Editing IPCC:AR6/WGII/Chapter-3 (section)

==== 3.4.2.3 Kelp Ecosystems ====

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Kelp are temperate, habitat-forming marine macroalgae or seaweeds, mostly of the order ''Laminariales'' , which extend across one-quarter of the world’s coastlines ( [[#Assis--2020|Assis et al., 2020]] ; [[#Jayathilake--2020|Jayathilake and Costello, 2020]] ). The perennial species form dense underwater forest canopies and three-dimensional habitat that provides refuge for fish, crustaceans, invertebrates and marine mammals ( [[#Filbee-Dexter--2016|Filbee-Dexter et al., 2016]] ; [[#Wernberg--2019|Wernberg et al., 2019]] ). Kelp ecosystems support fisheries, aquaculture, fertiliser and food provision, including for local and Indigenous Peoples, along with regulating services in the form of wave attenuation and habitat provision. Kelp aquaculture can also buffer against local acidification ( [[#Xiao--2021|Xiao et al., 2021]] ) and contribute to carbon storage ( [[#Froehlich--2019|Froehlich et al., 2019]] ).

Recent research ( [[#Straub--2019|Straub et al., 2019]] ; [[#Butler--2020|Butler et al., 2020]] ; [[#Filbee-Dexter--2020b|Filbee-Dexter et al., 2020b]] ; [[#Tait--2021|Tait et al., 2021]] ) supports the findings of previous assessments (Table 3.5) that kelp and other seaweeds in most regions are undergoing mass mortalities from high temperature extremes and range shifts from warming ( ''very high confidence'' ). Kelp are highly sensitive to the direct effect of high temperature on survival ( [[#Nepper-Davidsen--2019|Nepper-Davidsen et al., 2019]] ) and indirect impact of temperature on herbivorous species ( [[#Ling--2008|Ling, 2008]] ; [[#Vergés--2016|Vergés et al., 2016]] ), upwelling and nutrient availability ( [[#Carr--2015|Carr and Reed, 2015]] ; [[#Schiel--2015|Schiel and Foster, 2015]] ). Synergies between warming, storms, pollution and intensified herbivory (due to removal or loss of predators including sea stars and otters that constrain herbivory by fish and urchin populations) can also cause physiological stress and physical damage in kelp, reducing productivity and reproduction ( [[#Rogers-Bennett--2019|Rogers-Bennett and Catton, 2019]] ; [[#Beas-Luna--2020|Beas-Luna et al., 2020]] ; [[#McPherson--2021|McPherson et al., 2021]] ).

'''Table 3.5 |''' Summary of previous IPCC assessments of kelp ecosystems

{| class="wikitable"
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! Observations

! Projections

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| ''AR5 ( [[#Wong--2014|Wong et al., 2014]] )''

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| ‘Kelp forests have been reported to decline in temperate areas in both hemispheres, a loss involving climate change ( ''high confidence'' ). Decline in kelp populations attributed to ocean warming has been reported in southern Australia and the north coast of Spain.’

| ‘Kelp ecosystems will decline with the increased frequency of heatwaves and sea temperature extremes as well as through the impact of invasive subtropical species ( ''high confidence'' ).’

‘Climate change will contribute to the continued decline in the extent of [...] kelps in the temperate zone ( ''medium confidence'' ) and the range of [...] kelps in the Northern Hemisphere will expand poleward ( ''high confidence'' ) ''.'' ’

|-
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| ''SR15 ( [[#Hoegh-Guldberg--2018a|Hoegh-Guldberg et al., 2018a]] )''

Observed movement of kelp ecosystems not assessed.

| ‘In the transition to 1.5°C of warming, changes to water temperatures will drive some species (e.g., plankton, fish) to relocate to higher latitudes and cause novel ecosystems to assemble ( ''high confidence'' ). Other ecosystems (e.g., kelp forests, coral reefs) are relatively less able to move, however, and are projected to experience high rates of mortality and loss ( ''very high confidence'' ).’

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| ''SROCC ( [[#Bindoff--2019a|Bindoff et al., 2019a]] )''

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| ‘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'' ).’

‘The abundance of kelp forests has decreased at a rate of ~2% per year over the past half century, mainly due to ocean warming and marine heat waves [...], as well as from other human stressors ( ''high confidence'' ).’

‘Changes in ocean currents have facilitated the entry of tropical herbivorous fish into temperate kelp forests decreasing their distribution and abundance ( ''medium confidence'' ).’

‘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 forests will face moderate to high risk at temperatures above 1.5°C global sea surface 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'' ).’

‘Changes in ocean currents have facilitated the entry of tropical herbivorous fish into temperate kelp forests decreasing their distribution and abundance ( ''medium confidence'' ).’

