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

==== 3.4.2.7 Semi-Enclosed Seas ====

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This section assesses impacts on five SES, or seas larger than 200,000 km 2 with single entrances &lt;120 km wide, including the Persian Gulf, the Red Sea, the Black Sea, the Baltic Sea and the Mediterranean Sea. These SES are largely landlocked and are thus heavily influenced by surrounding landscapes, local and global climate-induced drivers, as well as non-climate drivers ( [[#3.1|Section 3.1]] ), making them highly vulnerable to cumulative threats. Key climate-induced drivers in SES are warming, increasing frequency and duration of MHWs, acidification and the increasing in size and number of OMZs (Figure 3.12; [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ). In AR5, SES were recognised as regionally significant for fisheries and tourism but highly exposed to both local and global stressors, offering limited options for organisms to migrate in response to climate change (Table 3.10).

'''Table 3.10 |''' Summary of past IPCC assessments of semi-enclosed seas (SES)

{| class="wikitable"
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! Observations

! Projections

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

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| ‘The surface waters of the SES exhibit significant warming from 1982, and most CBS [coastal boundary systems] show significant warming since 1950. Warming of the Mediterranean has led to the recent spread of tropical species invading from the Atlantic and Indian oceans.’

‘SES are highly vulnerable to changes in global temperature on account of their small [seawater] volume and landlocked nature. Consequently, SES will respond faster than most other parts of the ocean ( ''high confidence'' ).’

‘The impact of rising temperatures on SES is exacerbated by their vulnerability to other human influences such as over-exploitation, pollution and enhanced runoff from modified coastlines. Due to a mixture of global and local human stressors, key fisheries have undergone fundamental changes in their abundance and distribution over the past 50 years ( ''medium confidence'' ).’

| ‘Projected warming increases the risk of greater thermal stratification in some regions, which can lead to reduced O 2 ventilation [of underlying waters] and the formation of additional hypoxic zones, especially in the Baltic and Black seas ( ''medium confidence'' ).’

‘Changing rainfall intensity can exert a strong influence on the physical and chemical conditions within SES, and in some cases will combine with other climatic changes to transform these areas. These changes are ''likely'' to increase the risk of reduced bottom-water O 2 levels to Baltic and Black Sea ecosystems (due to reduced solubility, increased stratification, and microbial respiration), which is ''very likely'' to affect fisheries.’

Persian Gulf, Red Sea: ‘Extreme temperature events, such as heat waves, are projected to increase ( ''high confidence'' ) [... and] temperatures are ''very likely'' to increase above established thresholds for mass coral bleaching and mortality ( ''very high confidence'' ).’

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

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| Semi-enclosed seas were not assessed in this report.

| ‘Projections from multiple fish species distribution models for multiple fish species show hotspots of decrease in species richness in the Indo-Pacific region, and semi-enclosed seas such as the Red Sea and Persian Gulf ( ''medium evidence, high agreement'' ).

In addition, geographic barriers, such as land boundaries [...] or lower oxygen water in deeper waters, are projected to limit species range shifts in SES, resulting in a larger relative decrease in species richness ( ''medium confidence'' ).’

|}

Since AR5, there is evidence for increasing frequency and duration of MHWs, extreme-weather events and a diversity of threats across depth strata causing mass-mortality events, local extirpations and coral reef decline ( ''high confidence'' ) ( [[#3.4.2.1|Section 3.4.2.1]] ; SM3.3.2; [[#Buchanan--2016a|Buchanan et al., 2016a]] ; [[#Shlesinger--2018|Shlesinger et al., 2018]] ; [[#Wabnitz--2018b|Wabnitz et al., 2018b]] ; [[#Garrabou--2019|Garrabou et al., 2019]] ). In most SES, non-climate drivers, including pollution, habitat destruction and especially overfishing, are decreasing the local adaptive capacity of organisms and the ability of ecosystems to cope with climate-change impacts ( ''high confidence'' ) ( [[#Cramer--2018|Cramer et al., 2018]] ; [[#Hidalgo--2018|Hidalgo et al., 2018]] ; [[#Ben-Hasan--2019|Ben-Hasan and Christensen, 2019]] ). The SLR is accelerating faster than expected ( ''high confidence'' ) ( [[#Kulp--2019|Kulp and Strauss, 2019]] ), posing a key risk to SES’ coastal ecosystems and the services they provide in urban areas, including drinking water provision, housing and recreational activities, among others ( [[#Hérivaux--2018|Hérivaux et al., 2018]] ; [[#Reimann--2018|Reimann et al., 2018]] ).

