Editing IPCC:AR6/WGII/Chapter-16 (section)

===== 16.5.3.4.1 Hazard trends in deltas =====

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Deltas face multiple interacting hazards, many of which over the past decades have been intensified by local and regional anthropogenic developments (e.g., the construction of dams, groundwater extraction, or agricultural irrigation practices) and most of which are expected to be exacerbated by climate change ( ''high confidence'' ) ( [[#Giosan--2014|Giosan et al., 2014]] ; [[#Tessler--2015|Tessler et al., 2015]] ; [[#Tessler--2016|Tessler et al., 2016]] ; [[#Arto--2019|Arto et al., 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). The most important hazards include SLR, inundation, salinity intrusion, cyclones, storms and erosion, many of which occur in combination. The potential for flooding and inundation depends on the relative sea level rise (RSLR) which results from global and regional SLR as well as local subsidence within the deltas. Subsidence caused by natural and human drivers (mainly compaction and groundwater extraction) is currently the most important cause for RSLR in many deltas and can exceed the rate of climate-induced SLR by an order of magnitude ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). But in higher warming scenarios the relative importance of climate-driven SLR is expected to increase over time ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). In a global study covering 47 major deltas and assessing future trends of sediment delivery across four RCPs, three SSPs (1,2,3) and a projection of future dam construction, [[#Dunn--2019|Dunn et al. (2019)]] find most deltas (33 out of the 47) will experience a mean decline of 38% in sediment flux by the end of the century when considering the average of the scenarios. [[#Nienhuis--2020|Nienhuis et al. (2020)]] find in a global assessment that some deltas have gained land through increased sediment load (e.g., through deforestation), but recent land gains are unlikely to be sustained if SLR continues to accelerate. According to the latest assessments, it is ''virtually certain'' that global mean sea level will continue to rise over the 21st century, with SLR by 2100 ''likely'' to reach 0.28–0.55 m in a an SSP1–1.9 and 0.63–1.01 m in an SSP5–8.5 scenario relative to 1995–2014 ( [[#IPCC--2021|IPCC, 2021]] ). The combined effects of local subsidence and GMSL rise result in a significant increase in the potential for inundation of low-lying deltas across all RCPs, with some variation according to regional sea level change rates, without significant further adaptation measures ( ''very high confidence'' ).

In terms of salt-water intrusion and salinisation, global comparative studies are still lacking but the general processes are well understood (e.g., [[#White--2017|White and Kaplan, 2017]] ), and research on individual deltas is on the rise. In the Mekong Delta of Vietnam, one of the main rice-producing deltas globally, salinity intrusion has been observed to extend around 15 km inland during the rainy season and around 50 km during the dry season ( [[#Gugliotta--2017|Gugliotta et al., 2017]] ), resulting in rice yield losses of up to 4 t ha −1 yr −1 ( [[#Khat--2018|Khat et al., 2018]] ). SLR, along with the expansion of dams and dry season irrigation upstream, is expected to further increase the salinity intrusion into the delta. This creates additional risk for food production as rice and other crops might be pushed beyond their adaptation limits in terms of salt tolerance, potentially affecting many of the 282,000 agriculture-based livelihoods in the Mekong Delta and increasing the pressure for cost-intensive adaptation ( [[#Smajgl--2015|Smajgl et al., 2015]] ). [[#Genua-Olmedo--2016|Genua-Olmedo et al. (2016)]] find for the Ebro that in high scenario (RCP8.5, and SLR of almost 1 m by 2100), SLR-induced salinity intrusion will lead to almost a doubling of salinity levels and a decrease of mean rice productivity by over 20% in a high-SLR scenario with almost 1 m of SLR by the end of the century.

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