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

==== 5.4.1.2 Regulating Services ====

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Regulating services are those ecosystem functions, like climate regulation, that allow the environment to be in conditions conducive to human well-being and development (Costanza et al., 2017 <sup>[[#fn:r1304|1304]]</sup> ). AR5 WGII concluded that climate change will alter biological, chemical and physical processes in the ocean that provide feedback on the climate system through their effects on atmospheric composition ( ''high confidence'' ) (Pörtner et al., 2014 <sup>[[#fn:r1305|1305]]</sup> ). Sections 5.2 and 5.3 consider new evidence since AR5 regarding climate impacts on marine ecosystems and associated risks; their implications for regulating services are examined here.

A major regulating service provided by marine ecosystems is carbon sequestration. The observed net carbon uptake from the atmosphere to the global ocean varied between 1.0–2.5 GtC yr –1 between 2000 and 2012, with a ''very likely'' uptake of 30–38 Gt of anthropogenic C over the period 1994–2007 (Section 5.2.2.3, Gruber et al., 2019). Estimates of carbon sequestered in the deep ocean range from 0.4 GtC yr –1 (Rogers, 2015 <sup>[[#fn:r1306|1306]]</sup> ) to 1.6 GtC yr –1 (Armstrong et al., 2010) with the annual burial rate (permanent removal to sediment) around 0.2 GtC yr –1 (Armstrong et al., 2010 <sup>[[#fn:r1307|1307]]</sup> ).

Deep sea ecosystems also contribute to the removal of methane released from the beneath the seabed through microbial anaerobic oxidation and the sequestration of methane-derived carbon in carbonate (Marlow et al., 2014 <sup>[[#fn:r1309|1309]]</sup> ; Thurber et al., 2014 <sup>[[#fn:r1310|1310]]</sup> ). In coastal ecosystems, carbon is biologically sequestered in coastal sediments, commonly known as ‘blue carbon’ (Section 5.5.1). Tidal wetlands play disproportionately important roles in coastal carbon budgets, forming critical linkages between rivers, estuaries, and oceans (Najjar et al., 2018 <sup>[[#fn:r1311|1311]]</sup> ). Mean c arbon storage in the top meter of soil is estimated at 280 MgC ha –1 for mangroves, 250 MgC ha –1 for salt marshes, and 140 MgC ha –1 for seagrass meadows, with l ong-term rates of carbon accumulation in sediments of salt marshes, mangroves, and seagrasses ranging from 18–1713 gC m –2 yr –1 (Pendleton et al., 2012 <sup>[[#fn:r1312|1312]]</sup> ). These values are, however, highly variable (Section 5.5.1.2). The large space and time scales mean that there is a long time-lag between seafloor change and detectable changes in carbon sequestration. These large lags, in turn render assessment of climate impacts on regulatory services in the deep ocean having ''low confidence'' .

Under RCP2.6, CMIP5 ESMs project a reduced net ocean carbon uptake by 2080, to around 1.0 GtC yr –1 . Under RCP8.5, net ocean carbon uptake increases to a net sink of around 5.5 GtC yr –1 , but with variability between models (Lovenduski et al., 2016 <sup>[[#fn:r1313|1313]]</sup> ). Although the open ocean biological pump contributes only part of current carbon uptake (Boyd et al. 2019 <sup>[[#fn:r1314|1314]]</sup> ), the downward carbon flux at 1000 m is projected to decrease by 9–16% globally under RCP8.5 by 2100. A projected decrease in carbon sequestration in the North Atlantic by 27–41% has been estimated to represent a loss of 170‒3000 billion USD in abatement (mitigation) costs and 23–401 billion USD in social costs (Barange et al., 2017 <sup>[[#fn:r1315|1315]]</sup> ). Others have highlighted the declining value of open ocean carbon sequestration in the eastern tropical Pacific (Martin et al., 2016b <sup>[[#fn:r1316|1316]]</sup> ) and the Mediterranean (Melaku Canu et al., 2015 <sup>[[#fn:r1317|1317]]</sup> ). The open ocean therefore seems ''very likely'' to reduce its carbon uptake by the end of the 21st century, with the reduction ''very likely'' being greater under RCP8.5 than for RCP2.6; however, specific projections only have  ''medium confidence'' due to uncertainties associated with the structure of the models and with the future behaviour of the biological carbon pump (Section 5.2.2.3.1, 5.2.3) ''.''

