Editing IPCC:AR6/WGII/Cross-Chapter-Paper-6 (section)

==== CCP6.2.1.1 Warming and sea ice retreat cause shifts in distribution ranges of species ====

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In Arctic seas, warming and other climate impact drivers, primarily sea ice retreat, have led to range contractions of Arctic marine and ice-associated species and poleward expansions of boreal species ( ''very high confidence'' ) (Table CCP6.2) ( [[#Bouchard--2020|Bouchard and Fortier, 2020]] ; [[#Huntington--2020|Huntington et al., 2020]] ; [[#Mueter--2020|Mueter et al., 2020]] ) even though light and energetics at seasonal extremes may limit some range shifts ( ''limited evidence'' ) ( [[#Ljungström--2021|Ljungström et al., 2021]] ). Altered conditions allow more microorganisms to move poleward and provide opportunities for invasive species ( [[#Cavicchioli--2019|Cavicchioli et al., 2019]] ; [[#Nielsen--2020|Nielsen et al., 2020]] ; Mustonen, 2021). Phytoplankton communities harbour increasing numbers of taxa, including harmful species ( [[#Lovejoy--2017|Lovejoy et al., 2017]] ) and the coccolithophore ''Emiliania huxleyi'' , which meanwhile forms regular blooms in the Barents Sea ( [[#Neukermans--2018|Neukermans et al., 2018]] ; [[#Silkin--2020|Silkin et al., 2020]] ). Northward shifts of pelagic, benthic and demersal species and subsequent changes in Arctic community composition have been observed in the Bering, Greenland and Barents Seas ( [[#Grebmeier--2018|Grebmeier et al., 2018]] ; [[#Mueter--2020|Mueter et al., 2020]] ), as have higher numbers of economically important boreal species such as haddock and Pacific and Atlantic cod (CCP6.2.3). Cold-adapted Arctic fish species such as polar cod ( ''Boreogadus saida'' ) are expected to decline further and lose spawning habitats at GWL &gt;1.5°C, mainly due to a lack of phenotypic plasticity, as well as increasing interspecific competition with and predation from invading boreal species ( [[#Dahlke--2018|Dahlke et al., 2018]] ; [[#Marsh--2020|Marsh and Mueter, 2020]] ). Numerous mammals and sea birds respond to changes in the distribution of their preferred habitats and prey by shifting their range, altering the timing or pathways for migration or switching prey ( ''very high confidence'' ) ( [[#Hamilton--2017|Hamilton et al., 2017]] ; [[#Loseto--2018|Loseto et al., 2018]] ; [[#Meredith--2019|Meredith et al., 2019]] ). Ice-breeding seals (e.g., harp seals – ''Pagophilus groenlandicus'' ) often have little scope to shift distribution, leading to increases in strandings and pup mortality in years with little ice cover ( ''medium confidence'' ) (Table CCP6.2) ( [[#Boveng--2020|Boveng et al., 2020]] ). Recent studies confirm that polar bears ( ''Ursus maritimus'' ) are negatively affected by changing ice and snow conditions with decreases in denning, foraging, reproduction, genetic diversity and survival rates ( ''very high confidence'' ) (Table CCP6.2) ( [[#Boonstra--2020|Boonstra et al., 2020]] ; [[#Johnson--2020|Johnson and Derocher, 2020]] ; [[#Maduna--2021|Maduna et al., 2021]] ).

