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

==== 3.4.2.4 Estuaries, Deltas and Coastal Lagoons ====

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Estuaries, deltas and lagoons encounter environmental gradients over small spatial scales, generating diverse habitats that support myriad ecosystem services, including food provision, regulation of erosion, nutrient recycling, carbon sequestration, recreation and tourism, and cultural significance ( [[#D’Alelio--2021|D’Alelio et al., 2021]] ; [[#Keyes--2021|Keyes et al., 2021]] ). Although these coastal ecosystems have historically been sensitive to erosion-accretion cycles driven by sea level, drought and storms ( ''high confidence'' ) ( [[#Peteet--2018|Peteet et al., 2018]] ; [[#Wang--2018c|Wang et al., 2018c]] ; [[#Jones--2019b|Jones et al., 2019b]] ; [[#Urrego--2019|Urrego et al., 2019]] ; [[#Hapsari--2020|Hapsari et al., 2020]] ; [[#Zhao--2020b|Zhao et al., 2020b]] ), they were impacted for much of the 20th century primarily by non-climate drivers ( ''very high confidence'' ) ( [[#Brown--2018b|Brown et al., 2018b]] ; [[#Ducrotoy--2019|Ducrotoy et al., 2019]] ; [[#Elliott--2019|Elliott et al., 2019]] ; [[#He--2019|He and Silliman, 2019]] ; [[#Andersen--2020|Andersen et al., 2020]] ; [[#Newton--2020|Newton et al., 2020]] ; [[#Stein--2020|Stein et al., 2020]] ). Nevertheless, the influence of climate-induced drivers has become more apparent over recent decades ( ''medium confidence'' ) (Table 3.6).

'''Table 3.6 |''' Summary of previous IPCC assessments of estuaries, deltas and coastal lagoons

{| class="wikitable"
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! Observations

! Projections

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| ''AR5 ( [[#Wong--2014|Wong et al., 2014]] )''

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| Humans have impacted lagoons, estuaries and deltas ( ''high'' to ''very high confidence'' ), but non-climate drivers have been the primary agents of change ( ''very'' ''high confidence'' ).

In estuaries and lagoons, nutrient inputs have driven eutrophication, which has modified food-web structures ( ''high confidence'' ) and caused more-intense and longer-lasting hypoxia, more-frequent occurrence of harmful algal blooms and enhanced emissions of nitrous oxide ( ''high confidence'' ).

In deltas, land-use changes and associated disruption of sediment dynamics and land subsidence have driven changes that have been exacerbated by relative SLR and episodic events, including river floods and oceanic storm surges ( ''very high confidence'' ).

Increased coastal flooding, erosion and saltwater intrusions have led to degradation of ecosystems ( ''very high confidence'' ).

| Future changes in climate impact-drivers such as warming, acidification, waves, storms, sea level rise (SLR) and runoff will have consequences for ecosystem function and services in lagoons and estuaries ( ''high confidence'' ), but with regional differences in magnitude of change in impact drivers and ecosystem response.

Warming, changes in precipitation and changes in wind strength can interact to alter water-column salinity and stratification ( ''medium confidence'' ), which could impact water column oxygen content ( ''medium confidence'' ).

Land-use change, SLR and intensifying storms will alter deposition-erosion dynamics, impacting shoreline vegetation and altering turbidity ( ''medium confidence'' ). Together with warming, these drivers will alter the seasonal pattern of primary production and the distribution of biota throughout the ecosystems ( ''medium to high confidence'' ), impacting associated ecosystem services.

The projected impacts of climate change on deltas are associated mainly with pluvial floods and SLR, which will amplify observed impacts of interacting climate and non-climate drivers ( ''high confidence'' ).

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| ''SR15 ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] )''

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| Estuaries, deltas and lagoons were not assessed in this report.

| Under both a 1.5°C and 2°C of warming, relative to the pre-industrial era, deltas are expected to be highly threatened by SLR and localised subsidence ( ''high confidence'' ). The slower rate of SLR associated with 1.5°C of warming poses smaller risks of flooding and salinisation ( ''high confidence'' ), and facilitates greater opportunities for adaptation, including managing and restoring natural coastal ecosystems and infrastructure reinforcement ( ''medium confidence'' ).

[Intact coastal ecosystems] ‘may be effective in reducing the adverse impacts of rising sea levels and intensifying storms by protecting coastal and deltaic regions ( ''medium confidence'' ).’

‘Natural sedimentation rates are expected to be able to offset the effect of rising sea levels, given the slower rates of SLR associated with 1.5°C of warming ( ''medium confidence'' ). Other feedbacks, such as landward migration of wetlands and the adaptation of infrastructure, remain important ( ''medium confidence'' ).’

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| ''SROCC ( [[#Bindoff--2019a|Bindoff et al., 2019a]] )''

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| Increased seawater intrusion caused by SLR has driven upstream redistribution of marine biotic communities in estuaries ( ''medium confidence'' ) where physical barriers, such as the availability of benthic substrates, do not limit availability of suitable habitats ( ''medium confidence'' ).