The size and number of OMZs are increasing worldwide and in most SES ( ''high confidence'' ) ( [[#Global%20Ocean%20Oxygen%20Network--2018|Global Ocean Oxygen Network, 2018]] ), with growing impacts on fish species diversity and ecosystem functioning. In the Persian Gulf and Red Sea, increasing nutrient loads associated with coastal activities and warming has increased the size of OMZs ( ''high confidence'' ) ( [[#Al-Said--2018|Al-Said et al., 2018]] ; [[#Lachkar--2019|Lachkar et al., 2019]] ). OMZs represent an even greater problem in the Black and Baltic seas, with broad implications for ecosystem function and services ( [[#Levin--2009|Levin et al., 2009]] ), especially where actions to reduce nutrient loading from land have been unable to reduce the OMZ coverage ( ''high confidence'' ) ( [[#Carstensen--2014|Carstensen et al., 2014]] ; [[#Miladinova--2017|Miladinova et al., 2017]] ; [[#Global%20Ocean%20Oxygen%20Network--2018|Global Ocean Oxygen Network, 2018]] ). In the Baltic Sea, OMZs are affecting the extent of suitable spawning areas of cod, ''Gadus morhua'' ( ''high confidence'' ) ( [[#Hinrichsen--2016|Hinrichsen et al., 2016]] ), while in the Black Sea, the combined effect of OMZs and warming is influencing the distribution and physiology of fish species, and their migration and schooling behaviour in their overwintering grounds ( ''medium confidence'' ) ( [[#Güraslan--2017|Güraslan et al., 2017]] ). Cascading effects on food webs have been reported in the Baltic, where detrimental effects of changing oxygen levels on zooplankton production, pelagic and piscivorous fish are influencing seasonal succession and species composition of phytoplankton ( ''high confidence'' ) ( [[#Viitasalo--2015|Viitasalo et al., 2015]] ).

In the Mediterranean Sea (Cross-Chapter Paper 4), the increase in climate extremes and mass-mortality events reported in AR5 has continued ( ''very high confidence'' ) ( [[#Gómez-Gras--2021|Gómez-Gras et al., 2021]] ). Extreme-weather events (including deep convection; [[#González-Alemán--2019|González-Alemán et al., 2019]] ) and MHWs have become more frequent ( [[#Darmaraki--2019|Darmaraki et al., 2019]] ) and are associated with mass mortality of benthic sessile species across the basin ( ''high confidence'' ) ( [[#Garrabou--2019|Garrabou et al., 2019]] ; [[#Gómez-Gras--2021|Gómez-Gras et al., 2021]] ). Since AR5, in the Persian Gulf and Red Sea, extreme temperatures, together with disease and predation, have continued to cause bleaching-induced mortality of corals, along with declines in the average coral-colony size ( ''high confidence'' ) ( [[#Burt--2019|Burt et al., 2019]] ). Poleward migration and tropicalisation of species ( [[#3.4.2.3|Section 3.4.2.3]] ) has also continued in the Mediterranean, and these phenomena have also become an issue in the Black Sea ( ''high confidence'' ) ( [[#Boltachev--2014|Boltachev and Karpova, 2014]] ; [[#Hidalgo--2018|Hidalgo et al., 2018]] ). Climate impacts on phytoplankton production and phenology show high spatial heterogeneity across the Mediterranean Sea ( ''medium evidence'' ) ( [[#Marbà--2015b|Marbà et al., 2015b]] ; [[#Salgado-Hernanz--2019|Salgado-Hernanz et al., 2019]] ), with consequent effects on the diversity and abundance of zooplankton and fish species ( ''medium confidence'' ) ( [[#Peristeraki--2019|Peristeraki et al., 2019]] ). Changes in primary production and a decrease in river runoff have also altered the optimum habitats for small pelagic fish in the Mediterranean, from the local to the basin scale ( [[#Piroddi--2017|Piroddi et al., 2017]] ). Evidence of impacts from ocean acidification is increasing, with the rates of coral calcification showing major decline in the Red Sea ( ''medium confidence'' ) ( [[#3.4.2.1|Section 3.4.2.1]] ; [[#Steiner--2018|Steiner et al., 2018]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). In the Mediterranean Sea, evidence of acidification events have been reported at the local scale ( [[#Hassoun--2015|Hassoun et al., 2015]] ), with impacts on bivalves and coralligenous species ( ''medium confidence'' ) ( [[#Lacoue-Labarthe--2016|Lacoue-Labarthe et al., 2016]] ).

Climate models project increasing frequency and intensity of MHWs ( ''high confidence'' ) ( [[#3.2.2.1|Section 3.2.2.1]] ), which will exacerbate warming-driven impacts in the Red Sea and Persian Gulf regions, and erode the resilience of Red Sea coral reefs ( ''high confidence'' ) ( [[#Osman--2018|Osman et al., 2018]] ; [[#Genevier--2019|Genevier et al., 2019]] ; [[#Kleinhaus--2020|Kleinhaus et al., 2020]] ). In the Persian Gulf region, extreme temperatures, &gt;35°C ( [[#Pal--2016|Pal and Eltahir, 2016]] ), have been linked with high rates of extirpation and a decrease in fisheries catch potential ( ''medium confidence'' ) ( [[#Wabnitz--2018b|Wabnitz et al., 2018b]] ). In the Mediterranean Sea, east–west gradients in rates of warming are projected to trigger spatially different changes in primary production, which combined with the increasing arrival of non-indigenous species, may trigger biogeographic changes in fish diversity, increasing in the eastern and decreasing in the western Mediterranean ( ''medium to high confidence'' ) ( [[#Albouy--2013|Albouy et al., 2013]] ; [[#Macias--2015|Macias et al., 2015]] ). Projections also show greater impacts from SLR than originally expected in the Mediterranean and Baltic (e.g., [[#Dieterich--2019|Dieterich et al., 2019]] ; [[#Thiéblemont--2019|Thiéblemont et al., 2019]] ). In the Baltic Sea, under high nutrient load and warming climate scenarios, eutrophication is projected to increase in the future (2069–2098) compared with historical (1976–2005) periods. In contrast, under continued nutrient load reductions following present management regulations, environmental conditions and ecological state will continue to improve independently of the climate-warming scenarios ( ''low to medium confidence'' ) ( [[#Saraiva--2019|Saraiva et al., 2019]] ).

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