Editing IPCC:AR6/SR15/Chapter-3 (section)

=== 3.6.3 Implications Beyond the End of the Century ===

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==== 3.6.3.1 Sea ice ====

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Sea ice is often cited as a tipping point in the climate system (Lenton, 2012) <sup>[[#fn:r1356|1356]]</sup> . Detailed modelling of sea ice (Schröder and Connolley, 2007; Sedláček et al., 2011; Tietsche et al., 2011) <sup>[[#fn:r1357|1357]]</sup> , however, suggests that summer sea ice can return within a few years after its artificial removal for climates in the late 20th and early 21st centuries. Further studies (Armour et al., 2011; Boucher et al., 2012; Ridley et al., 2012) <sup>[[#fn:r1358|1358]]</sup> modelled the removal of sea ice by raising CO <sub>2</sub> concentrations and studied subsequent regrowth by lowering CO <sub>2</sub> . These studies suggest that changes in Arctic sea ice are neither irreversible nor exhibit bifurcation behaviour. It is therefore plausible that the extent of Arctic sea ice may quickly re-equilibrate to the end-of-century climate under an overshoot scenario.

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==== 3.6.3.2 Sea level ====

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Policy decisions related to anthropogenic climate change will have a profound impact on sea level, not only for the remainder of this century but for many millennia to come (Clark et al., 2016) <sup>[[#fn:r1359|1359]]</sup> . On these long time scales, 50 m of sea level rise (SLR) is possible (Clark et al., 2016) <sup>[[#fn:r1360|1360]]</sup> . While it is ''virtually certain'' that sea level will continue to rise well beyond 2100, the amount of rise depends on future cumulative emissions (Church et al., 2013) <sup>[[#fn:r1361|1361]]</sup> as well as their profile over time (Bouttes et al., 2013; Mengel et al., 2018) <sup>[[#fn:r1362|1362]]</sup> . Marzeion et al. (2018) <sup>[[#fn:r1363|1363]]</sup> found that 28–44% of present-day glacier volume is unsustainable in the present-day climate and that it would eventually melt over the course of a few centuries, even if there were no further climate change. Some components of SLR, such as thermal expansion, are only considered reversible on centennial time scales (Bouttes et al., 2013; Zickfeld et al., 2013) <sup>[[#fn:r1364|1364]]</sup> , while the contribution from ice sheets may not be reversible under any plausible future scenario (see below).

Based on the sensitivities summarized by Levermann et al. (2013) <sup>[[#fn:r1365|1365]]</sup> , the contributions of thermal expansion (0.20–0.63 m °C <sup>–1</sup> ) and glaciers (0.21 m °C <sup>–1</sup> but falling at higher degrees of warming mostly because of the depletion of glacier mass, with a possible total loss of about 0.6 m) amount to 0.5–1.2 m and 0.6–1.7 m in 1.5°C and 2°C warmer worlds, respectively. The bulk of SLR on greater than centennial time scales will therefore be caused by contributions from the continental ice sheets of Greenland and Antarctica, whose existence is threatened on multi-millennial time scales.

For Greenland, where melting from the ice sheet’s surface is important, a well-documented instability exists where the surface of a thinning ice sheet encounters progressively warmer air temperatures that further promote melting and thinning. A useful indicator associated with this instability is the threshold at which annual mass loss from the ice sheet by surface melt exceeds mass gain by snowfall. Previous estimates put this threshold at about 1.9°C to 5.1°C above pre-industrial temperatures (Gregory and Huybrechts, 2006) <sup>[[#fn:r1366|1366]]</sup> . More recent analyses, however, suggest that this threshold sits between 0.8°C and 3.2°C, with a best estimate at 1.6°C (Robinson et al., 2012) <sup>[[#fn:r1367|1367]]</sup> . The continued decline of the ice sheet after this threshold has been passed is highly dependent on the future climate and varies between about 80% loss after 10,000 years to complete loss after as little as 2000 years (contributing about 6 m to SLR). Church et al. (2013) <sup>[[#fn:r1368|1368]]</sup> were unable to quantify a ''likely'' range for this threshold. They assigned ''medium confidence'' to a range greater than 2°C but less than 4°C, and had ''low confidence'' in a threshold of about 1°C. There is insufficient new literature to change this assessment.

