Editing IPCC:AR6/WGII/Chapter-11 (section)

=== 11.3.2 Coastal and Ocean Ecosystems ===

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Australia’s EEZ covers over 8.1 million km 2 of marine territory, including 50,000 km of coastline ( [[#Dhanjal-Adams--2016|Dhanjal-Adams et al., 2016]] ), spanning sub-Antarctic islands in the south to tropical waters in the north. New Zealand’s marine territory extends from the sub-tropics to sub-Antarctic waters, encompassing an EEZ of 4 million km 2 , 18,000 km of coastline and 700 smaller islands and islets, in addition to the two main islands ( [[#Costello--2010a|Costello et al., 2010a]] ; [[#MfE--2016|MfE, 2016]] ).

The marine environment is important to the culture, health and well-being of the region’s diverse Indigenous Peoples, including those who had sovereign ownership, governance, resource rights, and stewardship over ‘Sea Country’ for many thousands of years before the current sea level stabilised approximately 6000 years ago and before current coastal ecosystems were established ( [[#Rist--2019|Rist et al., 2019]] ). Marine environments contribute AUD$69 billion per year to Australia’s economy (Eadie et al., 2011), and NZD$4 billion per year to New Zealand’s economy ( [[#MfE--2016|MfE, 2016]] ). They have a high proportion of rare and endemic species ( [[#Croxall--2012|Croxall et al., 2012]] ) and provide ecosystem services including food production, coastal protection, tourism and carbon sequestration ( [[#Croxall--2012|Croxall et al., 2012]] ; [[#Kelleway--2017|Kelleway et al., 2017]] ). Half of the species within New Zealand’s seas are endemic ( [[#Costello--2010b|Costello et al., 2010b]] ).

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==== 11.3.2.1 Observed Impacts ====

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Climate change is having major impacts on the region’s oceans ( ''very high confidence'' ) (Table 11.6) ( [[#Law--2016|Law et al., 2016]] ; [[#Sutton--2019|Sutton and Bowen, 2019]] ). Rising sea surface temperatures (SSTs) have exacerbated marine heatwaves, notably near western Australia in 2011, the GBR in 2016, 2017 and 2020 and the Tasman Sea in 2015/2016, 2017/2018 and 2018/2019 (Table 11.2) ( [[#BoM%20and%20CSIRO--2018|BoM and]] [[#CSIRO--2018|CSIRO, 2018]] ; [[#AMS--2019|AMS, 2019]] ; [[#NIWA--2019|NIWA, 2019]] ; [[#Salinger--2019b|Salinger et al., 2019b]] ; [[#Sutton--2019|Sutton and Bowen, 2019]] ; [[#BoM--2020|BoM, 2020]] ; [[#Salinger--2020|Salinger et al., 2020]] ; [[#Oliver--2021|Oliver et al., 2021]] ). Temperature anomalies ranged from 1.2°C to 4.0°C and durations ranged from 90–250 days (Table 11.2).

'''Table 11.6 |''' Observed climate-change-related changes in the marine ecosystems of Australia and New Zealand. Climate-related impacts have been documented at a range of scales from single-species or region-specific studies to multi-species or community-level changes.

{| class="wikitable"
|-
! Type of change

! Examples

! Climate-related Pressure

! Source

|-
| colspan="4"| '''Australia'''

|-
| Reduced activity and increased energetic demands

| Coral trout ( ''Plectropomus leopardus'' ), one of Australia’s most important commercial and recreational tropical finfish species

| Increased temperature (experimental laboratory study) and ocean warming

| ( [[#Johansen--2014|Johansen et al., 2014]] ; [[#Scott--2017|Scott et al., 2017]] )

|-
| Estuaries warming and freshening

| Australian lagoons and rivers warming and decreasing pH at a faster rate than predicted by climate models

| Warming and reduction in rainfall (leading to reduced flows and therefore being less frequently open to the sea)

| ( [[#Scanes--2020|Scanes et al., 2020]] )

|-
| Changes in life-history traits, behaviour or recruitment

| Reduced size of Sydney rock oysters (for commercial sale)

| Limited capacity to bio mineralise under acidification conditions

| ( [[#Fitzer--2018|Fitzer et al., 2018]] )

