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

==== 3.4.2.9 Upwelling Zones ====

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Eastern boundary upwelling systems (EBUS) comprise four important social–ecological systems in the Pacific (California and Peru-Humboldt) and Atlantic (Canary and Benguela) ocean basins. Each is characterised by high primary production, sustained by wind-driven upwelling that draws cold, nutrient-rich, generally low-pH and low-oxygen water to the surface ( [[#Bindoff--2019a|Bindoff et al., 2019a]] ). Despite their small relative size, the primary productivity in EBUS supports a vast biomass of marine consumers, including some of the world’s most productive fisheries ( [[#Pauly--2016|Pauly and Zeller, 2016]] ), along with many species of conservation significance ( [[#Bakun--2015|Bakun et al., 2015]] ).

Although upwelling is important in many other oceanic regions, we focus here on the most documented examples provided by the EBUS. Yet even here, observed changes in upwelling, temperature, acidification and loss of oxygen ( [[#Seabra--2019|Seabra et al., 2019]] ; [[#Abrahams--2021|Abrahams et al., 2021]] ; [[#Gallego--2021|Gallego et al., 2021]] ; [[#Varela--2021|Varela et al., 2021]] ) cannot be robustly attributed to anthropogenic climate change, and projected future changes in upwelling are expected to be relatively small and variable among and within EBUS ( [[#3.2.2.3|Section 3.2.2.3]] ; WGI AR6 Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ). We therefore have few updates to assessments provided by AR5 and SROCC (Table 3.13) and restrict our brief assessment to the limited amount of new evidence (Figure 3.12).

'''Table 3.13 |''' Summary of previous IPCC assessments of eastern boundary upwelling systems (EBUS)

{| class="wikitable"
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! Observations

! Projections

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

|
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| ‘[EBUS] are vulnerable to changes that influence the intensity of currents, upwelling and mixing (and hence changes in sea surface temperature, wind strength and direction), as well as O 2 content, carbonate chemistry, nutrient content and the supply of organic carbon to deep offshore locations ( ''high confidence'' ).’

Climate-change-induced intensification of ocean upwelling in some EBUS, as observed in past 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. Owing to contradictory observations, there is currently uncertainty about the future trends of major upwelling systems and how their drivers will shape ecosystem characteristics ( ''low confidence'' ).

‘Declining O 2 and shoaling of the aragonite saturation horizon through ocean acidification increase the risk of upwelling water being low in pH and O 2 , with impacts on coastal ecosystems and fisheries [...]. These risks and uncertainties are ''likely'' to involve significant challenges for fisheries and associated livelihoods along the west coasts of South America, Africa and North America ( ''low to medium confidence'' ).’

‘There is ''robust evidence'' and ''medium agreement'' that the California Current has experienced [...] an increase of the overall magnitude of upwelling events from 1967 to 2010 ( ''high confidence'' ). This is consistent with changes expected under climate change yet remains complicated by the influence of decadal-scale variability ( ''low confidence'' ).’ Declining oxygen concentrations and shoaling of the hypoxic boundary layer ''likely'' ‘reduced the available habitat for key benthic communities as well as fish and other mobile species. Together with the shoaling of the saturation horizon, these changes have increased the incidence of low O 2 and low pH water flowing onto the continental shelf ( ''high confidence'' ; 40 to 120 m), causing problems for industries such as the shellfish aquaculture industry.’

Despite its apparent sensitivity to environmental variability, there is ''limited evidence'' of ecological changes in the Benguela Current EBUS due to climate change.

| ‘Like other ocean sub-regions, [EBUS] are projected to warm under climate change, with increased stratification and intensified winds as westerly winds shift poleward ( ''likely'' ). However, cooling has also been predicted for some [EBUS], resulting from the intensification of wind-driven upwelling.’

‘There is ''medium agreement'' , despite ''limited evidence'' , that upwelling intensity and associated variables (e.g., temperature, nutrient and O 2 concentrations) from the Benguela system will change as a result of climate change.’

Any projected increase in upwelling intensity has potential disadvantages. ‘Elevated primary productivity may lead to decreasing trophic transfer efficiency, thus increasing the amount of organic carbon exported to the seabed, where it is ''virtually certain'' to increase microbial respiration and hence increase low O 2 stress.’

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

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| ‘Increasing ocean acidification and oxygen loss are negatively impacting two of the four major upwelling systems: the California Current and Humboldt Current ( ''high confidence'' ). Ocean acidification and decrease in oxygen level in the California Current upwelling system have altered ecosystem structure, with direct negative impacts on biomass production and species composition ( ''medium confidence'' ).’

‘Three out of the four major Eastern Boundary Upwelling Systems (EBUS) have shown large-scale wind intensification in the past 60 years ( ''high confidence'' ). However, the interaction of coastal warming and local winds may have affected upwelling strength, with the direction of changes [varying] between and within EBUS ( ''low confidence'' ). Increasing trends in ocean acidification in the California Current EBUS and deoxygenation in California Current and Humboldt Current EBUS are observed in the last few decades ( ''high confidence'' ), although there is ''low confidence'' to distinguish anthropogenic forcing from internal climate variability. The expanding California EBUS OMZ [oxygen minimum zone] has altered ecosystem structure and fisheries catches ( ''medium confidence'' ).’

‘Overall, EBUS have been changing with intensification of winds that drives the upwelling, leading to changes in water temperature and other ocean biogeochemistry ( ''medium confidence'' ).’

