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

==== 3.4.3.3 Changes in Community Composition and Biodiversity ====

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<span id="evidence-of-natural-adaptive-capacity-based-on-species-responses-to-past-climate-variability"></span>
===== 3.4.3.3.1 Evidence of natural adaptive capacity based on species’ responses to past climate variability =====

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Responses to abrupt climate change in the geological past suggest that adaptive capacity is limited for marine animals (Cross-Chapter Box PALEO in Chapter 1). Temperatures during the last Interglacial (~125 ka), which were warmer than today, led to poleward range shifts of reef corals ( ''medium confidence'' ) ( [[#Kiessling--2012|Kiessling et al., 2012]] ; [[#Jones--2019a|Jones et al., 2019a]] ). Temperature has also driven marine range shifts over multi-million-year time scales ( ''medium confidence'' ) ( [[#Gibbs--2016|Gibbs et al., 2016]] ; [[#Reddin--2018|Reddin et al., 2018]] ). Warming climates, even with low ocean-warming rates, inevitably decreased tropical marine biodiversity compared with middle latitudes ( ''high confidence'' ) ( [[#Mannion--2014|Mannion et al., 2014]] ; [[#Crame--2020|Crame, 2020]] ; [[#Yasuhara--2020|Yasuhara et al., 2020]] ; [[#Raja--2021|Raja and Kiessling, 2021]] ).

The paleorecord confirms that marine biodiversity has been vulnerable to climate warming both globally and regionally ( ''very high confidence'' ) (Cross-Chapter Box PALEO in Chapter 1; [[#Stanley--2016|Stanley, 2016]] ). In extreme cases of warming (e.g., &gt;5.2°C), marine mass extinctions occurred in the geological past, and there may be a relationship between warming magnitude and extinction toll ( ''medium confidence'' ) ( [[#Song--2021b|Song et al., 2021b]] ). A combination of warming and spreading anoxia caused marine extinctions in ancient episodes of rapid climate warming ( ''high confidence'' ) ( [[#Bond--2017|Bond and Grasby, 2017]] ; [[#Benton--2018|Benton, 2018]] ; [[#Penn--2018|Penn et al., 2018]] ; [[#Them%20II--2018|Them II et al., 2018]] ; [[#Chen--2019|Chen and Xu, 2019]] ). The role of ocean acidification in ancient extinctions is yet to be resolved ( ''low confidence'' ) ( [[#Clapham--2011|Clapham and Payne, 2011]] ; [[#Clarkson--2015|Clarkson et al., 2015]] ; [[#Jurikova--2020|Jurikova et al., 2020]] ; [[#Müller--2020|Müller et al., 2020]] ).

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<span id="observed-and-projected-changes-in-contemporary-community-structure-and-biodiversity"></span>
===== 3.4.3.3.2 Observed and projected changes in contemporary community structure and biodiversity =====

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Ocean temperature is a major driver of species richness in the global ocean at evolutionary time scales ( [[#Tittensor--2010|Tittensor et al., 2010]] ; [[#Chaudhary--2021|Chaudhary et al., 2021]] ). This, together with temperature-driven range and phenology shifts evident across taxa and ocean ecosystems (Sections 3.4.3.1, 3.4.3.2), suggests that recent ocean warming ( [[#3.2.2.1|Section 3.2.2.1]] ) should alter biodiversity at regional to global scales. Since previous assessments (Table 3.20), the most common evidence supporting these expected changes is replacement of cold-adapted species by warm-adapted species within an ecosystem as waters warm ( [[#Worm--2021|Worm and Lotze, 2021]] ). Known as tropicalisation ( [[#3.4.2.3|Section 3.4.2.3]] ), this phenomenon has been attributed to ocean warming ( ''medium to high confidence'' ) in communities as diverse as kelp, invertebrates, plankton and fish ( [[#Burrows--2019|Burrows et al., 2019]] ; [[#Flanagan--2019|Flanagan et al., 2019]] ; [[#Ajani--2020|Ajani et al., 2020]] ; [[#Villarino--2020|Villarino et al., 2020]] ; [[#Punzón--2021|Punzón et al., 2021]] ; [[#Smith--2021|Smith et al., 2021]] ).