|-
|
| Reduced growth in tiger flathead fish in equatorward range

| Ocean warming

| ( [[#Morrongiello--2015|Morrongiello and Thresher, 2015]] )

|-
|
| 55% of 335 fish species became smaller and 45% became larger as seas warmed around Australia

| Ocean warming (over three decades)

| ( [[#Audzijonyte--2020|Audzijonyte et al., 2020]] )

|-
|
| Rock lobster display reduced avoidance of predators at 23°C compared to 20°C

| Increased temperature (experimental laboratory study)

| ( [[#Briceño--2020|Briceño et al., 2020]] )

|-
|
| Analysis of stress rings in cores of corals from the GBR dating back to 1815 found that following bleaching events, the coral was less affected by subsequent marine heatwaves

| Heat events

| ( [[#DeCarlo--2019|DeCarlo et al., 2019]] )

|-
|
| Mortality and reductions in spawning stocks of fishery important abalone, prawns, rock lobsters

| 2011 marine heatwave

| ( [[#Caputi--2019|Caputi et al., 2019]] )

|-
|
| Recruitment of coral on GBR reduced to 11% of long-term average

| Warming-driven back-to-back global bleaching events

| ( [[#Hughes--2019b|Hughes et al., 2019b]] )

|-
|
| Green turtle hatchlings from southern GBR 65–69% female and hatchlings from northern GBR 100% female for last two decades

| Increased sand temperatures

| ( [[#Jensen--2018|Jensen et al., 2018]] )

|-
| New diseases, toxins

| First occurrence of virulent virus causing Pacific Oyster Mortality Syndrome (POMS), up to 90% of all farmed oysters died in impacted areas

| Detected during heatwave

| ( [[#de%20Kantzow--2017|de Kantzow et al., 2017]] )

|-
|
| Mussels, scallops, oysters, clams, abalone and rock lobsters on east coast of Tasmania found to have high levels of Paralytic Shellfish toxins, originating from a bloom of harmful ''Alexandrium tamarense''

| Warming and extension of the East Australian Current

| ( [[#Hallegraeff--2016|Hallegraeff and Bolch, 2016]] )

|-
|
| Range expansion of phytoplankton ''Noctiluca'' , which can be toxic

| Warming and extension of the East Australian Current

| ( [[#Hallegraeff--2020|Hallegraeff et al., 2020]] )

|-
|
| Mortality of fish following algal blooms in South Australia

| 2013 marine heatwave

| ( [[#Roberts--2019|Roberts et al., 2019]] )

|-
| Changes in species distributions

| Range extensions at the poleward range limit have been detected in: fish, cephalopods, crustaceans, nudibranchs, urchins, corals

| Ocean warming

| ( [[#Baird--2012|Baird et al., 2012]] ; [[#Robinson--2015|Robinson et al., 2015]] ; [[#Sunday--2015|Sunday et al., 2015]] ; [[#Ling--2018|Ling et al., 2018]] ; [[#Nimbs--2018|Nimbs and Smith, 2018]] ; [[#Ramos--2018|Ramos et al., 2018]] ; [[#Smith--2019|Smith et al., 2019]] ; [[#Caswell--2020|Caswell et al., 2020]] )

|-
|
| Contractions in range at the equatorward range edge have been detected in anemones, asteroids, gastropods, mussels, algae

| Ocean warming

| ( [[#Pitt--2010|Pitt et al., 2010]] ; [[#Poloczanska--2011|Poloczanska et al., 2011]] ; [[#Smale--2019|Smale et al., 2019]] )

|-
|
| Australia’s most southern dominant reef building coral, ''Plesiastrea versipora'' , in eastern Bass Strait, increasing in abundance at the poleward edge of the species’ range and in western Australia

| Ocean warming

| ( [[#Tuckett--2017|Tuckett et al., 2017]] ; [[#Ling--2018|Ling et al., 2018]] )

|-
|
| Southwestern Australia fish assemblages—warm-water fish increasing in density at poleward edge of distributions and cool-water species decreasing in density at equatorward edge of distributions; increase in warm-water habitat forming species leading to reduced habitat for invertebrate assemblages

