Polski
Science, Issue 7/2026
Oxygen levels in the world’s waters are declining. Researchers are sounding the alarm about the threat to Earth
Aug
Oxygen is disappearing from lakes, rivers, and oceans at a rate that scientists say exceeds safe limits. Researchers at the University of California, San Diego argue that water deoxygenation is not merely a consequence of climate change, but a distinct process that threatens the stability of the Earth system. Therefore, they propose recognizing it as the tenth planetary boundary—alongside threats such as global warming and biodiversity loss.
Oxygen Is Disappearing from Waters Around the World
The authors of the study, whose findings were published in the journal *Limnology and Oceanography*, examined the decline in the average concentration of dissolved oxygen in water caused by human activity. It turned out that the scale of these changes is very large.
Over the past 50 years, the average oxygen content in the oceans has decreased by about 2 percent, with the greatest declines recorded in inland waters, the polar regions, and the southeastern Atlantic, the Indian Ocean, and parts of the Pacific. Currently, more than 200 locations worldwide are experiencing persistent hypoxia, defined as oxygen concentrations below 2 mgO₂/L. In rivers, oxygen levels are falling by an average of 1–1.5 percent per decade, while lakes are among the ecosystems most affected by this phenomenon—in the temperate zone, the rate of decline inO₂ concentration in lakes reaches 1.5–5 percent per decade.
Deoxygenation Fuels Other Environmental Crises
The most important message of the researchers’ work is to show that oxygen loss does not occur in isolation. The authors describe in detail the numerous feedback loops between deoxygenation and other planetary boundaries.
Global warming reduces the solubility of oxygen while simultaneously accelerating its loss through rising temperatures and increased thermal stratification of the waters. It is estimated that approximately 50 percent of oxygen loss in the upper 1,000 m of the ocean results directly from a decrease in its solubility—an effect of rising temperatures. Under the high-emissions SSP5-8.5 scenario, by the end of the 21st century, air temperature could rise by 3.7°C, ocean surface temperature by 2.6°C, and the rate of deoxygenation in the 100–600-meter layer could triple. Even with ambitious emission reductions, this process will continue, albeit at a much slower rate.
A decrease in oxygen levels in water promotes the formation of nitrous oxide (N₂O), one of the most potent greenhouse gases and currently the most significant ozone-depleting substance. Under anaerobic conditions, the production of methane and hydrogen sulfide also increases, which further affects the climate and the functioning of the atmosphere. Deoxygenation thus becomes not only a consequence of climate change but also another factor driving it.
A Threat to Aquatic Life
The study’s authors point out that oxygen depletion is one of the most serious threats to biodiversity. In extreme cases, it leads to mass fish kills and the formation of so-called dead zones, but the negative effects appear much earlier. Even a slight decrease in oxygen concentration negatively affects the growth, survival, and reproduction of aquatic organisms, increases their susceptibility to disease, and disrupts the functioning of entire food webs.
The fossil record indicates a close link between past periods of severe ocean deoxygenation and mass extinctions. Contemporary observations show a similar trend—as oxygen levels decline, the number of species decreases, and ecosystems are dominated by organisms best adapted to life in oxygen-poor conditions. As a result, both the genetic and functional diversity of ecosystems declines.
How can global deoxygenation be measured?
One of the main challenges is to develop indicators that can determine when a safe planetary threshold has been exceeded. The authors propose several possible approaches. The most obvious approach is to monitor the concentration of dissolved oxygen. Hypoxia is typically defined as levels ≤2 mgO2/l, while the complete absence of oxygen is referred to as anoxia. At the same time, studies show that about half of the benthic organisms in temperate zones exhibit mortality even at concentrations exceeding 2 mgO₂/L; therefore, some scientists propose a more conservative threshold of 4.6 mgO₂/L.
The authors also consider the use of the percentage of oxygen saturation in water, indicator organisms, and the so-called metabolic index, which describes the relationship between oxygen availability and the organisms’ oxygen demand. Each of these approaches has its advantages, but none allows for the unambiguous determination of a global safety threshold—at least not yet.
We are already in a high-risk zone
Although there is not yet a quantitatively defined planetary boundary for water deoxygenation, according to the study’s authors, the current state should be considered a “zone of uncertainty”—that is, an area of increasing risk. Three main arguments support this view.
First, deoxygenation is strongly linked to all other planetary boundaries and amplifies their negative effects. Second, aquatic systems have a very long “memory”—even a complete halt toCO2 emissions would not stop the deoxygenation process, which will continue for hundreds of years, and in the deep ocean, even for millennia. Third, the capacity for natural oxygen replenishment is very limited, as the rate of oxygen loss is not offset by production through photosynthesis or oxygenation from the atmosphere via the mixing of water masses.
New scientific and political actions are needed
Scientists emphasize that recognizing water deoxygenation as the tenth planetary boundary could significantly increase interest in this issue in both the scientific and political communities. In their view, it is essential to develop global oxygen monitoring, refine models describing changes in water bodies, and better incorporate deoxygenation into international and national environmental protection strategies.
Main photo: Mostafa Ashraf Mostafa/Unsplash
Source:
Ferrer, E.M., Eddebbar, Y.A., Gangrade, S., McCormick, L.R., Pezner, A.K., Robinson, D., Garçon, V., Rose, K.C. and Levin, L.A. (2026), Abundant interactions and feedbacks between aquatic deoxygenation and the other planetary boundaries suggest “unsafe” levels of oxygen loss with far-reaching impacts. Limnol Oceanogr, 71: e70434. https://doi.org/10.1002/lno.70434
