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

==== 3.4.2.10 Polar Seas ====

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The polar seas cover ~20% of the global ocean and include the deep Arctic Ocean and surrounding shelf seas as well as the Southern Ocean south of the polar front. They play a significant role in ocean circulation and absorption of anthropogenic CO 2 ( [[#Meredith--2019|Meredith et al., 2019]] ). The Arctic is characterised by polar seas surrounded by land, while the Antarctic comprises continental Antarctica surrounded by the Southern Ocean. These high-latitude ecosystems share key properties, including strong seasonality in solar radiation and sea ice coverage. Sea ice regulates water-column physics, chemistry and biology, air–sea exchange and is a critical habitat for many species. In spring, when solar radiation returns and sea ice melts, intense phytoplankton blooms fuel food webs that include rich communities of both resident and summer-migrant species, with typically high dependency on a few key species for trophic transfer ( [[#Meredith--2019|Meredith et al., 2019]] ; [[#Rogers--2020|Rogers et al., 2020]] ). Over the past two decades, Arctic Ocean surface temperature has increased in line with the global average, while there has been no uniform warming across the Antarctic ( ''high confidence'' ) (WGI AR6 Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). Thus, the rate of change due to warming, and associated sea ice loss, is greater in the Arctic than in the Antarctic ( ''high confidence'' ) ( [[#3.2|Section 3.2]] ; Table 3.14; WGI AR6 Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). Both Arctic and Antarctic regions have a long history of living resource extraction, including some of the largest fisheries on the globe in terms of catches. However, only the Arctic hosts human populations, holding a rich Indigenous knowledge and local knowledge (IKLK) on these social–ecological systems (Cross-Chapter Paper 6; [[#Meredith--2019|Meredith et al., 2019]] ).

Previous assessments of polar seas (Table 3.14) concluded that climate change has already profoundly influenced polar ecosystems, through changing species distributions and abundances from primary producers to top predators, including both ecologically and economically important species ( ''high confidence'' ), and that it will continue to do so (Table 3.14).

'''Table 3.14 |''' Summary of previous IPCC assessments for polar seas

{| class="wikitable"
|-
! Observations

! Projections

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

|
|-
| Poleward species distributional shifts are due to climate warming ( ''medium to high confidence'' ).

Impacts of shifts in ocean conditions affect fish and shellfish abundances in the Arctic ( ''high confidence'' ).

Changes in sea ice and the physical environment to the west of the Antarctic Peninsula are altering phytoplankton stocks and productivity, and krill ( ''high confidence'' ).

| Some marine species will shift their ranges in response to changing ocean and sea ice conditions in the polar regions ( ''medium confidence'' ).

Loss of sea ice in summer and increased ocean temperatures are expected to impact secondary pelagic production in some regions of the Arctic Ocean, with associated changes in the energy pathways within the marine ecosystem ( ''medium confidence'' ).

Ocean acidification has the potential to inhibit embryo development and shell formation of some zooplankton and krill in the polar regions, with potentially far-reaching consequences to food webs in these regions ( ''medium confidence'' ).

Shifts in the timing and magnitude of seasonal biomass production could disrupt coupled phenologies in the food webs, leading to decreased survival of dependent species ( ''medium confidence'' ).

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

|
|-
| ‘A fundamental transformation is occurring in polar organisms and ecosystems, driven by climate change ( ''high confidence'' ).’

| ‘The losses in sea ice at 1.5°C and 2°C of warming will result in habitat losses for organisms such as seals, polar bears, whales and seabirds. There is ''high agreement'' and ''robust evidence'' that phytoplankton species will change because of sea ice retreat and related changes in temperature and radiation, and this is ''very likely'' to benefit fisheries productivity [in the Arctic spring bloom system].’

‘‘Unique and threatened systems’ (RFC1), [including Arctic and coral reefs], display a transition from high to very high risk of transition at temperatures between 1.5°C and 2°C of global warming, as opposed to at 2.6°C of global warming in AR5 ( ''high confidence'' ).’

