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

===== 5.5.1.2.2 Coastal vegetation: mangrove, salt marsh and seagrass ecosystems =====

Mangrove, salt marsh and seagrass habitats are widely recognised as blue carbon ecosystems with mitigation potential (Chmura et al., 2003 <sup>[[#fn:r1668|1668]]</sup> ; Duarte et al., 2005 <sup>[[#fn:r1669|1669]]</sup> ; Kennedy et al., 2010 <sup>[[#fn:r1670|1670]]</sup> ; McLeod et al., 2011 <sup>[[#fn:r1671|1671]]</sup> ). Although covering only ~0.1% of the Earth’s surface, these three ecosystems together have been estimated to support 1–10% of global marine primary production (Duarte et al., 2017 <sup>[[#fn:r1672|1672]]</sup> ). More than 150 countries contain at least one of these ecosystems; 71 countries contain all three (Herr and Landis, 2016 <sup>[[#fn:r1673|1673]]</sup> ), and 74 countries mention such coastal wetlands (five specifically as blue carbon) in their Nationally Determined Contributions (NDCs) to the Paris Agreement (Martin et al., 2016a <sup>[[#fn:r1674|1674]]</sup> ; Gallo et al., 2017 <sup>[[#fn:r1675|1675]]</sup> ).

These three vegetated coastal habitats are characterised by high, yet variable, organic carbon storage in their soils and sediments on a per unit area basis ( ''high confidence'' ). In the humid tropics, mangrove below-ground organic carbon is typically 500–1000 tC ha –1 (Donato et al., 2011 <sup>[[#fn:r1676|1676]]</sup> ; Alongi and Mukhopadhyay, 2015 <sup>[[#fn:r1677|1677]]</sup> ; Howard et al., 2017 <sup>[[#fn:r1678|1678]]</sup> )), although only ~50 tC ha –1 in arid regions (Almahasheer et al., 2017 <sup>[[#fn:r1679|1679]]</sup> ). Australian salt marshes show particularly wide variation in organic carbon storage, ranging from 15–1000 tC ha –1 (top 1 m) with mean of 165 tC ha –1 (Kelleway et al., 2016 <sup>[[#fn:r1680|1680]]</sup> ; Macreadie et al., 2017b <sup>[[#fn:r1681|1681]]</sup> ). For seagrass meadows, storage values are typically 400–1600 tC ha –1 but can exceed 2000 tC ha –1 (Serrano et al., 2014 <sup>[[#fn:r1682|1682]]</sup> ). These accumulations have occurred over decadal to millennial time scales (McKee et al., 2007 <sup>[[#fn:r1683|1683]]</sup> ; Lo Iacono et al., 2008 <sup>[[#fn:r1684|1684]]</sup> ). Such blue carbon stock values are similar to freshwater wetlands and peat, but higher than for most forest soils (Laffoley and Grimsditch, 2009 <sup>[[#fn:r1685|1685]]</sup> ; Pan et al., 2011 <sup>[[#fn:r1686|1686]]</sup> ) ( ''high confidence'' ).

When vegetated coastal ecosystems are disturbed, a proportion of their stored carbon is released back to the atmosphere, along with other greenhouse gases (Marba and Duarte, 2009 <sup>[[#fn:r1686|1686]]</sup> ; Duarte et al., 2010 <sup>[[#fn:r1686|1686]]</sup> ; Pendleton et al., 2012 <sup>[[#fn:r1688|1688]]</sup> ; Lovelock et al., 2017 <sup>[[#fn:r1689|1689]]</sup> ). Globally, around 25–50% of vegetated coastal habitats have already been lost or degraded due to coastal agricultural developments, urbanisation and other human disturbance during the past 100 years (McLeod et al., 2011 <sup>[[#fn:r1690|1690]]</sup> ). The highest historical losses (60–90%) have occurred in Europe and China (Jickells et al., 2015 <sup>[[#fn:r1691|1691]]</sup> ; Gu et al., 2018 <sup>[[#fn:r1692|1692]]</sup> ; Li et al., 2018a <sup>[[#fn:r1693|1693]]</sup> ). Current losses are estimated at 0.2–3.0% yr -1 , depending on vegetation type and location (FAO et al., 2014; Alongi and Mukhopadhyay, 2015 <sup>[[#fn:r1694|1694]]</sup> ; Atwood et al., 2017 <sup>[[#fn:r1695|1695]]</sup> ) ( ''medium confidence'' ). Associated global carbon emissions are estimated at 0.04–0.28 GtC yr –1  (Pendleton et al., 2012 <sup>[[#fn:r1696|1696]]</sup> ); 0.06–0.61 GtC yr –1 (Howard et al., 2017 <sup>[[#fn:r1697|1697]]</sup> ); 0.10–1.46 GtC yr –1 (Lovelock et al., 2017 <sup>[[#fn:r1698|1698]]</sup> ); and 0.007 GtC yr –1 (mangroves only) (Taillardat et al., 2018 <sup>[[#fn:r1699|1699]]</sup> ). This range of values reflects uncertainties regarding the global rate of habitat loss, and the proportion of carbon remineralised to CO 2 .

