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

==== 4.2.2.4 Local Coastal Sea Level ====

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Since the local coastal sea level (scale ~10 km) is affected by global, regional (scale ~100 km) and coastal scale features and processes like anthropogenic subsidence, it may differ substantially from the regional sea level. At the coast, the sea level change is additionally affected by wave run up, tidal level, wind forcing, sea level pressure (SLP), the dominant modes of climate variability, seasonal climatic periodicities, mesoscale eddies, changes in river flow, as well as anthropogenic subsidence (see also Box 4.1). These local contributions, combined with sea level events generated by storm surges and tides result in anomalous conditions (ESL) which last for a short time in contrast to the gradual increase over time from for instance ice mass loss. Flood risk due to ESL is exacerbated due to its interaction with RSL and hence physical vulnerability assessments combine uncertainties around ESL and RSL, both in terms of contemporary assessments and future projections (Little et al., 2015b <sup>[[#fn:r312|312]]</sup> ; Vousdoukas, 2016 <sup>[[#fn:r313|313]]</sup> ; Vousdoukas et al., 2016 <sup>[[#fn:r314|314]]</sup> ; Wahl et al., 2017 <sup>[[#fn:r315|315]]</sup> ). Changes in mean sea level have been dealt with in previous sections (e.g., Section 4.2.2.2.6). Here the focus is on some of the components of ESL that have been assessed in combination with changes in RSL. Church et al. (2013) concluded that change in sea level extremes is ''very'' ''likely'' to be caused by a RSL increase, and that storminess and surges will contribute towards these extremes; however, it was noted that there was ''low confidence'' in region-specific projections as there was only a limited number of studies with a poor geographical coverage available.

Recent advances in statistical and dynamical modelling of wave effects at the coast, storm surges and inundation risk have reduced the uncertainties around the inundation risks at the coast (Vousdoukas et al., 2016 <sup>[[#fn:r316|316]]</sup> ; Vitousek et al., 2017 <sup>[[#fn:r317|317]]</sup> ; Melet et al., 2018 <sup>[[#fn:r318|318]]</sup> ; Vousdoukas et al., 2018c <sup>[[#fn:r319|319]]</sup> ) and assessments of the resulting highly resolved coastal sea levels are now emerging (Cid et al., 2017 <sup>[[#fn:r320|320]]</sup> ; Muis et al., 2017 <sup>[[#fn:r321|321]]</sup> ; Wahl et al., 2017 <sup>[[#fn:r322|322]]</sup> ). This progress was facilitated due to the availability of, for example, the Global Extreme Sea Level Analysis (GESLA-2; Woodworth et al., 2016 <sup>[[#fn:r323|323]]</sup> ) high-frequency (hourly) datasets, advances in the Coordinated Ocean Wave Climate Project (COWCLIP; Hemer et al., 2013 <sup>[[#fn:r324|324]]</sup> ), coastal altimetry datasets (Cipollini et al., 2017 <sup>[[#fn:r325|325]]</sup> ), and the Global Tide and Surge Reanalysis (GTSR; Muis et al., 2016 <sup>[[#fn:r326|326]]</sup> ), while new analyses of datasets that have been available since before the publication of AR5 have continued (e.g., PSML; Holgate et al., 2012 <sup>[[#fn:r327|327]]</sup> ).

Although ESL is experienced episodically by definition, Marcos et al. (2015) <sup>[[#fn:r328|328]]</sup> examined the long-term behaviour of storm surge models and detected decadal and multidecadal variations in storm surge that are not related to changes in RSL. They found that, although 82% of their observed time series showed synchronous patterns at regional scales, the pattern tended to be non-linear, implying that it would be difficult to infer future behaviour unless the physical basis for the responses was understood. An analysis of the relative contributions of SLR and ESL due to storminess showed that in the US Pacific northwest since the early 1980s, increases in wave height and period have had a larger effect on coastal flooding and erosion than RSL (Ruggiero, 2012 <sup>[[#fn:r329|329]]</sup> ) since the early 1980s. This is also true in other regions distributed over the entire globe (Melet et al., 2016 <sup>[[#fn:r330|330]]</sup> ; Melet et al., 2018 <sup>[[#fn:r331|331]]</sup> ). Changes since 1990 in the sea level harmonics and seasonal phases and amplitudes of the wave period and significant wave height were found for the Gulf of Mexico coast and along the US east coast (Wahl et al., 2014 <sup>[[#fn:r332|332]]</sup> ; Wahl and Plant, 2015 <sup>[[#fn:r333|333]]</sup> ). These authors found that high waters have increased twice as much as one would expect from long-term SLR alone, because of additional changes in the seasonal cycle, yielding a 30% increase in risk of flooding. Such effects are ''likely'' to be highly dependent on the local conditions. For example, using WAVEWATCH III, TOPEX/Poseidon altimetry tide model data and atmospheric forcing physically downscaled using Delft3D-WAVE and Delft3D-FLOW in what they call the Coastal Storm Modeling System (CoSMoS), Vitousek et al. (2017) were able to detect local inundation hazards (at a scale of hundreds of metres) across regions along the Californian coast. Similarly, Castrucci and Tahvildari (2018) <sup>[[#fn:r334|334]]</sup> simulated the impact of SLR along the Mid-Atlantic region in the USA. A study for the Maldives shows that the contribution of wave setup is essential to estimate flood risks (Wadey et al., 2017 <sup>[[#fn:r335|335]]</sup> ).

