Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.4.2.3 Marine phenology =====

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Phenology is the study of the timing of important events in the annual life cycle of organisms (plants or animals; see also Annex VII: Glossary). The AR5 WGII noted that the timing of various seasonal biological events in the ocean had advanced by more than four days per decade over the previous 50-year period and concluded that there was ''high confidence'' in observed changes in the phenological metrics of marine organisms. The AR5 WGII further reported that, of those observations that showed a response, 81% of changes in phenology, distribution and abundance were consistent with anticipated responses to climate warming according to theoretical expectations, corroborated by updates in SROCC. The consequent current and future impacts on interactions between species, including competition and predator-prey dynamics, were noted with ''high confidence'' .

There are additional indications that phenological metrics related to different species are changing, but not always in a similar manner. For example, many seabirds are breeding earlier, while others are breeding later ( [[#Sydeman--2015|Sydeman et al., 2015]] ). Planktonic organisms in the North Atlantic are also responding differently to each other when subjected to the same environmental changes ( [[#Edwards--2004|Edwards and Richardson, 2004]] ). Furthermore, different factors could be responsible for triggering phenological responses in different stages in the life cycle of a single organism ( [[#Koeller--2009|Koeller et al., 2009]] ). The shift in the distribution of many benthic invertebrates on the North-west Atlantic shelf, including some commercially important shellfish, could be explained by phenology and larval transport, and the shift and contraction of species range have been associated with higher mortality ( [[#Fuchs--2020|Fuchs et al., 2020]] ). Changes in phytoplankton phenological indicators globally ( [[#Racault--2012|Racault et al., 2012]] ; [[#Sapiano--2012|Sapiano et al., 2012]] ) have been linked to indicators of climate variability, such as the multivariate ENSO Index ( [[#Racault--2017a|Racault et al., 2017a]] ), with responses varying across ecological provinces of the ocean ( [[#Longhurst--2007|Longhurst, 2007]] ).

Phenological links between multiple components of an ecosystem have to be maintained intact, to retain system integrity. Since all higher pelagic organisms depend on phytoplankton for their food, either directly or indirectly, a match favours survival, and a mismatch is antagonistic to survival. Match represents synchronicity in the phenological events of both prey and predator. There are indications from ocean-colour data used in conjunction with fisheries data that the survival rate of various larger marine organisms depends on phenological metrics related to the seasonality of phytoplankton growth. Such links have been demonstrated, for example, for haddock ( ''Melanogrammus aeglefinus'' ) in the North-west Atlantic ( [[#Platt--2003|Platt et al., 2003]] ); northern shrimp in the North Atlantic ( [[#Koeller--2009|Koeller et al., 2009]] ; [[#Ouellet--2011|Ouellet et al., 2011]] ); sardine ( ''Sardinella aurita'' ) off the Ivory coast ( [[#Kassi--2018|Kassi et al., 2018]] ); cod ( ''Gadus morhua'' ) and haddock ( ''Melanogrammus aeglefinus'' ) larvae in the North-West Atlantic ( [[#Trzcinski--2013|Trzcinski et al., 2013]] ); and oil sardine ( ''Sardinella longiceps'' ) off the south-west coast of India. [[#Borstad--2011|Borstad et al. (2011)]] showed that fledgling production rate of rhinoceros auklets ( ''Cerorhinca monocerata'' ) on a remote island in coastal north-eastern Pacific was related to seasonal values of chlorophyll-a biomass in the vicinity of the island.

In summary, new in situ data as well as satellite observations strengthen AR5 and SROCC findings that various phenological metrics for many species of marine organisms have changed in the last half century ( ''high confidence'' ), though many regions and many species of marine organisms remain under-sampled or even unsampled. The changes vary with location and with species ( ''high confidence'' ). There is a strong dependence of survival in higher trophic-level organisms (fish, exploited invertebrates, birds) on the availability of food at various stages in their life cycle, which in turn depends on phenologies of both ( ''high confidence'' ). There is a gap in our understanding of how the varied responses of marine organisms to climate change, from a phenological perspective, might threaten the stability and integrity of entire ecosystems.

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