Editing IPCC:AR6/WGIII/Chapter-7 (section)

==== 7.4.2.8 Reduce Conversion of Coastal Wetlands ====

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'''Activities, co-benefits, risks and implementation barriers.''' Reducing conversion of coastal wetlands, including mangroves, marshes and seagrass ecosystems, avoids emissions from above and below ground biomass and soil carbon through avoided degradation and/or loss. Coastal wetlands occur mainly in estuaries and deltas, areas that are often densely settled, with livelihoods closely linked to coastal ecosystems and resources ( [[#Moser--2012|Moser et al. 2012]] ). The carbon stocks of these highly productive ecosystems are sometimes referred to as ‘blue carbon’. Loss of existing stocks cannot be easily reversed over decadal time scales ( [[#Goldstein--2020|Goldstein et al. 2020]] ). The main drivers of conversion include intensive aquaculture, agriculture, salt ponds, urbanisation and infrastructure development, the extensive use of fertilisers, and extraction of water resources ( [[#Lovelock--2018|Lovelock et al. 2018]] ). Reduced conversion of coastal wetlands has many co-benefits, including biodiversity conservation, fisheries production, soil stabilisation, water flow and water quality regulation, flooding and storm surge prevention, and increased resilience to cyclones ( [[#Windham-Myers--2018|Windham-Myers et al. 2018]] a; [[#UNEP--2020|UNEP 2020]] ). Risks associated with the mitigation potential of coastal wetland conservation include uncertain permanence under future climate scenarios, including the effects of coastal squeeze, where coastal wetland area may be lost if upland area is not available for migration as sea levels rise ( [[#Lovelock--2020|Lovelock and Reef 2020]] ) (AR6 WGII, [[IPCC:Wg3:Chapter:Chapter-3#3.4.2|Section 3.4.2]] .5). Preservation of coastal wetlands also conflicts with other land use in the coastal zone, including aquaculture, agriculture, and human development; economic incentives are needed to prioritise wetland preservation over more profitable short-term land use. Integration of policies and efforts aimed at coastal climate mitigation, adaptation, biodiversity conservation, and fisheries, for example through integrated coastal zone management and marine spatial planning, will bundle climate mitigation with co-benefits and optimise outcomes ( [[#Herr--2017|Herr et al. 2017]] ).

'''Conclusions from AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL); mitigation potential, costs, and pathways.''' Coastal wetlands contain high, yet variable, organic carbon stocks, leading to a range of estimates of the global mitigation potential of reduced conversion. The SRCCL (Chapter 2) and SROCC (Chapter 5), report a technical mitigation potential of 0.15–5.35 GtCO 2 -eq yr –1 by 2050 ( [[#Pendleton--2012|Pendleton et al. 2012]] ; [[#Lovelock--2017|Lovelock et al. 2017]] ; [[#Howard--2017|Howard et al. 2017]] ; [[#Griscom--2017|Griscom et al. 2017]] ) '''.''' The mitigation potential is derived from quantification of losses of carbon stocks in vegetation and soil due to land conversion, shifts in GHG fluxes associated with land use, and alterations in net ecosystem productivity. The wide range in estimates mostly relate to the scope (all coastal ecosystems vs mangroves only) and different assumptions on decomposition rates. Loss rates of coastal wetlands have been estimated at 0.2–3% yr –1 , depending on the vegetation type and location ( [[#Atwood--2017|Atwood et al. 2017]] ; [[#Howard--2017|Howard et al. 2017]] ).

'''Developments since AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL).''' Global technical mitigation potential for conservation of coastal wetlands from recent literature have focused on protection of mangroves; estimates range from 0.06–2.25 GtCO 2 -eq yr –1 ( [[#Griscom--2020|Griscom et al. 2020]] ; [[#Bossio--2020|Bossio et al. 2020]] ) with 80% of the mitigation potential derived from improvements to soil carbon ( [[#Bossio--2020|Bossio et al. 2020]] ). Regional potentials ( [[#Roe--2021|Roe et al. 2021]] ) reflect mangrove protection; marsh and seagrass protection were not included due to lack of country-level data on marsh and seagrass distribution and conversion.

