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

==== 2.3.4.2 Marine Biosphere ====

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===== 2.3.4.2.1 Large-scale distribution of marine biota =====

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SROCC pointed out that long-term global observations of many key ocean variables, including phytoplankton, have not reached the density and accuracy necessary for detecting change. But SROCC noted the good comparability between short time-scale single-sensor ocean-colour products and a longer time-scale, climate-quality time series of multi-sensor, inter-sensor-bias-corrected, and error-characterized, global data on chlorophyll-a concentration in the surface layers of the ocean. With respect to oligotrophic gyres, AR5 WGII concluded that the oligotrophic subtropical gyres of the Atlantic and Pacific Oceans are expanding and that they indicate declining phytoplankton stocks in these waters ( ''limited evidence, low agreement'' ). With respect to distributions of marine organisms, AR5 WGII reported range shifts of benthic, pelagic, and demersal species and communities ( ''high confidence'' ), though the shifts were not uniform.

Phytoplankton are responsible for marine primary production through photosynthesis; they are a major player in the ocean carbon cycle. They have a high metabolic rate and respond fast to changes in environmental conditions (light, temperature, nutrients, mixing), and as such, serve as a key indicator for change in marine ecosystems. Concentration of chlorophyll-a, the major photosynthetic pigment in all phytoplankton, is often used as a measure of phytoplankton biomass. As primary producers, they are also food for organisms at higher trophic levels. The multi-sensor time series of chlorophyll-a concentration has now been updated ( [[#Sathyendranath--2019|Sathyendranath et al., 2019]] ) to cover 1998–2018. Figure 2.31 shows that global trends in chlorophyll-a for the last two decades are insignificant over large areas of the global ocean ( [[#von%20Schuckmann--2019|von Schuckmann et al., 2019]] ), but some regions exhibit significant trends, with positive trends in parts of the Arctic and the Antarctic waters (&gt;3% yr <sup>–1</sup> ), and both negative and positive trends (within ± 3% yr <sup>–1</sup> ) in parts of the tropics, subtropics and temperate waters. The interannual variability in chlorophyll-a data in many regions is strongly tied to indices of climate variability ( [[#2.4|Section 2.4]] and Annex IV) and changes in total concentration are typically associated with changes in phytoplankton community structure (e.g., [[#Brewin--2012|Brewin et al., 2012]] ; [[#Racault--2017b|Racault et al., 2017b]] ). Variability in community structure related to El Niño has, in turn, been linked to variability in fisheries, for example in the catch of anchovy ( ''Engraulis ringens'' ) in the Humboldt current ecosystem ( [[#Jackson--2011|Jackson et al., 2011]] ).

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[[File:8e2ab16ad7dde7fa160828a9cd447055 IPCC_AR6_WGI_Figure_2_31.png]]

'''Figure 2.''' '''31 |''' '''Phytoplankton dynamics in the ocean. (a)''' Climatology of chlorophyll-a concentration derived from ocean-colour data (1998–2018); '''(b)''' Linear trends in chlorophyll concentration. Trends are calculated using OLS regression with significance assessed following AR(1) adjustment after [[#Santer--2008|Santer et al. (2008)]] (‘×’ marks denote non-significant changes). '''(c)''' Histogram of linear trends in chlorophyll concentration, after area weighting and with per-pixel uncertainty estimates based on comparison with in situ data. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Since AR5 WGII, analysis of a longer time series of ocean-colour data (1998–2012) has shown ( [[#Aiken--2017|Aiken et al., 2017]] ) that the expansion of the low nutrient part of the North Atlantic oligotrophic gyre was significant, at 0.27 × 10 <sup>6</sup> km <sup>2</sup> per decade, but that the rate was much lower than that reported earlier by [[#Polovina--2008|Polovina et al. (2008)]] . Furthermore, [[#Aiken--2017|Aiken et al. (2017)]] reported no significant trend in the oligotrophic area of the South Atlantic Gyre. With the time series extended to 2016, [[#von%20Schuckmann--2018|von Schuckmann et al. (2018)]] reported that since 2007, there was a general decreasing trend in the areas of the North and South Pacific oligotrophic Gyres, while the North and South Atlantic oligotrophic Gyres remained stable, with little change in area, consistent with Aiken et al ''.'' (2017). The changing sign of trends in the areal extent of the oligotrophic gyres with increase in the length of the time series raises the possibility that these changes arise from interannual to multi-decadal variability. The time series of ocean-colour data is too short to discern any trend that might be superimposed on such variability.