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

|
|-
| Climate-induced changes in seasonal sea ice extent and thickness as well as ocean stratification are altering marine primary production ( ''high confidence'' ), with impacts on ecosystems ( ''medium confidence'' ).

Changes in the timing, duration and magnitude of primary production have occurred in both polar oceans, with marked regional or local variability ( ''high confidence'' ).

In both polar regions, climate-induced changes in ocean and sea ice conditions have expanded the range of temperate species and contracted the range of polar fish and ice-associated species ( ''high confidence'' ).

Ocean acidification will affect several key Arctic species ( ''medium confidence'' ).

| Future climate-induced changes in the polar oceans, sea ice, snow and permafrost will drive habitat and biome shifts, with associated changes in the ranges and abundance of ecologically important species ( ''medium confidence'' ).

Projected range expansion of sub-Arctic marine species will increase pressure for high-Arctic species ( ''medium confidence'' ), with regionally variable impacts.

Both polar oceans will be increasingly affected by CO 2 uptake, causing corrosive conditions for calcium carbonate shell-producing organisms ( ''high confidence'' ), with associated impacts on marine organisms and ecosystems ( ''medium confidence'' ).

The projected effects of climate-induced stressors on polar marine ecosystems present risks for commercial and subsistence fisheries, with implications for regional economies, cultures and the global supply of fish, shellfish, and Antarctic krill ( ''high confidence'' ).

|}

Since SROCC, evidence demonstrates that warmer oceans, less sea ice and increased advection results in increasing primary production in the Arctic, albeit with regional variation ( ''high confidence'' ), while trends remain spatially heterogeneous and less clear in the Antarctic ( ''medium confidence'' ) (Cross-Chapter Paper 6; [[#Del%20Castillo--2019|Del Castillo et al., 2019]] ; [[#Lewis--2020|Lewis et al., 2020]] ; [[#Pinkerton--2021|Pinkerton et al., 2021]] ; [[#Song--2021a|Song et al., 2021a]] ). Furthermore, climate warming influences key mechanisms determining energy transfer between trophic levels including (a) altered size spectra, (b) shifts in trophic pathways, (c) phenological mismatches and (d) increased top-down trophic regulation (Table 3.15); however, the scale of impacts from changes in these mechanisms on ecosystem productivity in warming polar oceans remains unresolved and is hence assigned ''low confidence'' .

'''Table 3.15 |''' Examples of mechanisms influencing the transfer of energy between lower trophic levels in warmer polar oceans

{| class="wikitable"
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! Mechanism

! Examples

! References

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| Altered size spectra

| Shifts towards smaller algal cells and zooplankton in warmer and more stratified oceans results in longer and less-efficient food chains, with lower lipid content.

| [[#Aarflot--2018|Aarflot et al. (2018)]] ; [[#Kimmel--2018|Kimmel et al. (2018)]] ; [[#Weydmann--2018|Weydmann et al. (2018)]] ; [[#Hop--2019|Hop et al. (2019)]] ; [[#Møller--2020|Møller and Nielsen (2020)]] ; [[#Spear--2020|Spear et al. (2020)]] ; but see [[#Dong--2021|Dong et al. (2021)]] and [[#Vernet--2017|Vernet et al. (2017)]] for opposite trends.

|-
| Shifts in trophic pathways

| Changes in microbial food-web interactions, including strengthening of the microbial loop, may reduce overall productivity. Transitions from sea ice algae to open-water phytoplankton production may reduce benthic–pelagic coupling and benthic production; transition from autotroph to heterotroph benthic production with increased water turbidity; shifts from krill-dominated to salp-dominated ecosystems in the Antarctic may have negative impacts on higher trophic levels.

