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

=== 13.4.1 Observed Impacts and Projected Risks ===

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==== 13.4.1.1 Observed Impacts ====

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Warming continues to be the key climate hazard for European seas (Figure 13.1). Interacting with other climatic and non-climatic drivers, it has detectable and attributable impacts at a wide range of biological and ecological organisational levels (Figure 13.11).

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[[File:23c8ea629db1257572940528be9203c7 IPCC_AR6_WGII_Figure_13_011.png]]

'''Figure 13.11 |''' '''Major impacts and risks for marine and coastal ecosystems in Europe for observed and projected 1''' '''.''' '''5°C and 3.0°C GWL''' (Table SM13.4)

Particularly habitat loss in shallow coastal waters and at the coasts themselves, and northward distribution shifts of populations and communities, are evident across all European marine sub-regions ( ''high confidence'' ) (Figure 13.11; Chapter 3). Marine heatwaves have had severe ecological impacts in SEUS ( ''high confidence'' ) (Cross-Chapter Paper 4), threatening sessile benthic biotas and coastal habitats ( [[#Munari--2011|Munari, 2011]] ; [[#Kersting--2013|Kersting et al., 2013]] ; [[#Rivetti--2014|Rivetti et al., 2014]] ; [[#Garrabou--2019|Garrabou et al., 2019]] ). Range contractions, extirpations ( ''medium confidence'' ) ( [[#Smale--2020|Smale, 2020]] ) and species redistributions have been observed ( ''high confidence'' ) in TEUS ( [[#Cottier-Cook--2017|Cottier-Cook et al., 2017]] ) and SEUS ( [[#Castellanos-Galindo--2020|Castellanos-Galindo et al., 2020]] ). Habitat losses, range shifts, species invasions and species thermal preferences have altered community compositions ( [[#Vasilakopoulos--2017|Vasilakopoulos et al., 2017]] ), resulting in the ‘subtropicalisation’ of TEUS and ‘tropicalisation’ of SEUS (Chapter 3; Cross-Chapter Paper 4) and temperature-dependent timing of abundance and reproduction cycles ( [[#Hjerne--2019|Hjerne et al., 2019]] ; [[#Polte--2021|Polte et al., 2021]] ; [[#Uriarte--2021|Uriarte et al., 2021]] ).

Reductions in growth and reproductive success of calcifying species are not yet unambiguously detected and attributed in European seas ( ''medium confidence'' ) (Figure 13.11), as many show resilience ( [[#Kroeker--2010|Kroeker et al., 2010]] ; [[#Wall--2015|Wall et al., 2015]] ). However, fish population sizes are shrinking ( [[#Queirós--2018|Queirós et al., 2018]] ; [[#Ikpewe--2021|Ikpewe et al., 2021]] ), and growth, reproduction and recruitment are negatively impacted ( [[#Lindegren--2018|Lindegren et al., 2018]] ; [[#Goldberg--2019|Goldberg et al., 2019]] ; [[#Hidalgo--2019|Hidalgo et al., 2019]] ; [[#Vieira--2019|Vieira et al., 2019]] ; [[#Denechaud--2020|Denechaud et al., 2020]] ; [[#Maynou--2020|Maynou et al., 2020]] ; [[#Polte--2021|Polte et al., 2021]] ), though positive effects also occur ( [[#Sguotti--2019|Sguotti et al., 2019]] ; [[#Tanner--2019|Tanner et al., 2019]] ). Biodiversity changes depend on region, habitat and taxon ( ''medium confidence'' ) (Figure 13.11) overall resulting in the redistribution of biodiversity in Europe ( [[#García%20Molinos--2016|García Molinos et al., 2016]] ), and biodiversity declines in some sub-regions ( ''high confidence'' ) ( [[#IPBES--2018|IPBES, 2018]] ).

Biological and ecological impacts have cascading effects for marine ecosystem functioning ( [[#Chivers--2017|Chivers et al., 2017]] ; [[#Baird--2019|Baird et al., 2019]] ) and biogeochemical cycling ( [[#Huete-Stauffer--2011|Huete-Stauffer et al., 2011]] ; [[#Munari--2011|Munari, 2011]] ; [[#Kersting--2013|Kersting et al., 2013]] ; [[#Rivetti--2014|Rivetti et al., 2014]] ; [[#Garrabou--2019|Garrabou et al., 2019]] ). In TEUS, increased water-column stratification ( [[#13.1|Section 13.1]] ) and decreasing eutrophication, result in reduced primary production ( ''high confidence'' ) (Figure 13.11; [[#Capuzzo--2018|Capuzzo et al., 2018]] ) and productivity at higher trophic levels ( ''high confidence'' ) ( [[#Free--2019|Free et al., 2019]] ), while in NEUS sea ice decline has resulted in primary production increase by 40–60% ( ''high confidence'' ) (Figure 13.11; [[#Arrigo--2015|Arrigo and van Dijken, 2015]] ; [[#Borsheim--2017|Borsheim, 2017]] ; [[#Lewis--2020|Lewis et al., 2020]] ). Climate-related deoxygenation impacts are small in most European waters ( ''medium confidence'' ) (Figure 13.11), expect for semi-enclosed seas such as the Baltic and Black seas ( [[#Frolov--2014|Frolov et al., 2014]] ; [[#Jacob--2014|Jacob et al., 2014]] ; [[#Reusch--2018|Reusch et al., 2018]] ). Here warming and eutrophication have altered ecosystem functioning ( ''high confidence'' ), reduced potential fish yield and increased harmful algal blooms ( [[#Alekseev--2014|Alekseev et al., 2014]] ; [[#Carstensen--2014|Carstensen et al., 2014]] ; [[#Berdalet--2017|Berdalet et al., 2017]] ; [[#Daskalov--2017|Daskalov et al., 2017]] ; [[#Riebesell--2018|Riebesell et al., 2018]] ; [[#Stanev--2018|Stanev et al., 2018]] ) along with the risks of ''Vibrio'' pathogens and vibriosis ( [[#13.7.1|Section 13.7.1]] ; [[#Baker-Austin--2017|Baker-Austin et al., 2017]] ; [[#Semenza--2017|Semenza et al., 2017]] ). Across all European seas there is only ''low confidence'' of a consistent change in provisioning ecosystem services (e.g., fishing yields) ( [[#13.5|Section 13.5]] ), because of inter-regional variability, but ''high confidence'' in the decrease in regulating services and coastal protection because of the cascading effects of ecosystem impacts (Figure 13.11).

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==== 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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