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

==== 3.4.5.4 Deltas and estuaries ====

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Observations of SLR and human influence are felt through salinization, which leads to mixing in deltas and estuaries, aquifers, leading to flooding (also enhanced by precipitation and river discharge), land degradation and erosion. Salinization is projected to impact freshwater sources and pose risks to ecosystems and human systems (Section 5.4; Wong et al., 2014) <sup>[[#fn:r786|786]]</sup> . For instance, in the Delaware River estuary on the east coast of the USA, upward trends of salinity (measured since the 1900s), accounting for the effects of streamflow and seasonal variations, have been detected and SLR is a potential cause (Ross et al., 2015) <sup>[[#fn:r787|787]]</sup> .

Z. Yang et al. (2015) <sup>[[#fn:r788|788]]</sup> found that future climate scenarios for the USA (A1B 1.6°C and B1 2°C in the 2040s) had a greater effect on salinity intrusion than future land-use/land-cover change in the Snohomish River estuary in Washington state (USA). This resulted in a shift in the salinity both upstream and downstream in low flow conditions. Projecting impacts in deltas needs an understanding of both fluvial discharge and SLR, making projections complex because the drivers operate on different temporal and spatial scales (Zaman et al., 2017; Brown et al., 2018b) <sup>[[#fn:r789|789]]</sup> . The mean annual flood depth when 1.5°C is first projected to be reached in the Ganges-Brahmaputra delta may be less than the most extreme annual flood depth seen today, taking into account SLR, surges, tides, bathymetry and local river flows (Brown et al., 2018b) <sup>[[#fn:r790|790]]</sup> . Further, increased river salinity and saline intrusion in the Ganges-Brahmaputra-Meghna is ''likely'' with 2°C of warming (Zaman et al., 2017) <sup>[[#fn:r791|791]]</sup> . Salinization could impact agriculture and food security (Cross-Chapter Box 6 in this chapter). For 1.5°C or 2°C stabilization conditions in 2200 or 2300 plus surges, a minimum of 44% of the Bangladeshi Ganges-Brahmaputra, Indian Bengal, Indian Mahanadi and Ghanese Volta delta land area (without defences) would be exposed unless sedimentation occurs (Brown et al., 2018b) <sup>[[#fn:r792|792]]</sup> . Other deltas are similarly vulnerable. SLR is only one factor affecting deltas, and assessment of numerous geophysical and anthropogenic drivers of geomorphic change is important (Tessler et al., 2018) <sup>[[#fn:r793|793]]</sup> . For example, dike building to reduce flooding and dam building (Gupta et al., 2012) <sup>[[#fn:r794|794]]</sup> restricts sediment movement and deposition, leading to enhanced subsidence, which can occur at a greater rate than SLR (Auerbach et al., 2015; Takagi et al., 2016) <sup>[[#fn:r795|795]]</sup> . Although dikes remain essential for reducing flood risk today, promoting sedimentation is an advisable strategy (Brown et al., 2018b) <sup>[[#fn:r796|796]]</sup> which may involve nature-based solutions. Transformative decisions regarding the extent of sediment restrictive infrastructure may need to be considered over centennial scales (Brown et al., 2018b) <sup>[[#fn:r797|797]]</sup> . Thus, in a 1.5°C or 2°C warmer world, deltas, which are home to millions of people, are expected to be highly threatened from SLR and localized subsidence ( ''high confidence'' ).

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