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

==== 3.4.2.5 Vegetated Blue Carbon Ecosystems ====

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Mangroves, salt marshes and seagrass beds (wetland ecosystems) are considered ‘blue carbon’ ecosystems due to their capacity to accumulate and store organic-carbon rich sediments (see Box 3.4; [[#Macreadie--2019|Macreadie et al., 2019]] ; [[#Rogers--2019|Rogers et al., 2019]] ) and provide an extensive range of other ecosystem services (see Box 3.4). Because these ecosystems are often found within estuaries and along sheltered coastlines, they share vulnerabilities, climate-induced drivers (Table 3.7) and non-climate drivers with estuaries and coastal lagoons ( [[#3.4.2.4|Section 3.4.2.4]] ).

'''Table 3.7 |''' Summary of previous IPCC assessments of mangroves, salt marshes and seagrass beds

{| class="wikitable"
|-
! Observations

! Projections

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

|
|-
| Seagrasses occurring close to their upper thermal limits are already stressed by climate change ( ''high confidence'' ).

‘Increased CO 2 concentrations have increased seagrass photosynthetic rates by 20% ( ''limited evidence, high agreement'' ).’

| Climate change will drive ongoing declines in the extent of seagrasses in temperate waters ( ''medium confidence'' ) as well as poleward range expansions of seagrasses and mangroves, especially in the Northern Hemisphere ( ''high confidence'' ).

Beneficial effects of elevated CO 2 will increase seagrass productivity and carbon burial rates in salt marshes during the first half of the 21st century, but there is ''limited evidence'' that this will improve their survival or resistance to warming.

As a result, interactions between climate change and non-climate drivers will continue to cause declines in estuarine vegetated systems ( ''very high confidence'' ).

|-
|
|-
| ''SR15 ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] )''

|
|-
| Vegetated blue carbon systems were not assessed in this report.

| Intact wetland ecosystems can reduce the adverse impacts of rising sea levels and intensifying storms by protecting shorelines ( ''medium confidence'' ), and their degradation could reduce remaining carbon budgets by up to 100 GtCO 2 .

Under 1.5°C of warming, natural sedimentation rates are projected to outpace SLR ( ''medium confidence'' ), but ‘other feedbacks, such as landward migration of wetlands and the adaptation of infrastructure, remain important ( ''medium confidence'' ).’

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

|
|-
| Coastal ecosystems, including salt marshes, mangroves, vegetated dunes and sandy beaches, can build vertically and expand laterally in response to SLR, though this capacity varies across sites ( ''high confidence'' ). These ecosystems provide important services that include coastal protection and habitat for diverse biota. However, as a consequence of human actions that fragment wetland habitats and restrict landward migration, coastal ecosystems progressively lose their ability to adapt to climate-induced changes and provide ecosystem services, including acting as protective barriers ( ''high confidence'' ).’

Warming and SLR-driven salinisation of wetlands are causing shifts in the distribution of plant species inland and poleward. Examples include mangrove encroachment into subtropical salt marshes ( ''high confidence'' ) and contraction in extent of low-latitude seagrass meadows ( ''high confidence'' ).

Plants with low tolerance to flooding and extreme temperatures are particularly vulnerable, increasing the risk of extirpation ( ''medium confidence'' ).

Extreme-weather events, including heatwaves, droughts and storms, are causing mass mortalities and changes in community composition in coastal wetlands ( ''high confidence'' ).

Severe disturbance of wetlands or transitions among wetland community types can favour invasive species ( ''medium confidence'' ).

The degradation or loss of vegetated coastal ecosystems reduces carbon storage, with positive feedbacks to the climate system ( ''high confidence'' ).

| ‘Seagrass meadows ( ''high confidence'' ) [...] will face moderate to high risk at temperature above 1.5°C global sea surface warming.’

‘The transition from undetectable to moderate risk in salt marshes [...] takes place between 0.7°C–1.2°C of global sea surface warming ( ''medium/high confidence'' ), and between 0.9°C–1.8°C ( ''medium confidence'' ) in sandy beaches, estuaries and mangrove forests.’

‘The ecosystems at moderate to high risk under future emission scenarios are mangrove forests (transition from moderate to high risk at 2.5°C–2.7°C of global sea surface warming), estuaries and sandy beaches (2.3°C–3.0°C) and salt marshes (transition from moderate to high risk at 1.8°C–2.7°C and from high to very high risk at 3.0°C–3.4°C) ( ''medium confidence'' ).’

‘Global coastal wetlands will lose between 20–90% of their area depending on emissions scenario with impacts on their contributions to carbon sequestration and coastal protection ( ''high confidence'' ).’