Mitigation through emission reduction can therefore be achieved by habitat protection, to greatly reduce or end the human-driven loss of mangrove, salt marsh and seagrass ecosystems. Such action could potentially produce nationally-significant mitigation (&gt;1% of fossil fuel emissions) for several countries (Taillardat et al., 2018 <sup>[[#fn:r1700|1700]]</sup> ). However, there are still many uncertainties in quantifying carbon release due to habitat degradation and loss (Lovelock et al., 2017 <sup>[[#fn:r1701|1701]]</sup> ), and hence in determining emission reductions. Furthermore, this mitigation option is not available to those countries where habitat loss is not currently occurring, for example, in Bangladesh (Taillardat et al., 2018 <sup>[[#fn:r1702|1702]]</sup> ). Since legal structures already exist in many countries to protect coastal wetlands, the main policy need may be the enforcement of national regulation and site-specific MPAs (Miteva et al., 2015 <sup>[[#fn:r1703|1703]]</sup> ; Herr et al., 2017 <sup>[[#fn:r1704|1704]]</sup> ; Howard et al., 2017 <sup>[[#fn:r1705|1705]]</sup> ).

The alternative mitigation approach using coastal blue carbon ecosystems is to enhance the natural carbon uptake of such habitats, not only by increasing their spatial coverage through habitat restoration and new habitat creation, but also by taking management measures to maximise the carbon uptake and storage for existing coastal ecosystems. Such measures include reducing anthropogenic nutrient inputs and other pollutants; restoring hydrology, by removing barriers to tidal flow and sediment delivery; and reinstating predators (to reduce carbon loss caused by some bioturbators) (Macreadie et al., 2017a <sup>[[#fn:r1706|1706]]</sup> ). Per unit area of habitat created, restored or rehabilitated, such actions may offer high rates of carbon removal: widely-quoted values are 226 ± 39 gC m -1 yr -1 for mangroves, 218 ± 24 gC m -1 yr -1 for salt marsh and 138 ± 38 gC m -1 yr -1 for seagrass ecosystems (McLeod et al., 2011 <sup>[[#fn:r1707|1707]]</sup> ; Isensee et al., 2019 <sup>[[#fn:r1708|1708]]</sup> ).

Around 90 restoration and rehabilitation projects for mangroves have been documented (López-Portillo et al., 2017 <sup>[[#fn:r1709|1709]]</sup> ), with associated development of a range of restoration evaluation methods (Zhao et al., 2016a <sup>[[#fn:r1710|1710]]</sup> ). Salt marsh restoration is reviewed by Adam (2019) <sup>[[#fn:r1711|1711]]</sup> and seagrass restoration by van Katwijk et al. (2016). Consistent conclusions, supported by other studies (Bayraktarov et al., 2016 <sup>[[#fn:r1712|1712]]</sup> ; Wylie et al., 2016 <sup>[[#fn:r1713|1713]]</sup> ) are that: natural regeneration increases the likelihood of longterm survival; higher success rates are achieved with strong stakeholder engagement; and it is critical that the (human) factors causing original loss and degradation have been properly addressed ( ''high confidence'' ).

