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

==== 7.4.2.9 Coastal Wetland Restoration ====

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'''Activities, co-benefits, risks and implementation barriers.''' Coastal wetland restoration involves restoring degraded or damaged coastal wetlands including mangroves, salt marshes, and seagrass ecosystems, leading to sequestration of ‘blue carbon’ in wetland vegetation and soil (SRCCL, Chapter 6; SROCC, Chapter 5). Successful approaches to wetland restoration include: (i) passive restoration, the removal of anthropogenic activities that are causing degradation or preventing recovery; and (ii) active restoration, purposeful manipulations to the environment in order to achieve recovery to a naturally functioning system ( [[#Elliott--2016|Elliott et al. 2016]] ) (IPCC AR6 WGII Chapter 3). Restoration of coastal wetlands delivers many valuable co-benefits, including enhanced water quality, biodiversity, aesthetic values, fisheries production (food security), and protection from rising sea levels and storm impacts ( [[#Barbier--2011|Barbier et al. 2011]] ; [[#Hochard--2019|Hochard et al. 2019]] ; [[#Sun--2020|Sun and Carson 2020]] ; [[#Duarte--2020|Duarte et al. 2020]] ). Of the 0.3 Mkm 2 coastal wetlands globally, 0.11 Mkm 2 of mangroves are considered feasible for restoration ( [[#Griscom--2017|Griscom et al. 2017]] ). Risks associated with coastal wetland restoration include uncertain permanence under future climate scenarios (IPCC AR6 WGII, Box 3.4), partial offsets of mitigation through enhanced methane and nitrous oxide release and carbonate formation, and competition with other land uses, including aquaculture and human settlement and development in the coastal zone (SROCC, Chapter 5). To date, many coastal wetland restoration efforts do not succeed due to failure to address the drivers of degradation (van [[#Katwijk--2016|Katwijk et al. 2016]] ). However, improved frameworks for implementing and assessing coastal wetland restoration are emerging that emphasise the recovery of ecosystem functions ( [[#Zhao--2016|Zhao et al. 2016]] ; [[#Cadier--2020|Cadier et al. 2020]] ). Restoration projects that involve local communities at all stages and consider both biophysical and socio-political context are more likely to succeed ( [[#Brown--2014|Brown et al. 2014]] ; [[#Wylie--2016|Wylie et al. 2016]] ).

'''Conclusions from AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL); mitigation potential, costs, and pathways.''' The SRCCL reported that mangrove restoration has the technical potential to mitigate 0.07 GtCO 2 yr –1 through rewetting ( [[#Crooks--2011|Crooks et al. 2011]] ) and take up 0.02–0.84 GtCO 2 yr –1 from vegetation biomass and soil enhancement through 2030 ( ''medium confidence'' ) ( [[#Griscom--2017|Griscom et al. 2017]] ). The SROCC concluded that cost-effective coastal blue carbon restoration had a potential of about 0.15–0.18 GtCO 2 -eq yr –1 , a low global potential compared to other ocean-based solutions but with extensive co-benefits and limited adverse side effects ( [[#Gattuso--2018|Gattuso et al. 2018]] ).

'''Developments since AR5 and IPCC Special Reports (SR1.5, SROCC and SRCCL).''' Recent studies emphasise the time frame needed to achieve the full mitigation potential ( [[#Duarte--2020|Duarte et al. 2020]] ; [[#Taillardat--2020|Taillardat et al. 2020]] ). The first project-derived estimate of the net GHG benefit from seagrass restoration found 1.54 tCO 2 -eq (0.42 MgC) ha –1 yr –1 10 years after restoration began ( [[#Oreska--2020|Oreska et al. 2020]] ); comparable to the default emission factor in the Wetlands Supplement ( [[#Kennedy--2014|Kennedy et al. 2014]] ). Recent studies of rehabilitated mangroves also indicate that annual carbon sequestration rates in biomass and soils can return to natural levels within decades of restoration ( [[#Cameron--2019|Cameron et al. 2019]] ; [[#Sidik--2019|Sidik et al. 2019]] ). A meta-analysis shows increasing carbon sequestration rates over the first 15 years of mangrove restoration with rates stabilising at 25.7 ± 7.7 tCO 2 -eq (7.0 ± 2.1 MgC) ha –1 yr –1 through forty years, although success depends on climate, sediment type, and restoration methods ( [[#Sasmito--2019|Sasmito et al. 2019]] ). Overall, 30% of mangrove soil carbon stocks and 50–70% of marsh and seagrass carbon stocks are unlikely to recover within 30 years of restoration, underscoring the importance of preventing conversion of coastal wetlands ( [[#Goldstein--2020|Goldstein et al. 2020]] ) ( [[#7.4.2.8|Section 7.4.2.8]] ).

