WASHINGTON, July 27, 2026 — A 2025 study in the journal Science documented that the ocean’s oxygen-starved zones have expanded by roughly one million square kilometers each decade since 1970. The finding, which draws on decades of satellite and shipboard measurements, confirms what researchers first warned of half a century ago: the world’s oceans are slowly suffocating. The phenomenon unfolds through two linked mechanisms.
Warmer water holds less dissolved gas — solubility drops by about 1.8 percent for every degree Celsius of warming. Meanwhile, nutrient runoff from agriculture and sewage — nitrogen and phosphorus in particular — fuels dense algal blooms.
When those blooms die and decompose, the process consumes oxygen faster than it can be replenished.
The mechanism
The planet has already warmed 1.1 degrees Celsius above pre-industrial levels, a figure from the Intergovernmental Panel on Climate Change’s Sixth Assessment Report, released in 2021. That warming, combined with changes in ocean circulation, creates stagnant, stratified layers of water. In a stratified column, oxygen from the surface can no longer mix downward to the depths where it is needed.
Oxygen minimum zones — or OMZs — form most persistently at depths between 200 and 1,000 meters. The eastern tropical Pacific and the Arabian Sea are the two largest and most studied regions.
There, dissolved oxygen concentrations fall below 0.5 milliliters per liter. The physical chemistry was first described in the 1970s by researchers including Dr. John P. Smiley and Dr. Thomas S.
Bianchi. But systematic monitoring only began after the launch of satellite-based ocean color sensors in the 1990s, which allowed scientists to track chlorophyll — a proxy for algal blooms — across entire basins.
The data
The 2025 Science study built on that monitoring record. Its authors concluded that cumulative dissolved oxygen loss is now detectable across every ocean basin, not just in the coastal dead zones that have long drawn public attention. The spatial spread of OMZs has expanded by roughly one million square kilometers per decade since 1970.
That consistency is itself a warning. On the timeline from initial scientific description in the 1970s to satellite-era monitoring in the 1990s to the 2025 synthesis, the trajectory has not bent.
The physical drivers — warming and nutrient loading — have not reversed, and the ocean has continued to lose oxygen.
A widespread threat
The implications extend beyond the marine food web. Low-oxygen zones compress the habitat of fish and shellfish, pushing commercially important species into shallower, often more vulnerable waters. Decomposition in oxygen-depleted water also produces nitrous oxide, a potent greenhouse gas, creating a feedback loop that further alters the ocean’s chemistry.
The record shows that no major ocean basin has been spared. The statement is sober — not breathless — but the cumulative weight of the data is clear.
What was once considered a localized phenomenon in coastal dead zones has become a basin-scale feature of the modern ocean. What to watch next: whether the expansion rate itself accelerates as the 1.1-degree-Celsius baseline rises. The 2025 study provides a benchmark; the next set of satellite sensors, scheduled for launch later this decade, will allow researchers to test whether the trend is steepening or holding.
For now, the numbers speak for themselves.
