Editing IPCC:AR6/WGI/Chapter-5 (section)

===== 5.6.2.2.2 Ocean-based biological CDR methods =====

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Both ocean biological and physical processes drive the CO <sub>2</sub> exchange between the ocean and atmosphere. However, the ocean physical processes that remove CO <sub>2</sub> from the atmosphere, such as large-scale circulation, cannot be feasibly altered, so ocean CDR methods focus on increasing the productivity of ocean ecosystems, and subsequent sequestration of carbon ( [[#GESAMP--2019|GESAMP, 2019]] ). There has been no change to the assessment of SROCC (SROCC [[#5.5.1|Section 5.5.1]] ): there is ''low confidence'' that nutrient addition to the open ocean, either through artificial ocean upwelling or iron fertilization, could contribute to climate change mitigation, due to its inconclusive effect on carbon sequestration and risks of adverse side effects on marine ecosystems (Figure 5.36, Table 5.9; Supplementary Materials Text 5.SM.3 and Table 5.SM.4; AR6 WGIII Chapter 12; [[#Gattuso--2018|Gattuso et al., 2018]] ; [[#Boyd--2019|Boyd and Vivian, 2019]] ; [[#Feng--2020|Feng et al., 2020]] ). In addition, ocean fertilization is currently prohibited by the LondonProtocol ( [[#Dixon--2014|Dixon et al., 2014]] ; [[#GESAMP--2019|GESAMP, 2019]] ).

Restoration of vegetated coastal ecosystems (sometimes referred to as ‘blue carbon’ – see Glossary) refers to the potential for increasing carbon sequestration by plant growth and burial of organic carbon in the soil of coastal wetlands (including salt marshes and mangroves) and seagrass ecosystems. Wider usage of the term blue carbon occurs in the literature, for example, including seaweeds (macroalgae), shelf sea sediments and open ocean carbon exchanges. However, such systems are less amenable to management, with many uncertainties relating to the permanence of their carbon stores ( [[#Windham-Myers--2018|Windham-Myers et al., 2018]] ; [[#Lovelock--2019|Lovelock and Duarte, 2019]] ; SROCC, [[#5.5.1.1|Section 5.5.1.1]] ).

Coastal wetlands and seagrass meadows store significant amounts of carbon and are among the most productive ecosystems per unit area ( [[#Griscom--2017|Griscom et al., 2017]] , 2020; [[#Ortega--2019|Ortega et al., 2019]] ; [[#Serrano--2019|Serrano et al., 2019]] ). These rates could be reduced in the future, since these habitats are vulnerable to changing conditions, such as temperature, salinity, sediment supply, storm severity and continued coastal development ( [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#NASEM--2019|NASEM, 2019]] ). These ecosystems are under threat from anthropogenic conversion and degradation and are being lost at rates between 0.7% and 7% per annum with consequent CO <sub>2</sub> emissions (e.g., [[#Atwood--2017|Atwood et al., 2017]] ; [[#Howard--2017|Howard et al., 2017]] ; [[#Hamilton--2018|Hamilton and Friess, 2018]] ; [[#Sasmito--2019|Sasmito et al., 2019]] ). Although sea level rise might lead to greater carbon sequestration in coastal wetlands ( [[#Rogers--2019|Rogers et al., 2019]] ), there is ''high confidence'' that the frequency and intensity of marine heatwaves will increase (Cross-Chapter Box 9.1; [[#Frölicher--2018|Frölicher and Laufkötter, 2018]] ; [[#Laufkötter--2020|Laufkötter et al., 2020]] ),which poses a more immediate threat to the integrity of coastal carbon stocks ( [[#Smale--2019|Smale et al., 2019]] ). Blue carbon restoration seeks to increase the rate of carbon sequestration, although restoration may be challenging, because of ongoing use of coastal land for human settlement, conversion to agriculture and aquaculture, shoreline hardening and port development.

Biogeochemical factors affecting reliable quantification of the climatic benefits of coastal vegetation include the variable production of CH <sub>4</sub> and N <sub>2</sub> O by such ecosystems ( [[#Adams--2012|Adams et al., 2012]] ; [[#Keller--2018|Keller, 2018]] ; [[#Rosentreter--2018|Rosentreter et al., 2018]] ), uncertainties regarding the provenance of the carbon that they accumulate ( [[#Macreadie--2019|Macreadie et al., 2019]] ), and the release of CO <sub>2</sub> by biogenic carbonate formation in seagrass ecosystems ( [[#Kennedy--2018|Kennedy et al., 2018]] ). While coastal habitat restoration potentially provides significant mitigation of national emissions for some countries ( [[#Taillardat--2018|Taillardat et al., 2018]] ; [[#Serrano--2019|Serrano et al., 2019]] ), the global sequestration potential of blue carbon approaches is &lt;0.02 PgC yr <sup>–1</sup> ( ''medium confidence'' ) (Figure 5.36; SROCC, [[#5.5.1.2|Section 5.5.1.2]] ; [[#Griscom--2017|Griscom et al., 2017]] ; [[#Gattuso--2018|Gattuso et al., 2018]] ; [[#GESAMP--2019|GESAMP, 2019]] ; [[#NASEM--2019|NASEM, 2019]] ).

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