Editing IPCC:AR6/SROCC/Chapter-5 (section)

===== 5.2.2.2.3 Tides and coastal physical changes in a changing climate =====

Coastal systems are subject to the same large-scale warming trends as the open ocean, but the local response may be dominated by a complex of localised changes in factors such as circulation, mixing, river plumes or the seasonal upwelling of cold water. Using ESMs to project how these factors will interact often requires much finer resolution than is currently affordable in global models, however regional high-resolution models can be effective, especially in marginal seas like the Mediterranean with restricted interactions with the open ocean and that respond primarily to local forcing (Adloff et al., 2015 <sup>[[#fn:r135|135]]</sup> ). High resolution regional models have also been used to project robust localised ocean climate changes in wide shelf seas with more extensive interactions with the open ocean, like those in northwestern Europe (Tinker et al., 2016 <sup>[[#fn:r136|136]]</sup> ). The technical difficulties of using nested regional models are much greater in coasts adjacent to energetic large-scale currents like the Gulf Stream, Kuroshio, and Agulhas, and projecting detailed coastal climate change such places may require the use of expensive high resolution global models (Saba et al., 2016 <sup>[[#fn:r137|137]]</sup> ). These physical coastal changes have consequences that cascade through ecosystems to people, as is illustrated in detail for eastern boundary upwelling systems in Box 5.2.

Both human structures and ecological systems in the coastal zone are directly impacted by tidal amplitudes, which contribute to high-water levels and the tidal flushing rates of estuaries, embayments, marshes and mangroves. The tides are the response of a forced-damped-resonance system (Arbic et al., 2009 <sup>[[#fn:r138|138]]</sup> ). The M 2 tide is the dominant tidal constituent in most places, with a period of half a lunar day, or 12 hours, 25 minutes; the M 2 tides are created by the differential motion of the solid Earth and oceans in response to the gravitational attraction of the moon (Newton, 1687 <sup>[[#fn:r139|139]]</sup> ; Laplace, 1799 <sup>[[#fn:r140|140]]</sup> ). The astronomical forcing evolves only slowly, however the tidal damping and basin resonance at tidal frequencies can change in response to changes in sea level, stratification and coastal conditions (Müller, 2012 <sup>[[#fn:r141|141]]</sup> ; Schindelegger et al., 2018 <sup>[[#fn:r142|142]]</sup> ). Several recent studies have analysed historical coastal tide gauge data and found amplitude trends of order 1 – 4% per century (Ray, 2009; Woodworth, 2010; Müller et al., 2011). In some locations, the changes in the tides have been of comparable importance to changes in mean sea level for explaining changes in high water levels (Jay, 2009). For many individual tide gauges, the trends in tidal amplitude are strongly positively or negatively correlated with local time-mean sea level trends (Devlin et al., 2017 <sup>[[#fn:r145|145]]</sup> ). Another source of secular tidal changes, changes in oceanic stratification, modifies the rate of energy conversion from the barotropic tides to the internal tides (Jayne and St. Laurent, 2001 <sup>[[#fn:r146|146]]</sup> ), the vertical profile of turbulent viscosity on shelves (Müller, 2012 <sup>[[#fn:r147|147]]</sup> ), and the propagation speed of the internal tides (Zhao, 2016 <sup>[[#fn:r148|148]]</sup> ). For example, Colosi and Munk (2006) found an increase in the amplitude of the principal lunar semidiurnal tide M 2 in Honolulu of about 1 cm over the past 100 years, which they attributed primarily to changes in oceanic stratification bringing about local changes in relative phases of the internal and external M 2 tides, increasing constructive interference. Both sea level and stratification are expected to exhibit robust secular positive trends in the coming century due to climate change, at rates that are significantly larger than historical trends, and people may choose to replace natural beaches and marshes with sea-walls in response to rising sea levels. As a result, it is ''very likely'' that the majority of coastal regions will experience statistically significant changes in tidal amplitudes over the course of the 21st century.

Because coastal tides are near resonance in many locations, small changes in sea level and bay shape can change the local tides significantly. For example, the insertion of tidal power plants can have a significant impact on the local tides (Ward et al., 2012 <sup>[[#fn:r149|149]]</sup> ). Various observational and modeling studies demonstrate that SLR has spatial heterogeneous impacts on the tides, with some locations experiencing decreased tidal amplitudes and others experiencing increased tidal amplitudes (Pickering et al., 2012 <sup>[[#fn:r150|150]]</sup> ; Devlin et al., 2017 <sup>[[#fn:r151|151]]</sup> ; Pickering et al., 2017 <sup>[[#fn:r152|152]]</sup> ). Projections of tidal changes indicate that the patterns and even the sign of changes in tidal amplitudes depend on whether the coastlines are allowed to recede with rising sea levels or are held in place (Pickering et al., 2017 <sup>[[#fn:r153|153]]</sup> ; Schindelegger et al., 2018 <sup>[[#fn:r154|154]]</sup> ) . Pelling et al. (2013) <sup>[[#fn:r155|155]]</sup> and Hwang et al. (2014) <sup>[[#fn:r156|156]]</sup> demonstrate that the rapid coastline changes in China’s Bohai Sea have already altered the tides in that region and throughout the Yellow Sea (Hwang et al., 2014 <sup>[[#fn:r157|157]]</sup> ). Pelling and Green (2014) examine the impact of flood defenses as well as SLR on tides on the European Shelf. Such tidal changes have implications for designing flood defenses, for tidal renewable energy, for tidal flushing timescales of estuaries and embayments, and for navigational dredging requirements (Pickering et al., 2012 <sup>[[#fn:r158|158]]</sup> ) (Section 5.4.2). The sign and amplitude of local changes to tides are ''very likely'' to be impacted by both human coastal adaptation measures and climate drivers (listed above).

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