| Cross-Chapter Paper 6; [[#Fujiwara--2016|Fujiwara et al. (2016)]] ; [[#Onda--2017|Onda et al. (2017)]] ; [[#Vernet--2017|Vernet et al. (2017)]] ; [[#Grebmeier--2018|Grebmeier et al. (2018)]] ; [[#Moore--2018b|Moore et al. (2018b)]] ; Cavan et al. (2019); [[#Vaqué--2019|Vaqué et al. (2019)]] ; [[#Yurkowski--2020|Yurkowski et al. (2020)]] ; Braekcman et al. (2021)

|-
| Phenological mismatches

| Mismatches in timing arise between spring phytoplankton blooms and zooplankton recruits.

| [[#Søreide--2010|Søreide et al. (2010)]] ; [[#Renaud--2018|Renaud et al. (2018)]] ; [[#Dezutter--2019|Dezutter et al. (2019)]]

|-
| Increased top-down trophic regulation

| Increased predation efficiency and top-down regulation of zooplankton by zooplanktivorous fish (due to more light with less sea ice) disconnects zooplankton and phytoplankton production.

| [[#Langbehn--2017|Langbehn and Varpe (2017)]] ; [[#Kaartvedt--2018|Kaartvedt and Titelman (2018)]] ; [[#Hobbs--2021|Hobbs et al. (2021)]]

|}

Major community shifts, both gradual and abrupt, are observed in polar oceans in response to warming trends and MHWs (Arctic only) ( ''high confidence'' ) (Figure 3.12; Cross-Chapter Paper 6; [[#Beaugrand--2019|Beaugrand et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Huntington--2020|Huntington et al., 2020]] ). In general, abundances and ranges of Arctic fish species are declining and contracting, while ranges of boreal fish species are expanding, both geographically and in terms of feeding interactions and ecological roles ( ''high confidence'' ) ( [[#Huserbråten--2019|Huserbråten et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Huntington--2020|Huntington et al., 2020]] ; [[#Pecuchet--2020a|Pecuchet et al., 2020a]] ), with variable outcomes for large commercial fish stocks (Cross-Chapter Paper 6; [[#Kjesbu--2014|Kjesbu et al., 2014]] ; [[#Holsman--2018|Holsman et al., 2018]] ; [[#Free--2019|Free et al., 2019]] ). The extreme seasonal solar radiation cycles of these high latitudes may both act as a barrier for species immigration and change predator–prey dynamics in previously ice-covered areas, factors not currently considered in projections ( ''limited evidence'' ) ( [[#Kaartvedt--2018|Kaartvedt and Titelman, 2018]] ; [[#Ljungström--2021|Ljungström et al., 2021]] ). Responses by marine mammals and birds to the ongoing changes in polar ecosystems are both positive and negative ( [[#Meredith--2019|Meredith et al., 2019]] ; [[#Bestley--2020|Bestley et al., 2020]] ). Phenological, behavioural, physiological and distributional changes are observed in marine mammals and birds in response to altered ecological interactions and habitat degradation, especially to loss of sea ice ( ''high confidence'' ) (see Box 3.2; Cross-Chapter Paper 6; [[#Beltran--2019|Beltran et al., 2019]] ; [[#Cusset--2019|Cusset et al., 2019]] ; [[#Descamps--2019|Descamps et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Huntington--2020|Huntington et al., 2020]] ). Reproductive failures and declining abundances attributed to warmer polar oceans and less sea ice cover are observed in populations of polar bears, ''Ursus maritimus'' , seals, whales and marine birds ( ''high confidence'' ) (see Box 3.2; Duffy- [[#Anderson--2019|Anderson et al., 2019]] ; [[#Ropert-Coudert--2019|Ropert-Coudert et al., 2019]] ; [[#Bestley--2020|Bestley et al., 2020]] ; [[#Chambault--2020|Chambault et al., 2020]] ; [[#Molnár--2020|Molnár et al., 2020]] ; [[#Stenson--2020|Stenson et al., 2020]] ). The ongoing changes in polar marine ecosystems can lead to temporary increases in biodiversity and functional diversity (e.g., due to immigration of boreal species in the Arctic, ''high confidence'' ), but reduced trophic-transfer efficiencies and functional redundancy, with uncertain consequences for ecosystem resilience and vulnerability ( ''limited evidence'' , ''low agreement'' ) ( [[#Griffith--2019b|Griffith et al., 2019b]] ; [[#Alabia--2020|Alabia et al., 2020]] ; [[#du%20Pontavice--2020|du Pontavice et al., 2020]] ; [[#Alabia--2021|Alabia et al., 2021]] ; [[#Frainer--2021|Frainer et al., 2021]] ).

