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

==== 16.5.3.4 Illustration: Risk and Adaptation Pathways in Densely Populated and Agricultural Deltas ====

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Large deltas, which are very dynamic risk hotspots of global importance and interest ( [[#Wigginton--2015|Wigginton, 2015]] ; [[#Hill--2020|Hill et al., 2020]] ; [[#Nicholls--2020|Nicholls et al., 2020]] ), serve well to illustrate how risk pathways develop over time, determined by climatic as well as non-climatic risk drivers and by adaptation. Deltas occupy less than 0.5% of the global land area but host over 5% of the global population ( [[#Dunn--2019|Dunn et al., 2019]] ) and contribute major fractions of food production in many world regions ( [[#Kuenzer--2020|Kuenzer et al., 2020]] ). Future risk in these areas is heavily driven by climate change but also greatly depends on past, current and future socioeconomic changes which influence future trends in exposure, vulnerability and adaptive capacity of natural and human systems ( ''high confidence'' ) ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). From a risk perspective, trends over the past decades have been unfavourable for many deltas, as most of them have experienced a simultaneous intensification of hazards, rise in exposure and stagnation or only limited reduction in vulnerability, particularly in low-income countries ( ''high confidence'' ) ( [[#Day--2016|Day et al., 2016]] ; [[#Tessler--2016|Tessler et al., 2016]] ; [[#Loucks--2019|Loucks, 2019]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Hill--2020|Hill et al., 2020]] ).

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===== 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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===== 16.5.3.4.2 Exposure trends in deltas =====

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Next to the trends in hazards, future exposure of and in deltas is shaped particularly by the increase of population and infrastructure and the intensification of land use. Over the recent years, the population has been rising in major deltas, roughly along with overall national population trends ( [[#Szabo--2016|Szabo et al., 2016]] ). In 2017, 339 million people lived in deltas with a high exposure to flooding, cyclones and other coastal hazards ( [[#Edmonds--2020|Edmonds et al., 2020]] ). Over 40% of the global population exposed to flooding from tropical cyclones lived in deltas, more than 90% of which in developing countries and emerging economies (ibid.). Looking into the future, population in low-elevation coastal zones is expected to increase by 2050 across all SSPs with diverging developments in the second half of the century, and at the end of the century will reach well over 1 billion people in SSP3 ( [[#Jones--2016|Jones and O’Neill, 2016]] ; [[#Merkens--2016|Merkens et al., 2016]] ). A major part of this population is expected to reside in deltas with large cities or mega-urban agglomerations such as the Pearl River Delta, China. One of the first studies using the SSP-RCP framework on the delta scale suggests a strong increase in intensive agricultural land by the middle of the century in three SSPs (2, 3, 5) in the Volta Delta, Ghana, while the Mahanadi, India, and the Ganges–Brahmaputra–Meghna do not show a significant further increase ( [[#Kebede--2018|Kebede et al., 2018]] ). Hence, the amount of population and infrastructure as well as agricultural land is expected to rise further under certain SSPs, further increasing the exposure to future climate hazards.

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===== 16.5.3.4.3 Vulnerability trends in deltas =====

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Deltas are characterised by multi-faceted vulnerabilities of their environment and human populations. Over 200 indicators are being used in the literature to characterise and analyse vulnerability in deltas, spanning social, ecological and economic aspects ( [[#Sebesvari--2016|Sebesvari et al., 2016]] ). However, only a few studies model or dynamically assess trends in vulnerability, particularly for the future, at global scale, or take a comparative approach. But overall, a global trend assessment suggests that social vulnerability to climate hazards has been improving over the past years in all world regions hosting major deltas apart from Oceania, yet with emerging economies and developing countries in Africa showing less improvement than the Americas, Asia and Europe ( [[#Feldmeyer--2017|Feldmeyer et al., 2017]] ). An analysis of 48 major deltas finds that vulnerability therefore is a less dominant source of future increase in risk than exposure ( [[#Haasnoot--2012|Haasnoot et al., 2012]] ). However, case study research from individual deltas suggests that delta populations, particularly those with agriculture-based livelihoods, have seen more limited vulnerability reduction due in particular to the impacts of environmental hazards, stress and disasters ( ''high confidence'' ). In the Mekong Delta, for instance, the strong economic growth since the beginning of Vietnam’s reform process has not led to a reduction of vulnerability across the board for all socioeconomic groups ( [[#Garschagen--2015|Garschagen, 2015]] ). Rather, issues such as widespread landlessness or continued poverty have maintained and, in some respect, increased social vulnerability.

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