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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minutePacific oxygen is not falling at one rate everywhere. Oxygen-poor layers sit at different depths in the tropical and northern Pacific, long-term trends differ from one region to the next, and climate cycles such as El Niño and La Niña can shift oxygen levels in the tropical Pacific from one year to the next. The clearest coastal case in the published record is the US Pacific Northwest shelf, where near-bottom hypoxia has become more widespread over the long-term record and covered an unusually large area in summer 2021.
Any claim about “Pacific oxygen” should be read with three questions attached: where, how deep, and over what timescale. The sections below work through each of those axes.
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Why one verdict does not fit the whole Pacific
The table compares the regions, depths and timescales covered by the published studies. Figures are those each source reports. Where a study does not state a value, the table says so.
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| Area | Depth or layer | Main signal reported | Drivers or context reported | Source type and date |
|---|---|---|---|---|
| Upper tropical Pacific | Upper ocean, analysed at 50–300 m | Strong oxygen-deficient layer; non-linear long-term trends; size of the trend not stated in the summary | Large interannual ENSO shifts; PDO influence in tropical and eastern records | Observational time series since 1950; Stramma et al., Biogeosciences, 2020 |
| Eastern and central tropical Pacific | Upper ocean | More oxygen during El Niño; less during La Niña | Vertical advection, biological consumption, vertical mixing and lateral advection, partly compensating | Observations and global ocean–sea-ice simulations; Eddebbar et al., Journal of Climate, 2026 |
| Western tropical Pacific | Upper ocean | Tends toward the opposite of the eastern pattern during ENSO events | Large-scale thermocline shallowing during El Niño | Eddebbar et al., 2026; NOAA PMEL summary, July 14, 2026 |
| North Pacific | Deeper oxygen-deficient layer than in the upper tropics; exact depth not stated in the summary | Non-linear trends shaped by decadal modes; size of the trend not stated | NPGO influence; in the Oyashio region, several modes and an 18.6-year tidal cycle | Stramma et al., 2020 |
| Northern tropical Pacific | Not stated in the summary | Proposed multidecadal deoxygenation linked to North Atlantic temperature variability | Stratification, trade winds, equatorial circulation and the Equatorial Undercurrent, affecting oxygen supply and biological oxygen demand | Proposal in a 2024 Nature Communications paper |
| US Pacific Northwest shelf | Near-bottom water inshore of the 200 m isobath | Hypoxic share of the upwelling-season average rose from 2% (1950–1980) to 24% (2009–2018); 56% in summer 2021 | Increased upwelling-favorable wind forcing, per the authors, under climate change | Observations and maps since 1950; Barth et al., Scientific Reports, 2024 |
Tropical Pacific: El Niño and La Niña move the oxygen field
A 2026 study in the Journal of Climate by Eddebbar, Hoffman, Sharp, Whitt, Subramanian and Stevenson, titled “ENSO-Driven Variability of Oxygen Content and Distribution in the Tropical Pacific,” combines observations with global ocean–sea-ice simulations. Its main reported pattern is more oxygen in the eastern and central tropical Pacific during El Niño and less during La Niña. Regional results differ among the data products examined, so the pattern describes an overall tendency rather than identical values everywhere.
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| ENSO phase | Eastern and central tropical Pacific | Western tropical Pacific |
|---|---|---|
| El Niño | More oxygen | Opposite pattern: less oxygen |
| La Niña | Less oxygen | Opposite pattern: more oxygen |
How El Niño reshapes the water column
NOAA’s Pacific Marine Environmental Laboratory (PMEL) summary, “Oxygen in the Tropical Pacific responds to El Niño Forcing,” dated July 14, 2026, explains the mechanism in physical terms. During El Niño the eastern Pacific thermocline deepens, which reduces the upwelling of oxygen-poor deep water. The western Pacific experiences a large-scale thermocline shallowing. La Niña tends toward the opposite arrangement.
Four pathways that partly offset each other
The 2026 analysis describes a balance among several processes. Thermocline adjustment and vertical advection move water up and down; biological consumption draws oxygen down as organisms respire; vertical mixing and lateral transport redistribute it. The authors report substantial contributions from each, and some of them compensate for one another. A net change in oxygen therefore reflects the sum of processes that can push in opposite directions.
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An event signal is not a trend
The 2026 authors caution against reading an oxygen increase during an El Niño phase as proof that a long-term decline has reversed. A 2020 analysis of Pacific oxygen and nutrient time series since 1950, covering 50–300 m (Stramma and colleagues, Biogeosciences 17(3), 813–831), reaches a related conclusion: El Niño and La Niña often shape eastern tropical Pacific oxygen distribution during an event, but they do not have a multi-year influence on trends. Event-scale swings and decadal trends therefore need separate analysis.
