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

=== 5.3.6 Kelp Forests ===

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Kelp forests are three-dimensional, highly productive coastal ecosystems with a reported global NPP between 1.02‒1.96 GtC yr –1 (Krause-Jensen and Duarte, 2016 <sup>[[#fn:r1151|1151]]</sup> ). They cover about 25% of the world’s coastline (Filbee-Dexter et al., 2016 <sup>[[#fn:r1152|1152]]</sup> ), mostly temperate and polar (Steneck et al., 2003 <sup>[[#fn:r1153|1153]]</sup> ). Canopy-forming macroalgae provide habitat for many associated invertebrates and fish communities (Pessarrodona et al., 2019 <sup>[[#fn:r1154|1154]]</sup> ). This assessment synthesises new evidence since SR15 on climate risks and impacts, and their interactions with non-climatic drivers on ecosystem biodiversity, structure and functioning.

Observational and experimental evidence since SR15 (Hoegh-Guldberg et al. 2018 <sup>[[#fn:r1155|1155]]</sup> ) supports that report’s conclusions that kelp forests are already experiencing large-scale changes, and that critical thresholds occur for some forests at 1.5°C of global warming ( ''high confidence'' ). Due to their low capacity to relocate and high sensitivity to warming, kelp forests are projected to experience higher frequency of mass mortality events as the exposure to extreme temperature rises ( ''very high confidence'' ). Moreover, changes in ocean currents have facilitated the entry of tropical herbivorous fish into temperate kelp forests decreasing their distribution and abundance ( ''medium confidence'' ). More evidence from model projections in the 21st century supports this observed range contraction of kelp forests at the warm end of their distributional margins and expansion at the poleward end with the rate being faster for high emission scenarios ( ''high confidence'' ).

New global estimates show that the abundance of kelp forests has decreased at a rate of ~2% per year over the past half century (Wernberg et al., 2019 <sup>[[#fn:r1156|1156]]</sup> ), mainly due to ocean warming and marine heat waves (e.g., in western Australia a mean loss of 43% in area followed a marine heat weave in summer 2010–2011 (Wernberg et al., 2016), Section 6.4.2.1), as well as from other human stressors ( ''high confidence'' ) (Filbee-Dexter and Wernberg, 2018 <sup>[[#fn:r1157|1157]]</sup> ). At some localities, human-driven environmental changes such as coastal eutrophication and pollution is causing severe deterioration of kelp forests adding to the loss of these ecosystems from warming, storms and heat weaves (Andersen et al., 2013 <sup>[[#fn:r1158|1158]]</sup> ; Filbee-Dexter and Wernberg, 2018 <sup>[[#fn:r1159|1159]]</sup> ).

Two global datasets and one dataset covering European coastlines (Araujo et al., 2016 <sup>[[#fn:r1160|1160]]</sup> ; Krumhans et al., 2016 <sup>[[#fn:r1161|1161]]</sup> ; Poloczanska et al., 2016 <sup>[[#fn:r1162|1162]]</sup> ) identify large local and regional variations in kelp abundance over the past half century with 38% of these ecoregions showing a decline, 27% an increase and 35% no change (Krumhans et al., 2016 <sup>[[#fn:r1163|1163]]</sup> ). These data reflect the high spatio-temporal variability and resilience of kelp forests (Reed et al., 2016 <sup>[[#fn:r1164|1164]]</sup> ; Wernberg et al., 2018 <sup>[[#fn:r1165|1165]]</sup> ). For example, a 34 year dataset of kelp canopy biomass along the California coastline does not yet show a significant response to global warming because this ecosystem responds to low frequency marine climate oscillations (Bell et al., 2018c <sup>[[#fn:r1166|1166]]</sup> ). However, between 1950‒2010 regional warming caused consistent negative responses in abundance, phenology, demography and calcification of macroalgae for the northeast Atlantic and southeast Indian Ocean (Poloczanska et al., 2016 <sup>[[#fn:r1167|1167]]</sup> ). Declines in kelp forest abundance attributed to climate change and not related to sea urchin overgrazing (which is a major driver of decline and regime shift; Ling et al. (2014)) have been documented since the 1970s and evidence has increased within the last two decades (Filbee-Dexter and Wernberg, 2018 <sup>[[#fn:r1168|1168]]</sup> ). Despite a lack of data from some regions such as South America (Pérez-Matus et al., 2017 <sup>[[#fn:r1169|1169]]</sup> ), observational evidence since SR15 supports with ''very'' ''high confidence'' that warming is driving a contraction of kelp forests at low latitudes (Franco et al., 2018b <sup>[[#fn:r1170|1170]]</sup> ; Casado-Amezúa et al., 2019 <sup>[[#fn:r1171|1171]]</sup> ; Pessarrodona et al., 2019 <sup>[[#fn:r1172|1172]]</sup> ) and expansion in polar regions ( ''medium confidence'' ) (Section 3.2.3.1.2) (Bartsch et al., 2016 <sup>[[#fn:r1173|1173]]</sup> ; Paar et al., 2016 <sup>[[#fn:r1174|1174]]</sup> ).