‘Kelp forests at low latitudes [...] will continue to retreat as a result of intensified extreme temperatures, and their low dispersal ability will elevate the risk of local extinction under RCP8.5 ( ''high confidence'' ).’

|}

Trends in kelp abundance since 1950 are uneven globally ( [[#Krumhansl--2016|Krumhansl et al., 2016]] ; [[#Wernberg--2019|Wernberg et al., 2019]] ), with population declines (e.g., giant kelp ''Macrocystis pyrifera'' in Tasmania, [[#Butler--2020|Butler et al., 2020]] ; and sugar kelp ''Saccharina latissima'' in the North Atlantic, [[#Filbee-Dexter--2020b|Filbee-Dexter et al., 2020b]] ) more common than increases or no change (e.g., giant kelp ''Macrocystis pyrifera'' in southern Chile; [[#Friedlander--2020|Friedlander et al., 2020]] ). Warming is driving range contraction and extirpation at the warm edge of species’ ranges and expansions at the cold range edge ( ''very high confidence'' ) ( [[#Smale--2019|Smale, 2019]] ; [[#Filbee-Dexter--2020b|Filbee-Dexter et al., 2020b]] ). Local declines in populations of kelp and other canopy-forming seaweeds driven by MHWs and other stressors have caused irreversible shifts to turf- or urchin-dominated ecosystems, with lower productivity and biodiversity ( ''high confidence'' ) ( [[#Filbee-Dexter--2014|Filbee-Dexter and Scheibling, 2014]] ; [[#Filbee-Dexter--2018|Filbee-Dexter and Wernberg, 2018]] ; [[#Rogers-Bennett--2019|Rogers-Bennett and Catton, 2019]] ; [[#Beas-Luna--2020|Beas-Luna et al., 2020]] ; Stuart- [[#Smith--2021|Smith et al., 2021]] ), ecosystems dominated by warm-affinity seaweeds or coral ( ''high confidence'' ) ( [[#Vergés--2019|Vergés et al., 2019]] ), and loss of genetic diversity ( [[#Coleman--2020a|Coleman et al., 2020a]] ; [[#Gurgel--2020|Gurgel et al., 2020]] ).

Species distribution models of kelp project range shifts and local extirpations with increasing levels of warming (Japan: [[#Takao--2015|Takao et al., 2015]] , [[#Sudo--2020|Sudo et al., 2020]] ; Australia: see Table 11.6, and [[#Assis--2018|Assis et al., 2018]] , [[#Martínez--2018|Martínez et al., 2018]] , [[#Castro--2020|Castro et al., 2020]] ; Europe: [[#de%20la%20Hoz--2019|de la Hoz et al., 2019]] ; North America: [[#Wilson--2019|Wilson et al., 2019]] ; South America: see Figure 12.3). There is ''high agreement'' on the direction but not the magnitude of change ( [[#Martínez--2018|Martínez et al., 2018]] ; [[#Castro--2020|Castro et al., 2020]] ), but effects of MHWs are not simulated. Where the length of higher-latitude coastlines is limited, range contractions are projected to occur, even with 2°C of global warming (i.e., SSP1-2.6) due to loss of habitat at the warm edge of species’ ranges ( [[#Martínez--2018|Martínez et al., 2018]] ). Poleward expansion of warm-affinity herbivores, including urchins, could further reduce warm-edge kelp populations ( [[#Castro--2020|Castro et al., 2020]] ; [[#Mulders--2020|Mulders and Wernberg, 2020]] ). Evidence from natural temperate CO 2 seeps suggests that ocean acidification at levels above those in RCP4.5 in 2100 could offset the increase in urchin abundance ( [[#Coni--2021|Coni et al., 2021]] ). Genetic analyses suggest that kelp populations at the midpoint of species’ ranges will have lower tolerance of warming than that implied by species distribution models, without assisted gene flow from warm-edge populations ( [[#King--2019|King et al., 2019]] ; [[#Wood--2021|Wood et al., 2021]] ).

While reducing non-climate drivers can help prevent kelp loss from warming and MHWs, there is limited potential for restoration of kelp ecosystems after transition to urchin-dominant ecosystems ( ''high confidence'' ). Current restoration efforts are generally small scale (&lt;0.1 km 2 ) and less advanced than those in ecosystems like coral reefs ( [[#Coleman--2020b|Coleman et al., 2020b]] ; [[#Eger--2020|Eger et al., 2020]] ; [[#Layton--2020|Layton et al., 2020]] ). Although abundance of herbivores limits kelp populations, there is ''limited evidence'' that restoring predators of herbivores by creating marine reserves, or directly removing grazing species, will increase kelp forest resilience to warming and extremes ( [[#Vergés--2019|Vergés et al., 2019]] ; [[#Wernberg--2019|Wernberg et al., 2019]] ). Active reseeding of wild kelp populations through transplantation and propagation of warm-tolerant genotypes ( [[#Coleman--2020b|Coleman et al., 2020b]] ; [[#Alsuwaiyan--2021|Alsuwaiyan et al., 2021]] ) can overcome low dispersal rates of many kelp species and facilitate effective restoration ( ''medium confidence'' ) ( [[#Morris--2020c|Morris et al., 2020c]] ).

Building on the conclusions of SROCC, this assessment finds that kelp ecosystems are expected to decline and undergo changes in community structure in the future due to warming and increasing frequency and intensity of MHWs ( ''high confidence'' ). Risk of loss of kelp ecosystems and shifts to turf- or urchin-dominated ecosystems are highest at the warm edge of species’ ranges ( ''high confidence'' ) and risks increase under RCP6.0 and RCP8.5 by the end of the century ( ''high confidence'' ).

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