'''Table 3.20 |''' Summary of previous IPCC assessments of community composition and biodiversity

{| class="wikitable"
|-
! Observations

! Projections

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

|
|-
| The paleoecological record shows that global climate changes comparable in magnitudes to those projected for the 21st century under all scenarios resulted in large-scale biome shifts and changes in community composition, and that for rates projected under RCP6 and 8.5 those changes were associated with species extinctions in some groups ( ''high confidence'' ).

Loss of corals due to bleaching has a potentially critical influence on the maintenance of marine biodiversity in the tropics ( ''high confidence'' ).

| Spatial shifts of marine species due to projected warming will cause high-latitude invasions and high local-extinction rates in the tropics and semi-enclosed seas ( ''medium confidence'' ).

Species richness and fisheries catch potential are projected to increase, on average, at mid and high latitudes ( ''high confidence'' ) and decrease at tropical latitudes ( ''medium confidence'' ).

‘Shifts in the geographical distributions of marine species [...] cause changes in community composition and interactions [...]. Thereby, climate change will reassemble communities and affect biodiversity, with differences over time and between biomes and latitudes ( ''high confidence'' ).’

‘Models are currently useful for developing scenarios of directional changes in net primary productivity, species distributions, community structure, and trophic dynamics of marine ecosystems, as well as their implications for ecosystem goods and services under climate change. However, specific quantitative projections by these models remain imprecise ( ''low confidence'' ).’

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

|
|-
| ‘Ocean warming has contributed to observed changes in biogeography of organisms ranging from phytoplankton to marine mammals ( ''high confidence'' ), consequently changing community composition ( ''high confidence'' ), and in some cases altering interactions between organisms and ecosystem function ( ''medium confidence'' ).’

| Poleward range shifts are projected to decrease species richness in tropical oceans, counterbalanced by increases in mid- to high-latitude regions, leading to global-scale species turnover ( ''medium confidence'' on trends, ''low confidence'' on magnitude because of model uncertainties and the limited number of published model simulations). ‘The projected intensity of species turnover is lower under low-emission scenarios ( ''high confidence'' ).’

‘Projections from multiple fish species distribution models 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, [bounding the] poleward species range edge in semi-enclosed seas or low-oxygen water in deeper waters are projected to limit range shifts, resulting in a larger relative decrease in species richness ( ''medium confidence'' ).’

‘The large variation in sensitivity between different zooplankton taxa to future conditions of warming and ocean acidification suggests elevated risk on community structure and inter-specific interactions of zooplankton in the 21st century ( ''medium confidence'' ).’

|}

At local to regional scales, tropicalisation often increases species richness where warm-water species extend their ranges to overlap with existing communities and decreases species richness where warming waters extirpate species ( ''medium to high confidence'' ) ( [[#Friedland--2020a|Friedland et al., 2020a]] ; [[#Chaudhary--2021|Chaudhary et al., 2021]] ; [[#Worm--2021|Worm and Lotze, 2021]] ). Latitudinal estimates from catalogued observations show declining species richness in equatorial waters over the past 50 years, with concomitant increases in species richness at mid-latitudes; the pattern is especially prominent in free-swimming pelagic species (Figure 3.18; [[#Chaudhary--2021|Chaudhary et al., 2021]] ). Similar patterns among marine animals have been described previously for historical warming events ( [[#Song--2020b|Song et al., 2020b]] ). Tropicalisation is associated with increased representation of herbivorous species ( [[#Vergés--2016|Vergés et al., 2016]] ; [[#Zarco-Perello--2020|Zarco-Perello et al., 2020]] ; [[#Smith--2021|Smith et al., 2021]] ), although observations and theory suggest that dietary generalism can also favour range-shifting species ( [[#Monaco--2020|Monaco et al., 2020]] ; [[#Wallingford--2020|Wallingford et al., 2020]] ).

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[[File:775d59d4d5b1b9873ed3788209234bbd IPCC_AR6_WGII_Figure_3_018.png]]

'''Figure 3.18 |''' '''Changes in the latitudinal distribution of marine species richness.'''

'''(a)''' Observed species richness for three historical periods. The observed latitudinal patterns in species richness are for a suite of taxonomic groups based on 48,661 marine species ( [[#Chaudhary--2021|Chaudhary et al., 2021]] ).

