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

==== 3.5.5.5 Regulation of Carbon Cycling in Ocean and Coastal Ecosystems ====

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Current and future total carbon storage and cycling in the ocean are governed by past and future CO 2 emissions trajectories (Table 3.26), but regional ocean and coastal carbon stocks and cycling vary over time and space due to processes being altered by climate, including ocean circulation, sea ice cover, coastal upwelling and thermal stratification ( [[#3.2.2.3|Section 3.2.2.3]] ); ocean primary production and export (Sections 3.2.3, 3.4.4); and marine ecosystem biodiversity ( ''high confidence'' ) ( [[#3.5.2|Section 3.5.2]] ; Figure 3.22). Quantifying regional carbon fluxes and stocks is still challenging and relies on indirect measures (e.g., [[#Fennel--2019|Fennel et al., 2019]] ; [[#Clay--2020|Clay et al., 2020]] ), especially in coastal ecosystems where drivers interact. Carbon cycling and storage co-occurs with other regulating services such as habitat provision, water-quality maintenance and coastal protection ( [[#Ouyang--2018|Ouyang et al., 2018]] ), particularly in vegetated coastal ecosystems (see Box 3.4). Adaptations to support regional carbon cycling and storage generally focus on area-based management and conservation ( [[#3.6.3.2|Section 3.6.3.2]] ), but interventions to enhance ocean carbon storage are being explored for mitigation (WGIII AR6 Chapter 7).

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'''Box 3.4 | Blue Carbon Ecosystems'''
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Climate change and other anthropogenic drivers, including eutrophication, land-use changes and overexploitation, directly and indirectly threaten blue carbon ecosystems (Annex II: Glossary). Commonly considered blue carbon ecosystems include vegetated coastal ecosystems (Sections 3.4.2.3–3.4.2.5), whose mangroves, salt marshes and seagrass beds host rooted, vascular plants known to store large amounts of carbon for long periods and to be amenable to management ( [[#Lovelock--2019|Lovelock and Duarte, 2019]] ). Other ocean and coastal taxa, including rooted or floating macroalgae (e.g., non-vascular multicellular kelp or seaweed genera such as ''Macrocystis'' spp., ''Sargassum'' spp. or ''Laminaria'' spp. ( [[#Filbee-Dexter--2020|Filbee-Dexter and Wernberg, 2020]] ), phytoplankton and even pelagic fauna (e.g., finfish or whales; [[#Chami--2019|Chami et al., 2019]] ), have also been proposed as blue carbon ecosystems. Terrestrial vascular-plant-derived material can also carry and store significant amounts of carbon in marine environments ( [[#Cragg--2020|Cragg et al., 2020]] ). There is increasing evidence about the coverage and carbon content of macroalgal, planktonic and faunal taxa, but ''low agreement'' about their long-term carbon-storage potential and manageability ( [[#Alongi--2018b|Alongi, 2018b]] ; [[#Wernberg--2018|Wernberg and Filbee-Dexter, 2018]] ; [[#Lovelock--2019|Lovelock and Duarte, 2019]] ; [[#Ortega--2019|Ortega et al., 2019]] ; [[#Pfister--2019|Pfister et al., 2019]] ; [[#Queirós--2019|Queirós et al., 2019]] ; [[#Filbee-Dexter--2020a|Filbee-Dexter et al., 2020a]] ; [[#Gallagher--2020|Gallagher, 2020]] ; [[#Mariani--2020|Mariani et al., 2020]] ; [[#Thorhaug--2020|Thorhaug et al., 2020]] ; [[#van%20Son--2020|van Son et al., 2020]] ; [[#Bach--2021|Bach et al., 2021]] ; [[#Bayley--2021|Bayley et al., 2021]] ; [[#Cavanagh--2021|Cavanagh et al., 2021]] ; [[#Frontier--2021|Frontier et al., 2021]] ; [[#Martin--2021|Martin et al., 2021]] ; [[#Pedersen--2021|Pedersen et al., 2021]] ; [[#Weigel--2021|Weigel and Pfister, 2021]] ). This section focuses on the array of ecosystem services and adaptation opportunities provided by vegetated coastal blue carbon ecosystems, where consensus and evidence are most abundant. Mitigation potential of blue carbon ecosystems is assessed with land-based mitigation options in WGIII AR6 [[IPCC:Wg2:Chapter:Chapter-7#7.4|Section 7.4]] .

