Editing IPCC:AR6/SROCC/Chapter-4 (section)

===== 4.2.2.2.4 Greenland and Antarctic ice sheets =====

Frequent observations of ice sheet mass changes have only been available since the advent of space observations (see Section 3.3.1). In the pre-satellite era, mass balance was geodetically reconstructed only for the GIS (Kjeldsen et al., 2015 <sup>[[#fn:r217|217]]</sup> ) . These geodetic reconstructions empirically constrain the contribution of the GIS to SLR between 1900 and 1983 to 17.2 (10.7 – 23.2; Kjeldsen et al., 2015 <sup>[[#fn:r218|218]]</sup> ) . During the satellite era, three approaches have been developed to estimate ice sheet mass balance: 1) Mass loss is estimated by direct measurements of ice sheet height changes with satellite laser or radar altimetry in combination with climatological/glaciological models for firn density and compaction, 2) the input–output method combines measurements of ice flow velocities estimated from satellite (synthetic aperture radar or optical imagery) across key outlets with estimates of net surface balance derived from ice thickness data, 3) space gravimetry data yields direct estimate of the mass changes by inversion of the anomalies in the gravity field (see Section 3.3.1 for more details). AR5 concluded that the three space-based methods give consistent results. They agree in showing that the rate of SLR due to the GIS and AIS’ contributions has increased since the early 1990s. Since AR5, up-to-date observations confirm this statement with increased confidence for both ice sheets (Rignot et al., 2019 <sup>[[#fn:r219|219]]</sup> ; see Section 3.3.1) . The assessment of the literature since AR5 made in Section 3.3.1 shows that the contribution from Greenland to SLR over 2012–2016 (0.68 (0.64 – 0.72) mm yr <sup>–1</sup> ) was similar to the contribution over 2002–2011 (0.73 (0.67 – 0.79) mm yr <sup>–1</sup> ) and ''extremely likely'' greater than over 1992–2001 (0.02 (0.21 – 0.25) mm yr <sup>–1</sup> ). The contribution from Antarctica over 2012–2016 (0.55 (0.48 – 0.62) mm yr <sup>–1</sup> ) was ''extremely likely'' greater than over the 2002–2011 period (0.23 (0.16 – 0.30) mm yr <sup>–1</sup> ) and ''likely'' greater than over the period 1992–2001 (0.14 (0.12 – 0.16); see Section 3.3.1 for more details).

Here, the approach of Section 3.3.1 is followed, using the two multi-method assessments from Bamber et al. (2018) <sup>[[#fn:r220|220]]</sup> and the IMBIE team (2018) to evaluate the contribution of ice sheet mass loss to SLR over 1993–2015 and 2006–2015 (see Table 4.1). These two studies agree with results from the WCRP Global Sea Level Budget Group (2018) . For the estimation of the AIS contribution, Bamber et al. (2018) <sup>[[#fn:r221|221]]</sup> and the The IMBIE team (2018) use similar but not identical data sources and processing. Both studies find consistent results within uncertainties over both periods. In Table 4.1, the results of these two studies were averaged, and weighted the average on the basis of their uncertainties, because there is no apparent reason to discount either study. For the estimation of the GIS contribution only the Bamber et al. (2018) <sup>[[#fn:r222|222]]</sup> estimate is used, as there is no other multi-method assessment available.

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