'''(b)''' Projected changes in species richness under RCP4.5 and RCP8.5 are calculated as differences by grid cell by 2100 relative to 2006. Latitudinal global median (5° moving average). (Based on Figure 1b,c in [[#García%20Molinos--2016|García Molinos et al., 2016]] .) The projected latitudinal patterns in changes in species richness under climate change are based on a numerical model that includes species-specific information across a suite of taxonomic groups, based on 12,796 marine species ( [[#García%20Molinos--2016|García Molinos et al., 2016]] ).

At the community level, the magnitude and shape of projected future biodiversity changes differ depending on which groups are considered ( ''medium confidence'' ) ( [[#Chaudhary--2021|Chaudhary et al., 2021]] ). Molecular-based richness measures indicate that the most dramatic increases in diversity relative to current conditions are expected for photosynthetic eukaryotes and copepods in the Arctic Ocean ( [[#Ibarbalz--2019|Ibarbalz et al., 2019]] ). However, component eukaryotic taxa, for example diatoms ( [[#Busseni--2020|Busseni et al., 2020]] ), are projected to lose diversity by 2100 under RCP8.5. Ecosystem models project a decline in nutrient supply that drives the disappearance of less-competitive and larger phytoplankton types, leading to extinction of up to 30% of diatom types, particularly in the Northern Hemisphere, by 2100 under RCP8.5 ( [[#Henson--2021|Henson et al., 2021]] ). Models further suggest that high latitudes are ''likely'' to encounter entirely novel phytoplankton communities by 2100 under RCP8.5 (100% change in community composition; [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ; [[#Reygondeau--2020|Reygondeau et al., 2020]] ). At the polar edges, the increased richness is projected to coincide with high species turnover and increasing dominance of smaller phytoplankton types ( [[#Henson--2021|Henson et al., 2021]] ). These imply pronounced changes to both the oceans’ ecological and biogeochemical function, as regions dominated by small phytoplankton typically support less-productive food webs ( [[#3.4.3.4|Section 3.4.3.4]] ; [[#Stock--2017|Stock et al., 2017]] ; [[#Armengol--2019|Armengol et al., 2019]] ) and sequester less particulate organic carbon (POC) in the deep ocean ( [[#3.4.3.5|Section 3.4.3.5]] ; [[#Mouw--2016|Mouw et al., 2016]] ; [[#Cram--2018|Cram et al., 2018]] ) than areas dominated by larger size classes ( ''high confidence'' ).

The profound climatic and environmental changes projected for the Arctic region by 2100 (Cross-Chapter Paper 6) are also anticipated to alter the composition of apex assemblages like marine mammals (see Box 3.2; [[#Albouy--2020|Albouy et al., 2020]] ). Under both RCP2.6 and 8.5 scenarios the most vulnerable marine mammal species will be the North Pacific right whale ( ''Eubalaena japonica'' , listed as an endangered species; [[#IUCN--2020|IUCN, 2020]] ) and the grey whale ( ''Eschrichtius robustus'' , which has critically endangered subpopulations; [[#IUCN--2020|IUCN, 2020]] ). The extinction of the most-vulnerable species will disproportionately eliminate unique and important evolutionary lineages as well as functional diversity, with consequent impacts throughout the entire marine ecosystem ( [[#3.3.4|Section 3.3.4]] ). More generally, future warming and acidification simulated in mesocosm experiments support projections of a substantial increase in biomass and productivity of primary producers and secondary consumers, but a decrease by &gt;40% of primary consumers ( [[#Nagelkerken--2020|Nagelkerken et al., 2020]] ). On longer time scales, alteration of energy flow through marine food webs may lead to ecological tipping points ( [[#Wernberg--2016|Wernberg et al., 2016]] ; [[#Harley--2017|Harley et al., 2017]] ) after which the food web collapses into shorter, bottom-heavy trophic pyramids ( ''medium confidence'' ).