Carbon storage and burial in mangroves, salt marshes and seagrass meadows (see Table Box 3.4.1) help regulate ocean and coastal carbon cycling and may contribute to nature-based mitigation, although regional estimates vary widely based on climatic and edaphic conditions (WGIII AR6 [[IPCC:Wg2:Chapter:Chapter-7#7.4|Section 7.4]] ). In addition, coastal vegetated ecosystems provide substantial and interdependent regulating, provisioning and cultural ecosystem services. These services include: (a) disproportionately high biodiversity per unit area ( [[#Pörtner--2021a|Pörtner et al., 2021a]] ); (b) abundant habitat ( [[#3.5.5.1|Section 3.5.5.1]] ) and nurseries for aquatic, terrestrial, aerial and microbial species; (c) natural filtration of waste and stormwater runoff into the coastal ocean (Sections 3.5.5.3, 4.2.7; Cross-Chapter Box ILLNESS in Chapter 2); (d) coastal protection ( [[#3.5.5.4|Section 3.5.5.4]] ; [[#Ouyang--2018|Ouyang et al., 2018]] ; [[#Quevedo--2020|Quevedo et al., 2020]] ); (e) food and natural materials (Sections 3.5.3, 3.5.4); and (f) support for tourism, livelihoods and cultural activities ( [[#3.5.6|Section 3.5.6]] ). Global estimates of services provided by coastal blue carbon ecosystems depend on the quality of available mapping, which is currently best developed for mangroves ( [[#Macreadie--2019|Macreadie et al., 2019]] ), and improving for salt marshes and seagrasses ( [[#McOwen--2017|McOwen et al., 2017]] ; [[#McKenzie--2020|McKenzie et al., 2020]] ; [[#Young--2021|Young et al., 2021]] ).

'''Table Box 3.4.1 |''' Estimates of organic carbon storage and burial rates in mangroves, salt marshes and seagrass meadows a

{| class="wikitable"
|-
!
! Mangroves

! Salt marshes

! Seagrass meadows

|-
| Carbon stocks (MgC ha –1 )

| 856 ± 64.2 [79–2208] ( [[#Kauffman--2020|Kauffman et al., 2020]] )

| 317.2 ± 38.2 [27–1900] ( [[#Alongi--2018c|Alongi, 2018c]] )

| 139.7 [9.1–628] ( [[#Fourqurean--2012|Fourqurean et al., 2012]] ; [[#Alongi--2018d|Alongi, 2018d]] )

|-
| Carbon burial rate (g C m –2 yr –1 )

| 194 ± 30 [6.2–1722] ( [[#Wang--2020|Wang et al., 2020]] )

| 168 ± 14 [1.2–1167.5] ( [[#Wang--2020|Wang et al., 2020]] )

| 220.7 ± 40.2 [–2094 to 2124] ( [[#Alongi--2018d|Alongi, 2018d]] )

|-
| Global carbon burial rate (TgC yr –1 )

| 41 ( [[#Wang--2020|Wang et al., 2020]] )

| 12.63 ( [[#Wang--2020|Wang et al., 2020]] )

| 35.31 ( [[#Alongi--2020|Alongi, 2020]] )

|-
| Global areal coverage (Mha)

| 13.7 ( [[#Richards--2020|Richards et al., 2020]] )

| 5.5 ( [[#McOwen--2017|McOwen et al., 2017]] )

| 16 ( [[#McKenzie--2020|McKenzie et al., 2020]] )

|}

(a) Estimates are the mean ± 95% confidence interval, where available (indicating the ''extremely likely'' range) and range. Carbon stocks for mangroves include above- and below-ground storage up to 3 m depth (sampling period 2007–2017). The estimates for salt-marsh and seagrass stocks are soil stocks up to 1 m depth (observations spanning 1983–2016 for salt marshes and until 2016 for seagrass meadows). Date ranges for the burial rates are: 1989–2020, 1975–2020 and 1956–2016 for mangroves, salt marshes and seagrass meadows, respectively.