| Combination of increased temperatures and changes in habitat-forming algal species

| ( [[#Shalders--2018|Shalders et al., 2018]] ; [[#Teagle--2018|Teagle et al., 2018]] )

|-
|
| Predicted reduction range of rare W ''ilsonia humilis'' herb in Tasmanian saltmarsh but no change in rest of community

| Wetter and drier climate

| ( [[#Prahalad--2019|Prahalad and Kirkpatrick, 2019]] )

|-
| Changes in abundance

| Shift towards a zooplankton community dominated by warm-water small copepods in southeast Australia

| Ocean warming

| ( [[#Kelly--2016|Kelly et al., 2016]] )

|-
|
| Diebacks of tidal wetland mangroves

| 2015–2016 heatwaves combined with moisture stress

| ( [[#Duke--2017|Duke et al., 2017]] )

|-
|
| Decline in giant kelp in Tasmania, Australia, less than 10% remaining; loss of kelp Australia-wide totalling at least 140,187 hectares

| Ocean warming and change in East Australian Current (lower nutrients)

| ( [[#Wahl--2015|Wahl et al., 2015]] ; [[#Butler--2020|Butler et al., 2020]] ; [[#Filbee-Dexter--2020|Filbee-Dexter and Wernberg, 2020]] )

|-
|
| Regional loss of seagrass in Shark Bay World Heritage Area, western Australia

| High air and water temperatures during 2011 heatwave

| ( [[#Strydom--2020|Strydom et al., 2020]] )

|-
|
| Increased annual dugong and inshore dolphin mortality across Queensland

| Sustained low air temperature and increased freshwater discharge during high Southern Oscillation Index (SOI) (ENSO) index

| ( [[#Meager--2014|Meager and Limpus, 2014]] )

|-
|
| Predicted equatorward decline and poleward shift of sea urchin in eastern Australia

| Ocean warming

| ( [[#Castro--2020|Castro et al., 2020]] )

|-
|
| Increasing mortality of Australian fur seal pups in low-lying colonies

| Storm surges and high tides amplified by ongoing SLR

| ( [[#McLean--2018|McLean et al., 2018]] ) (Box 11.6)

|-
| Rapid shifts in community composition, structure and integrity

| Community-wide tropicalisation in Australian temperate reef communities; temperate species replaced by seaweeds, invertebrates, corals, and fishes characteristic of sub-tropical and tropical waters

| Extreme marine heatwaves led to 100-km range contraction of extensive kelp forests

| ( [[#Vergés--2016|Vergés et al., 2016]] ; [[#Wernberg--2016|Wernberg et al., 2016]] )

|-
|
| Ongoing declines in habitat-forming seaweeds

| Climate-driven shift of tropical herbivores

| ( [[#Thomson--2015|Thomson et al., 2015]] ; [[#Nowicki--2017|Nowicki et al., 2017]] ; [[#Zarco-Perello--2017|Zarco-Perello et al., 2017]] ; [[#Wernberg--2016|Wernberg et al., 2016]] )

|-
|
| Dieback of temperate seagrass in Shark Bay, Australia, subsequently replaced by tropical early successional seagrass with seagrass-associated megafauna (sea turtles) declining in health status

| 2011 marine heatwave

| ( [[#Strydom--2020|Strydom et al., 2020]] )

|-
|
| Increased herbivory by fish on tropicalised reefs of western Australia

| Change in species composition due to ocean warming

| ( [[#Zarco-Perello--2019|Zarco-Perello et al., 2019]] )

|-
|
| No recovery 2 years after coral bleaching and macroalgae mortality in western Australia

| 2011 marine heatwave

| ( [[#Bridge--2014|Bridge et al., 2014]] )

|-
|
| Mass mortality of particular coral species on affected reefs during heatwaves on GBR (Eastern Australia) led to altered coral reef structure and species composition 8 months later.