Calcareous polar organisms are among the groups most sensitive to ocean acidification ( ''high confidence'' ) ( [[#3.3.2|Section 3.3.2]] ). [[#Niemi--2021|Niemi et al. (2021)]] reports that &gt;80% of sampled sea snail, ''Limacina helicina'' , a key species in pelagic food webs, displayed signs of shell dissolution in the Amundsen Gulf. However, bacteria, phytoplankton, zooplankton and benthic communities are found to be detrimentally impacted, resilient or even positively influenced by ocean acidification in observational and experimental studies ( [[#3.3|Section 3.3]] ; [[#Hildebrandt--2016|Hildebrandt et al., 2016]] ; [[#Thor--2018|Thor et al., 2018]] ; [[#Ericson--2019|Ericson et al., 2019]] ; [[#McLaskey--2019|McLaskey et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Petrou--2019|Petrou et al., 2019]] ; [[#Renaud--2019|Renaud et al., 2019]] ; [[#Brown--2020|Brown et al., 2020]] ; [[#Hancock--2020|Hancock et al., 2020]] ; [[#Henley--2020|Henley et al., 2020]] ; [[#Johnson--2020|Johnson and Hofmann, 2020]] ; [[#Torstensson--2021|Torstensson et al., 2021]] ). While fish larval stages may be sensitive, adult fish are expected to have low vulnerability to projected acidification levels ( [[#3.3.3|Section 3.3.3]] ; [[#Hancock--2020|Hancock et al., 2020]] ), although reduced swimming capacity in polar cod in an ocean acidification experiment has been observed ( [[#Kunz--2018|Kunz et al., 2018]] ). Polar organisms’ sensitivity to ocean acidification may increase with increasing light levels due to the loss of sea ice (algae; [[#Donahue--2019|Donahue et al., 2019]] ; [[#Kvernvik--2020|Kvernvik et al., 2020]] ), temperature stress (pteropods; [[#Johnson--2020|Johnson and Hofmann, 2020]] ) or indirectly via increased heterotrophic bacterial productivity ( ''limited evidence'' ) ( [[#Vaqué--2019|Vaqué et al., 2019]] ). Due to limited mechanistic understanding of observed effects, and mixed responses among Arctic species, future impacts of ocean acidification are assigned ''medium confidence'' for polar species and ''low confidence'' for outcomes for polar ecosystems ( [[#Meredith--2019|Meredith et al., 2019]] ; [[#Green--2021b|Green et al., 2021b]] ).

While levels of pollutants in biota (e.g., persistent organic pollutants, mercury) have generally declined over the past decades, recent increasing levels are associated with release from reservoirs in ice, snow and permafrost, and through changing food webs and pathways for trophic amplification ( ''medium confidence'' ) (see Box 3.2; [[#Ma--2016|Ma et al., 2016]] ; [[#Amélineau--2019|Amélineau et al., 2019]] ; [[#Foster--2019|Foster et al., 2019]] ; [[#Bourque--2020|Bourque et al., 2020]] ; [[#Kobusińska--2020|Kobusińska et al., 2020]] ). Also, a warmer climate, altered ocean currents and increased human activities elevate the risk of invasive species in the Arctic ( ''medium confidence'' ), potentially changing ecosystems in this region ( ''high confidence'' ) ( [[#Chan--2019|Chan et al., 2019]] ; [[#Goldsmit--2020|Goldsmit et al., 2020]] ). In the remote Antarctic, there is a lower risk of invasive species ( ''limited evidence'' ) ( [[#McCarthy--2019|McCarthy et al., 2019]] ; [[#Holland--2021|Holland et al., 2021]] ).