Global estimates show mangroves have the largest per hectare carbon stocks (see IPCC AR6 WGII Box 3.4 for estimates of carbon stocks, burial rates and ecosystem extent for coastal wetland ecosystems). Mean ecosystem carbon stock in mangroves is 3131 tCO 2 -eq ha –1 among the largest carbon stocks on Earth. Recent studies emphasise the variability in total ecosystem carbon stocks for each wetland type, based on species and climatic and edaphic conditions ( [[#Kauffman--2020|Kauffman et al. 2020]] ; [[#Bedulli--2020|Bedulli et al. 2020]] ; [[#Ricart--2020|Ricart et al. 2020]] ; Alongi et al. 2020; F. [[#Wang--2021|Wang et al. 2021]] ), and highlight the vulnerability of soil carbon below 1 m depth ( [[#Arifanti--2019|Arifanti et al. 2019]] ). Sea level strongly influences coastal wetland distribution, productivity, and sediment accretion; therefore, sea level rise will impact carbon accumulation and persistence of existing carbon stocks ( [[#Macreadie--2019|Macreadie et al. 2019]] ) (IPCC AR6 WGII Box 3.4).

Recent loss rates of mangroves are 0.16–0.39% yr –1 and are highest in South-East Asia ( [[#Hamilton--2016|Hamilton and Casey 2016]] ; [[#Friess--2019|Friess et al. 2019]] ; [[#Hamilton--2016|Hamilton and Casey 2016]] ). Assuming loss of soil carbon to 1 m depth after deforestation, avoiding mangrove conversion has the technical potential to mitigate approximately 23.5–38.7 MtCO 2 -eq yr –1 ( [[#Ouyang--2020|Ouyang and Lee 2020]] ); note, this potential is additional to reduced conversion of forests ( [[#Griscom--2020|Griscom et al. 2020]] ) ( [[#7.4.2.1|Section 7.4.2.1]] ). Regional estimates show that about 85% of mitigation potential for avoided mangrove conversion is in South-East Asia and Pacific (32 MtCO 2 -eq yr –1 at USD100 tCO 2 –1 ), 10% is in Latin American and the Caribbean (4 MtCO 2 -eq yr –1 ), and approximately 5% in other regions ( [[#Griscom--2020|Griscom et al. 2020]] ; [[#Roe--2021|Roe et al. 2021]] ).

Key uncertainties remain in mapping extent and conversion rates for salt marshes and seagrasses ( [[#McKenzie--2020|McKenzie et al. 2020]] ). Seagrass loss rates were estimated at 1–2% yr –1 ( [[#Dunic--2021|Dunic et al. 2021]] ) with stabilisation in some regions ( [[#de%20los%20Santos--2019|de los]] [[#Santos--2019|Santos et al. 2019]] ) (AR6 WGII, [[IPCC:Wg3:Chapter:Chapter-3#3.4.2|Section 3.4.2]] .5); however, loss occurs non-linearly and depends on site-specific context. Tidal marsh extent and conversion rates remains poorly estimated, outside of the USA, Europe, South Africa, and Australia ( [[#Mcowen--2017|Mcowen et al. 2017]] ; [[#Macreadie--2019|Macreadie et al. 2019]] ).

'''Critical assessment and conclusion.''' There is ''medium confidence'' that coastal wetland protection has a technical potential of 0.8 (0.06–5.4) GtCO 2 -eq yr –1 of which 0.17 (0.06–0.27) GtCO 2 -eq yr –1 is available up to USD100 tCO 2 –1 . There is a ''high certainty'' ( ''robust evidence'' , ''high agreement'' ) that coastal ecosystems have among the largest carbon stocks of any ecosystem. As these ecosystems provide many important services, reduced conversion of coastal wetlands is a valuable mitigation strategy with numerous co-benefits. However, the vulnerability of coastal wetlands to climatic and other anthropogenic stressors may limit the permanence of climate mitigation.

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