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

===== 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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