Editing IPCC:AR6/WGII/Chapter-13 (section)

==== 13.4.1.2 Projected Risks ====

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Risks to marine and coastal European ecosystems are ''very likely'' to intensify (Figure 13.11) in response to projected further warming. Since the capacity of natural systems for autonomous adaptation is limited ( ''medium confidence'' ) ( [[#Thomsen--2017|Thomsen et al., 2017]] ; [[#Miller--2018|Miller et al., 2018]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ), pronounced changes in community composition and biodiversity patterns are projected by 2100 for TEUS and the eastern Mediterranean Sea (SEUS) for &gt;3°C GWL ( [[#García%20Molinos--2016|García Molinos et al., 2016]] ), challenging conservation efforts ( [[#Corrales--2018|Corrales et al., 2018]] ; [[#Cramer--2018|Cramer et al., 2018]] ; [[#Kim--2019|Kim et al., 2019]] ). At 1.5°C GWL, particularly in winter, Mediterranean coastal fish communities are projected to lose ~10% of species, increasing to ~60% at 4°C GWL ( [[#Dahlke--2020|Dahlke et al., 2020]] ), exacerbating regime shifts linked to overexploitation ( ''medium confidence'' ) ( [[#Clark--2020|Clark et al., 2020]] ). Warming at this level will threaten many species currently living in marine protected areas (MPAs) in TEUS and NEUS ( [[#Bruno--2018|Bruno et al., 2018]] ). Increasing marine heatwaves (MWHs), particularly in SEUS at 4°C GWL ( [[#Darmaraki--2019a|Darmaraki et al., 2019a]] ), elevate risks for species ( [[#Galli--2017|Galli et al., 2017]] ), coastal biodiversity, and ecosystem functions, goods and services ( [[#Smale--2019|Smale et al., 2019]] ); however, MWH-related risk levels differ among biotas ( [[#Pansch--2018|Pansch et al., 2018]] ) and across European seas ( [[#Smale--2015|Smale et al., 2015]] ).

Marine primary production is projected to further decrease by 2100 in most European seas between 0.3% at 1.5°C GWL to 2.7% at 4°C GWL ( ''high confidence'' ) (Figure 13.11), mainly caused by stratification-driven reductions in nutrient availability, impacting food webs ( [[#Doney--2012|Doney et al., 2012]] ; [[#Laufkoetter--2015|Laufkoetter et al., 2015]] ; [[#Wakelin--2015|Wakelin et al., 2015]] ; [[#Salihoglu--2017|Salihoglu et al., 2017]] ; [[#Holt--2018|Holt et al., 2018]] ; [[#Bryndum-Buchholz--2019|Bryndum-Buchholz et al., 2019]] ; [[#Carozza--2019|Carozza et al., 2019]] ; [[#Kwiatkowski--2019|Kwiatkowski et al., 2019]] ). In the Barents Sea, however, largely stable primary production is projected under all scenarios in response to sea ice decline ( [[#Slagstad--2011|Slagstad et al., 2011]] ) and in the eastern Mediterranean due to reduced stratification ( [[#Macias--2015|Macias et al., 2015]] ; [[#Moullec--2019|Moullec et al., 2019]] ). These changes in productivity are projected to increase fish and macroinvertebrate biomass between 5 and 22% ( [[#Moullec--2019|Moullec et al., 2019]] ). Decreasing net primary production will impact higher trophic levels ( [[#13.5.1|Section 13.5.1]] ), for example, in TEUS ( [[#Holt--2016|Holt et al., 2016]] ; [[#Holt--2018|Holt et al., 2018]] ). Marine animal biomass is projected to ''likely'' decline in most European waters, with decreases &lt;10% under all scenarios until the 2030s but losses growing to 25% at 2°C GWL and 50% at 4°C GWL in coastal waters of the northeast Atlantic ( [[#Lotze--2019|Lotze et al., 2019]] ; [[#Bryndum-Buchholz--2020|Bryndum-Buchholz et al., 2020]] ).