The Antarctic ice sheet, in contrast, loses the mass gained by snowfall as outflow and subsequent melt to the ocean, either directly from the underside of floating ice shelves or indirectly by the melting of calved icebergs. The long-term existence of this ice sheet will also be affected by a potential instability (the marine ice sheet instability, MISI), which links outflow (or mass loss) from the ice sheet to water depth at the grounding line (i.e., the point at which grounded ice starts to float and becomes an ice shelf) so that retreat into deeper water (the bedrock underlying much of Antarctica slopes downwards towards the centre of the ice sheet) leads to further increases in outflow and promotes yet further retreat (Schoof, 2007) <sup>[[#fn:r1369|1369]]</sup> . More recently, a variant on this mechanism was postulated in which an ice cliff forms at the grounding line and retreats rapidly though fracture and iceberg calving (DeConto and Pollard, 2016) <sup>[[#fn:r1370|1370]]</sup> . There is a growing body of evidence (Golledge et al., 2015; DeConto and Pollard, 2016) <sup>[[#fn:r1371|1371]]</sup> that large-scale retreat may be avoided in emissions scenarios such as Representative Concentration Pathway (RCP)2.6 but that higher-emissions RCP scenarios could lead to the loss of the West Antarctic ice sheet and sectors in East Antarctica, although the duration (centuries or millennia) and amount of mass loss during such a collapse is highly dependent on model details and no consensus exists yet. Schoof (2007) <sup>[[#fn:r1372|1372]]</sup> suggested that retreat may be irreversible, although a rigorous test has yet to be made. In this context, overshoot scenarios, especially of higher magnitude or longer duration, could increase the risk of such irreversible retreat.

Church et al. (2013) <sup>[[#fn:r1373|1373]]</sup> noted that the collapse of marine sectors of the Antarctic ice sheet could lead to a global mean sea level (GMSL) rise above the ''likely'' range, and that there was ''medium confidence'' that this additional contribution ‘would not exceed several tenths of a metre during the 21st century’.

The multi-centennial evolution of the Antarctic ice sheet has been considered in papers by DeConto and Pollard (2016) <sup>[[#fn:r1374|1374]]</sup> and Golledge et al. (2015) <sup>[[#fn:r1375|1375]]</sup> . Both suggest that RCP2.6 is the only RCP scenario leading to long-term contributions to GMSL of less than 1.0 m. The long-term committed future of Antarctica and the GMSL contribution at 2100 are complex and require further detailed process-based modelling; however, a threshold in this contribution may be located close to 1.5°C to 2°C of global warming.

In summary, there is ''medium confidence'' that a threshold in the long-term GMSL contribution of both the Greenland and Antarctic ice sheets lies around 1.5°C to 2°C of global warming relative to pre-industrial; however, the GMSL associated with these two levels of global warming cannot be differentiated on the basis of the existing literature.

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==== 3.6.3.3 Permafrost ====

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The slow rate of permafrost thaw introduces a lag between the transient degradation of near-surface permafrost and contemporary climate, so that the equilibrium response is expected to be 25–38% greater than the transient response simulated in climate models (Slater and Lawrence, 2013) <sup>[[#fn:r1376|1376]]</sup> . The long-term, equilibrium Arctic permafrost loss to global warming was analysed by Chadburn et al. (2017) <sup>[[#fn:r1377|1377]]</sup> . They used an empirical relation between recent mean annual air temperatures and the area underlain by permafrost coupled to Coupled Model Intercomparison Project Phase 5 (CMIP5) stabilization projections to 2300 for RCP2.6 and RCP4.5. Their estimate of the sensitivity of permafrost to warming is 2.9–5.0 million km <sup>2</sup> °C <sup>–1</sup> (1 standard deviation confidence interval), which suggests that stabilizing climate at 1.5°C as opposed to 2°C would reduce the area of eventual permafrost loss by 1.5 to 2.5 million km <sup>2</sup> (stabilizing at 56–83% as opposed to 43–72% of 1960–1990 levels). This work, combined with the assessment of Collins et al. (2013) <sup>[[#fn:r1378|1378]]</sup> on the link between global warming and permafrost loss, leads to the assessment that permafrost extent would be appreciably greater in a 1.5°C warmer world compared to in a 2°C warmer world ( ''low to medium confidence'' ).

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