Coastal vegetated ecosystems are vulnerable to harm from multiple climate-induced and non-climate drivers, and together these have reduced wetland area globally ( ''high confidence'' ) ( [[#3.4.2.5|Section 3.4.2.5]] ) and endangered the services provided by these ecosystems ( ''high confidence'' ). Loss of coastal vegetated ecosystems changes biodiversity (Sections 3.5.2, 3.4.2.3–3.4.2.5; [[#Numbere--2019|Numbere, 2019]] ; [[#Parreira--2021|Parreira et al., 2021]] ), increases risk of damage and erosion from SLR and storms (Sections 3.4.2.3–3.4.2.5; Cross-Chapter Box SLR in Chapter 3; [[#Galeano--2017|Galeano et al., 2017]] ) and impacts provisioning (Sections 3.5.3–3.5.4; [[#Li--2018b|Li et al., 2018b]] ; [[#Maina--2021|Maina et al., 2021]] ). These changes also strongly determine the quantity and longevity of blue carbon storage ( ''high confidence'' ) ( [[#Macreadie--2019|Macreadie et al., 2019]] ; [[#Lovelock--2020|Lovelock and Reef, 2020]] ). Specific site characteristics and ecosystem responses to climate change will determine future local blue carbon storage or loss ( ''high confidence'' ) (see Table Box 3.4.2). For instance, poleward migration of mangroves to areas dominated by salt marshes is expected to increase carbon storage ( [[#Kelleway--2016|Kelleway et al., 2016]] ); however, this change in the dominant vegetation and associated faunal changes can modify carbon stocks and sequestration, as well as other ecosystem services ( [[#Martinetto--2016|Martinetto et al., 2016]] ; [[#Kelleway--2017|Kelleway et al., 2017]] ; [[#Smee--2017|Smee et al., 2017]] ; [[#Macreadie--2019|Macreadie et al., 2019]] ; [[#Macy--2019|Macy et al., 2019]] ). Landward range expansion of mangroves, marshes and seagrass in response to gradual RSLR can enhance carbon sequestration ( [[#3.4.2.5|Section 3.4.2.5]] ; Cross-Chapter Box SLR in Chapter 3; [[#Macreadie--2019|Macreadie et al., 2019]] ), but coastal squeeze can limit this ( [[#Phan--2015|Phan et al., 2015]] ; [[#Schuerch--2018|Schuerch et al., 2018]] ) and RSLR can either submerge and bury or erode and release stored blue carbon ( [[#3.4.2.5|Section 3.4.2.5]] ; [[#Macreadie--2019|Macreadie et al., 2019]] ; [[#Lovelock--2020|Lovelock and Reef, 2020]] ). Gains and losses of mangrove habitat area (and therefore carbon storage) projected for nations under RCP4.5 and RCP8.5 depend primarily on the combination of SLR rate, adaptation scenario (including coastal development) and island or continental status ( [[#Lovelock--2020|Lovelock and Reef, 2020]] ). The influence of warming, MHWs and acidification on seagrass meadows ( [[#Kendrick--2019|Kendrick et al., 2019]] ; [[#Strydom--2020|Strydom et al., 2020]] ), and associated coralligenous reefs ( [[#Zunino--2019|Zunino et al., 2019]] ), suggests that future warming and especially MHWs will cause more widespread loss of services from these ecosystems ( [[#3.4.2.5|Section 3.4.2.5]] ). Loss of blue carbon ecosystems will not only halt carbon storage but also release stored carbon: emissions after 2000 due to global mangrove deforestation have been estimated at 23.5–38.7 Tg Cyr –1 ( [[#Ouyang--2020|Ouyang and Lee, 2020]] ). Mitigation estimates for avoided conversion and restoration of coastal wetlands and the implications of the impacts of climate change are assessed in WGIII AR6 [[IPCC:Wg2:Chapter:Chapter-7#7.4|Section 7.4]] .

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Box 3.4

To date, initiatives aiming to restore coastal wetland ecosystems primarily address ecosystem characteristics other than carbon storage ( [[#Herr--2017|Herr et al., 2017]] ; [[#de%20los%20Santos--2019|de los Santos et al., 2019]] ; [[#Lovelock--2019|Lovelock and Duarte, 2019]] ; [[#Friess--2020a|Friess et al., 2020a]] ). But recovery of coastal vegetated ecosystems is expected to bring back the full suite of ecosystem services they provide, not just carbon storage ( ''medium confidence'' ) ( [[#Marbà--2015a|Marbà et al., 2015a]] ; [[#Burden--2019|Burden et al., 2019]] ; [[#Friess--2020a|Friess et al., 2020a]] ), making coastal restoration a low-risk action that offers both adaptation and mitigation benefits ( [[#Steven--2020|Steven et al., 2020]] ; [[#Gattuso--2021|Gattuso et al., 2021]] ). Successful restoration requires using appropriate plant species in suitable environmental settings ( [[#Wodehouse--2019|Wodehouse and Rayment, 2019]] ; [[#Friess--2020a|Friess et al., 2020a]] ) with favourable geomorphology and biophysical conditions ( [[#Cameron--2019|Cameron et al., 2019]] ; [[#Ochoa-Gómez--2019|Ochoa-Gómez et al., 2019]] ) and considering social, economic, policy and operational constraints ( [[#3.6.3.2.2|Section 3.6.3.2.2]] ; Cross-Chapter Box NATURAL in Chapter 2), now and in the future ( ''high confidence'' ) ( [[#Duarte--2020|Duarte et al., 2020]] ; [[#Lovelock--2020|Lovelock and Reef, 2020]] ). Nevertheless, restored spaces may not store carbon at rates equal to those of undisturbed spaces ( [[#Yang--2020|Yang et al., 2020]] ), and it may take decades to determine or achieve carbon-storage outcomes of restoration ( [[#Sasmito--2019|Sasmito et al., 2019]] ; [[#Duarte--2020|Duarte et al., 2020]] ; [[#Oreska--2020|Oreska et al., 2020]] ). Integration improves efforts to restore or conserve coastal wetland ecosystems to accomplish both adaptation and mitigation outcomes ( [[#Steven--2020|Steven et al., 2020]] ). Government-led conservation of blue carbon ecosystems as part of national and subnational climate strategies (e.g., [[#Friess--2020a|Friess et al., 2020a]] ; [[#Kelleway--2020|Kelleway et al., 2020]] ; [[#Wedding--2021|Wedding et al., 2021]] ) benefits from coordination with private activities, such as incentivising conservation with payments for ecosystem services ( [[#Muenzel--2018|Muenzel and Martino, 2018]] ; [[#Friess--2020a|Friess et al., 2020a]] ). Moreover, successful area-based protection measures consider both environmental and social issues ( [[#3.6.3.2|Section 3.6.3.2]] ). Continued integration and alignment of policies at international to local levels ( [[#3.6|Section 3.6.5]] ) will also support achieving the adaptation and mitigation benefit of blue carbon spaces ( [[#Friess--2020a|Friess et al., 2020a]] ; [[#Steven--2020|Steven et al., 2020]] ; [[#Wu--2020a|Wu et al., 2020a]] ).