| 2016 marine heatwave

| ( [[#Hughes--2018c|Hughes et al., 2018c]] )

|-
|
| Community-wide restructuring along GBR 1 year after the 2016 mass bleaching event

| 2016 marine heatwave

| (Stuart- [[#Smith--2018|Smith et al., 2018]] )

|-
| colspan="4"| '''New Zealand'''

|-
| Changes in life-history

| Alteration of shell of pāua (black footed abalone, ''Haliotis iris'' ) under lowered pH (calcite layer thinner, greater etching of external shell surface)

| Lowered pH (experimental laboratory study)

| ( [[#Cummings--2019|Cummings et al., 2019]] )

|-
|
| Decline in maximum swimming performance of kingfish and snapper

| Elevated CO 2 (experimental laboratory study)

| ( [[#Watson--2018|Watson et al., 2018]] ; [[#McMahon--2020|McMahon et al., 2020]] )

|-
|
| Increased mortality and faster growth in juvenile kingfish

| Increased temperature

| ( [[#Watson--2018|Watson et al., 2018]] )

|-
|
| Earlier spawning of snapper in South Island

| 2017–2018 heatwave

| ( [[#Salinger--2019b|Salinger et al., 2019b]] )

|-
| Increase in mortality

| Heat stress mortality in salmon farms off Marlborough, New Zealand, where 20% of salmon stocks died

| 2017–2018 marine heatwave

| ( [[#Salinger--2019b|Salinger et al., 2019b]] )

|-
| Changes in species distributions

| Species increasingly caught further south (e.g., snapper and kingfish)

| Ocean warming and 2017–2018 marine heatwave

| ( [[#Salinger--2019b|Salinger et al., 2019b]] )

|-
|
| Non-breeding distribution of New Zealand nesting seabird (Antarctic prion) shifting south with long-term climate inferred from stable isotopes

| Climate warming

| ( [[#Grecian--2016|Grecian et al., 2016]] )

|-
|
| Less phytoplankton production in Tasman Sea but more on sub-tropical front

| Ocean warming

| ( [[#Chiswell--2020|Chiswell and Sutton, 2020]] )

|-
|
| Loss of bull kelp ( ''Durvillaea'' ) populations in southern New Zealand subsequently replaced by introduced kelp ''Undaria''

| 2017–2018 heatwave when sea and air temperatures exceeded 23°C and 30°C respectively

| ( [[#Salinger--2019b|Salinger et al., 2019b]] ; [[#Thomsen--2019|Thomsen et al., 2019]] ; [[#Salinger--2020|Salinger et al., 2020]] )

|}

Ocean carbon storage and acidification has led to decreased surface pH in the region (Table 11.2), including the sub-Antarctic waters off the East Coast of New Zealand’s South Island ( ''very high confidence'' ) ( [[#Law--2016|Law et al., 2016]] ). The depth of the Aragonite Saturation Horizon has shallowed by 50–100 m over much of New Zealand, which may limit and/or increase the energetic costs of growth of calcifying species ( ''low confidence'' ) ( [[#Anderson--2015|Anderson et al., 2015]] ; [[#Bostock--2015|Bostock et al., 2015]] ; [[#Mikaloff-Fletcher--2017|Mikaloff-Fletcher et al., 2017]] ).

In the estuaries of southwestern Australia, sustained warming and drying trends have caused dramatic declines in freshwater flows of up to 70% since the 1970s and increased frequency and severity of hypersaline conditions, enhanced water column stratification and hypoxia and reduced flushing and greater retention of nutrients ( [[#Hallett--2017|Hallett et al., 2017]] ).

Extensive changes in the life history and distribution of species have been observed in Australia’s ( ''very high confidence'' ) ( [[#Gervais--2021|Gervais et al., 2021]] ) and New Zealand’s marine systems ( ''medium confidence'' ) (Table 11.6) (Cross-Chapter box MOVING SPECIES in Chapter 5). New occurrences or increased prevalence of disease, toxins and viruses are evident ( [[#de%20Kantzow--2017|de Kantzow et al., 2017]] ; [[#Condie--2019|Condie et al., 2019]] ), along with heat stress mortalities and changes in community composition ( [[#Wernberg--2016|Wernberg et al., 2016]] ; [[#Zarco-Perello--2017|Zarco-Perello et al., 2017]] ; [[#Thomsen--2019|Thomsen et al., 2019]] ). Extreme climatic events in Australia from 2011 to 2017 led to abrupt and extensive mortality of key habitat-forming organisms — corals, kelps, seagrasses and mangroves — along over 45% of the continental coastline of Australia ( ''high confidence'' ) ( [[#Babcock--2019|Babcock et al., 2019]] ).

