Projections of the SMB contribution to sea level tend to be slightly more negative since AR5, due at least in part to the higher range in equilibrium climate sensitivity values in CMIP6 ( [[#Payne--2021|Payne et al., 2021]] ). Mean and ±1 standard deviation ranges for grounded Antarctic Ice Sheet SMB changes from 2000 to 2100 computed from CMIP5 models are –0.08 (–0.13 to –0.04) m sea level equivalent (SLE) for RCP8.5 and, similarly for CMIP6 models, are –0.07 (–0.11 to –0.03) m for SSP5-8.5 ( [[#Gorte--2020|Gorte et al., 2020]] ). The general circulation models (GCMs) used to drive ice-sheet models in ISMIP6 (Box 9.3) project mean grounded AIS SMB changes from 2005 to 2100 of –0.06 (range –0.08 to –0.03) m SLE under RCP8.5 for the six CMIP5 models ( [[#Seroussi--2020|Seroussi et al., 2020]] ) and –0.09 (range –0.10 to –0.07) m SLE under SSP5-8.5 for the four CMIP6 models, which have climate sensitivity values of 4.8°C –5.3°C ( [[#Payne--2021|Payne et al., 2021]] ). We apply the AR5 parametric AIS SMB model ( [[#9.6.3.2|Section 9.6.3.2]] ) to updated projections of global mean temperature from a two-layer energy budget emulator (Supplementary Material 7.SM.2), which gives a median –0.05 (5–95% range –0.07 to –0.02) m SLE for SSP5-8.5 ( [[#9.4.2.5|Section 9.4.2.5]] , Table 9.3), that is, similar to the AR5 assessment and slightly smaller than the CMIP6 estimate. This estimate is used to augment the LARMIP-2 dynamic projections (Box 9.3) in Sections 9.4.2.5 and 9.4.2.6. Overall, CMIP5 and CMIP6 GCM simulations of sea level fall by 2100 due to Antarctic SMB increases are around 2–4 cm greater than estimates derived with the statistical method used in AR5. Further details about projections of Antarctic temperature, precipitation and SMB are provided in Section Atlas.11.1.4, which assesses that, due to the challenges of model evaluation ( [[#9.4.2.2|Section 9.4.2.2]] ) and the possibility of increased meltwater runoff ( [[#Kittel--2021|Kittel et al., 2021]] ), there is only ''medium confidence'' that the future contribution of Antarctic SMB to sea level this century will be negative under all greenhouse gas emissions scenarios. Longer time scales are discussed in 9.4.2.6.

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