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

===== 3.4.3.4.1 Observed changes =====

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Observed changes in biomass in the global ocean, beyond those for phytoplankton (Table 3.23), have not routinely been attributed to climate-induced drivers, but rather to the compound effects of multiple drivers, especially fishing ( [[#Christensen--2014|Christensen et al., 2014]] ; [[#Palomares--2020|Palomares et al., 2020]] ). We therefore do not assess observed changes in ocean biomass here.

'''Table 3.23 |''' Summary of previous IPCC assessments of changes in open ocean and deep-sea biomass

{| class="wikitable"
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! Measure

! Observations

! Projections

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| ''AR5 WGII ( [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ; [[#Pörtner--2014|Pörtner et al., 2014]] )''

|
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| Chlorophyll- ''a'' /phytoplankton biomass

| ‘Phytoplankton biomass: the approximately 15-year archived time series of satellite-chlorophyll (as a proxy of phytoplankton biomass) is too short to reveal trends over time and their causes’ (WGII AR5 [[IPCC:Wg2:Chapter:Chapter-6#6.1.2|Section 6.1.2]] ; [[#Pörtner--2014|Pörtner et al., 2014]] ).

‘Chlorophyll concentrations measured by satellites have decreased in the subtropical gyres of the North Pacific, Indian and North Atlantic oceans by 9, 12 and 11%, respectively, over and above the inherent seasonal and interannual variability from 1998 to 2010 ( ''high confidence'' ; ''p'' ≤ 0.05). Significant warming over this period has resulted in increased water-column stratification, reduced mixed-layer depth and possibly decreases in nutrient availability and ecosystem productivity ( ''limited evidence, medium agreement'' ). The short time frame of these studies against well-established patterns of long-term variability leads to the conclusion that these changes are about as ''likely'' as not due to climate change’ (WGII AR5 Chapter 30; [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ).

| ‘Owing to contradictory observations there is currently uncertainty about the future trends of major upwelling systems and how their drivers (enhanced productivity, acidification and hypoxia) will shape ecosystem characteristics ( ''low confidence'' )’ (WGII AR5 [[IPCC:Wg2:Chapter:Chapter-6|Chapter 6]] Executive Summary; [[#Pörtner--2014|Pörtner et al., 2014]] ).

|-
| Animal biomass

| Observed changes in animal biomass were not assessed in this report.

| ‘The climate-change-induced intensification of ocean upwelling in some eastern boundary systems, as observed in the last decades, may lead to regional cooling, rather than warming, of surface waters and cause enhanced productivity ( ''medium confidence'' ), but also enhanced hypoxia, acidification and associated biomass reduction in fish and invertebrate stocks’ (WGII AR5 [[IPCC:Wg2:Chapter:Chapter-6|Chapter 6]] Executive Summary; [[#Pörtner--2014|Pörtner et al., 2014]] ).

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

|
|-
| Chlorophyll- ''a'' /phytoplankton biomass

| ‘[Changes reported] in overall open-ocean chlorophyll levels (a proxy of phytoplankton biomass) of less than ±1% yr –1 for individual time periods. Regionally, trends of ±4% between 2002 and 2015 for different regions are found when different satellite products are merged, with increases at high latitudes and moderate decreases at low latitudes’ (SROCC [[IPCC:Wg2:Chapter:Chapter-5#5.2.2|Section 5.2.2.6]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

| Projected changes in chlorophyll- ''a'' /phytoplankton biomass were not assessed in this report.

|-
| Animal biomass

| Observed changes in open-ocean and deep-sea biomass were not assessed in this report.

| ‘There is ''high agreement'' in model projections that global zooplankton biomass will ''very likely'' reduce in the 21st century, with projected decline under RCP8.5 almost doubled that of RCP2.6 ( ''very likely'' ). However, the strong dependence of the projected declines on phytoplankton production ( ''low confidence'' ) and simplification in representation of the zooplankton communities and food web render their projections having ''low confidence'' .’

The global biomass of marine animals, including those that contribute to fisheries, is projected to decrease by 4.3 ± 2.0% (95% confidence interval) and 15.0 ± 5.9% under RCP2.6 and RCP8.5, respectively, by 2080–2099 relative to 1986–2005, while the decrease is around 4.9% by 2031–2050 across all RCP2.6 and RCP8.5 ( ''very likely'' ). Regionally, total animal biomass decreases largely in tropical and mid-latitude oceans ( ''very likely'' ).

‘Projected decrease in upper-ocean export of organic carbon to the deep seafloor is expected to result in a loss of animal biomass on the deep seafloor by 5.2–17.6% by 2090–2100 compared to the present (2006–2015) under RCP8.5 with regional variations ( ''medium confidence'' ). Some increases are projected in the polar regions, due to enhanced stratification in the surface ocean, reduced primary production and shifts towards small phytoplankton ( ''medium confidence'' ). The projected impacts on biomass in the abyssal seafloor are larger under RCP8.5 than RCP4.5 ( ''very likely'' ).’

|-
| ''WGI AR6 [[IPCC:Wg2:Chapter:Chapter-2|Chapter 2]] ( [[#Gulev--2021|Gulev et al., 2021]] )''

|
|-
| Chlorophyll- ''a'' /phytoplankton biomass

| The multi-sensor time series of chlorophyll- ''a'' concentration has been updated to cover two decades (1998–2018).

‘Global trends in chlorophyll- ''a'' for the last two decades are insignificant over large areas of the global oceans, but some regions exhibit significant trends, with positive trends in parts of the Arctic and the Antarctic waters (&gt;3% yr –1 ), and both negative and positive trends (within ±3% yr –1 ) in parts of the tropics, subtropics and temperate waters.’

‘In the last two decades, the concentration of phytoplankton at the base of the marine food web, as indexed by chlorophyll concentration, has shown weak and variable trends in low and mid-latitudes and an increase in high latitudes ( ''medium confidence'' ).’

| Projected changes in open-ocean and deep-sea biomass were not assessed in this report.

|}

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