Estuarine wetlands will remain resilient to modest rates of SLR where their sediment dynamics are unconstrained. But SLR and warming are projected to drive global loss of up to 90% of vegetated wetlands by the end of the century under the RCP8.5 ( ''medium confidence'' ), especially if landward migration and sediment supply are limited by human modification of shorelines and river flows ( ''medium confidence'' ).

‘Moreover, pervasive coastal squeeze and human-driven habitat deterioration will reduce the natural capacity of these ecosystems to adapt to climate impacts ( ''high confidence'' ).’

|}

Since AR5 and SROCC, syntheses have emphasised that the vulnerability of rooted wetland ecosystems to climate-induced drivers is exacerbated by non-climate drivers ( ''high confidence'' ) ( [[#Elliott--2019|Elliott et al., 2019]] ; [[#Ostrowski--2021|Ostrowski et al., 2021]] ; [[#Williamson--2021|Williamson and Guinder, 2021]] ) and climate variability ( ''high confidence'' ) ( [[#Day--2019|Day and Rybczyk, 2019]] ; [[#Kendrick--2019|Kendrick et al., 2019]] ; [[#Shields--2019|Shields et al., 2019]] ). Global rates of mangrove loss have been extensive but are slowing ( ''high confidence'' ) at least partially due to management interventions ( [[#Friess--2020b|Friess et al., 2020b]] ; [[#Goldberg--2020|Goldberg et al., 2020]] ). From 2000 to 2010 mangrove loss averaged 0.16% yr –1 , globally, but with greatest loss in Southeast Asia ( ''high confidence'' ) ( [[#Hamilton--2016|Hamilton and Casey, 2016]] ; [[#Friess--2019|Friess et al., 2019]] ; [[#Goldberg--2020|Goldberg et al., 2020]] ) and ubiquitous fragmentation leaving few mangroves intact (Bryan- [[#Brown--2020|Brown et al., 2020]] ). Salt-marsh ecosystems have also suffered extensive losses (up to 60% in places since the 1980s), especially in developed and rapidly developing countries ( ''medium confidence'' ) (Table 12.3; [[#Gu--2018|Gu et al., 2018]] ; [[#Stein--2020|Stein et al., 2020]] ). Similarly, 29% of seagrass meadows were lost from 1879–to 2006 due primarily to coastal development and degradation of water quality, with climate-change impacts escalating since 1990 ( ''medium confidence'' ) ( [[#Waycott--2009|Waycott et al., 2009]] ; [[#Sousa--2019|Sousa et al., 2019]] ; [[#Derolez--2020|Derolez et al., 2020]] ; [[#Green--2021a|Green et al., 2021a]] ). Local examples of habitat stability or growth (e.g., [[#de%20los%20Santos--2019|de los Santos et al., 2019]] ; [[#Laengner--2019|Laengner et al., 2019]] ; [[#Sousa--2019|Sousa et al., 2019]] ; [[#Suyadi--2019|Suyadi et al., 2019]] ; [[#Derolez--2020|Derolez et al., 2020]] ; [[#Goldberg--2020|Goldberg et al., 2020]] ; [[#McKenzie--2020|McKenzie and Yoshida, 2020]] ) indicate some resilience to climate change in the absence of non-climate drivers ( ''high confidence'' ). Nevertheless, previous declines have left wetland ecosystems more vulnerable to impacts from climate-induced drivers and non-climate drivers ( ''high confidence'' ) ( [[#Friess--2019|Friess et al., 2019]] ; [[#Williamson--2021|Williamson and Guinder, 2021]] ).