In the Southern Ocean, southward range shifts are expected to result from increased warming coupled with the narrow thermal tolerance of cold-adapted Antarctic species ( [[#Convey--2019|Convey and Peck, 2019]] ; [[#Morley--2019|Morley et al., 2019]] ; [[#Gutt--2021|Gutt et al., 2021]] ). Such shifts have so far only been detected for Antarctic krill ( ''Euphausia superba'' ), with a poleward contraction of the highest densities of krill in the Atlantic sector ( ''medium confidence'' ) (Table CCP6.2); ( [[#Atkinson--2019|Atkinson et al., 2019]] ). Ocean warming is expected to put pressure on Antarctic phytoplankton ( [[#Pinkerton--2021|Pinkerton et al., 2021]] ) and fish species unable to move further south in shelf areas, including waters off sub-Antarctic islands ( ''low confidence'' ) (Table CCP6.2) ( [[#Caccavo--2021|Caccavo et al., 2021]] ). Off the Antarctic Peninsula and sub-Antarctic islands, invasive benthic invertebrates and macroalgae have already been detected ( ''medium confidence'' ) ( [[#Fraser--2018|Fraser et al., 2018]] ; [[#Avila--2020|Avila et al., 2020]] ; [[#Brasier--2021|Brasier et al., 2021]] ), and projected changes will further favour the spread of invasive species ( [[#Fraser--2020|Fraser et al., 2020]] ; [[#Macaya--2020|Macaya et al., 2020]] ). On a local to regional scale, the benthic recolonisation of the newly exposed seabed after the disintegration of ice shelves shows typical succession patterns, with mass occurrences of few pioneer species followed by gradual shifts to a more diverse typical shelf community, driven by increasing pelagic primary production upon ice-shelf collapse and strengthening of the pelagic–benthic coupling ( ''high confidence'' ) ( [[#Brasier--2021|Brasier et al., 2021]] ; [[#Gutt--2021|Gutt et al., 2021]] ). Range changes of Antarctic birds and marine mammals have been observed, which vary among sub-regions and are mostly attributable to changes in sea ice extent and food availability ( ''high confidence'' ) (Table CCP6.2) ( [[#Gutt--2018|Gutt et al., 2018]] ; [[#Convey--2019|Convey and Peck, 2019]] ; [[#Bestley--2020|Bestley et al., 2020]] ). With projected sea ice retreat and associated change in prey distribution ( [[#Henley--2020|Henley et al., 2020]] ), foraging areas of sub-Antarctic sea birds and marine mammals will shift southwards, leading to elevated pressure on populations due to higher foraging costs during the breeding season ( ''medium confidence'' ) ( [[#Ropert-Coudert--2018|Ropert-Coudert et al., 2018]] ; [[#Bestley--2020|Bestley et al., 2020]] ; [[#Hindell--2020|Hindell et al., 2020]] ; [[#Hückstädt--2020|Hückstädt et al., 2020]] ; [[#Wege--2021|Wege et al., 2021]] ). These changes are particularly impacting emperor penguins ( ''Aptenodytes forsteri'' ) (Table CCP6.2), with the projected population declining close to extinction by 2100 under Business-As-Usual climate scenarios ( ''medium confidence'' ) ( [[#Jenouvrier--2020|Jenouvrier et al., 2020]] ; [[#Trathan--2020|Trathan et al., 2020]] ; [[#Jenouvrier--2021|Jenouvrier et al., 2021]] ), whereas population decline is halted by 2060 under the 1.5°C climate scenario ( ''low confidence'' ) ( [[#Jenouvrier--2020|Jenouvrier et al., 2020]] ).

'''Table CCP6.2 |''' Summary of observed impacts (and projected risks of climate change for polar marine, terrestrial and freshwater ecosystems identified in [[IPCC:Wg2:Chapter:Chapter-3#3.2.3|Section 3.2.3]] and Box 3.4 in [[IPCC:Wg2:Chapter:Chapter-3|Chapter 3]] of the IPCC SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ).

{| class="wikitable"
|-
! '''Affected system'''

! '''Hazard'''

'''*Cascading effect'''

! '''Observed impacts, future risks and natural adaptations identified in SROCC (confidence level)'''

|-
| colspan="3"| ''Arctic marine ecosystems''

|-
| Primary producers (PP-1)

| Sea ice loss

\* Freshening

\* Stratification

| Impact: timing (earlier and later blooms), distribution and magnitude (&gt;30% increase in annual net primary production since 1998) ( ''high confidence'' )

|-
|
| Acidification

| Adaptation: phytoplankton may compensate for decrease in pH

|-
| Zooplankton

| \* PP-1

| Impact: changing production and community composition ( ''medium confidence'' )

|-
| Benthos

| \* PP-1

| Impact: changing production and biodiversity ( ''medium confidence'' )

|-
|
| Acidification

| Risk: effects on zooplankton and pteropods depends on climate scenario and species’ sensitivity/adaptive capacity

|-
| Fish

| Warming

\* Prey changes

| Impact: northward expanding ranges of sub-Arctic/boreal species (e.g., Atlantic cod) in Bering Sea (Detection— ''high confidence'' , Attribution— ''medium confidence'' ) negatively affecting Arctic polar cod ( ''medium confidence'' )