In 2016 and 2017, the GBR experienced consecutive occurrences of the most severe coral bleaching in recorded history ( ''very high confidence'' ) (Box 11.2), with shallow-water reef in the top two-thirds of the GBR affected and the severity of bleaching on individual reefs tightly correlated with the level of local heat exposure ( [[#Hughes--2018b|Hughes et al., 2018b]] ; [[#Hughes--2019c|Hughes et al., 2019c]] ). Mass mortality of corals from these two unprecedented events resulted in larval recruitment in 2018 declining by 89% compared to historical levels ( [[#Hughes--2019b|Hughes et al., 2019b]] ). southern reefs were also affected by warming, although significantly less than in the north ( [[#Kennedy--2018|Kennedy et al., 2018]] ). Coral reefs in Australia are at very high risk of continued negative effects on ecosystem structure and function ( ''very high confidence'' ) ( [[#Hughes--2019b|Hughes et al., 2019b]] ), cultural well-being ( ''very high confidence'' ) ( [[#Goldberg--2016|Goldberg et al., 2016]] ; [[#Lyons--2019|Lyons et al., 2019]] ), food provision ( ''medium confidence'' ) ( [[#Hoegh-Guldberg--2017|Hoegh-Guldberg et al., 2017]] ), coastal protection ( ''high confidence'' ) ( [[#Ferrario--2014|Ferrario et al., 2014]] ) and tourism ( ''high confidence'' ) ( [[#Deloitte%20Access%20Economics--2017|Deloitte Access Economics, 2017]] ; [[#Prideaux--2018|Prideaux and Pabel, 2018]] ; [[#GBRMPA--2019|GBRMPA, 2019]] ). If bleaching persists, an estimated 10,000 jobs and AUD$1 billion in revenue would be lost per year from declines in tourism alone ( [[#Swann--2016|Swann and Campbell, 2016]] ).

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==== 11.3.2.2 Projected Impacts ====

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Future ocean warming, coupled with periodic extreme heat events, is projected to lead to the continued loss of ecosystem services and ecological functions ( ''high confidence'' ) ( [[#Smale--2019|Smale et al., 2019]] ) as species further shift their distributions and/or decline in abundance ( [[#Day--2018|Day et al., 2018]] ). Compounding climate-driven changes in the distribution of habitat-forming species, invasive macroalgae are predicted to exhibit higher growth under all higher pCO 2 and lower pH conditions ( [[#Roth-Schulze--2018|Roth-Schulze et al., 2018]] ). Corals and mangroves around northern Australia and kelp and seagrass around southern Australia are of critical importance for ecosystem structure and function, fishery productivity, coastal protection and carbon sequestration; these ecosystem services are therefore ''extremely likely'' [[#footnote-000|2]] to decline with continued warming. Equally, many species provide important ecosystem structure and function in New Zealand’s seas including in the deep sea ( [[#Tracey--2019|Tracey and Hjorvarsdottir, 2019]] ). The future level of sustainable exploitation of fisheries is dependent on how climate change impacts these ecosystems. Native kelp is projected to further decline in southeastern New Zealand with warming seas (Table 11.6). Climate change could affect New Zealand fisheries’ productivity ( [[#Cummings--2021|Cummings et al., 2021]] ), and both ocean warming and acidification may directly affect shellfish culture ( [[#Cunningham--2016|Cunningham et al., 2016]] ; [[#Cummings--2019|Cummings et al., 2019]] ) and indirectly through changes in phytoplankton production ( [[#Pinkerton--2017|Pinkerton, 2017]] ).

