Editing IPCC:AR6/SROCC/Chapter-4 (section)

==== 4.3.3.5 Ecosystems and Ecosystem Services ====

<div id="section-4-3-3-5ecosystems-and-ecosystem-services-block-1"></div>

<span id="tidal-wetlands"></span>
===== 4.3.3.5.1 Tidal wetlands =====

<div id="section-4-3-3-5ecosystems-and-ecosystem-services-block-2"></div>

Global coastal wetlands have been reduced by a half since the pre-industrial period due to the impacts of both climatic and non-climatic drivers such as flooding, coastal urbanisation, alterations in drainage and sediment supply. (Sections 4.3.2.3, 5.3.2). Potentially one of the most important of the eco-morphodynamic feedback allowing for relatively stable morphology under SLR is the ability of marsh and mangrove systems to enhance the trapping of sediment, which in turn allows tidal wetlands to grow and increase the production and accumulaGlobal coastal wetlands have been reduced by a half since the pre-industrial period due to the impacts of both climatic and non-climatic drivers such as flooding, coastal urbanisation, alterations in drainage and sediment supply. (Sections 4.3.2.3, 5.3.2). Potentially one of the most important of the eco-morphodynamic feedbacks allowing for relatively stable morphology under SLR is the ability of marsh and mangrove systems to enhance the trapping of sediment, which in turn allows tidal wetlands to grow and increase the production and accumulation of organic material (Kirwan and Megonigal, 2013 <sup>[[#fn:r1272|1272]]</sup> ). When ecosystem health is maintained and sufficient sediment exists to support their growth, this particular feedback has generally allowed marshes and mangrove systems to build vertically at rates equal to or greater than SLR up to the present day (Kirwan et al., 2016 <sup>[[#fn:r1273|1273]]</sup> ; Woodroffe et al., 2016 <sup>[[#fn:r1274|1274]]</sup> ).

