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

==== 3.4.3.1 Biogeography and Species Range Shifts ====

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===== 3.4.3.1.1 Observed species range shifts =====

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Since previous assessments (Table 3.16), poleward range shifts have remained a ubiquitous response to climate change ( ''high confidence'' ), moving species from warmer regions into higher-latitude ecosystems ( [[#Fossheim--2015|Fossheim et al., 2015]] ; [[#Kumagai--2018|Kumagai et al., 2018]] ; [[#Burrows--2019|Burrows et al., 2019]] ; [[#Lenoir--2020|Lenoir et al., 2020]] ).

'''Table 3.16 |''' Summary of previous IPCC assessments of biogeography and species range shifts

{| class="wikitable"
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! Observations

! Projections

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| ''AR5 ( [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ; [[#Pörtner--2014|Pörtner et al., 2014]] )''

|
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| The distribution and abundance of many fishes and invertebrates have shifted poleward and/or to deeper, cooler waters ( ''high confidence'' ).

On average, species’ distributions have shifted poleward by 72.0 ± 0.35 km per decade ( ''high confidence'' ).

| Spatial shifts of marine species due to projected warming will cause high-latitude invasions and high local-extinction rates in the tropics and semi-enclosed seas ( ''medium confidence'' ).

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

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| ‘Ocean warming has contributed to observed changes in biogeography of organisms ranging from phytoplankton to marine mammals ( ''high confidence'' ).’

‘The direction of the majority of the shifts of epipelagic organisms are consistent with a response to warming ( ''high confidence'' )’ but are also shaped by oxygen concentrations and ocean currents across depth, latitudinal and longitudinal gradients ( ''high confidence'' ).

Geographic ranges have shifted since the 1950s by 51.5 ± 33.3 km per decade (mean and ''very likely'' range) and 29.0 ± 15.5 km per decade for organisms in the epipelagic and seafloor ecosystems, respectively.

| ‘Recent model projections since AR5 and SR15 continue to support global-scale range shifts of marine fishes at rates of tens to hundreds of km per decade in the 21st century, with rate of shifts being substantially higher under RCP8.5 than RCP2.6.’

|}

Thermal tolerances of epipelagic populations drive biogeographic change (Figures 3.10, 3.15), but the strength and direction of range shifts tend to be modulated by both climate-induced and non-climate drivers ( [[#Pinsky--2020b|Pinsky et al., 2020b]] ), including: (a) interactive effects of hypoxia and ocean acidification ( [[#Sampaio--2021|Sampaio et al., 2021]] ); (b) oceanic dispersal barriers ( [[#Choo--2021|Choo et al., 2021]] ), food and critical habitat availability ( [[#Alabia--2020|Alabia et al., 2020]] ; [[#Tanaka--2021|Tanaka et al., 2021]] ); (c) geographic position, including depth ( [[#Mardones--2021|Mardones et al., 2021]] ); and (d) ocean currents ( [[#Sunday--2015|Sunday et al., 2015]] ; [[#Chapman--2020|Chapman et al., 2020]] ; [[#Fuchs--2020|Fuchs et al., 2020]] ). The difference between physiological thermal tolerances ( [[#3.3.2|Section 3.3.2]] ) and local environmental conditions determines safety margins against future climate warming in ectotherms ( [[#Pinsky--2019|Pinsky et al., 2019]] ). Acclimation and evolution ( [[#3.3.4|Section 3.3.4]] ) and life-history stage ( [[#3.3.3|Section 3.3.3]] ) also alter species’ thermal tolerances. Biogeographic responses are further modulated by other interacting factors (Table 3.17).