Coastal blue carbon ecosystems provide climate regulatory services through their carbon removal and storage (Section 5.3.3). The current rates of loss of blue carbon ecosystems, partly due to climate change (Section 5.3) results in release of their stored CO 2 to the atmosphere (Section 5.5.1.2.2). However, increases in carbon sequestration are also possible; for example, temperature-driven displacement of salt marsh plants by mangrove trees may increase carbon uptake in coastal wetlands (Megonigal et al., 2016 <sup>[[#fn:r1318|1318]]</sup> ). Different rates of SLR may have opposite effects, with potential increases in net carbon uptake for slowly rising sea levels (assuming inland habitat migration is possible), but net carbon release for more rapid SLR (Figure 5.19). Such contrasting feedbacks between scenarios arise from the different responses of plant biomass, sediment accretion and inundation that control the overall response of vegetated coastal ecosystems to rising sea level (Gonneea et al., 2019 <sup>[[#fn:r1319|1319]]</sup> ). Thus, under high emission scenarios, SLR and warming are expected to reduce carbon sequestration by vegetated coastal ecosystems ( ''medium confidence'' ); however, under conditions of slow SLR, there may be net increase in carbon uptake by some coastal wetlands ( ''medium confidence'' )

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'''Figure 5.19'''

<span id="figure-5.19-biogeomorphic-climate-feedbacks-involving-plant-biomass-sediment-accretion-and-inundation-that-control-the-response-of-vegetated-coastal-ecosystems-to-rising-sea-levels.-a-under-high-rates-of-soil-formation-plants-are-able-to-offset-gradual-sea-level-rise-slr-and-may-produce-a-negative-feedback-by-increasing-the-uptake-of-atmospheric-co2."></span>
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'''Figure 5.19 | Biogeomorphic climate feedbacks involving plant biomass, sediment accretion and inundation that control the response of vegetated coastal ecosystems to rising sea levels. (A) Under high rates of soil formation plants are able to offset gradual sea level rise (SLR) and may produce a negative feedback by increasing the uptake of atmospheric CO2. […]'''
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Figure 5.19 | Biogeomorphic climate feedbacks involving plant biomass, sediment accretion and inundation that control the response of vegetated coastal ecosystems to rising sea levels. (A) Under high rates of soil formation plants are able to offset gradual sea level rise (SLR) and may produce a negative feedback by increasing the uptake of atmospheric CO2. In addition, below ground root production contributes to the formation of new soils and consolidates the seabed substrates. (B) Under low rate of soil formation, and when SLRs exceed critical thresholds, plants become severely stressed by inundation leading to less organic accretion and below ground subsidence and decay, producing a positive feedback by net CO2 outgassing. This figure does not consider landward movements, controlled by topography and human land-use.

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Coastal vegetation-rich ecosystems such as mangrove, coral reefs and salt marshes reduce storm impacts, protect the coastline from erosion, and help buffer the impacts of SLR, wave action and even moderate-sized tsunamis (Orth et al., 2006 <sup>[[#fn:r1320|1320]]</sup> ; Ferrario et al., 2014 <sup>[[#fn:r1321|1321]]</sup> ; Rao et al., 2015 <sup>[[#fn:r1322|1322]]</sup> ) (Section 5.5.2.2). Their loss or degradation under climate change (Sections 5.3) would therefore reduce the benefits of these regulatory services to coastal human communities (Perry et al., 2018 <sup>[[#fn:r1323|1323]]</sup> ), increasing the risk of damage and mortality from natural disasters (Rao et al., 2015 <sup>[[#fn:r1324|1324]]</sup> ) ( ''high confidence'' ). In some locations where climate-induced range expansion of coastal wetlands occurs, regulatory services such as storm protection and nutrient storage may be enhanced; however, the replacement of an existing ecosystem by others (e.g., salt marshes replaced by mangroves) may reduce habitat availability for fauna requiring specific vegetation structure and consequently other types of ecosystem services (Kelleway et al., 2017b <sup>[[#fn:r1325|1325]]</sup> ; Sheng and Zou, 2017 <sup>[[#fn:r1326|1326]]</sup> ).

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