Quantification of the climatic benefits of such actions is, however, not straightforward. Measurements of carbon burial rates show high site-specific variability, being strongly affected by a wide range of environmental factors for mangroves (Adame et al., 2017 <sup>[[#fn:r1714|1714]]</sup> ; Schile et al., 2017 <sup>[[#fn:r1715|1715]]</sup> ), seagrasses (Lavery et al., 2013 <sup>[[#fn:r1716|1716]]</sup> ) and salt marshes (Kelleway et al., 2017b <sup>[[#fn:r1717|1717]]</sup> ). The reliable determination of sediment accumulation rates is a key consideration, with associated uncertainties not fully reflected in the McLeod et al. (2011) <sup>[[#fn:r1718|1718]]</sup> estimates given above. In particular, geochemical-based studies have indicated that seagrass carbon burial may have been greatly overestimated (Johannessen and Macdonald, 2016 <sup>[[#fn:r1719|1719]]</sup> ). These issues are contentious (Johannessen and Macdonald, 2018a <sup>[[#fn:r1720|1720]]</sup> ; Johannessen and Macdonald, 2018b <sup>[[#fn:r1721|1721]]</sup> ; Macreadie et al., 2018 <sup>[[#fn:r1722|1722]]</sup> ; Oreska et al., 2018 <sup>[[#fn:r1723|1723]]</sup> ); their scientific resolution is highly desirable. Additional complexities relating to the mitigation role of coastal blue carbon ecosystems include the following:

* Emissions of other greenhouse gases also need to be taken into account (Keller, 2019b <sup>[[#fn:r1724|1724]]</sup> ). Methane release from mangrove habitats can reduce the scale of their climatic benefits by 18–22% (Adams et al., 2012 <sup>[[#fn:r1725|1725]]</sup> ; Chen and Ganapin, 2016 <sup>[[#fn:r1726|1726]]</sup> ; Chmura et al., 2016 <sup>[[#fn:r1727|1727]]</sup> ; Rosentreter et al., 2018 <sup>[[#fn:r1728|1728]]</sup> ; Cameron et al., 2019 <sup>[[#fn:r1729|1729]]</sup> ) and nitrous oxide and methane together may offset salt marsh CO 2 uptake by 24–31% (Adams et al., 2012 <sup>[[#fn:r1730|1730]]</sup> ). Nitrous oxide emissions are strongly affected by nutrient loading (Chmura et al., 2016 <sup>[[#fn:r1731|1731]]</sup> ); under pristine conditions, mangroves can provide a sink rather than a source (Maher et al., 2016 <sup>[[#fn:r1732|1732]]</sup> ). Note that values of the ‘offset’ depend on the metrics used for determining CO 2 equivalents.
* Carbonate formation, releasing CO 2 , may also reduce the benefits of carbon storage by similar proportions (Howard et al., 2017 <sup>[[#fn:r1733|1733]]</sup> ; Macreadie et al., 2017a <sup>[[#fn:r1734|1734]]</sup> ; Kennedy et al., 2018 <sup>[[#fn:r1735|1735]]</sup> ; Saderne et al., 2019 <sup>[[#fn:r1736|1736]]</sup> ).
* Lateral transfers are not well-quantified. Whilst some of the carbon stored in coastal marine sediments may be recalcitrant carbon from terrestrial or atmospheric sources (and should therefore be excluded) (Chew and Gallagher, 2018 <sup>[[#fn:r1737|1737]]</sup> ), export of dissolved organic carbon, inorganic carbon and alkalinity may be considered as additional sequestration (Maher et al., 2018 <sup>[[#fn:r1738|1738]]</sup> ; Santos et al., 2019 <sup>[[#fn:r1739|1739]]</sup> ).
* The permanence of vegetated coastal systems, even if well-protected, cannot be assumed under future temperature regimes (Ward et al., 2016 <sup>[[#fn:r1740|1740]]</sup> ; Duke et al., 2017 <sup>[[#fn:r1741|1741]]</sup> ; Jennerjahn et al., 2017 <sup>[[#fn:r1742|1742]]</sup> ; Nowicki et al., 2017 <sup>[[#fn:r1743|1743]]</sup> )
* Responses to future SLR are also uncertain and complex (Kirwan and Megonigal, 2013 <sup>[[#fn:r1744|1744]]</sup> ; Spencer et al., 2016 <sup>[[#fn:r1745|1745]]</sup> ) . However, impacts are not necessarily negative: carbon sequestration capacity may increase where totally new habitats are created (Barnes, 2017 <sup>[[#fn:r1746|1746]]</sup> ), or if mangroves replace salt marshes (Kelleway et al., 2016 <sup>[[#fn:r1747|1747]]</sup> ).