'''Table Box 3.4.2 |''' Examples of vegetated blue carbon ecosystem carbon-storage gains and losses in response to climate-induced drivers, and key actions contributing to maintained and/or increased carbon storage a

{| class="wikitable"
|-
!
! Mangroves

! Salt marshes

! Seagrasses

|-
| ''Sea level rise''

|
|-
| Landward expansion by vegetation

| +C

| +C

| +C

|-
| Coastal squeeze

| −C

| −C

| −C

|-
| Loss of low-lying or submerged land or vegetation

| −C

| −C

| −C

|-
| Human adaptation to increase accommodation space

| +C

| +C

|
|-
| ''Extreme storms''

|
|-
| Erosion, loss of area, subsidence

| −C

| −C

| 0 to −C

|-
| Enhanced sedimentation

| +C

| +C

| +C

|-
| Vegetation damage and mortality

| −C to +C

|
| −C

|-
| ''Warming''

|
|-
| Increased productivity

| +C

|
| +C

|-
| Vegetation mortality

|
| −C

|-
| Increased decomposition of soil

| −C

| −C to +C

|
|-
| Poleward expansion of mangroves

| +C

| −C

|
|-
| Poleward expansion of seagrasses

|
| +C

|-
| Poleward expansion of bioturbators

| ∆C

|
|-
| Change in dominant species

| ∆C

|
|-
| ''Rising concentrations of atmospheric CO'' 2

|
|-
| Increased productivity of some species

| ∆C

| ∆C

| +C

|-
| Biodiversity loss

|
| −C

|-
| ''Altered precipitation''

|
|-
| Vegetation mortality

| −C

|
|-
| Reduced productivity

| −C

| −C

|
|-
| Increased productivity

| +C

|
| +C

|-
| Increased remineralisation

| −C

| −C

|
|-
| Low-salinity events

|
| 0 to −C

|-
| ''Key actions to sustain blue carbon storage''

|
|-
| Protect ecosystems

| X

| X

| X

|-
| Develop alternative livelihoods

| X

|
|-
| Provide space for landward migration

| X

| X

|
|-
| Restore hydrological connections

| X

| X

|
|-
| Maintain or restore sediment supply

| X

| X

|
|-
| Restore ecosystems

| X

|
| X

|-
| Plant indigenous species

|
| X

|
|-
| Reduce nutrient inputs

|
| X

|}

(a) ‘+C’ indicates potential positive effects on blue carbon stocks, ‘−C’ indicates potential negative effects, ‘0’ indicates no effects and ‘∆C’ indicates positive potential or negative effects. Effects on carbon stocks are from [[#3.4.2.5|Section 3.4.2.5]] , [[#Macreadie--2019|Macreadie et al. (2019)]] , [[#Lovelock--2020|Lovelock and Reef (2020)]] and [[#Wang--2020|Wang et al. (2020)]] . Key actions to sustain blue carbon storage are from [[#Duarte--2020|Duarte et al. (2020)]] and [[#Wedding--2021|Wedding et al. (2021)]] .

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Box 3.4

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