In deltas, the local sea level can be dominated by anthropogenic subsidence more than by the processes outlined above. It is often a primary driver of elevated local SLR and increased flood hazards in those regions. This is particularly true for deltaic systems, where fertile soils, low-relief topography, freshwater access, and strategic ports have encouraged the development of many of the world’s most densely populated coastlines and urban centres. For example, globally, one in fourteen humans resides in mid-to-low latitude deltas (Day et al., 2016 <sup>[[#fn:r336|336]]</sup> ). Although in these areas RSL is dominated by anthropogenic subsidence, climate effects need to be included for estimating risks associated with RSL (Syvitski et al., 2009 <sup>[[#fn:r337|337]]</sup> ).

Deltas are formed by the accumulation of unconsolidated river born sediments and porous organic material, both of which are particularly prone to compaction. It is the compaction which causes a drop in land elevation that increases the rate of local SLR above what would be observed along a static coastline or one where only climatological forced processes control the RSL. Under stable deltaic conditions, the accumulation of fluvially-sourced surficial sediment and organic matter offsets this natural subsidence (Syvitski and Saito, 2007 <sup>[[#fn:r338|338]]</sup> ); however, in many cases this natural process of delta construction has been disturbed by reductions in fluvial sediment supply via upstream dams and fluvial channelisation (Vörösmarty et al., 2003 <sup>[[#fn:r339|339]]</sup> ; Syvitski and Saito, 2007 <sup>[[#fn:r340|340]]</sup> ; Syvitski et al., 2009 <sup>[[#fn:r341|341]]</sup> ; Luo et al., 2017 <sup>[[#fn:r342|342]]</sup> ). Further, the extraction of fluids and gas that fill the pore space of deltaic sediments and provide support for overlying material has significantly increased the rate of compaction and resultant anthropogenic subsidence along many populated deltas (Higgins, 2016 <sup>[[#fn:r343|343]]</sup> ). In addition, Nicholls (2011) pointed to anthropogenic subsidence by the weight of buildings in megacities in South-East Asia.

Average natural and anthropogenic subsidence rates of 6–9 mm yr <sup>–1</sup> are reported for the highly populated areas of Ganges-Brahmaputra-Meghna delta in the urban centres of Kolkata and Dhaka (Brown and Nicholls, 2015 <sup>[[#fn:r344|344]]</sup> ). A fraction of these subsidence rates might be caused by long-term processes of increased sediment loading during the Holocene resulting from changes in the monsoon system (Karpytchev et al., 2018 <sup>[[#fn:r345|345]]</sup> ). Subsidence rates are expected to decrease in the Ganges-Brahmaputra-Meghna delta in the near future due to planned dam projects and an estimated 21% drop in resulting sediment supply (Tessler et al., 2018 <sup>[[#fn:r346|346]]</sup> ). Observations of enhanced natural and anthropogenic subsidence on the Ganges-Brahmaputra-Meghna are common to most heavily populated deltaic systems. Coastal mega-cities that have been particularly prone to human-enhanced subsidence include Bangkok, Ho Chi Minh city (Vachaud et al., 2018 <sup>[[#fn:r347|347]]</sup> ), Jakarta, Manila, New Orleans, West Netherlands and Shanghai (Yin et al., 2013 <sup>[[#fn:r348|348]]</sup> ; Cheng et al., 2018 <sup>[[#fn:r349|349]]</sup> ). On a global scale, observed rates of modern deltaic anthropogenic subsidence range from 6–100 mm yr <sup>–1</sup> (Bucx et al., 2015 <sup>[[#fn:r350|350]]</sup> ; Higgins, 2016 <sup>[[#fn:r351|351]]</sup> ). Rates of recent deltaic subsidence over the last few decades have been at least twice the 3 mm yr <sup>–1</sup> rate of GMSL rise observed over this same interval (Higgins, 2016 <sup>[[#fn:r352|352]]</sup> ; Tessler et al., 2018 <sup>[[#fn:r353|353]]</sup> ). Numerical models that have reproduced these observed rates of anthropogenic deltaic subsidence by considering human-induced compaction and reduced sediment supply, support anthropogenic causes for elevated rates of subsidence (Tessler et al., 2018 <sup>[[#fn:r354|354]]</sup> ).

In summary, ESL interacts with RSL rise including anthropogenic subsidence in many vulnerable areas (see Box 4.1). Therefore, it is concluded with ''high confidence'' that the inclusion of local processes (wave effects, storm surges, tides, erosion, sedimentation and compaction) is essential to estimate local, relative and changes in ESL events. Although the effect of anthropogenic subsidence may be very large locally, it is not accounted for in the projection sections of this chapter as no global data sets are available which are consistent with RCP scenarios, and because the scale at which these processes take place is often smaller than the spatial scale used in climate models.

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