Warming has driven poleward range shifts in species’ distributions among estuaries ( ''medium confidence'' ).

Interactions between warming, eutrophication and hypoxia have increased the incidence of harmful algal blooms ( ''high confidence'' ), pathogenic bacteria, such as ''Vibrio'' species, ( ''low confidence'' ) and mortalities of invertebrates and fish communities ( ''medium confidence'' ).

| ‘Salinisation and expansion of hypoxic conditions will intensify in eutrophic estuaries, especially in mid and high latitudes with microtidal regimes ( ''high confidence'' ).’

‘The effects of warming will be more pronounced in high-latitude and temperate shallow estuaries with limited exchange with the open ocean [...] and seasonality that already leads to dead zone development [...] ( ''medium confidence'' ).’

Interaction between SLR and changes in precipitation will have greater impacts on shallow than deep estuaries ( ''medium confidence'' ).

Estuaries characterised by large tidal exchanges and associated well-developed sediments will be more resilient to projected SLR and changes in river flow ( ''medium confidence'' ). Human activities that inhibit sediment dynamics in coastal deltas increase their vulnerability to SLR ( ''medium confidence'' ).

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Estuarine biota are sensitive to warming ( ''high confidence'' ), with recent responses including changes in abundance of some fish stocks ( [[#Erickson--2021|Erickson et al., 2021]] ; [[#Woodland--2021|Woodland et al., 2021]] ), poleward shifts in distributions of fish species, communities and associated biogeographic transition zones (Table 12.3; [[#Franco--2020|Franco et al., 2020]] ; [[#Troast--2020|Troast et al., 2020]] ), recruits of warm-affinity species persisting into winter ( [[#Kimball--2020|Kimball et al., 2020]] ) and changes in seasonal timing of peaks in species abundance ( [[#Kimball--2020|Kimball et al., 2020]] ). MHWs can be more severe in estuaries than in adjacent coastal seas ( [[#Lonhart--2019|Lonhart et al., 2019]] ), causing conspicuous impacts ( ''very high confidence'' ), including mass mortality of intertidal vegetation ( [[#3.4.2.5|Section 3.4.2.5]] ), range shifts in algae and animals ( [[#Lonhart--2019|Lonhart et al., 2019]] ) and reduced spawning success among invertebrates ( [[#Shanks--2020|Shanks et al., 2020]] ).

Relative SLR extends the upstream limit of saline waters ( ''high confidence'' ) ( [[#Harvey--2020|Harvey et al., 2020]] ; [[#Jiang--2020|Jiang et al., 2020]] ) and alters tidal ranges ( ''high confidence'' ) ( [[#Idier--2019|Idier et al., 2019]] ; [[#Talke--2020|Talke et al., 2020]] ). Elevated water levels also alter submergence patterns for intertidal habitat ( ''high confidence'' ) ( [[#Andres--2019|Andres et al., 2019]] ), moving high-water levels inland ( ''high confidence'' ) ( [[#Peteet--2018|Peteet et al., 2018]] ; [[#Appeaning%20Addo--2020|Appeaning Addo et al., 2020]] ; [[#Liu--2020e|Liu et al., 2020e]] ) and increasing the salinity of coastal water tables and soils ( ''high confidence'' ) ( [[#Eswar--2021|Eswar et al., 2021]] ). These processes favour inland and/or upstream migration of intertidal habitat, where it is unconstrained by infrastructure, topography or other environmental features ( ''high confidence'' ) ( [[#Kirwan--2019|Kirwan and Gedan, 2019]] ; [[#Parker--2019|Parker and Boyer, 2019]] ; [[#Langston--2020|Langston et al., 2020]] ; [[#Magolan--2020|Magolan and Halls, 2020]] ; [[#Saintilan--2020|Saintilan et al., 2020]] ). The spread of ‘ghost forests’ along the North American east coast ( [[#Kirwan--2019|Kirwan and Gedan, 2019]] ) and elsewhere ( [[#Grieger--2020|Grieger et al., 2020]] ) illustrates this phenomenon. Along estuarine shorelines, changing submergence patterns and upstream penetration of saline waters interact synergistically to stress intertidal plants, changing species composition and reducing above-ground biomass, in some cases favouring invasive species ( [[#Xue--2018|Xue et al., 2018]] ; [[#Buffington--2020|Buffington et al., 2020]] ; [[#Gallego-Tévar--2020|Gallego-Tévar et al., 2020]] ). Overall, changing salinity and submergence patterns decrease the ability of shoreline vegetation to trap sediment ( [[#Xue--2018|Xue et al., 2018]] ), reducing accretion rates and increasing the vulnerability of estuarine shorelines to submergence by SLR and erosion by wave action ( ''medium confidence'' ) ( [[#Zhu--2020b|Zhu et al., 2020b]] ).

