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

==== 5.5.1.1 Context for Blue Carbon and Overview Assessment ====

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There is political and scientific agreement on the need for a wide range of mitigation actions to avoid dangerous climate change (UNEP, 2017 <sup>[[#fn:r1616|1616]]</sup> ; IPCC, 2018 <sup>[[#fn:r1617|1617]]</sup> ). Opportunities to reduce emissions by the greater use of ocean renewable energy are identified in Section 5.4.2.3.2. Here, in accordance with the approved scoping of this report, the assessment of mitigation options is limited to the management of natural ocean processes, that is, requiring policy intervention, with a focus on ‘blue carbon’. Natural processes ''per se'' , although important to the climate system and the global carbon cycle, are not a mitigation response. Two management approaches are possible: first, actions to maintain the integrity of natural carbon stores, thereby decreasing their potential release of greenhouse gases, whether caused by human or climate-drivers; and second, through actions that enhance the longterm (century-scale) removal of greenhouse gases from the atmosphere by marine systems, primarily by biological means.

These mitigation approaches match those proposed using terrestrial natural processes (Griscom et al., 2017 <sup>[[#fn:r1618|1618]]</sup> ), with extensive afforestation and reforestation included in all climate models that limit future warming to 1.5⁰C (de Coninck et al., 2018). As on land, reliable carbon accounting is a critical consideration (Grassi et al., 2017 <sup>[[#fn:r1619|1619]]</sup> ), together with confidence in the longterm security of carbon storage. The feasibility of climatically-significant (and societally acceptable) mitigation using marine natural processes therefore depends on a robust quantitative understanding of how human actions can affect the uptake and release of greenhouse gases from different marine environments, interacting with natural biological, physical and chemical processes. Whilst CO 2 is the most important greenhouse gas, marine fluxes of methane and nitrous oxide can also be important, for both coastal regions and the open ocean (Arévalo-Martínez et al., 2015 <sup>[[#fn:r1620|1620]]</sup> ; Borges et al., 2016 <sup>[[#fn:r1621|1621]]</sup> ; Hamdan and Wickland, 2016 <sup>[[#fn:r1622|1622]]</sup> ).

The term ‘blue carbon’ was originally used to cover biological carbon in all marine ecosystems (Nellemann et al., 2009 <sup>[[#fn:r1623|1623]]</sup> ). Subsequent use of the term has focused on carbon-accumulating coastal habitats structured by rooted plants, such as mangroves, tidal salt marshes and seagrass meadows, that are relatively amenable to management (McLeod et al., 2011 <sup>[[#fn:r1624|1624]]</sup> ; Pendleton et al., 2012 <sup>[[#fn:r1625|1625]]</sup> ; Thomas, 2014 <sup>[[#fn:r1626|1626]]</sup> ; Macreadie et al., 2017a <sup>[[#fn:r1627|1627]]</sup> ; Alongi, 2018 <sup>[[#fn:r1628|1628]]</sup> ; Windham-Myers et al., 2019 <sup>[[#fn:r1629|1629]]</sup> ; Lovelock and Duarte, 2019 <sup>[[#fn:r1630|1630]]</sup> ). Comparisons across the full range of freshwater and saline wetland types are assisted by standardised approaches (Nahlik and Fennessy, 2016 <sup>[[#fn:r1631|1631]]</sup> ; Vázquez-González et al., 2017 <sup>[[#fn:r1632|1632]]</sup> ). Seaweeds (macroalgae) can also be considered as coastal blue carbon (Krause-Jensen and Duarte, 2016 <sup>[[#fn:r1633|1633]]</sup> ; Krause-Jensen et al., 2018 <sup>[[#fn:r1634|1634]]</sup> ; Raven, 2018 <sup>[[#fn:r1635|1635]]</sup> ), however, because of differences in their carbon processing, their climate mitigation potential is assessed separately within Section 5.5.1.2 below.