In many areas worldwide where the distribution range of kelp has contracted due to climatic and non-climatic drivers, it has been replaced by a less diverse and less complex turf-dominated ecosystem (Filbee-Dexter and Wernberg, 2018 <sup>[[#fn:r1175|1175]]</sup> ) ( ''high confidence'' ). Kelp supports other ecosystem components by providing food, substrate for spawning and habitat that mediate trophic interactions (O’Brien et al., 2018); its degradation therefore reduces species richness, biomass production and dependent flora and fauna species (Teagle and Smale, 2018 <sup>[[#fn:r1176|1176]]</sup> ; Pessarrodona et al., 2019 <sup>[[#fn:r1177|1177]]</sup> ). In the northeast Atlantic, the warm water species ''Laminaria ochroleuca'' is expanding poleward into regions previously dominated by the cold water species ''L. hyperborea'' which is retreating at its southern edge. These two kelp species are similar in morphology, but the cold water ''L. hyperborea'' hosts sessile communities of algae and invertebrates 12 times more diverse and richer in biomass than the warm water kelp species (Teagle and Smale, 2018 <sup>[[#fn:r1178|1178]]</sup> ). Climate-driven shifts in the species composition also affect carbon cycling, because warm-temperate kelps produce larger pools of organic matter than cold-temperate species, and their detritus is degraded faster (Pessarrodona et al., 2019 <sup>[[#fn:r1179|1179]]</sup> ).

New empirical eco-physiological studies in combination with field surveys support the evidence for climate change causing kelp forest degradation and range shifts (Franco et al., 2018b <sup>[[#fn:r1180|1180]]</sup> ; Wernberg et al., 2018 <sup>[[#fn:r1181|1181]]</sup> ). For example, interactive effects of ocean warming and acidification cause kelp degradation and disease-like symptoms, with detrimental effects on photosynthetic efficiency (Qiu et al., 2019 <sup>[[#fn:r1182|1182]]</sup> ). Enhanced herbivory due to warming and the establishment of herbivorous fish species in temperate kelp forest has been observed to enhance ecosystem degradation (Vergés et al., 2016 <sup>[[#fn:r1183|1183]]</sup> ). However, invader seaweed species driven by warming can create more complex trophic interactions, reducing the consumption by herbivorous gastropods (Miranda et al., 2019 <sup>[[#fn:r1184|1184]]</sup> ). Increased physical stress by storm events also alters the kelps community, affecting the recruitment time of kelp species. The resulting dominance of younger stages favors species with a year-round spore production or an opportunistic life strategy, reducing the kelp canopy (Pereira et al., 2017 <sup>[[#fn:r1185|1185]]</sup> ).

Projections of future distribution of kelp species based on their physiological thresholds show major species-specific range shifts under different emission scenarios. For example, under RCP2.6, laminaria and other canopy-forming seaweed species in the Northwest Atlantic are projected to show northward range shifts at their southern (warm) edge of ≤40 km, with some equatorial range expansion from 2050 to 2100. That northward range shift increases to 406 km under RCP8.5 (at 13–19 km per decade, including contractions of their warmer edges) (Wilson et al., 2019 <sup>[[#fn:r1186|1186]]</sup> ). Whilst no changes in species richness are projected under RCP2.6, more than 50% richness loss is projected under RCP8.5 in some areas (Wilson et al., 2019 <sup>[[#fn:r1187|1187]]</sup> ). Overall, model projections show that worldwide range contractions of kelps can be expected to continue at the warm end of distributional margins and range expansions at their poleward end ( ''high confidence'' ) (Raybaud et al., 2013 <sup>[[#fn:r1188|1188]]</sup> ; Assis et al., 2016 <sup>[[#fn:r1189|1189]]</sup> ; Assis et al., 2018 <sup>[[#fn:r1190|1190]]</sup> ; Wilson et al., 2019 <sup>[[#fn:r1191|1191]]</sup> ).

In summary, kelp forests have experienced large-scale habitat loss and degradation of ecosystem structure and functioning over the past half century, implying a moderate to high level of risk at present conditions of global warming ( ''high confidence'' ) (Section 5.3.7). The loss of kelp forests is followed by the colonisation of turfs, which contributes to the reduction in habitat complexity, carbon storage and diversity ( ''high confidence'' ). Kelp ecosystems are expected to continue to decline in temperate regions driven by ocean warming and intensification of extreme climate events ( ''high confidence'' ). The level of risk for the ecosystem is projected to rise to very high under RCP8.5 scenario by 2100 ( ''high confidence'' )

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