Climate-change-related temperature and acidification may affect species sex ratios and, thus, population viability ( ''medium confidence'' ) (Table 11.3) ( [[#Law--2016|Law et al., 2016]] ; [[#Tait--2016|Tait et al., 2016]] ; [[#Mikaloff-Fletcher--2017|Mikaloff-Fletcher et al., 2017]] ). Acidification may alter sex determination (e.g., in the oyster ''Saccostrea glomerate'' ), resulting in changes in sex ratios ( [[#Parker--2018|Parker et al., 2018]] ), and may thus affect reproductive success ( ''low confidence'' ). Decreasing river flows ( [[#Chiew--2017|Chiew et al., 2017]] ) are projected to cause periodically open estuaries across southwest Australia to remain closed for longer periods, inhibiting the extent to which marine taxa can access these systems ( [[#Hallett--2017|Hallett et al., 2017]] ) and with warming predicted to constrain activity in some large fish ( [[#Scott--2019b|Scott et al., 2019b]] ). Major knowledge gaps include environmental tolerances of key life stages, sources of recruitment, population linkages, critical ecological (e.g., predator–prey interactions) or phenological relationships and projected responses to lowered pH ( [[#Fleming--2014|Fleming et al., 2014]] ; [[#Fogarty--2019|Fogarty et al., 2019]] ).

Black-browed albatrosses breeding on Macquarie Island may be more vulnerable to future climate-driven changes to weather patterns in the Southern Ocean and potential latitudinal shifts in the sub-Antarctic Front ( [[#Cleeland--2019|Cleeland et al., 2019]] ). New Zealand coastal ecosystems face risks from sea level rise (SLR) and extreme weather events ( [[#MfE--2020a|MfE, 2020a]] ).

Nutrient availability and productivity in the sub-tropical waters of New Zealand are projected to decline due to increased SST and strengthening of the thermocline, but they may increase in sub-Antarctic waters, potentially bringing some benefit to fish and other species ( ''low confidence'' ) ( [[#Law--2018b|Law et al., 2018b]] ). For New Zealand waters as a whole, declines in net primary productivity of 1.2% and 4.5% are projected under RCP4.5 and RCP8.5 respectively by 2100, and declines in the primary production of surface waters by an average 6% from the present day under RCP8.5, with sub-tropical waters experiencing the largest decline ( [[#Tait--2016|Tait et al., 2016]] ).

The pH of surface waters around New Zealand is projected to decline by 0.33 under RCP 8.5 by 2090 ( [[#Tait--2016|Tait et al., 2016]] ), and the depth at which carbonate dissolves is projected to be significantly shallower ( [[#Mikaloff-Fletcher--2017|Mikaloff-Fletcher et al., 2017]] ), affecting the distribution of some species of calcifying cold water corals ( ''medium confidence'' ) ( [[#Law--2016|Law et al., 2016]] ). However, model projections suggest that the top of the Chatham Rise may provide temporary refugia for scleractinian stony corals from ocean acidification because the Chatham Rise sits above the aragonite saturation horizon ( [[#Anderson--2015|Anderson et al., 2015]] ; [[#Bostock--2015|Bostock et al., 2015]] ). For sub-tropical corals, skeletal formation will be vulnerable to the changes in ocean pH, with implications for their longer-term growth and resilience ( [[#Foster--2015|Foster et al., 2015]] ).

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==== 11.3.2.3 Adaptation ====

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Climate change adaptation opportunities and pathways have been identified across aquaculture, fisheries, conservation and tourism sectors in the region ( [[#MacDiarmid--2013|MacDiarmid et al., 2013]] ; [[#Fleming--2014|Fleming et al., 2014]] ; [[#MPI--2015|MPI, 2015]] ; [[#Jennings--2016|Jennings et al., 2016]] ; [[#MfE--2016|MfE, 2016]] ; [[#Royal%20Society%20Te%20Apārangi--2017|Royal Society Te Apārangi, 2017]] ; [[#Ling--2019|Ling and Hobday, 2019]] ), and some stakeholders are already autonomously adapting ( [[#Pecl--2019|Pecl et al., 2019]] ). Some fishing and aquaculture industries use seasonal forecasts of environmental conditions to improve decision-making, risk management and business planning ( [[#Hobday--2016|Hobday et al., 2016]] ), with the potential to use 5-yearly forecasts similarly ( [[#Champion--2019|Champion et al., 2019]] ). Shifts in the distribution and availability of target species (e.g., oceanic tuna) would impact the ability of domestic fishing vessels to continue current fishing practices, with potential social and economic adjustment costs ( [[#Dell--2015|Dell et al., 2015]] ), including disruption to supply chains ( [[#Fleming--2014|Fleming et al., 2014]] ; [[#Plagányi--2014|Plagányi et al., 2014]] ) (Cross-Chapter Box MOVING SPECIES in Chapter 5). Species abundance data are insufficient to enable projections of climate impacts on fishery productivity. However, fishery and aquaculture industries are considering adaptation strategies, such as changing harvests and relocating farms ( [[#Pinkerton--2017|Pinkerton, 2017]] ). Thus, while climate change is ''extremely likely'' to affect the abundance and distribution of marine species around New Zealand, insufficient monitoring means there is ''limited evidence'' of ecosystem level change in biodiversity to date and no quantitative projections of which species may win and lose to climate change (Table 11.6) ( [[#Law--2018a|Law et al., 2018a]] ; [[#Law--2018b|Law et al., 2018b]] ).