Warming and MHWs have affected the range, species composition and survival of some wetland ecosystems. Warming is allowing some, but not all ( [[#Rogers--2018|Rogers and Krauss, 2018]] ; [[#Saintilan--2018|Saintilan et al., 2018]] ), mangrove species to expand their ranges poleward ( ''high confidence'' ) ( [[#Friess--2019|Friess et al., 2019]] ; [[#Whitt--2020|Whitt et al., 2020]] ). This expansion can affect species interactions ( [[#Guo--2017|Guo et al., 2017]] ; [[#Friess--2019|Friess et al., 2019]] ), and enhance sediment accretion and carbon storage rates in some instances ( ''medium confidence'' ) ( [[#Guo--2017|Guo et al., 2017]] ; [[#Kelleway--2017|Kelleway et al., 2017]] ; [[#Chen--2018b|Chen et al., 2018b]] ; [[#Coldren--2019|Coldren et al., 2019]] ; [[#Raw--2019|Raw et al., 2019]] ). Drought, low sea levels and MHWs can cause significant die-offs among mangroves ( ''medium confidence'' ) ( [[#Lovelock--2017b|Lovelock et al., 2017b]] ; [[#Duke--2021|Duke et al., 2021]] ). Seagrasses are similarly vulnerable to warming ( ''high confidence'' ) ( [[#Repolho--2017|Repolho et al., 2017]] ; [[#Duarte--2018|Duarte et al., 2018]] ; [[#Jayathilake--2018|Jayathilake and Costello, 2018]] ; [[#Savva--2018|Savva et al., 2018]] ), which has been attributed as one cause of observed changes in distribution and community structure ( ''medium confidence'' ) ( [[#Hyndes--2016|Hyndes et al., 2016]] ; [[#Nowicki--2017|Nowicki et al., 2017]] ). MHWs, together with storm-driven turbidity and structural damage, can cause seagrass die-offs ( ''high confidence'' ) ( [[#Arias-Ortiz--2018|Arias-]] [[#Ortiz--2018|Ortiz et al., 2018]] ; [[#Kendrick--2019|Kendrick et al., 2019]] ; [[#Smale--2019|Smale et al., 2019]] ; [[#Strydom--2020|Strydom et al., 2020]] ), shifts to small, fast-growing species ( ''high confidence'' ) ( [[#Kendrick--2019|Kendrick et al., 2019]] ; [[#Shields--2019|Shields et al., 2019]] ; [[#Strydom--2020|Strydom et al., 2020]] ) and ecosystem collapse ( [[#Serrano--2021|Serrano et al., 2021]] ).

The sensitivity of salt marshes and mangroves to RSLR depends on whether they accrete inorganic sediment and/or organic material at rates equivalent to rising water levels ( ''very high confidence'' ) ( [[#Peteet--2018|Peteet et al., 2018]] ; [[#FitzGerald--2019|FitzGerald and Hughes, 2019]] ; [[#Friess--2019|Friess et al., 2019]] ; [[#Gonneea--2019|Gonneea et al., 2019]] ; [[#Leo--2019|Leo et al., 2019]] ; [[#Marx--2020|Marx et al., 2020]] ; [[#Saintilan--2020|Saintilan et al., 2020]] ). Otherwise, wetland ecosystems must migrate either inland or upstream, or face gradual submergence in deeper, increasingly saline water ( ''very high confidence'' ) ( [[#3.4.2.4|Section 3.4.2.4]] ; [[#Andres--2019|Andres et al., 2019]] ; [[#Jones--2019b|Jones et al., 2019b]] ; [[#Cohen--2020|Cohen et al., 2020]] ; [[#Mafi-Gholami--2020|Mafi-Gholami et al., 2020]] ; [[#Magolan--2020|Magolan and Halls, 2020]] ; [[#Sklar--2021|Sklar et al., 2021]] ). Ability to migrate depends on local topography, the positioning of anthropogenic infrastructure and structures placed to defend such infrastructure ( [[#Schuerch--2018|Schuerch et al., 2018]] ; [[#Fagherazzi--2020|Fagherazzi et al., 2020]] ; [[#Cahoon--2021|Cahoon et al., 2021]] ). Submergence drives changes in community structure ( ''high confidence'' ) ( [[#Jones--2019b|Jones et al., 2019b]] ; [[#Yu--2019|Yu et al., 2019]] ; [[#Douglass--2020|Douglass et al., 2020]] ; [[#Langston--2020|Langston et al., 2020]] ) and functioning ( ''high confidence'' ) ( [[#Charles--2019|Charles et al., 2019]] ; [[#Buffington--2020|Buffington et al., 2020]] ; [[#Stein--2020|Stein et al., 2020]] ), and will eventually lead to extirpation of the most sensitive vegetation ( ''medium confidence'' ) ( [[#Schepers--2017|Schepers et al., 2017]] ; [[#Scalpone--2020|Scalpone et al., 2020]] ) and associated animals ( ''low confidence'' ) ( [[#Rosencranz--2018|Rosencranz et al., 2018]] ). The impacts of storms on wetlands are variable and described in SM3.3.1.