Global projections anticipate a ''likely'' future reorganisation of marine life of variable magnitude, contingent on emission scenario ( [[#Beaugrand--2015|Beaugrand et al., 2015]] ; [[#Jones--2015|Jones and Cheung, 2015]] ; [[#Barton--2016|Barton et al., 2016]] ; [[#García%20Molinos--2016|García Molinos et al., 2016]] ; [[#Nagelkerken--2020|Nagelkerken et al., 2020]] ; [[#Henson--2021|Henson et al., 2021]] ). Marine organism redistributions projected under RCP4.5 and RCP8.5 include extirpations and range contractions in the tropics, strongly decreasing tropical biodiversity, and range expansions at higher latitudes, associated with increased diversity and homogenisation of marine communities (Figure 3.18b). Under continuing climate change, the projected loss of biodiversity may ultimately threaten marine ecosystem stability ( ''medium confidence'' ) ( [[#Albouy--2020|Albouy et al., 2020]] ; [[#Nagelkerken--2020|Nagelkerken et al., 2020]] ; [[#Henson--2021|Henson et al., 2021]] ), altering both the functioning and structure of marine ecosystems and thus affecting service provisioning ( ''medium confidence'' ) ( [[#3.5|Section 3.5]] ; [[#Ibarbalz--2019|Ibarbalz et al., 2019]] ; [[#Righetti--2019|Righetti et al., 2019]] ).

However, biodiversity observations remain sparse, and statistical and modelling tools can provide conflicting diversity information (e.g., [[#Righetti--2019|Righetti et al., 2019]] ; [[#Dutkiewicz--2020|Dutkiewicz et al., 2020]] ) because correlative approaches assume that the modern-day relationship between marine species distribution and environmental conditions remains the same into the future, whereas mechanistic models permit marine species to respond dynamically to changing environmental forcing. Moreover, existing global projections of future biodiversity disproportionately focus on the effects sea surface temperature ( [[#Thomas--2012|Thomas et al., 2012]] ), typically overlooking other factors such as ocean acidification, deoxygenation and nutrient availability ( [[#3.2.3|Section 3.2.3]] ), and often failing to account for natural adaptation (e.g., [[#3.3.4|Section 3.3.4]] ; see Box 3.1; [[#Barton--2016|Barton et al., 2016]] ; [[#Henson--2021|Henson et al., 2021]] ).

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<span id="abrupt-ecosystem-shifts-and-extreme-events"></span>
===== 3.4.3.3.3 Abrupt ecosystem shifts and extreme events =====

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Climate-change-driven changes in ocean characteristics and the frequency and intensity of extreme events ( [[#3.2|Section 3.2]] ) increase the risk of persistent, rapid and abrupt ecosystem change ( ''very high confidence'' ), often referred to as ecosystem collapses or regime shifts (AR6 WGI Chapter 9; [[#Collins--2019a|Collins et al., 2019a]] ; [[#Canadell--2021|Canadell and Jackson, 2021]] ; [[#Ma--2021|Ma et al., 2021]] ). Such abrupt changes include altering ecosystem structure, function and biodiversity outside the range of natural fluctuations ( [[#Collins--2019a|Collins et al., 2019a]] ; [[#Canadell--2021|Canadell and Jackson, 2021]] ). They can involve mass-mortality events and ‘tipping points’ or ‘critical transitions’, where strong positive feedbacks within an ecosystem lead to self-sustaining change (Figure 3.19a; [[#Scheffer--2012|Scheffer et al., 2012]] ; [[#Möllmann--2015|Möllmann et al., 2015]] ; [[#Biggs--2018|Biggs et al., 2018]] ). Abrupt ecosystem shifts have been observed in both large open-ocean ecosystems and coastal ecosystems ( [[#3.4.2|Section 3.4.2]] ), with dramatic social consequences through significant loss of diverse ecosystem services ( ''high confidence'' ) ( [[#3.5|Section 3.5]] ; [[#Biggs--2018|Biggs et al., 2018]] ; [[#Pinsky--2018|Pinsky et al., 2018]] ; [[#Beaugrand--2019|Beaugrand et al., 2019]] ; [[#Collins--2019a|Collins et al., 2019a]] ; [[#Filbee-Dexter--2020b|Filbee-Dexter et al., 2020b]] ; [[#Huntington--2020|Huntington et al., 2020]] ; [[#Trisos--2020|Trisos et al., 2020]] ; [[#Turner--2020b|Turner et al., 2020b]] ; [[#Canadell--2021|Canadell and Jackson, 2021]] ; [[#Ma--2021|Ma et al., 2021]] ; [[#Ruthrof--2021|Ruthrof et al., 2021]] ). A summary of previous assessments of abrupt ecosystem shifts and extreme events is provided in Table 3.21.

