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

===== 4.3.3.5.1 Tidal wetlands =====

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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> ).

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