‘The direction and magnitude of observed changes vary among and within EBUS, with uncertainties regarding the driving mechanisms behind this variability. Moreover, the high natural variability of EBUS and their insufficient representation by global ESMs [Earth system models] gives ''low confidence'' that these observed changes can be attributed to anthropogenic causes.’

| ‘Anthropogenic changes in EBUS will emerge primarily in the second half of the 21st century ( ''medium confidence'' ). EBUS will be impacted by climate change in different ways, with strong regional variability with consequences for fisheries, recreation and climate regulation ( ''medium confidence'' ). The Pacific EBUS are projected to have calcium carbonate undersaturation in surface waters within a few decades RCP8.5 ( ''high confidence'' ); combined with warming and decreasing oxygen levels, this will increase the impacts on shellfish larvae, benthic invertebrates, and demersal fishes ( ''high confidence'' ) and related fisheries and aquaculture ( ''medium confidence'' ).’

‘The inherent natural variability of EBUS, together with uncertainties in present and future trends in the intensity and seasonality of upwelling, coastal warming and stratification, primary production and biogeochemistry of source waters poses large challenges in projecting the response of EBUS to climate change and to the adaptation of governance of biodiversity conservation and living marine resources in EBUS ( ''high confidence'' ).’

‘Given the high sensitivity of the coupled human–natural EBUS to oceanographic changes, the future sustainable delivery of key ecosystem services from EBUS is at risk under climate change; those that are most at risk in the 21st century include fisheries ( ''high confidence'' ), aquaculture ( ''medium confidence'' ), coastal tourism ( ''low confidence'' ) and climate regulation ( ''low confidence'' ).’

‘For vulnerable human communities with a strong dependence on EBUS services and low adaptive capacity, such as those along the Canary Current system, unmitigated climate-change effects on EBUS (complicated by other non-climatic stresses such as social unrest) have a high risk of altering their development pathways ( ''high confidence'' ).’

|}

The California EBUS is arguably the best-studied of the four ecosystems in terms of robust projections of climate change, although even here, there is ''limited evidence'' and ''low agreement'' among projections. For example, trends in outputs from high-resolution, downscaled models in the California EBUS generally reflect those from underlying coarser-scale ESMs, but projections for physical variables are more convergent among modelling approaches than are those for biogeochemical variables ( ''high confidence'' ) ( [[#Howard--2020a|Howard et al., 2020a]] ; [[#Pozo%20Buil--2021|Pozo Buil et al., 2021]] ). Models agree on general warming in the California EBUS, with concomitant declines in oxygen content ( ''medium confidence'' ) ( [[#Howard--2020b|Howard et al., 2020b]] ; [[#Fiechter--2021|Fiechter et al., 2021]] ; [[#Pozo%20Buil--2021|Pozo Buil et al., 2021]] ). But implications for the future spatial distribution of species, including for some fisheries resources ( [[#Howard--2020b|Howard et al., 2020b]] ; [[#Fiechter--2021|Fiechter et al., 2021]] ), are confounded by local-scale oceanographic processes ( [[#Siedlecki--2021|Siedlecki et al., 2021]] ) and by lateral input of anthropogenic land-based nutrients ( [[#Kessouri--2021|Kessouri et al., 2021]] ), suggesting that such projections should be accorded ''low confidence'' .

More generally, changes in upwelling intensity are observed to affect organismal metabolism, population productivity and recruitment, and food-web structure ( ''medium confidence'' ) ( [[#van%20der%20Sleen--2018|van der Sleen et al., 2018]] ; [[#Brodeur--2019|Brodeur et al., 2019]] ; [[#Ramajo--2020|Ramajo et al., 2020]] ). But ''low confidence'' in projected trends in upwelling make it difficult to extrapolate these results to understand potential changes in the ecology of EBUS. Projected changes in fish biomass within EBUS ( [[#Carozza--2019|Carozza et al., 2019]] ) are therefore accorded ''low confidence'' . Finally, although MHWs are an important emerging hazard in the global ocean, with intensity, frequency and duration increasing strongly ( [[#3.2.2.1|Section 3.2.2.1]] ), the number of MHW days yr –1 within EBUS has been increasing more slowly (or decreasing faster, in the case of the Peru-Humboldt system) than in surrounding waters ( [[#Varela--2021|Varela et al., 2021]] ). Notwithstanding these trends, EBUS remain vulnerable both to MHWs ( ''high confidence'' ) (Sen [[#Gupta--2020|Gupta et al., 2020]] ) and to their long-lasting impacts ( ''high confidence'' ) ( [[#Arafeh-Dalmau--2019|Arafeh-Dalmau et al., 2019]] ; [[#Harvell--2019|Harvell et al., 2019]] ; [[#McPherson--2021|McPherson et al., 2021]] ). On this basis, the suggestion that EBUS may represent refugia from MHWs is accorded ''low confidence'' .

Despite ''low confidence'' in detailed projections for ecological changes in EBUS, the WGI assessment (WGI AR6 Chapter 9; [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ) that upwelling-favourable winds will weaken (or be present for shorter durations) at low latitude but intensify at high latitude ( ''high confidence'' ), albeit by no more than 20% in either case ( ''medium confidence'' ), presents some key risks to associated EBUS ecosystems. These risks include potential decreases in provisioning services, including fisheries and marine aquaculture ( [[#Bertrand--2018|Bertrand et al., 2018]] ; [[#Kifani--2018|Kifani et al., 2018]] ; [[#Lluch-Cota--2018|Lluch-Cota et al., 2018]] ; [[#van%20der%20Lingen--2018|van der Lingen and Hampton, 2018]] ), and cultural services such as nature-based tourism ( [[#3.5|Section 3.5]] ).

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