'''Table 3.21 |''' Summary of previous IPCC assessments of observed and projected abrupt ecosystem shifts and extreme events

{| class="wikitable"
|-
! Observations

! Projections

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

|
|-
| Observations of abrupt ecosystem shifts and extreme events were not assessed in this report.

| ‘Warming and acidification will lead to coral bleaching, mortality, and decreased constructional ability ( ''high confidence'' ), making coral reefs the most vulnerable marine ecosystem with little scope for adaptation. Temperate seagrass and kelp ecosystems will decline with the increased frequency of heatwaves and sea temperature extremes as well as through the impact of invasive subtropical species ( ''high confidence'' ).’

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

|
|-
| ‘Marine heatwaves (MHWs), periods of extremely high ocean temperatures, have negatively impacted marine organisms and ecosystems in all ocean basins over the last two decades, including critical foundation species such as corals, seagrasses and kelps ( ''very high confidence'' ).’

| ‘Marine heatwaves are projected to further increase in frequency, duration, spatial extent and intensity (maximum temperature) ( ''very high confidence'' ). Climate models project increases in the frequency of marine heatwaves by 2081–2100, relative to 1850–1900, by approximately 50 times under RCP8.5 and 20 times under RCP2.6 ( ''medium confidence'' ).’

‘Extreme El Niño and La Niña events are projected to ''likely'' increase in frequency in the 21st century and to ''likely'' intensify existing hazards, with drier or wetter responses in several regions across the globe. Extreme El Niño events are projected to occur about twice as often under both RCP2.6 and RCP8.5 in the 21st century when compared to the 20th century ( ''medium confidence'' ).’

‘Limiting global warming would reduce the risk of impacts of MHWs, but critical thresholds for some ecosystems (e.g., kelp forests, coral reefs) will be reached at relatively low levels of future global warming ( ''high confidence'' ).’

|}

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[[File:8606e2823dc86adf18dafb7ba38045b4 IPCC_AR6_WGII_Figure_3_019.png]]

'''Figure 3.19 |''' '''Observed ecological regime shifts and their drivers in the oceans.'''

'''(a)''' A conceptual representation of ecosystem resilience and regime shifts. Shift from Regime 1 to Regime 2 can be triggered by either a large shock (i.e., an abrupt environmental transition) or gradual internal or external change that erodes the dominant balancing feedbacks, reducing ecosystem resilience (indicated by the shallower dotted line, relative to the deeper ‘valley’ reflecting higher resilience). (Based on [[#Biggs--2018|Biggs et al., 2018]] ).

'''(b)''' The sum of the magnitude and extent of the abrupt community shifts that has been estimated at each geographic cell in the global ocean during 1960–2014, calculated as the ratio of the amplitude of the change in a particular year to the average magnitude of the change over the entire time series (thus, is dimensionless). (Based on [[#Beaugrand--2019|Beaugrand et al., 2019]] ).

Abrupt ecosystem shifts are associated with large-scale patterns of climate variability ( [[#Alheit--2019|Alheit et al., 2019]] ; [[#Beaugrand--2019|Beaugrand et al., 2019]] ; [[#Lehodey--2020|Lehodey et al., 2020]] ), some of which are projected to intensify with climate change ( ''medium confidence'' ) (WGI AR6 Chapter 1; [[#Wang--2017a|Wang et al., 2017a]] ; [[#Collins--2019a|Collins et al., 2019a]] ; [[#Chen--2021|Chen et al., 2021]] ). Over the past 60 years, abrupt ecosystem shifts have generally followed El Niño/Southern Oscillation events of any strength, but some periods had geographically limited ecological shifts (~0.25% of the global ocean in 1984–1987) and others more extensive shifts (14% of the global ocean in 2012–2015) ( ''medium confidence'' ) (Figure 3.19b; [[#Beaugrand--2019|Beaugrand et al., 2019]] ). Typically, interacting drivers, such as eutrophication and overharvest, reduce ecosystem resilience to climate extremes (e.g., MHWs, cyclones) or gradual warming, and hence promote ecosystem shifts ( ''high confidence'' ) (Figure 3.19a; [[#Rocha--2015|Rocha et al., 2015]] ; [[#Biggs--2018|Biggs et al., 2018]] ; [[#Babcock--2019|Babcock et al., 2019]] ; [[#Turner--2020b|Turner et al., 2020b]] ; [[#Bergstrom--2021|Bergstrom et al., 2021]] ; [[#Canadell--2021|Canadell and Jackson, 2021]] ; [[#Tait--2021|Tait et al., 2021]] ). Also, shifts in different ecosystems may be connected through common drivers or through cascading effects ( ''medium confidence'' ) ( [[#Rocha--2018a|Rocha et al., 2018a]] ).