According to recent data, the technical mitigation potential for global coastal wetland restoration is 0.04–0.84 GtCO 2 -eq yr –1 ( [[#Griscom--2020|Griscom et al. 2020]] ; [[#Bossio--2020|Bossio et al. 2020]] ; [[#Roe--2021|Roe et al. 2021]] ) with 60% of the mitigation potential derived from improvements to soil carbon ( [[#Bossio--2020|Bossio et al. 2020]] ). Regional potentials based on country-level estimates from [[#Griscom--2020|Griscom et al. (2020)]] show the technical and economic (up to USD100 tCO 2 –1 ) potential of mangrove restoration; seagrass and marsh restoration was not included due to lack of country-level data on distribution and conversion (but see [[#McKenzie--2020|McKenzie et al. 2020]] for updates on global seagrass distribution). Although global potential is relatively moderate, mitigation can be quite significant for countries with extensive coastlines (e.g., Indonesia, Brazil) and for small island states where coastal wetlands have been shown to comprise 24–34% of their total national carbon stock ( [[#Donato--2012|Donato et al. 2012]] ). Furthermore, non-climatic co-benefits can strongly motivate coastal wetland restoration worldwide ( [[#UNEP--2021a|UNEP 2021a]] ). Major successes in both active and passive restoration of seagrasses have been documented in North America and Europe ( [[#Lefcheck--2018|Lefcheck et al. 2018]] ; [[#de%20los%20Santos--2019|de los]] [[#Santos--2019|Santos et al. 2019]] ; [[#Orth--2020|Orth et al. 2020]] ); passive restoration may also be feasible for mangroves ( [[#Cameron--2019|Cameron et al. 2019]] ).

There is high site-specific variation in carbon sequestration rates and uncertainties regarding the response to future climate change ( [[#Jennerjahn--2017|Jennerjahn et al. 2017]] ; [[#Nowicki--2017|Nowicki et al. 2017]] ) (IPCC AR6 WGII Box 3.4). Changes in distributions ( [[#Kelleway--2017|Kelleway et al. 2017]] ; [[#Wilson--2019|Wilson and Lotze 2019]] ) ''',''' methane release (Al-Haj and Fulweiler 2020), carbonate formation ( [[#Saderne--2019|Saderne et al. 2019]] ), and ecosystem responses to interactive climate stressors are not well-understood ( [[#Short--2016|Short et al. 2016]] ; Fitzgerald and Hughes 2019; [[#Lovelock--2020|Lovelock and Reef 2020]] ).

'''Critical assessment and conclusion.''' There is ''medium confidence'' that coastal wetland restoration has a technical potential of 0.3 (0.04–0.84) GtCO 2 -eq yr –1 of which 0.1 (0.05–0.2) GtCO 2 -eq yr –1 is available up to USD100 tCO 2 –1 . There is ''high confidence'' that coastal wetlands, especially mangroves, contain large carbon stocks relative to other ecosystems and ''medium confidence'' that restoration will reinstate pre-disturbance carbon sequestration rates. There is ''low confidence'' on the response of coastal wetlands to climate change; however, there is ''high confidence'' that coastal wetland restoration will provide a suite of valuable co-benefits.

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