In the open ocean, the biological carbon pump is driven by the combination of photosynthesis by phytoplankton and downward transfer of particulate carbon by a variety of processes (Henson et al., 2010 <sup>[[#fn:r1636|1636]]</sup> ; DeVries et al., 2017 <sup>[[#fn:r1637|1637]]</sup> ); it results in large-scale transfer of around 10 GtC yr -1 carbon from near-surface waters to the ocean interior (Boyd et al., 2019 <sup>[[#fn:r1638|1638]]</sup> ). Most of this carbon is respired in the mesopelagic and contributes to the 37,000 GtC inventory of DIC, with around ~0.1 GtC yr -1 eventually being permanently removed in deep sea sediments (Cartapanis et al., 2018 <sup>[[#fn:r1639|1639]]</sup> ). In addition, the microbial carbon pump (Jiao et al., 2010 <sup>[[#fn:r1640|1640]]</sup> ) produces refractory dissolved organic molecules throughout the water column at a rate of around 0.4 GtC yr -1 (Jiao et al., 2014b <sup>[[#fn:r1641|1641]]</sup> ), which due to their residence time of hundreds to thousands of years maintain the 700 GtC inventory of dissolved organic carbon in the ocean (Jiao et al., 2010 <sup>[[#fn:r1642|1642]]</sup> ; Jiao et al., 2014a <sup>[[#fn:r1643|1643]]</sup> ; Legendre et al., 2015 <sup>[[#fn:r1644|1644]]</sup> ; Jiao et al., 2018a <sup>[[#fn:r1645|1645]]</sup> ). The natural removal of carbon by the various carbon pumps is closely balanced by upwelling and outgassing, with the ocean a moderate source of CO 2 under pre-industrial conditions (Ciais et al., 2013 <sup>[[#fn:r1646|1646]]</sup> ). The mitigation potential of managing natural processes in the open ocean is only briefly assessed here (Section 5.5.1.3).

Gattuso et al. (2018) <sup>[[#fn:r1647|1647]]</sup> provide an overview assessment of the environmental, technical and societal feasibilities of using a range of ocean management actions to reduce climate change and its impacts. Their results for nine actions based on natural processes are summarised in Figure 5.23, also including marine renewable energy (wind, wave and tidal) for comparison. Eight semi-quantitative criteria were used to assess each action: maximum potential effectiveness by 2100 in reducing climatic drivers (ocean warming, ocean acidification and SLR), assuming full theoretical implementation; technological readiness and lead time to full potential effectiveness (subsequently combined as technical feasibility); duration of benefits; co-benefits; trade-offs (originally described as dis-benefits); cost-effectiveness; and governability (capability of implementation, and management of any associated conflicts). Here, governability is considered as a constraint (governability challenges) reversing the scoring scale used by Gattuso et al. (2018) <sup>[[#fn:r1647|1647]]</sup>

Global measures (circles in Figure 5.23) can be regarded as mitigation, reducing drivers; local measures (rectangles), are primarily EbA, reducing impacts (Section 5.5.2), although they may also contribute to mitigation; two actions were considered at both scales. Gattuso et al. (2018) did not consider the effects of actions on ocean oxygenation, notwithstanding the importance of deoxygenation as a component of climate change. Additional detail is given in SM5.4.

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'''Figure 5.23'''

<span id="summary-of-potential-benefits-and-constraints-of-ocean-based-risk-reduction-options-using-natural-processes-from-literature-based-expert-assessments-by-gattuso-et-al.-2018.-mitigation-effectiveness-was-quantified-relative-to-representative-concentration-pathway-rcp8.5-assuming-maximum-theoretical-implementation-with-reduction-of-climate-related-drivers-considered-at-either-global-or-local-100-km2-scale-shown-as-circles-or-rectangles"></span>
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'''Summary of potential benefits and constraints of ocean-based risk-reduction options using natural processes, from literature-based expert assessments by Gattuso et al. (2018). Mitigation effectiveness was quantified relative to Representative Concentration Pathway (RCP)8.5, assuming maximum theoretical implementation, with reduction of climate-related drivers considered at either global or local (&lt;100 km2) scale, shown as circles or rectangles […]'''
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[[File:a072007cf5429aa42751d06d8f804f17 SROCC_Ch05_Figure5.23-scaled.jpg]]

Summary of potential benefits and constraints of ocean-based risk-reduction options using natural processes, from literature-based expert assessments by Gattuso et al. (2018). Mitigation effectiveness was quantified relative to Representative Concentration Pathway (RCP)8.5, assuming maximum theoretical implementation, with reduction of climate-related drivers considered at either global or local (&lt;100 km2) scale, shown as circles or rectangles respectively. Impact reduction, co-benefits and trade-offs are in the context of eight sensitive marine ecosystems and ecosystem services. ‘Technical issues to overcome’ is based on scores for technological readiness, lead time for full implementation and duration of effects. Cost is based on USD per tonne of CO2 either not released or removed from the atmosphere (for global measures) or per hectare of coastal area with action implemented (for local measures). ‘Governance challenges’ shows the potential difficulty of implementation by the international community. NA, not assessed. Additional information on scoring methods is given in SM5.4, Tables SM5.9a and SM5.9b.

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