Recent MHWs ( [[#3.2.2.1|Section 3.2.2.1]] ) have caused major ecosystem shifts and mass mortality in oceanic and coastal ecosystems, including corals, kelp forests and seagrass meadows (Sections 3.4.2.1, 3.4.2.3, 3.4.2.5, 3.4.2.6, 3.4.2.10; Cross-Chapter Box MOVING SPECIES in Chapter 5; Cross-Chapter Box EXTREMES in Chapter 2), with dramatic declines in species foundational for habitat formation or trophic flow, biodiversity declines, and biogeographic shifts in fish stocks ( ''very high confidence'' ) (Table 3.15; Cross-Chapter Box MOVING SPECIES in Chapter 5; [[#Canadell--2021|Canadell and Jackson, 2021]] ). Three major bleaching episodes on Australia’s Great Barrier Reef in 5 years corresponded with extreme temperatures in 2016, 2017 and 2020 ( [[#Pratchett--2021|Pratchett et al., 2021]] ). Between 1981 and 2017, MHWs have increased more than 20-fold due to anthropogenic climate change ( [[#3.2.2.1|Section 3.2.2.1]] ; WGI AR6 Chapter 9; [[#Laufkötter--2020|Laufkötter et al., 2020]] ; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), increasing the risk of abrupt ecosystem shifts ( ''high confidence'' ) (Figure 3.19a; Cross-Chapter Box EXTREMES in Chapter 2; [[#van%20der%20Bolt--2018|van der Bolt et al., 2018]] ; [[#Garrabou--2021|Garrabou et al., 2021]] ; [[#Wernberg--2021|Wernberg, 2021]] ).

Ecosystems can recover from abrupt shifts (e.g., [[#Babcock--2019|Babcock et al., 2019]] ; [[#Christie--2019|Christie et al., 2019]] ; [[#Medrano--2020|Medrano et al., 2020]] ). However, where climate change is a dominant driver, ecosystem collapses increasingly cause permanent transitions ( ''high confidence'' ), although the extents of such transitions depend on the emission scenario ( [[#Trisos--2020|Trisos et al., 2020]] ; [[#Garrabou--2021|Garrabou et al., 2021]] ; [[#Klein--2021|Klein et al., 2021]] ; [[#Pratchett--2021|Pratchett et al., 2021]] ; [[#Wernberg--2021|Wernberg, 2021]] ). Over the coming decades, MHWs are projected to ''very likely'' become more frequent under all emission scenarios ( [[#3.2|Section 3.2]] ; WGI AR6 Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ), with intensities and rates too high for recovery of degraded foundational species, habitats or biodiversity ( ''medium confidence'' ) ( [[#Babcock--2019|Babcock et al., 2019]] ; [[#Garrabou--2021|Garrabou et al., 2021]] ; [[#Klein--2021|Klein et al., 2021]] ; [[#Serrano--2021|Serrano et al., 2021]] ; [[#Wernberg--2021|Wernberg, 2021]] ). Emission pathways that result in temperature overshoot above 1.5 o C will increase the risks of abrupt and irreversible shifts in coral reefs and other vulnerable ecosystems ( [[#3.4.4|Section 3.4.4]] ).

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<span id="time-of-emergence-species-exposure-to-altered-environments"></span>
===== 3.4.3.3.4 Time of emergence: species exposure to altered environments =====

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Since SROCC, more studies have assessed the time of emergence for climate-induced drivers ( [[#3.2.3|Section 3.2.3]] ) and the ecosystem attributes through which the impacts manifest. However, as in previous assessments (Table 3.22), the time of emergence for a given driver or ecosystem attribute depends on the reference period, the definition of the signal emergence threshold and the spatio-temporal scales considered (see Box 5.1 in SROCC; [[#Kirtman--2013|Kirtman et al., 2013]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

'''Table 3.22 |''' Conclusions from previous IPCC assessments about projected time of emergence on coastal, ocean and deep-sea systems