While recent reviews suggest that mangroves’ surface accretion rate will keep pace with a high SLR scenario (RCP8.5) up to years 2055 and 2070 in fringe and basin mangrove settings, respectively (Sasmito et al., 2016 <sup>[[#fn:r1275|1275]]</sup> ), process-based models of vertical marsh growth that incorporate biological and physical feedbacks support survival under rates of SLR as high as 1–5 cm yr–1 before drowning (Kirwan et al., 2016 <sup>[[#fn:r1276|1276]]</sup> ). Threshold rates of SLR before marsh drowning however vary significantly from site-to-site and can be substantially lower than 1 cm yr–1 in micro-tidal regions where the tidal trapping of sediment is reduced and/or in areas with low sediment availability (Lovelock et al., 2015 <sup>[[#fn:r1277|1277]]</sup> ; Ganju et al., 2017 <sup>[[#fn:r1278|1278]]</sup> ; Jankowski et al., 2017 <sup>[[#fn:r1279|1279]]</sup> ; Watson et al., 2017 <sup>[[#fn:r1280|1280]]</sup> ). Global environmental change may also to lead to changes in growth rates, productivity and geographic distribution of different mangrove and marsh species, including the replacement of environmentally sensitive species by those possessing greater climatic tolerance (Krauss et al., 2014 <sup>[[#fn:r1281|1281]]</sup> ; Reef and Lovelock, 2014 <sup>[[#fn:r1282|1282]]</sup> ; Coldren et al., 2019 <sup>[[#fn:r1283|1283]]</sup> ). Processes impacting lateral changes at the marsh boundary including wave erosion are just as important, if not more, than vertical accretion rates in determining coastal wetland survival (e.g., Mariotti and Carr, 2014). For most low-lying coastlines, a seaward loss of wetland area due to marsh retreat could be offset by a similar landward migration of coastal wetlands (Kirwan and Megonigal, 2013 <sup>[[#fn:r1284|1284]]</sup> ; Schile et al., 2014 <sup>[[#fn:r1285|1285]]</sup> ), this landward migration having the potential to maintain and even increase the extent of coastal wetlands globally (Morris et al., 2012 <sup>[[#fn:r1286|1286]]</sup> ; Kirwan et al., 2016 <sup>[[#fn:r1287|1287]]</sup> ; Schuerch et al., 2018 <sup>[[#fn:r1288|1288]]</sup> ). This natural process will however be constrained in areas with steep topography or hard engineering structures (i.e., coastal squeeze, Section 4.3.2.4). Seawalls, levees and dams can also prevent the fluvial and marine transport of sediment to wetland areas and reduce their resilience further (Giosan, 2014 <sup>[[#fn:r1289|1289]]</sup> ; Tessler et al., 2015 <sup>[[#fn:r1290|1290]]</sup> ; Day et al., 2016 <sup>[[#fn:r1291|1291]]</sup> ; Spencer et al., 2016 <sup>[[#fn:r1292|1292]]</sup> ). When ecosystem health is maintained and sufficient sediment exists to support their growth, this particular feedback has generally allowed marshes and mangrove systems to build vertically at rates While recent reviews suggest that mangroes’ surface accretion rate will keep pace with a high SLR scenario (RCP8.5) up to years 2055 and 2070 in fringe and basin mangrove settings, respectively (Sasmito et al., 2016 <sup>[[#fn:r1275|1275]]</sup> ), process-based models of vertical marsh growth that incorporate biological and physical feedbacks support survival under rates of SLR as high as 1–5 cm yr–1 before drowning (Kirwan et al., 2016 <sup>[[#fn:r1276|1276]]</sup> ). Threshold rates of SLR before marsh drowning however vary significantly from site-to-site and can be substantially lower than 1 cm yr–1 in micro-tidal regions where the tidal trapping of sediment is reduced and/or in areas with low sediment availability (Lovelock et al., 2015 <sup>[[#fn:r1277|1277]]</sup> ; Ganju et al., 2017 <sup>[[#fn:r1278|1278]]</sup> ; Jankowski et al., 2017 <sup>[[#fn:r1279|1279]]</sup> ; Watson et al., 2017 <sup>[[#fn:r1280|1280]]</sup> ). Global environmental change may also to lead to changes in growth rates, productivity and geographic distribution of different mangrove and marsh species, including the replacement of environmentally sensitive species by those possessing greater climatic tolerance (Krauss et al., 2014 <sup>[[#fn:r1281|1281]]</sup> ; Reef and Lovelock, 2014 <sup>[[#fn:r1282|1282]]</sup> ; Coldren et al., 2019 <sup>[[#fn:r1283|1283]]</sup> ). Processes impacting lateral changes at the marsh boundary including wave erosion are just as important, if not more, than vertical accretion rates in determining coastal wetland survival (e.g., Mariotti and Carr, 2014). For most low-lying coastlines, a seaward loss of wetland area due to marsh retreat could be offset by a similar landward migration of coastal wetlands (Kirwan and Megonigal, 2013 <sup>[[#fn:r1284|1284]]</sup> ; Schile et al., 2014 <sup>[[#fn:r1285|1285]]</sup> ), this landward migration having the potential to maintain and even increase the extent of coastal wetlands globally (Morris et al., 2012 <sup>[[#fn:r1286|1286]]</sup> ; Kirwan et al., 2016 <sup>[[#fn:r1287|1287]]</sup> ; Schuerch et al., 2018 <sup>[[#fn:r1288|1288]]</sup> ). This natural process will however be constrained in areas with steep topography or hard engineering structures (i.e., coastal squeeze, Section 4.3.2.4). Seawalls, levees and dams can also prevent the fluvial and marine transport of sediment to wetland areas and reduce their resilience further (Giosan, 2014 <sup>[[#fn:r1289|1289]]</sup> ; Tessler et al., 2015 <sup>[[#fn:r1290|1290]]</sup> ; Day et al., 2016 <sup>[[#fn:r1291|1291]]</sup> ; Spencer et al., 2016 <sup>[[#fn:r1292|1292]]</sup> ).