'''Table 3.8 |''' Estimates of vulnerability of coastal wetlands to sea level rise (SLR) on the basis of sediment cores

{| class="wikitable"
|-
! Region

! Habitat

! Reference

! colspan="2"| Rates of SLR at which habitat loss is

! colspan="2"| WGI AR6 Table 9.9 median estimate (and ''likely'' range) of SLR ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] )

|-
!
! ''Likely''

! ''Very likely''

! 2040–2060

! 2080–2100

|-
| Global

| Mangrove

| [[#Saintilan--2020|Saintilan et al. (2020)]]

| 4.2 a

| 6.1

| SSP1-1.9:

4.2 (2.9–6.1) mm yr –1

| 4.3 (2.5–6.6) mm yr –1

|-
| Southeastern USA

| Salt marsh

| [[#Törnqvist--2020|Törnqvist et al. (2020)]]

| 3.5 b

| 4.2 b

| SSP5-8.5:

7.3 (5.7–9.8) mm yr –1

| 12.2 (8.8–17.7) mm yr –1

|-
| UK

| Salt marsh

| [[#Horton--2018|Horton et al. (2018)]]

| 4.6 a

| 7.1 a

| colspan="2"|
|}

Notes:

(a) Estimate digitised from published figure

(b) Published figure digitised and remodelled as binomial generalised linear model (number drowned as compared with not drowned)

As noted in SROCC, given the diversity of coastal wetlands as well as the dependence of their future vulnerability to climate change on adaptation pathways ( [[#Krauss--2021|Krauss, 2021]] ; [[#Rogers--2021|Rogers, 2021]] ), projections of future impacts based on shoreline elevation estimated from satellite data and CMIP5 projections ( [[#Spencer--2016|Spencer et al., 2016]] ; [[#Schuerch--2018|Schuerch et al., 2018]] ) vary greatly. Although all approaches have individual strengths and weaknesses ( [[#Törnqvist--2021|Törnqvist et al., 2021]] ), paleorecords provide some clarity because they yield estimates of wetland responses to changes in climate in the absence of other anthropogenic drivers and are therefore inherently conservative. On the basis of paleorecords (Table 3.8), we assess that mangroves and salt marshes are ''likely'' at high risk from future SLR, even under SSP1-1.9, with impacts manifesting in the mid-term ( ''medium confidence'' ). Under SSP5-8.5, wetlands are ''very likely'' at high risk from SLR, with larger impacts manifesting before 2040 ( ''medium confidence'' ). By 2100, these ecosystems are at high risk of impacts under all scenarios except SSP1-1.9 ( ''high confidence'' ), with impacts most severe along coastlines with gently sloping shorelines, limited sediment inputs, small tidal ranges and limited space for inland migration ( ''very high confidence'' ) (Cross-Chapter Box SLR in Chapter 3; [[#Schuerch--2018|Schuerch et al., 2018]] ; [[#FitzGerald--2019|FitzGerald and Hughes, 2019]] ; [[#Leo--2019|Leo et al., 2019]] ; [[#Schuerch--2019|Schuerch et al., 2019]] ; [[#Raw--2020|Raw et al., 2020]] ; [[#Saintilan--2020|Saintilan et al., 2020]] ).

For seagrasses, recent projections for climate-change impacts vary by species and region. Warming is projected to increase the habitat available to ''Zostera marina'' on the east coast of the USA by 2100 but contract its southern range edge by 150–650 km under RCP2.6 and RCP8.5, respectively ( [[#Wilson--2019|]] [[#Wilson--2019|Wilson and Lotze, 2019]] ). Other species, such as ''Posidonia oceanica'' in the Mediterranean, might lose as much as 75% of their habitat by 2050 under RCP8.5 and become functionally extinct ( ''low confidence'' ) by 2100 ( [[#Chefaoui--2018|Chefaoui et al., 2018]] ). Observed impacts of MHWs ( [[#Kendrick--2019|Kendrick et al., 2019]] ; [[#Strydom--2020|Strydom et al., 2020]] ; [[#Serrano--2021|Serrano et al., 2021]] ) indicate that increasing intensity and frequency of MHWs ( [[#3.2.2.1|Section 3.2.2.1]] ) will have escalating impacts on seagrass ecosystems ( ''high confidence'' ). Habitat suitability can also be reduced by moderate RSLR, due to its impact on light attenuation ( ''medium confidence'' ) ( [[#Aoki--2020|Aoki et al., 2020]] ; [[#Ondiviela--2020|Ondiviela et al., 2020]] ; [[#Scalpone--2020|Scalpone et al., 2020]] ).

Overall, warming will drive range shifts in wetland species ( ''medium to high confidence'' ), but SLR poses the greatest risk for mangroves and salt marshes, with significant losses projected under all future scenarios by mid-century ( ''medium confidence'' ) and substantially greater losses by 2100 under all scenarios except SSP1-1.9 ( ''high confidence'' ). MHWs pose the greatest risk to seagrasses ( ''high confidence'' ). In all cases, losses will be greatest where accommodation space is constrained or where other non-climate drivers exacerbate risk from climate-induced drivers ( ''very high confidence'' ).

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