{| class="wikitable"
|-
! Oceanic systems and chapter subsection

! Projections

|-
| Coastal ( [[#3.4.2|Section 3.4.2]] )

| ‘Multiple [climate-impact drivers] will emerge [...] in the 21st century under RCP8.5, while the time of emergence will be later and with less [climatic hazards] under RCP2.6. [Non-climate] impacts such as eutrophication add to, and in some cases, exacerbate these large-scale slow climate drivers beyond biological thresholds at local scale (e.g., deoxygenation)’ ( [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.7]] in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

|-
| Epipelagic ( [[#3.4.3|Section 3.4.3]] )

| ‘Observed range shifts in response to climate change in some regions such as the north Atlantic are strongly influenced by warming due to both multi-decadal [climate change and] variability, suggesting that there is a longer time of emergence of range shifts from natural variability and a need for longer biological time series for robust attribution’ ( [[IPCC:Wg2:Chapter:Chapter-5#5.2|Section 5.2.3.1.1]] in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

|-
| Open ocean ( [[#3.4.3|Section 3.4.3]] )

| ‘[The timing] for five primary drivers of marine ecosystem change (surface warming and acidification, oxygen loss, nitrate concentration and net primary production change) are all prior to 2100 for over 60% of the ocean area under RCP8.5 and over 30% under RCP2.6 ( ''very likely'' )’ (Figure 1 in Box 5.1 in SROCC, Box 5.1 in SROCC, Executive Summary in SROCC Chapter 5; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

‘Anthropogenic signals are expected to remain detectable over large parts of the ocean, even for the RCP2.6 scenario for pH and SST, but are ''likely'' [to be less conspicuous] for nutrients and NPP [net primary production] in the 21st century. For example, for the open ocean, the anthropogenic pH signal in Earth System Models’ (ESM) historical simulations is ''very likely'' to have emerged for three-quarters of the ocean prior to 1950, and it is ''very likely'' over 95% of the ocean has already been affected, with little discernible difference between scenarios’ (Executive Summary in SROCC Chapter 5, Box 5.1 in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). ‘The climate signal of oxygen loss will ''very likely'' emerge by 2050 with a ''very likely'' range of 59–80% by 2031–2050 and rising with a ''very likely'' range of 79–91% of the ocean area by 2081–2100 (RCP8.5 emissions scenario). The emergence of oxygen loss is smaller in area for the RCP2.6 scenario in the 21st century and by 2090 the [area where emergence is evident is declining].’ It has also been shown that signatures of altered oxygen solubility or utilisation may emerge earlier than for oxygen levels (Executive Summary in SROCC Chapter 5, Box 5.1 in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

|-
| Deep sea (Box 3.3)

| ‘Emergence of risk is expected to occur later at around the mid-21st century under RCP8.5 for abyssal plain and chemosynthetic ecosystems (vents and seeps)’ ( [[IPCC:Wg2:Chapter:Chapter-5#5.2|Section 5.2.5]] in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). ‘All deep seafloor ecosystems are expected to be subject to at least moderate risk under RCP8.5 by the end of the 21st century, with cold water corals undergoing a transition from moderate to high risk below 3°C’ (SM5.2 in SROCC; [[#Bindoff--2019b|Bindoff et al., 2019b]] ).

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Anthropogenically driven changes in chlorophyll- ''a'' concentrations across an ensemble of 30 ESMs are expected to exceed natural variability under RCP8.5 by 2100 in ~65–80% of the global oceans, when the natural variability is calculated using the ensemble’s standard deviation ( [[#Schlunegger--2020|Schlunegger et al., 2020]] ); however, if two standard deviations are used, then significant trends in chlorophyll- ''a'' concentration are expected under RCP8.5 across ~31% of the global oceans by 2100 ( [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ). In contrast, the anthropogenic signal in phytoplankton community structure, which has a lower natural variability, will emerge under RCP8.5 across 63% of the ocean by 2100 when two standard deviations are used ( ''limited evidence'' ) ( [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ).