<div id="section-4-3-3-5ecosystems-and-ecosystem-services-block-4"></div>

<span id="coral-reefs"></span>
===== 4.3.3.5.2 Coral reefs =====

<div id="section-4-3-3-5ecosystems-and-ecosystem-services-block-5"></div>

Coral reefs are considered to be the marine ecosystem most threatened by climate-related ocean change, especially ocean warming and acidification, even under an RCP2.6 scenario (Gattuso et al., 2015 <sup>[[#fn:r1293|1293]]</sup> ; Albright et al., 2018 <sup>[[#fn:r1294|1294]]</sup> ; Hoegh-Guldberg et al., 2018 <sup>[[#fn:r1295|1295]]</sup> ; Díaz et al., 2019 <sup>[[#fn:r1296|1296]]</sup> ; Section 5.3.4). AR5 concluded that ‘a number of coral reefs could […] keep up with the maximum rate of SLR of 15.1 mm yr–1 projected for the end of the century […] (medium confidence) [but a future net accretion rate lower] than during the Holocene (Perry et al., 2013 <sup>[[#fn:r1297|1297]]</sup> ) and increased turbidity (Storlazzi et al., 2011 <sup>[[#fn:r1298|1298]]</sup> ) will weaken this capability (very high confidence)’ (Wong et al., 2014: 379 <sup>[[#fn:r1299|1299]]</sup> ). Subsequently, some studies suggested that SLR may have negligible impacts on coral reefs’ vertical growth because the projected rate and magnitude of SLR by 2100 are within the potential accretion rates of most coral reefs (van Woesik et al., 2015 <sup>[[#fn:r1300|1300]]</sup> ). Other studies, however, stressed that the overall net vertical accretion of reefs may decrease after the first 30 years of rise in a 1.2 m SLR scenario (Hamylton et al., 2014 <sup>[[#fn:r1301|1301]]</sup> ), and that most reefs will not be able to keep up with SLR under RCP4.5 and beyond (Perry et al., 2018 <sup>[[#fn:r1302|1302]]</sup> ). The SR1.5 also concludes that coral reefs ‘are projected to decline by a further 70–90% at 1.5°C (high confidence) with larger losses (&gt;99%) at 2°C (very high confidence)’ (Hoegh-Guldberg et al., 2018: 10 <sup>[[#fn:r1303|1303]]</sup> ). A key point is that SLR will not act in isolation of other drivers. Cumulative impacts, including anthropogenic drivers, are estimated to reduce the ability of coral reefs to keep pace with future SLR (Hughes et al., 2017 <sup>[[#fn:r1304|1304]]</sup> ; Yates et al., 2017 <sup>[[#fn:r1305|1305]]</sup> ) and thereby reduce the capacity of reefs to provide sediments and protection to coastal areas. For example, the combination of reef erosion due to acidification and human-induced mechanical destruction is altering seafloor topography, increasing risks from SLR in carbonate sediment dominated regions (Yates et al., 2017 <sup>[[#fn:r1306|1306]]</sup> ). Both ocean acidification (Albright et al., 2018 <sup>[[#fn:r1307|1307]]</sup> ; Eyre et al., 2018 <sup>[[#fn:r1308|1308]]</sup> ) and ocean warming (Perry and Morgan, 2017 <sup>[[#fn:r1309|1309]]</sup> ) have been considered to slow future growth rates and reef accretion (Section 5.3.4). Recent literature also shows that alterations of coral reef 3D structure from changes in growth, breakage, disease or acidification can profoundly affect their ability to buffer waves impacts (through wave breaking and wave energy damping), and therefore keep-up with SLR (Yates et al., 2017 <sup>[[#fn:r1310|1310]]</sup> ; Harris et al., 2018 <sup>[[#fn:r1311|1311]]</sup> ). Such prospects contribute to raise concerns about the future ability of atoll islands to adjust naturally to SLR and persist (Section 4.3.3.3, Cross-Chapter Box 9). Another concern is that locally, even minimal SLR can increase turbidity on fringing reefs, reducing light and, therefore, photosynthesis and calcification. SLR-induced turbidity can be caused by increased coastal erosion and the transfer of sediment to nearby reefs and enhanced sediment resuspension (Field et al., 2011 <sup>[[#fn:r1312|1312]]</sup> ).

<div id="section-4-3-3-5ecosystems-and-ecosystem-services-block-6"></div>

<span id="seagrasses"></span>
===== 4.3.3.5.3 Seagrasses =====

<div id="section-4-3-3-5ecosystems-and-ecosystem-services-block-7"></div>

Due to their natural capacity to enhance accretion and in the absence of mechanical or chemical destruction by human activities, seagrasses are not expected to be severely affected by SLR, except indirectly through the increase of the impacts of extreme weather events and waves on coastal morphology (i.e., erosion) as well as through changes in light levels and through effects on adjacent ecosystems (Saunders et al., 2013 <sup>[[#fn:r1313|1313]]</sup> ). Extreme ﬂooding events have also been shown to cause large-scale losses of seagrass habitats (Bandeira and Gell, 2003 <sup>[[#fn:r1314|1314]]</sup> ), for example seagrasses in Queensland, Australia, were lost in a disastrous ﬂooding event (Campbell and McKenzie, 2004 <sup>[[#fn:r1315|1315]]</sup> ). Changes in ocean currents can have either positive or negative effects on seagrasses, creating new space for seagrasses to grow or eroding seagrass beds (Bjork et al., 2008 <sup>[[#fn:r1316|1316]]</sup> ). But overall, seagrass will primarily be negatively affected by the direct effects of increased sea temperature on growth rates and the occurrence of disease (Marba and Duarte, 2010 <sup>[[#fn:r1317|1317]]</sup> ; Burge et al., 2013 <sup>[[#fn:r1318|1318]]</sup> ; Koch et al., 2013 <sup>[[#fn:r1319|1319]]</sup> ; Thompson et al., 2015 <sup>[[#fn:r1320|1320]]</sup> ; Chefaoui et al., 2018 <sup>[[#fn:r1321|1321]]</sup> ; Gattuso et al., 2018 <sup>[[#fn:r1322|1322]]</sup> ; Section 5.3.2) as well as by heavy rains that may dilute the seawater to a lower salinity. Such impacts will be exacerbated by major causes of seagrass decline including coastal eutrophication, siltation and coastal development (Waycott et al., 2009 <sup>[[#fn:r1323|1323]]</sup> ). Noteworthy is that some positive impacts are expected, as ocean acidification is expected to benefit photosynthesis and growth rates of seagrass (Repolho et al., 2017 <sup>[[#fn:r1324|1324]]</sup> ).