Fisheries are largely sustainably managed yet are expanding polewards following sea ice melt in the Arctic ( ''high confidence'' ) ( [[#Fauchald--2021|Fauchald et al., 2021]] ) and possibly in the Antarctic ( ''limited evidence'' ) ( [[#Santa%20Cruz--2018|Santa Cruz et al., 2018]] ). Tourism is increasing and expanding in both polar regions, while shipping and hydrocarbon exploration are growing in the Arctic, increasing the risks of compound effects on vulnerable and already stressed populations and ecosystems ( ''high confidence'' ) (Sections 3.6.3.1.3, 3.6.3.1.4; Cross-Chapter Paper 6; [[#Hauser--2018|Hauser et al., 2018]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Helle--2020|Helle et al., 2020]] ; [[#Rogers--2020|Rogers et al., 2020]] ; [[#Cavanagh--2021|Cavanagh et al., 2021]] ).

Ensemble global model projections indicate future increases in primary production and total animal biomass towards 2100 under RCP2.6 (~5 and 50%, respectively) and RCP8.5 (~10 and 70%, respectively), in the Arctic ( [[#Bryndum-Buchholz--2019|Bryndum-Buchholz et al., 2019]] ; [[#Lotze--2019|Lotze et al., 2019]] ; [[#Nakamura--2019|Nakamura and Oka, 2019]] ), highlighting opportunities for, and possibly conflicts over, new ecosystem services ( [[#3.5|Section 3.5]] ). For the Southern Ocean, no overall trends are apparent, but greater variability in both primary production and total animal biomass are projected under RCP2.6, with an ~5 and 15% increase in primary production and total animal biomass under RCP8.5, respectively ( [[#Bryndum-Buchholz--2019|Bryndum-Buchholz et al., 2019]] ; [[#Lotze--2019|Lotze et al., 2019]] ; [[#Nakamura--2019|Nakamura and Oka, 2019]] ). All projections presented exhibit high inter-model variability and hence uncertainty ( [[#Heneghan--2021|Heneghan et al., 2021]] ). Furthermore, regional models project significant distributional shifts and wide-ranging trends (i.e., relatively stable, increasing and declining) in productivity for key ecological and commercial species, and functional groups, with weak to strong dependence on emission scenarios, indicating ''low confidence'' in future outcomes for polar marine ecosystems and associated ecosystem services ( [[#3.5|Section 3.5]] ; [[#Piñones--2016|Piñones and Fedorov, 2016]] ; [[#Griffiths--2017|Griffiths et al., 2017]] ; [[#Klein--2018|Klein et al., 2018]] ; [[#Hansen--2019|Hansen et al., 2019]] ; [[#Meredith--2019|Meredith et al., 2019]] ; [[#Steiner--2019|Steiner et al., 2019]] ; [[#Tai--2019|Tai et al., 2019]] ; [[#Alabia--2020|Alabia et al., 2020]] ; [[#Holsman--2020|Holsman et al., 2020]] ; [[#Reum--2020|Reum et al., 2020]] ; [[#Veytia--2020|Veytia et al., 2020]] ; [[#Sandø--2021|Sandø et al., 2021]] ). Potentially highly influential tipping points associated with Arctic sea ice melt and Antarctic ocean circulation change adds to this uncertainty (Cross-Chapter Paper 6; [[#Heinze--2021|Heinze et al., 2021]] ). Nevertheless, increasing evidence supports that sustainable and adaptive ecosystem-based fisheries practices can reduce detrimental impacts of climate change on harvested populations ( ''medium confidence'' ) ( [[#3.6.3.1.2|Section 3.6.3.1.2]] ; [[#Klein--2018|Klein et al., 2018]] ; [[#Free--2019|Free et al., 2019]] ; [[#Hansen--2019|Hansen et al., 2019]] ; [[#Holsman--2020|Holsman et al., 2020]] ).

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