'''Table 3.17 |''' Synthesis of selected processes conditioned by multiple environmental drivers that interact with warming to ultimately define range-shift responses

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! Factor

! Effect

! Example references

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| Evolution and acclimation

| Evolution of thermal tolerances and acclimation under local climatic conditions can increase resilience to future climate warming, slowing the loss of species at trailing (warm) range edges.

| [[#Palumbi--2014|Palumbi et al. (2014)]] ; [[#Miller--2020a|Miller et al. (2020a)]]

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| rowspan="4"| Marine heatwaves (MHWs)

| MHWs can influence the evolution of thermal tolerances by eliminating genotypes that are intolerant of elevated temperatures.

| [[#Buckley--2016|Buckley and Huey (2016)]] ; [[#Sunday--2019|Sunday et al. (2019)]]

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| MHWs can produce widespread die-offs of shallow-water benthic organisms triggering extensive contractions of their ranges.

| [[#Smale--2013|Smale and Wernberg (2013)]]

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| MHWs can facilitate range expansions by opening niches and/or enhancing recruitment of warm-affiliated species.

| [[#Leriorato--2019|Leriorato and Nakamura (2019)]] ; [[#Thomsen--2019|Thomsen et al. (2019)]] ; [[#Monaco--2021|Monaco et al. (2021)]]

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| Cold waves can halt or even reverse range expansions at leading edges.

| [[#Leriorato--2019|Leriorato and Nakamura (2019)]]

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| rowspan="3"| Ocean currents

| Ocean currents can influence range dynamics through their effect on dispersal, depending on their magnitude, direction and seasonal patterns.

| Hunt et al. (2016); [[#Kumagai--2018|Kumagai et al. (2018)]] ; [[#Fuchs--2020|Fuchs et al. (2020)]]

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| Where currents align with spatial gradients of warming, range expansions track thermal changes more closely. Conversely, directional mismatches result in consistently slower expansion rates and larger response lags, an effect more acute for benthic organisms relying on passive dispersion of larvae and propagules.

| García Molinos et al. (2017)

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| Rates of range contraction across taxa decreased (increased) under directional agreement (mismatch) with ocean currents, possibly associated with enhanced (reduced) flows of adaptive genes to warming in downstream (upstream) populations within the distributional range.

| García Molinos et al. (2017)

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| rowspan="2"| Climatic refugia

| Areas of locally stable climatic conditions, such as deeper waters or regions with internal tides or localised upwelling, can buffer the effects of regional warming, facilitating species persistence and conserving genetic diversity at rear-edge populations.

| [[#Smith--2014|Smith et al. (2014)]] ; [[#Assis--2016|Assis et al. (2016)]] ; [[#Lourenço--2016|Lourenço et al. (2016)]] ; [[#Wyatt--2020|Wyatt et al. (2020)]]

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| Distributional shifts into deeper, cooler habitats can offer an effective alternative response to latitudinal shifts, because sharper thermal gradients mean that vertical displacements, needed to compensate for the same amount of warming, are several orders of magnitude smaller than planar displacements.

| [[#Smith--2014|Smith et al. (2014)]] ; [[#Assis--2016|Assis et al. (2016)]] ; [[#Lourenço--2016|Lourenço et al. (2016)]]

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| rowspan="2"| Oxygen availability

| Oxygen supersaturation may extend ectotherm survival to extreme temperatures and increase thermal tolerances by compensating for the increasing metabolic demand at high temperatures.

| [[#Giomi--2019|Giomi et al. (2019)]]

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| Oxygen deprivation increases metabolic demand and respiration rates. Shallowing of oxygen-dead zones and subsequent hypoxic avoidance can render deep thermal refuges unsuitable for organisms.

| [[#Brown--2015|Brown and Thatje (2015)]] ; [[#Roman--2019|Roman et al. (2019)]] ; [[#Hughes--2020|Hughes et al. (2020)]]

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| Habitat availability and quality

| The availability and quality of habitat (underwater light conditions, adequate substrate, nutrient and food supply) set limits to the distribution of organisms and range-shift dynamics (e.g., resilience of populations to climate warming and the consolidation of range expansions).

| Krause- [[#Jensen--2019|Jensen et al. (2019)]] ; [[#Tamir--2019|Tamir et al. (2019)]]

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| rowspan="2"| Biotic interactions, including food availability

| Species interactions can confer resilience to warming by retarding habitat degradation and buffering the impacts of warming on organisms.