<div id="section-4-3-3-5ecosystems-and-ecosystem-services-block-8"></div>

<span id="coastal-protection-by-coastal-and-marine-ecosystems"></span>
===== 4.3.3.5.4 Coastal protection by coastal and marine ecosystems =====

<div id="section-4-3-3-5ecosystems-and-ecosystem-services-block-9"></div>

Major ‘protection’ benefits derived from the above-mentioned coastal ecosystems include wave attenuation and shoreline stabilisation, for example, by coral reefs (Elliff and Silva, 2017 <sup>[[#fn:r1325|1325]]</sup> ; Siegle and Costa, 2017 <sup>[[#fn:r1326|1326]]</sup> ), mangroves (Zhang et al., 2012 <sup>[[#fn:r1327|1327]]</sup> ; Barbier, 2016 <sup>[[#fn:r1328|1328]]</sup> ; Menéndez et al., 2018 <sup>[[#fn:r1329|1329]]</sup> ) or salt marshes (Möller et al., 2014 <sup>[[#fn:r1330|1330]]</sup> ; Hu et al., 2015 <sup>[[#fn:r1331|1331]]</sup> ). Recently, a global meta-analysis of 69 studies demonstrated that, on average, these ecosystems together reduced wave heights between 35–71% at the limited locations considered (Narayan et al., 2016 <sup>[[#fn:r1332|1332]]</sup> ), with coral reefs, salt marshes, mangroves and seagrass/kelp beds reducing wave heights by 54–81%, 62–79%, 25–37% and 25–45% respectively (see Narayan et al., 2016 for map of locations considered). Additional studies suggest greater wave attenuation in mangrove systems (Horstman et al., 2014 <sup>[[#fn:r1333|1333]]</sup> ), and highlight broader complexities in wave attenuation related to total tidal wetland extent, water depth, and species. Global analyses show that natural and artificial seagrasses can attenuate wave height and energy by as much as 40% and 50%, respectively (Fonseca and Cahalan, 1992 <sup>[[#fn:r1334|1334]]</sup> ; John et al., 2015 <sup>[[#fn:r1335|1335]]</sup> ), while coral reefs have been observed to reduce total wave energy by 94–98% (n = 13; Ferrario et al., 2014 <sup>[[#fn:r1336|1336]]</sup> ) and wave driven flooding volume by 72% (Beetham et al., 2017 <sup>[[#fn:r1337|1337]]</sup> ). In addition, storm surge attenuation based on a recent literature review by Stark et al. (2015) <sup>[[#fn:r1338|1338]]</sup> range from -2–25 cm km <sup>–1</sup> length of marsh, where the negative value denotes actual amplification. Other ecosystems provide coastal protection, including macroalgae, oyster and mussel beds, and also beaches, dunes and barrier islands, but there is less understanding of the level of protection conferred by these other organisms and habitats (Spalding et al., 2014 <sup>[[#fn:r1339|1339]]</sup> ).

While there is little literature on the extent to which SLR specifically will affect coastal protection by coastal and marine ecosystems, it is estimated that SLR may reduce this ecosystem service ( ''limited evidence, high agreement'' ) through the above-described impacts on the ecosystems themselves, and in combination with the impacts of other climate-related changes to the ocean (e.g., ocean warming and acidification; Sections 5.3.1 to 5.3.6, 5.4.1). Wave attenuation by coral reefs, for example, is estimated to be negatively affected in the near future by changes in coral reefs’ structural complexity more than by SLR (Harris et al., 2018 <sup>[[#fn:r1340|1340]]</sup> ); changes in mean and ESL events will rather add a layer of stress. Beck et al. (2018) estimate that under RCP8.5 by 2100, a 1 m loss in coral reefs’ height will increase the global area flooded under a 100-year storm event by 116% compared to today, against +66% with no reef loss.

<div id="section-4-3-3-6human-activities"></div>

<span id="human-activities"></span>