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'''Box 11.2 | The Great Barrier Reef in Crisis'''
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The GBR is the world’s largest coral reef system, comprising 3863 reefs over an area of 348,700 km 2 , stretching for 2300 km. The GBR is a central cornerstone of the beliefs, knowledges, lores, languages and ways of living for over 70 geographically and culturally diverse Traditional Owner groups spanning the length of the GBR ( [[#Dale--2018|Dale et al., 2018]] ), and it contributes an estimated AUD$6.4 billion per year (pre-COVID) to the Australian economy, mainly via tourism. As the world’s most extensive coral reef ecosystem, the GBR is a globally outstanding and significant entity, with practically the entire ecosystem inscribed as a World Heritage Site in 1981 (UNESCO, 1981).

The GBR is already severely impacted by climate change, particularly ocean warming, through more frequent and severe coral bleaching ( ''very high confidence'' ) ( [[#Hughes--2018b|Hughes et al., 2018b]] ; [[#Hughes--2019c|Hughes et al., 2019c]] ). The worst coral bleaching event on record affected over 90% of reefs in 2016 ( [[#Hughes--2018b|Hughes et al., 2018b]] ). In the most northern 700-km-long section of the GBR in which the heat exposure was the most extreme, 50% of the coral cover on reef crests was lost within 8 months ( [[#Hughes--2018c|Hughes et al., 2018c]] ). Throughout the entire GBR, including the southern third where heat exposure was minimal, the cover of corals declined by 30% between March and November 2016 ( [[#Hughes--2018b|Hughes et al., 2018b]] ). In 2017, the central third of the reef was the most severely affected and the back-to-back regional-scale bleaching events has led to an unprecedented shift in the composition of GBR coral assemblages, transforming the northern and middle sections of the reef system ( [[#Hughes--2018c|Hughes et al., 2018c]] ) to a highly degraded state ( ''very high confidence'' ). Coral recruitment to the GBR in 2018 was reduced to only 11% of the long-term average ( [[#Hughes--2019b|Hughes et al., 2019b]] ). A mass bleaching event also occurred in 2020, making it the third event in 5 years ( [[#BoM--2020|BoM, 2020]] ) (Figure Boxes 11.2.1 and 11.2.2).

[[File:3c30996eb6ef6e557ce34b67d7fba9c3 IPCC_AR6_WGII_Figure_11_Box_11_2_1.png]]

'''Figure Box 11.2.1 |''' '''Top panels: spatial patterns in heat exposure along the GBR in 2016 (left pair) and 2017 (right pair), measured from satellites as Degree Heating Weeks (DHW, °C-weeks).''' Middle panels: geographic footprint of recurrent coral bleaching in 2016 (left) and again in 2017 (right), measured by aerial assessments of individual reefs (adapted from ( [[#Hughes--2019c|Hughes et al., 2019c]] )]). Bottom panels: density of coral recruits (mean per recruitment panel on each reef), measured over three decades, from 1996 to 2016 ( ''n'' = 47 reefs, 1784 panels) (left), compared to the density of coral recruits in 2018 after the mass mortality of corals in 2016 and 2017 due to the back-to-back bleaching events ( ''n'' = 17 reefs, 977 panels) (right). The area of each circle is scaled to the overall recruit density of spawners and brooders combined. Yellow and blue indicate the proportion of spawners and brooders respectively (from ( [[#Hughes--2019b|Hughes et al., 2019b]] )]).