| [[#Falkenberg--2015|Falkenberg et al. (2015)]] ; [[#Giomi--2019|Giomi et al. (2019)]]

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| Changes in biotic interactions (e.g., altered predation rates, food availability, competition or trophic mismatches) induced by climate warming can modify range-shift dynamics.

| [[#Selden--2018|Selden et al. (2018)]] ; [[#Westerbom--2018|Westerbom et al. (2018)]] ; Figueira et al. (2019); Pinsky et al. (2020b); [[#Monaco--2021|Monaco et al. (2021)]]

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'''Figure 3.15 |''' '''Range-shift dynamics in marine ectotherms in response to climate warming.''' As the ocean warms, conditions at the edge of the species’ distribution may become warmer than the maximum thermal tolerance of the species (Figure 3.9), causing local populations to undergo a gradual decline in performance, a decreasing population size and ultimately their extirpation, resulting in a range contraction. Conversely, at the cool extreme of the distribution, habitats beyond the current range of the species will become thermally suitable in the future (i.e., within the species’ thermal tolerance range) and, providing the species can disperse to those locations, allow for the colonisation and consolidation of new populations and subsequent range expansion. These are processes conditioned by multiple drivers that interact with warming to ultimately define range-shift responses, some of which are described in Table 3.17. Note that physiological thermal tolerances relate to body temperatures of the organism rather than ambient temperatures.

A global meta-analysis of range shifts ( [[#Lenoir--2020|Lenoir et al., 2020]] ) that included data from 951 species (over half of which exhibited median range shifts consistent with climate change) estimates that marine species are moving poleward at a rate of 59.2 km per decade ( ''very likely'' range: 43.7–74.7 km per decade), closely matching the local climate velocity ( ''high confidence'' ). In some cases, warming-related distribution shifts were followed by density-dependent use of these areas, influencing associated fisheries ( [[#Baudron--2020|Baudron et al., 2020]] ), and in others, warming influenced competitive interactions: in the Arctic-Boreal Barents Sea, warming-induced increases in cod ( ''Gadus morhua'' ) abundance reduces haddock ( ''Melanogrammus aeglefinus'' ) abundance ( [[#Durant--2020|Durant et al., 2020]] ).

Biogeographic shifts lead to novel communities and biotic interactions ( ''high confidence'' ) ( [[#Zarco-Perello--2017|Zarco-Perello et al., 2017]] ; [[#Pecuchet--2020b|Pecuchet et al., 2020b]] ), with concomitant changes in ecosystem functioning and servicing ( ''high confidence'' ) ( [[#Vergés--2019|Vergés et al., 2019]] ; [[#Nagelkerken--2020|Nagelkerken et al., 2020]] ; [[#Peleg--2020|Peleg et al., 2020]] ). For instance, temperature-driven changes in distribution and abundance of copepods, the dominant zooplankton, were observed between 1960 and 2014 in the North Atlantic. These changes subsequently affect biogenic carbon cycling through alteration of microbial remineralisation and carbon sequestration in deep water ( ''medium confidence'' ) ( [[#3.4.3|Section 3.4.3.6]] ; [[#Pitois--2006|Pitois and Fox, 2006]] ; [[#Brun--2019|Brun et al., 2019]] ).

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===== 3.4.3.1.2 Observed vertical redistributions =====

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Epipelagic isotherms have recently (1980–2015) deepened at an average of 6.6 ± 18.8 m per decade ( [[#Pinsky--2019|Pinsky et al., 2019]] ), but there is ''low agreement'' on whether species move deeper in pursuit of thermal refuge. Prior studies suggested range shifts to depth ( [[#Dulvy--2008|Dulvy et al., 2008]] ; [[#Pinsky--2013|Pinsky et al., 2013]] ; [[#Yemane--2014|Yemane et al., 2014]] ), but increasing evidence suggests that fish and planktonic communities across large parts of the North Atlantic, sub-Arctic and northeast Pacific Ocean redistribute horizontally with horizontal climate velocity, except where vertical temperature gradients are particularly steep. There is ''low confidence'' for temperature-driven depth shifts in the epipelagic zone ( [[#Burrows--2019|Burrows et al., 2019]] ; [[#Campana--2020|Campana et al., 2020]] ; [[#Caves--2021|Caves and Johnsen, 2021]] ). At the same time, decreasing oxygen concentrations and the vertical expansion of OMZs have already decreased suitable habitat of pelagic fishes, including tuna and billfishes, by ~15% primarily due to vertical compression of environmental niches ( [[#Stramma--2012|Stramma et al., 2012]] ; [[#Deutsch--2015|Deutsch et al., 2015]] ).