The time of emergence of climate impacts on ecosystems will be modulated jointly by species-specific adaptation potential ( [[#3.3.4|Section 3.3.4]] ; [[#Jones--2018|Jones and Cheung, 2018]] ; [[#Collins--2020|Collins et al., 2020]] ; [[#Gamliel--2020|Gamliel et al., 2020]] ; [[#Miller--2020a|Miller et al., 2020a]] ), speed of range shifts and spatial reorganisation ( ''high confidence'' ) (Sections 3.3, 3.4.2, 3.4.3). These ecosystem responses complicate projections of the time of emergence of environmental properties that impact biogeochemical cycling ( [[#Schlunegger--2019|Schlunegger et al., 2019]] ; [[#Schlunegger--2020|Schlunegger et al., 2020]] ; [[#Wrightson--2020|Wrightson and Tagliabue, 2020]] ), ecosystem structure and biodiversity (Figure 3.20a,c; [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ; [[#Trisos--2020|Trisos et al., 2020]] ), and higher trophic levels, including fisheries targets ( [[#Cheung--2020|Cheung and Frölicher, 2020]] ). Better accounting for multiple interacting factors in ESMs (see Box 3.1) will provide insight into how marine ecosystems will respond to future climate ( ''high confidence'' ).

The time of emergence of ecosystem responses supports planning for specific time-bound actions to reduce risks to ecosystems ( [[#3.6.3.2|Section 3.6.3.2.1]] ; [[#Bruno--2018|Bruno et al., 2018]] ; [[#Trisos--2020|Trisos et al., 2020]] ). Although under RCP8.5, climate refugia from SST after 2050 are primarily in the Southern Ocean in tropical waters, these refugia are mainly from deoxygenation ( [[#Bruno--2018|Bruno et al., 2018]] ). Marine assemblages in these places will be exposed to unprecedented temperatures after 2050, peaking in 2075 (Figure 3.20a,b; [[#Trisos--2020|Trisos et al., 2020]] ). In contrast, changes in phytoplankton community structure will emerge earlier, primarily in the Pacific Ocean subtropics and through much of the North Atlantic Ocean (Figure 3.20c,d; [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ). Under RCP8.5, changes in phytoplankton community structure and, to a lesser extent, exposure of marine species to unprecedented temperatures, will emerge earlier in marine protected areas (MPAs), covering ~7.7% of the global oceans ( [[#3.6.2.3|Section 3.6.2.3.2.1]] ; UNEP-WCMC and [[#IUCN--2020|IUCN, 2020]] ; [[#UNEP-WCMC%20and%20IUCN--2021|UNEP-WCMC and IUCN, 2021]] ) as compared with non-MPAs (Figure 3.20b,d). Such assessment can support planning for future MPA placement and extent. Because MPAs can serve as refugia from non-climate drivers (Sections 3.6.2.3, 3.6.3.2.1), they facilitate opportunities for adaptation among marine species and communities in coastal oceans ( [[#3.4.2|Section 3.4.2]] ).

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[[File:8f6c3fd05dd924e5d2f20a2442625694 IPCC_AR6_WGII_Figure_3_020.png]]

'''Figure 3.20 |''' '''Time of exposure to altered environments.'''

'''(a)''' Simulated spatial variation in the time of exposure of marine species to unprecedented temperatures under RCP8.5. Time of exposure is quantified as the median year after which local species are projected to encounter temperatures warmer than the historical maximum within their full geographic range for a period of at least 5 years. This estimate is based on 22 Coupled Model Intercomparison Project 5 (CMIP5) models, and is drawn from data presented by Trisos et al. (2020). Only regions that have times of emergence by 2100 are shown.

'''(b)''' The distribution in the time of exposure to unprecedented temperatures within marine assemblages ( [[#Trisos--2020|Trisos et al., 2020]] ) under RCP8.5 in marine protected areas (in turquoise) and in non-marine protected areas (in purple). Values were calculated after regridding to equal-area 0.5° hexagons.

'''(c)''' Time of emergence for phytoplankton community-structure changes (based on a proxy–ecosystem-model reflectance at 500 nm) under RCP8.5. Only regions with statistically significant ( ''p'' &lt; 0.05) trends that are presently largely ice free and have times of emergence by 2100 are shown. (Based on the results of one numerical model from [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ).

'''(d)''' The distribution in the time of emergence for changes in phytoplankton community structure (same proxy as in panel c) ( [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ) under RCP8.5 in marine protected areas (in turquoise) and in non-marine protected areas (in purple). Values were calculated after regridding to equal-area 0.5° hexagons.

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