Ocean acidification and its biological and ecological risks are projected to rise in European waters by impeding growth and reproductive success of vulnerable calcifying organisms ( ''medium confidence'' ) (Figure 13.11). Coralline algae are projected to reduce skeletal performance at 3°C GWL, with negative consequences for habitat formation ( ''medium confidence'' ) ( [[#Ragazzola--2016|Ragazzola et al., 2016]] ). Regionally ( [[#Brodie--2014|Brodie et al., 2014]] ), differences in species-specific vulnerability will result in community shifts from calcifying macroalgae ( ''medium confidence'' ) ( [[#Ragazzola--2013|Ragazzola et al., 2013]] ) to non-calcifying macroalgae ( ''high confidence'' ) ( [[#Gordillo--2016|Gordillo et al., 2016]] ). Experimental studies demonstrated high resilience of some important habitat formers, such as the deep-water coral ''Lophelia pertusa'' ( [[#Wall--2015|Wall et al., 2015]] ; [[#Morato--2020|Morato et al., 2020]] ), and habitat engineers, such as Mediterranean limpets ( [[#Langer--2014|Langer et al., 2014]] ), facilitated by energy reallocation. However, if not supported by sufficient food availability ( [[#Thomsen--2013|Thomsen et al., 2013]] ; [[#Clements--2018|Clements and Darrow, 2018]] ), such energy reallocation will negatively impact growth or reproduction ( ''medium confidence'' ) ( [[#Thomsen--2013|Thomsen et al., 2013]] ; [[#Büscher--2017|Büscher et al., 2017]] ). This suggests that acidification risks will be amplified by increased stratification and reduced primary production ( ''medium confidence'' ). The emergence of harmful algal blooms and pathogens at higher GWLs is unclear across all European seas ( ''low confidence'' ) (Figure 13.11).

Risks to marine biotas and ecosystems in European seas are projected to impact important ecosystem services (Figure 13.11). Elevated CO 2 levels predicted at 4°C GWL will affect the C/N ratio of organic-matter export and, hence, the efficiency of the biological pump ( ''low confidence'' ), depending on the shifts in plankton composition and, hence, food-web structure ( [[#Taucher--2020|Taucher et al., 2020]] ). Atlantic herring ( ''Clupea harengus'' ) will benefit with enhanced larval growth and survival from indirect food-web effects ( [[#Sswat--2018a|Sswat et al., 2018a]] ), whereas Atlantic cod ( ''Gadus morhua'' ) will face overall negative impacts ( ''medium confidence'' ) ( [[#13.5|Section 13.5]] ; [[#Stiasny--2018|Stiasny et al., 2018]] ; [[#Stiasny--2019|Stiasny et al., 2019]] ). Anoxic dead zones in the Black ( [[#Altieri--2015|Altieri and Gedan, 2015]] ) and the Baltic ( [[#Jokinen--2018|Jokinen et al., 2018]] ; [[#Reusch--2018|Reusch et al., 2018]] ) seas are projected to increase, for example, by 5% in the Baltic Sea at 4°C GWL ( [[#Saraiva--2019|Saraiva et al., 2019]] ). Europe’s coastal vegetated ‘blue carbon’ ecosystems (subtidal seagrass meadows and intertidal salt marshes) are highly vulnerable ( [[#Spencer--2016|Spencer et al., 2016]] ; [[#Schuerch--2018|Schuerch et al., 2018]] ; [[#Spivak--2019|Spivak et al., 2019]] ), particularly in microtidal areas such as the Baltic and Mediterranean coast. Losses are projected for ''Posidonia oceanica'' seagrass habitats in the Mediterranean by up to 75% at 2.5°C GWL ( ''low confidence'' ) (Chapter 3). The Wadden Sea, the world’s largest system of intertidal flats, is projected to reduce in surface area and height, as the sediment transport capacity limits the possibility of growth with rapidly rising sea levels ( [[#Wang--2018|Wang et al., 2018]] ; [[#Jiang--2020|Jiang et al., 2020]] ). For the Dutch Wadden Sea, the critical rate of 6–10 mm yr –1 , at which intertidal flats will start to ‘drown’, will be reached by 2030 at 1.5°C GWL ( ''medium confidence'' ), or even earlier through subsidence due to human activities ( [[#van%20der%20Spek--2018|van der Spek, 2018]] ). European coastal zones provided a total of 494 billion EUR of ecosystem services in 2018, and 4.2–5.1% of this value will be lost due to coastal erosion by 2100 at 2.5°C and 4.6°C GWL, respectively ( ''medium confidence'' ) ( [[#Paprotny--2021|Paprotny et al., 2021]] ).

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