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

===== 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"
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! Observations

! Projections

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

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| 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'' ).’

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

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| ‘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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