[[File:7a3030f3f364d57e289d95e3a1dcc9ca IPCC_AR6_WGII_Figure_11_Box_11_2_2.png]]

'''Figure Box 11.2.2 | Variation in the severity of mass-bleaching episodes recorded on Australia’s GBR over the last four decades (1980–2020).''' The overall number of reefs surveyed was substantially higher in 1998, 2002, 2016, 2017 and 2020 when aerial surveys were undertaken, whereas the severity of other more localised bleaching episodes was documented with in-water surveys (adapted from ( [[#Pratchett--2021|Pratchett et al., 2021]] ). Extent of bleaching in 2020 was similar in severity to that of 2016 but more geographically widespread and included southern reefs.

Increased heat exposure also affects the abundance and distribution of associated fish, invertebrates and algae ( ''high confidence'' ) (Stuart- [[#Smith--2018|Smith et al., 2018]] ). Thus, coral bleaching is an indicator of thermal effects on coral habitat, fauna and flora. Bleaching is expected to continue for the GBR and Australia’s other coral reef systems ( ''virtually certain'' ). Bleaching conditions are projected to occur twice each decade from 2035, annually after 2044 under RCP8.5 and annually after 2051 under RCP4.5 ( [[#Heron--2017|Heron et al., 2017]] ). Global warming of 3°C would result in over six times the 2016 level of thermal stress ( [[#Lough--2018|Lough et al., 2018]] ).

Increases in cyclone intensity projected for this century, and other extreme weather events, will greatly accelerate coral reef degradation ( [[#Osborne--2017|Osborne et al., 2017]] ). Additionally, through interactions between elevated ocean temperature and coastal runoff (nutrient and sediment), extreme weather events may contribute to an increased frequency and/or amplitude of crown-of-thorns starfish outbreaks ( [[#Uthicke--2015|Uthicke et al., 2015]] ), further reducing the spatial distribution of coral.

Recovery of coral reefs following repeated disturbance events is slow ( [[#Hughes--2019b|Hughes et al., 2019b]] ; [[#IPCC--2019b|IPCC, 2019b]] ), and it takes at least a decade after each bleaching event for the very fastest growing corals to recover ( ''high confidence'' ) ( [[#Gilmour--2013|Gilmour et al., 2013]] ; [[#Osborne--2017|Osborne et al., 2017]] ). Estimates of future levels of thermal stress, measured as degree heating months, which incorporates both the magnitude and duration of warm season SST anomalies, suggest that achieving the 1.5°C Paris Agreement target would be insufficient to prevent more frequent mass bleaching events ( ''very high confidence'' ) ( [[#Lough--2018|Lough et al., 2018]] ), although it may reduce their occurrence ( [[#Heron--2017|Heron et al., 2017]] ), and occurrences of warming events similar to 2016 bleaching could be reduced by 25% ( [[#King--2017|King et al., 2017]] ).

Tourist motivations for visiting the GBR are changing, with a recent survey finding that two-thirds of tourists were visiting ‘before it was gone’ and a similar number were reporting damage to the reef—an example of ‘last chance tourism’ ( [[#Piggott-McKellar--2016|Piggott-McKellar and McNamara, 2016]] ). The Australian government is investing AUD$1.9 billion to support the GBR through science and practical environmental outcomes, including reducing other anthropogenic pressures, which can suppress natural adaptive capacity ( [[#CoA--2019b|CoA, 2019b]] ; [[#GBRMPA--2019|GBRMPA, 2019]] ). However, adaptation efforts on the GBR aimed specifically at climate impacts, for example coral restoration following marine heatwave impacts ( [[#Boström-Einarsson--2020|Boström-Einarsson et al., 2020]] ), may slow the impacts of climate change in small discrete regions of the reef or reduce short-term socioeconomic ramifications, but they will not prevent widespread bleaching (Condie et al. 2021).

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