North and eastern Pacific: decadal modes overlay the trend
The 2020 Stramma study describes a strong oxygen-deficient layer in the upper tropical Pacific and a deeper one in the North Pacific. Its trends are non-linear, and climate modes exert strong effects on them rather than leaving one simple trajectory.
Eastern Pacific and the Pacific Decadal Oscillation
Stramma and colleagues attribute PDO influence to tropical and eastern Pacific records. In most eastern Pacific regions within the studied layer, oxygen increases and nutrients decrease during negative PDO phases, with the opposite tendencies during positive phases. Because PDO phases persist for years to decades, a record that covers only one phase can look like a trend when it is not one.
North Pacific and the Oyashio region
In North Pacific records the study attributes a particularly strong influence to the North Pacific Gyre Oscillation (NPGO). In the Oyashio region, several climate modes and an 18.6-year tidal cycle overlay the long-term trend, which makes that trend harder to read from a short record.
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Proposed mechanisms still under study
Beyond ENSO and decadal modes, the published literature discusses warming, changes in ventilation and circulation, oxygen supply, and biological consumption. Two specific studies add distinct proposals.
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A North Atlantic link to the northern tropical Pacific (2024)
A 2024 Nature Communications paper, “North Atlantic temperature control on deoxygenation in the northern tropical Pacific,” proposes a multidecadal connection between North Atlantic temperature variability and deoxygenation in the northern tropical Pacific. The mechanism it describes runs through stratification, trade winds, equatorial circulation and the Equatorial Undercurrent, which together can affect both oxygen supply and biological oxygen demand. The paper presents this as a mechanism advanced by its own analyses. The full causal chain from the Atlantic to the Pacific is a proposal, not an established consensus.
Aerosol pollution and iron deposition (2016)
A 2016 Nature Geoscience study, “Acceleration of oxygen decline in the tropical Pacific over the past decades by aerosol pollutants,” reports from its model and sensitivity experiments that aerosol pollutants can accelerate mid-depth oxygen decline in the tropical Pacific. The mechanism is pollution-enhanced iron deposition, which raises regional productivity and respiration, interacting with natural climate variability. This shows that biogeochemical pathways can contribute. It does not show that aerosols explain every region or all of the observed decline.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.The Pacific Northwest shelf: a measured coastal case
The most detailed coastal record comes from Barth and colleagues, “Widespread and increasing near-bottom hypoxia in the coastal ocean off the United States Pacific Northwest,” Scientific Reports 14, 3798 (2024). NOAA’s Ocean Acidification Program hosted the study’s page, dated February 15, 2024.
What summer 2021 measured
Summer 2021 featured unusually strong upwelling and unusually extensive near-bottom low-oxygen water off the US Pacific Northwest. The study defines near-bottom hypoxia as dissolved oxygen below 61 µmol kg−1. It was observed over nearly half of the continental shelf inside the 200 m isobath, an area of 15,500 km². A mid-shelf ribbon about 450 km long, off north-central Oregon and Washington, had oxygen below 50 µmol kg−1. These figures describe one shelf during one event, not the whole Pacific.
The long-term record since 1950
The authors’ maps since 1950 show a consistent trend toward lower oxygen near the seafloor, although the spatial pattern depends on shelf width and other regional features. The study reports the average share of near-bottom water inshore of the 200 m isobath that was hypoxic during the summer upwelling season:
| Period | Average hypoxic share, summer upwelling season | Context |
|---|---|---|
| 1950–1980 | 2% | Nearly absent |
| 2009–2018 | 24% | Multi-year period average |
| Summer 2021 | 56% | Unusually strong upwelling season |
The authors state that the longer-term increase is consistent with increased upwelling-favorable wind forcing under climate change. In the study’s words: “Widespread and increasing near-bottom hypoxia is consistent with increased upwelling-favorable wind forcing under climate change.”
Quick Recap
What the evidence does not yet establish
- No harmonized, current basin-wide rate of Pacific oxygen loss appears in these studies. Any single Pacific-wide percentage would go beyond the published evidence.
- Hypoxic shares on the Pacific Northwest shelf cannot be compared directly with open-ocean oxygen inventories, because they measure different things.
- Each mechanism discussed above comes from an individual study. None of them ranks the drivers for the whole Pacific.
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