|-
|
| \* Prey declines

| Risk: decreasing production of walleye pollock, Pacific cod and arrowtooth flounder, due to declines in large copepods ( ''medium confidence'' )

|-
| Birds and marine mammals

| Sea ice loss

| Impact: phenological, behavioural, physiological and distributional changes; endemic marine mammals have little scope to move northwards in response to warming ( ''high confidence'' )

|-
| Polar bears

| Sea ice timing, distribution, thickness

| Impact: phenological shifts, and changes in distribution, denning, foraging behaviour and survival rates ( ''hi'' ''gh co'' ''nfidence'' )

|-
| colspan="3"| ''Antarctic marine ecosystems''

|-
| Primary productivity

| Sea ice loss

\* Freshening

\* Stratification

| Impact: little overall change in biomass at circumpolar scale from 1998 to 2006, but sub-regional differences ( ''medium confidence'' ); changes difficult to detect and attribute to climate change

|-
| Microbes

| Acidification

| Impact: detrimental effect on primary production and changes to the structure and function of microbial communities ( ''medium confidence'' )

|-
| Antarctic krill

| Warming

| Impact: declines in abundance in the South Atlantic sector ( ''medium confidence'' ); may not represent a long-term, climate-driven trend but a decline following a period of anomalous peak abundance ( ''low confidence'' )

|-
|
| Risk: southward range shift due to changes in the location of the optimum conditions for growth and recruitment, with decreases most apparent in the areas with the most rapid warming, such as the southwest Atlantic/Weddell Sea region ( ''medium confidence'' )

|-
| Zooplankton

| Acidification

| Risk: vulnerability of pteropods through effects on eggs ( ''medium confidence'' )

|-
| Benthos

| Sea ice loss

| Risk: increase of biomass on the Antarctic continental shelf as productivity from longer phytoplankton blooms outweighs ice-scour mortality ( ''low confidence'' )

|-
|
| Sea ice loss

| Risk: shallow-water communities may become dominated by macroalgae due to increases in the amount of light (possible loss of endemic species by 12% due to warming temperatures) ( ''low confidence'' )

|-
| Fish

| Warming

| Risk: icefish may be displaced from shallow regions around sub-Antarctic islands ( ''low confidence'' )

|-
| Birds and marine mammals

| Sea ice cover

| Impact: predictability of foraging grounds and sea ice cover associated with climate are main drivers of population changes: increases for gentoo penguins (decreases for Adélie, chinstrap, king and Emperor penguins) ( ''high confidence'' )

|-
| colspan="3"| ''Arctic terrestrial and freshwater ecosystems''

|-
| Vegetation

| Warming

| Impact: greening ( ''high confidence'' )

|-
|
| Risk: decrease in tundra areal extent &gt;50% by 2050; wood shrubs expected to increase ( ''medium confidence'' )

|-
| Vertebrates

| Warming

| Impact: expanding range into Arctic

|-
| Freshwater primary productivity

| \* Increased runoff

\* Increased permafrost thaw

| Impact: increased productivity in rivers, lakes and coastal areas

|-
|
| Risk: expected to mobilise stores of pollutants

|-
| Pathogens

| Warming

| Impact: expanding range into Arctic

|-
|
| Risk: mobilisation may increase in high latitudes, including anthrax from frozen carcasses possibly released from permafrost

|-
| Fish

| \* Freshwater winter habitat

\* Increased discharge

| Risk: disruption of the life history of Arctic freshwater fish

|-
|
| \* Warming freshwater

| Risk: may make some surface waters inhospitably warm for cold water fish species

|-
| Biodiversity

| Warming

| Impact: sub-Arctic biodiversity expanding into Arctic

|-
| Reindeer/caribou

| Climate factors

| Impact: reindeer/caribou declined overall without adaptation ( ''high confidence'' ), with climate affecting many aspects of their life history ( ''medium confidence'' )

|-
|
| Risk: domesticated reindeer/caribou can be affected by fire, which reduces pasture, as well as by increased ice-on-snow, which can cause starvation

|-
| colspan="3"| ''Antarctic terrestrial and freshwater ecosystems''

|-
| Terrestrial biota

| \* Increased coastal ice melt

| Impact: increasing coastal ice-free areas available for colonisation ( ''high confidence'' )

|-
| Alien species

| Warming

| Risk: barriers to alien species reduce, affecting terrestrial biodiversity ( ''medium confidence'' )

|}

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