Similarly, there is limited consistent and long-term information on large-scale distributions of marine organisms at higher trophic levels. But there are increased indications since AR5 and SROCC that the distributions of various higher trophic-level organisms are shifting both polewards and to deeper levels ( [[#Edwards--2016|Edwards et al., 2016]] ; [[#Haug--2017|Haug et al., 2017]] ; [[#Atkinson--2019|Atkinson et al., 2019]] ; [[#Lenoir--2020|Lenoir et al., 2020]] ; [[#Pinsky--2020|Pinsky et al., 2020]] ), mostly consistent with changes in temperature. However observations also show a smaller set of counter-intuitive migrations towards warmer and shallower waters, which could be related to changes in phenology and in larval transport by currents ( [[#Fuchs--2020|Fuchs et al., 2020]] ). There are also strengthening indications of greater representation by species with warm-water affinity in marine communities, consistent with expectations under observed warming ( [[#Burrows--2019|Burrows et al., 2019]] ). There are indications that pre-1850 CE plankton communities are different from their modern counterparts globally ( [[#Jonkers--2019|Jonkers et al., 2019]] ). Indicators of geographical distributions of species (mostly from coastal waters) suggest that the rates at which some species are leaving or arriving at an ecosystem are variable, leading to changes in community composition ( [[#Blowes--2019|Blowes et al., 2019]] ), with ''likely'' greater representation of warm-water species in some locations ( [[#Burrows--2019|Burrows et al., 2019]] ).

In summary, there is ''high confidence'' that the latitudinal and depth limits of the distribution of various organisms in the marine biome are changing. There is ''medium confidence'' that there are differences in the responses of individual species relative to each other, such that the species compositions of ecosystems are changing. There is ''medium confidence'' that chlorophyll concentration in the surface shows weak negative and positive trends in parts of low and mid latitudes, and weak positive trends in some high-latitude areas. There is ''medium confidence'' that the large-scale distribution of the oligotrophic gyre provinces is subject to significant inter-annual variations, but ''low confidence'' in the long-term trends in the areal extent of these provinces because of insufficient length of direct observations.

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===== 2.3.4.2.2 Marine primary production =====

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SROCC expressed ''low confidence'' in satellite-based estimates of trends in marine primary production, citing insufficient length of the time series and lack of corroborating in situ measurements and independent validation time series. The report also cites significant mismatches in absolute values and decadal trends in primary production when different satellite-based products are compared.

Recent model-based results with assimilation of satellite data ( [[#Gregg--2019|Gregg and Rousseaux, 2019]] ), show global annual mean marine primary production of around 38 (±1.13) PgC yr <sup>–1</sup> over 1998–2015. This new result lies towards the low end of values reported in earlier, satellite-based, studies (range 36.5–67 PgC yr <sup>–1</sup> , reported in [[#Sathyendranath--2020|Sathyendranath et al. (2020)]] ). Reconciling the results of [[#Gregg--2019|Gregg and Rousseaux (2019)]] with earlier satellite-based studies leads to a mean of 47 (±7.8) PgC yr <sup>–1</sup> . There is a strong correlation between interannual regional variability in marine primary production and climate variability ( [[#Racault--2017b|Racault et al., 2017b]] ; [[#Gregg--2019|Gregg and Rousseaux, 2019]] ). The increase in primary production in the Arctic has been associated with retreating sea ice and with increases in nutrient supply and chlorophyll concentration ( [[#Lewis--2020|Lewis et al., 2020]] ). [[#Gregg--2019|Gregg and Rousseaux (2019)]] reported a decreasing trend in marine primary production, of –0.8 PgC (–2.1%) per decade globally. There is ''low confidence'' in this trend because of the small number of studies and the short length of the time series (&lt;20 years).

In conclusion, there is ''low confidence'' because of the small number of recent studies and the insufficient length of the time series analysed that marine primary production is 47 (± 7.8) PgC yr <sup>–1</sup> . A small decrease in productivity is evident globally for the period 1998–2015, but regional changes are larger and of opposing signs ( ''low confidence'' ).

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===== 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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