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===== 3.4.3.1.3 Projected changes in species range shifts =====

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Continued changes in the biogeography of marine predators and prey are anticipated under future climate change, with climate velocity in the epipelagic zone during 2050–2100 under RCP8.5 projected to be sevenfold faster than that during 1955–2005 ( ''medium confidence'' ) (Figure 3.4; [[#Brito-Morales--2020|Brito-Morales et al., 2020]] ). This has substantial ecological implications, as projections suggest near elimination of overlaps between the distributions of certain predator–prey pairs in the northeast Atlantic Ocean when their current joint distributions (1989–2014) are compared with those projected (2037–2062) under RCP8.5 ( [[#Sadykova--2020|Sadykova et al., 2020]] ).

Deepening of epipelagic isotherms is projected to accelerate over 2006–2100 to rates of 8.5 m per decade under RCP4.5 and 32 m per decade under RCP8.5 ( [[#Jorda--2020|Jorda et al., 2020]] ). Although vertical redistribution of thermal niches is three to four orders of magnitude slower than horizontal displacement, maximum depth limits imposed by the seafloor and photic layer (both of which are projected to be reached in this century) will ''likely'' vertically compress suitable habitat for most marine organisms ( ''medium confidence'' ) ( [[#Dueri--2014|Dueri et al., 2014]] ; [[#Jorda--2020|Jorda et al., 2020]] ).

Projections from coupled biogeochemical and ecosystem models suggest a general decline in mesopelagic biomass ( [[#Lefort--2015|Lefort et al., 2015]] ), although this may vary among ocean basins. The volume of OMZs have been expanding at many locations ( ''high confidence'' ), and the oxygen content of the subsurface ocean is projected to decline to historically unprecedented conditions over the 21st century ( ''medium confidence'' ) ( [[#3.2.3|Section 3.2.3.2]] ; WGI AR6 [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.3.2]] ; [[#Canadell--2021|Canadell et al., 2021]] ) at a rate of 10–15 µM per decade in OMZs ( [[#3.2.3|Section 3.2.3.2]] ; [[#Breitburg--2018|Breitburg et al., 2018]] ). Oxygen availability and the effects of ocean acidification (Sections 3.3, 3.4.2) on zooplankton might become a dominant constraint in the upper ocean’s metabolic index, which is projected to decrease globally by 20% by 2100 ( [[#Deutsch--2015|Deutsch et al., 2015]] ; [[#Steinberg--2017|Steinberg and Landry, 2017]] ). In addition, extremely rapid acceleration of climate velocities projected in the mesopelagic under all emissions scenarios suggest that species in this ocean stratum will be even more exposed to future warming than species in the epipelagic (Figure 3.4; [[#Brito-Morales--2020|Brito-Morales et al., 2020]] ). But projections also suggest that warming-related increases in trophic efficiency lead to a 17% increase in the biomass of the deep-scattering layer (zooplankton and fish in the mesopelagic) by 2100 (low ''confidence'' ) ( [[#Bindoff--2019a|Bindoff et al., 2019a]] ). Observational studies appear to show that mesopelagic fishes adapted to warm water increased in abundance and distribution in the California Current associated with warming and the expansion of OMZ ( [[#Koslow--2019|Koslow et al., 2019]] ), suggesting that some mesopelagic fish stocks might be resilient to a changing climate ( ''medium confidence'' ).

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