Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.4.2.6 Long-Term Radiative Feedbacks Associated with Ice Sheets ====

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Although long-term radiative feedbacks associated with ice sheets are not included in our definition of ECS (Box 7.1), the relevant feedback parameter is assessed here because the time scales on which these feedbacks act are relatively uncertain, and the long-term temperature response to CO <sub>2</sub> forcing of the entire Earth system may be of interest.

Earth’s ice sheets (Greenland and Antarctica) are sensitive to climate change ( [[IPCC:Wg1:Chapter:Chapter-9#9.4|Section 9.4]] ; [[#Pattyn--2018|Pattyn et al., 2018]] ). Their time evolution is determined by both their surface mass balance and ice dynamic processes, with the latter being particularly important for the West Antarctic Ice Sheet. Surface mass balance depends on the net energy and hydrological fluxes at their surface, and there are mechanisms of ice-sheet instability that depend on ocean temperatures and basal melt rates ( [[IPCC:Wg1:Chapter:Chapter-9#9.4.1.1|Section 9.4.1.1]] ). The presence of ice sheets affects Earth’s radiative budget, hydrology, and atmospheric circulation due to their characteristic high albedo, low roughness length, and high altitude, and they influence ocean circulation through freshwater input from calving and melt (e.g., [[#Fyke--2018|Fyke et al., 2018]] ). Ice-sheet changes also modify surface albedo through the attendant change in sea level and therefore land area ( [[#Abe-Ouchi--2015|Abe-Ouchi et al., 2015]] ). The time scale for ice sheets to reach equilibrium is of the order of thousands of years ( [[#Clark--2016|Clark et al., 2016]] ). Due to the long time scales involved, it is a major challenge to run coupled climate–ice sheet models to equilibrium, and as a result, long-term simulations are often carried out with lower complexity models, and/or are asynchronously coupled.

In AR5, it was described that both the Greenland and Antarctic ice sheets would continue to lose mass in a warming world (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ), with a continuation in sea level rise beyond the year 2500 assessed as ''virtually certain'' . However, there was ''low confidence'' in the associated radiative feedback mechanisms, and as such, there was no assessment of the magnitude of long-term radiative feedbacks associated with ice sheets. That assessment is consistent with SROCC, wherein it was stated that ‘with limited published studies to draw from and no simulations run beyond 2100, firm conclusions regarding the net importance of atmospheric versus ocean melt feedbacks on the long-term future of Antarctica cannot be made.’

The magnitude of the radiative feedback associated with changes to ice sheets can be quantified by comparing the global mean long-term equilibrium temperature response to increased CO <sub>2</sub> concentrations in simulations that include interactive ice sheets with that of simulations that do not include the associated ice sheet–climate interactions ( [[#Swingedouw--2008|Swingedouw et al., 2008]] ; [[#Vizcaíno--2010|Vizcaíno et al., 2010]] ; [[#Goelzer--2011|Goelzer et al., 2011]] ; [[#Bronselaer--2018|Bronselaer et al., 2018]] ; [[#Golledge--2019|Golledge et al., 2019]] ). These simulations indicate that on multi-centennial time scales, ice-sheet mass loss leads to freshwater fluxes that can modify ocean circulation ( [[#Swingedouw--2008|Swingedouw et al., 2008]] ; [[#Goelzer--2011|Goelzer et al., 2011]] ; [[#Bronselaer--2018|Bronselaer et al., 2018]] ; [[#Golledge--2019|Golledge et al., 2019]] ). This leads to reduced surface warming (by about 0.2°C in the global mean after 1000 years; [[#7.4.4.1.1|Section 7.4.4.1.1]] ; [[#Goelzer--2011|Goelzer et al., 2011]] ), although other work suggests no net global temperature effect of ice-sheet mass loss ( [[#Vizcaíno--2010|Vizcaíno et al., 2010]] ). However, model simulations in which the Antarctic Ice Sheet is removed completely in a paleoclimate context indicate a positive global mean feedback on multi-millennial time scales due primarily to the surface-albedo change ( [[#Goldner--2014a|Goldner et al., 2014a]] ; [[#Kennedy-Asser--2019|Kennedy-Asser et al., 2019]] ); in ( [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] [[IPCC:Wg1:Chapter:Chapter-9#9.6.3|Section 9.6.3]] ) it is assessed that such ice-free conditions could eventually occur given 7°C–13°C of warming. This net positive feedback from ice-sheet mass loss on long time scales is also supported by model simulations of the mid-Pliocene Warm Period (MPWP; Cross-chapter Box 2.1) in which the volume and area of the Greenland and West Antarctic ice sheets are reduced in model simulations in agreement with geological data ( [[#Chandan--2018|Chandan and Peltier, 2018]] ), leading to surface warming. As such, overall, on multi-centennial time scales the feedback parameter associated with ice sheets is ''likely'' negative ( ''medium confidence'' ), but on multi-millennial time scales by the time the ice sheets reach equilibrium, the feedback parameter is ''very likely'' positive ( ''high confidence'' ) (Table 7.10). However, a relative lack of models carrying out simulations with and without interactive ice sheets over centennial to millennial time scales means that there is currently not enough evidence to quantify the magnitude of these feedbacks, or the time scales on which they act.

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