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

==== 4.2.1.5 Geodynamic Processes ====

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Changing distributions of water mass between land, ice and ocean reservoirs cause nearly instantaneous changes in the Earth’s gravity field and rotation, and elastic deformation of the solid Earth. These processes combine to produce spatially varying patterns of sea level change (Mitrovica et al., 2001 <sup>[[#fn:r35|35]]</sup> ; Mitrovica et al., 2011 <sup>[[#fn:r36|36]]</sup> ). For example, adjacent to an ice sheet losing mass, reduced gravitational attraction between the ice and nearby ocean causes RSL to fall, despite the rise in GMSL from the input of melt water to the ocean. The opposite effect is found far from the ice sheet, where RSL rise can be enhanced as much as 30% relative to the global average.

On time scales longer than the elastic Earth response, redistributions of water and ice cause time-dependent, visco-elastic deformation. This is observed in regions previously covered by ice during the Last Glacial Maximum (LGM), including much of Scandinavia and parts of North America (Lambeck et al., 1998 <sup>[[#fn:r37|37]]</sup> ; Peltier, 2004 <sup>[[#fn:r38|38]]</sup> ), where glacio-isostatic adjustment (GIA) is causing uplift and a lowering of RSL that continues today. In other locations proximal to the previous ice load, and where a glacial forebulge once existed, the relaxing forebulge can contribute to a relative SLR, as currently being experienced along the coastline of the northeast United States. Water being syphoned to high latitudes as the peripheral bulges collapse leads to a widespread RSL fall in equatorial regions, while the overall loading of ocean crust by melt water can cause uplift of land areas near continental margins, far from the location of previous ice loading (Mitrovica and Milne, 2003 <sup>[[#fn:r39|39]]</sup> ; Milne and Mitrovica, 2008 <sup>[[#fn:r40|40]]</sup> ). Rates of modern VLM associated with these post-glacial processes are generally on the order of a few mm yr <sup>–1</sup> or less, but can exceed 1 cm yr <sup>–1</sup> in some places. Because these gravity, rotation, and deformation (GRD) processes control spatial patterns of SLR from melting land ice, they need to be accounted for in regional-to-local sea level assessments. GRD processes are also important for marine-based ice sheets themselves, because they locally reduce RSL at retreating grounding lines which can slow and reduce retreat (Gomez et al., 2015 <sup>[[#fn:r41|41]]</sup> ; see 4.3.3.1.2 and Cross-chapter Box 8 in Chapter 3; Larour et al., 2019 <sup>[[#fn:r42|42]]</sup> ).

VLM from tectonics and dynamic topography associated with viscous mantle processes also affect spatial patterns of relative sea level change. These geological processes are important for reconstructing ancient sea levels based on geological indicators (Austermann and Mitrovica, 2015 <sup>[[#fn:r43|43]]</sup> ; see SM4.1). Along with other natural and anthropogenic processes including volcanism, compaction, and anthropogenic subsidence from ground water extraction (Section 4.2.2.4) these geodynamic processes can be locally important, producing rates of VLM comparable to or greater than recent climate-driven rates of GMSL change (Wöppelmann and Marcos, 2016 <sup>[[#fn:r44|44]]</sup> ). In this chapter, GIA and anthropogenic subsidence are used, and other components of VLM are ignored unless explicitly stated.Changing distributions of water mass between land, ice and ocean reservoirs cause nearly instantaneous changes in the Earth’s gravity field and rotation, and elastic deformation of the solid Earth. These processes combine to produce spatially varying patterns of sea level change (Mitrovica et al., 2001; Mitrovica et al., 2011). For example, adjacent to an ice sheet losing mass, reduced gravitational attraction between the ice and nearby ocean causes RSL to fall, despite the rise in GMSL from the input of melt water to the ocean. The opposite effect is found far from the ice sheet, where RSL rise can be enhanced as much as 30% relative to the global average.

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