In summary, a combination of both conservation and restoration of mangrove, salt marsh and seagrass habitats can contribute to national mitigation effort for those countries with relatively large coastlines where such ecosystems naturally occur (Murdiyarso et al., 2015 <sup>[[#fn:r1748|1748]]</sup> ; Atwood et al., 2017 <sup>[[#fn:r1749|1749]]</sup> ). However, the associated current uncertainties in quantifying relevant carbon storage and flows are expected to be problematic for reliable measurement, reporting and verification ( ''high confidence'' ).

At the global scale, synthesis studies have estimated the potential additional sequestration achieved by cost effective coastal blue carbon restoration as ~0.05 GtC yr –1 (Griscom et al., 2017 <sup>[[#fn:r1750|1750]]</sup> ) and 0.04 GtC yr –1 (National Academies of Sciences, Engineering, and Medicine, 2019), assuming that a relatively high proportion of vegetated ecosystems can be re-instated to their 1980–1990 extents. These values compare to current net anthropogenic emissions from all sources of 10.0 GtC yr –1 (Le Quéré et al., 2018), and are consistent with the ‘very low’ scores by (Gattuso et al., 2018) for the climate mitigation benefits of conserving and restoring coastal vegetation (Figure 5.23). Coastal ecosystem restoration could theoretically achieve higher sequestration, around ~0.2 GtC yr-1 (Griscom et al., 2017 <sup>[[#fn:r1752|1752]]</sup> ), but would be challenging, because of the semi-permanent and on-going nature of most coastal land-use change, such as human settlement, conversion to agriculture and aquaculture, shoreline hardening and port development (Gittman et al., 2015 <sup>[[#fn:r1753|1753]]</sup> ; Li et al., 2018a <sup>[[#fn:r1754|1754]]</sup> ).

Restoration costs could also be an important constraint for large-scale application. Based on published data from 246 observations, Bayraktarov et al. (2016) <sup>[[#fn:r1755|1755]]</sup> estimated median total costs for restoration of one hectare of mangrove, salt marsh and seagrass habitat to be ~2,508, 151,129 and 383,672 respectively, in 2010 USD. For each ecosystem, there was high variability in costs according to the economy of the country where the restoration projects were carried out, and the restoration technique applied. Assessment of coastal conservation and restoration costs is also given in Section 4.4.2.3, in Box 5.5 (in the context of coral reef restoration costs) and Section 5.5.2.5.

Measures to protect and restore coastal blue carbon habitats provide many other societal benefits in addition to climate regulation (Section 5.4.1). In particular, there is ''high confidence'' that coastal wetlands benefit local fisheries, enhance biodiversity, give storm protection, reduce coastal erosion, improve water quality and support local livelihoods (Costanza et al., 2008 <sup>[[#fn:r1756|1756]]</sup> ; Spalding et al., 2014 <sup>[[#fn:r1757|1757]]</sup> ). Coastal ecosystems may keep pace with sufficiently gradual SLR, and may be more cost-effective in flood protection than hard infrastructure like seawalls (Temmerman et al., 2013 <sup>[[#fn:r1758|1758]]</sup> ; Möller, 2019 <sup>[[#fn:r1759|1759]]</sup> ). Coastal blue carbon can therefore be considered as a ‘no regrets’ mitigation option at the national level in many countries, in addition to (not a replacement for) more effective mitigation measures. Additional research is needed over the full range of environmental conditions to improve knowledge and understanding of the complex carbon dynamics of coastal vegetation and associated systems, to enable well-quantified and cost-effective carbon sequestration enhancement (Vázquez-González et al., 2017 <sup>[[#fn:r1760|1760]]</sup> ; Windham-Myers et al., 2019 <sup>[[#fn:r1761|1761]]</sup> ).

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