Drought and freshwater abstraction can reduce freshwater inflows to estuaries and lagoons, increasing salinity, reducing water quality ( [[#Brooker--2020|Brooker and Scharler, 2020]] ) and depleting resident macrophyte communities ( [[#Scanes--2020b|Scanes et al., 2020b]] ). Changes in freshwater input and SLR, combined with land-use change, can alter inputs of land-based sediments, causing expansion ( [[#Suyadi--2019|Suyadi et al., 2019]] ; [[#Magolan--2020|Magolan and Halls, 2020]] ) or contraction ( [[#Andres--2019|Andres et al., 2019]] ; [[#Appeaning%20Addo--2020|Appeaning Addo et al., 2020]] ; [[#Li--2020b|Li et al., 2020b]] ) of intertidal habitats. The same phenomena alter salinity gradients, which are the primary drivers of estuarine species distributions ( ''high confidence'' ) ( [[#Douglass--2020|Douglass et al., 2020]] ; [[#Lauchlan--2020|Lauchlan and Nagelkerken, 2020]] ). Extreme reduction of freshwater input can extend residence time of estuarine water, leading to persistent HABs ( [[#Lehman--2020|Lehman et al., 2020]] ) and converting estuaries to lagoons if the mouth clogs with sediment ( [[#Thom--2020|Thom et al., 2020]] ).

Acidification of estuarine water is a growing hazard ( ''medium confidence'' ) ( [[#Doney--2020|Doney et al., 2020]] ; [[#Scanes--2020a|Scanes et al., 2020a]] ; [[#Cai--2021|Cai et al., 2021]] ), and resident organisms display sensitivity to altered pH in laboratory settings ( ''medium confidence'' ) ( [[#Young--2019a|Young et al., 2019a]] ; [[#Morrell--2020|Morrell and Gobler, 2020]] ; [[#Pardo--2021|Pardo and Costa, 2021]] ). However, attribution of the biological effects of acidification is difficult because many biogeochemical processes affect estuarine carbon chemistry (Sections 3.2.3.1, 3.3.2). Warming can exacerbate the impacts of both acidification and hypoxia on estuarine organisms ( [[#Baumann--2018|Baumann and Smith, 2018]] ; [[#Collins--2019b|Collins et al., 2019b]] ; [[#Ni--2020|Ni et al., 2020]] ). These effects are further complicated by eutrophication, with high nitrogen loads associated with lower pH ( [[#Rheuban--2019|Rheuban et al., 2019]] ). Warming (including MHWs) and eutrophication interact to decrease estuarine oxygen content and pH, increasing the vulnerability of animals to MHWs ( [[#Brauko--2020|Brauko et al., 2020]] ) and exacerbating the incidence and impact of dead zones ( ''medium confidence'' ) ( [[#Altieri--2015|Altieri and Gedan, 2015]] ). The impacts of storms on estuaries are variable and are described in SM3.3.1.

All these impacts are projected to escalate under future climate change, but their magnitude depends on the amount of warming, the socioeconomic development pathway and implementation of adaptation strategies ( ''medium confidence'' ). Modelling studies ( [[#Lopes--2019|Lopes et al., 2019]] ; [[#Rodrigues--2019|Rodrigues et al., 2019]] ; [[#White--2019|White et al., 2019]] ; [[#Zhang--2019|Zhang and Li, 2019]] ; [[#Hong--2020|Hong et al., 2020]] ; [[#Krvavica--2020|Krvavica and Ružić, 2020]] ; [[#Liu--2020e|Liu et al., 2020e]] ; [[#Shalby--2020|Shalby et al., 2020]] ) suggest that responses of estuaries to SLR will be complex and context dependent ( [[#Khojasteh--2021|Khojasteh et al., 2021]] ), but project that salinity, tidal range, storm-surge amplitude, depth and stratification will increase with SLR ( ''medium confidence'' ), and that marine-dominated waters will penetrate farther upstream ( ''high confidence'' ). Without careful management of freshwater inputs, sediment augmentation and/or the restoration of shorelines to more natural states, transformation and loss of intertidal areas and wetland vegetation will increase with SLR ( ''high confidence'' ) ( [[#Doughty--2019|Doughty et al., 2019]] ; [[#Leuven--2019|Leuven et al., 2019]] ; [[#Yu--2019|Yu et al., 2019]] ; [[#Raw--2020|Raw et al., 2020]] ; [[#Shih--2020|Shih, 2020]] ; [[#Stein--2020|Stein et al., 2020]] ), with small, shallow microtidal estuaries being more vulnerable to impacts than deeper estuaries with well-developed sediments ( ''medium confidence'' ) ( [[#Leuven--2019|Leuven et al., 2019]] ; [[#Williamson--2021|Williamson and Guinder, 2021]] ). Warming and MHWs will enhance stratification and deoxygenation in shallow lagoons ( ''medium confidence'' ) ( [[#Derolez--2020|Derolez et al., 2020]] ) and will continue to drive range shifts among estuarine biota ( ''medium confidence'' ) ( [[#Veldkornet--2019|Veldkornet and Rajkaran, 2019]] ; [[#Zhang--2020c|Zhang et al., 2020c]] ), resulting in extirpations where thermal habitat is lost and potentially generating new habitat for warm-affinity species ( ''limited evidence, medium agreement'' ) ( [[#Veldkornet--2019|Veldkornet and Rajkaran, 2019]] ).

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