Global warming is steadily stripping oxygen from the world’s rivers, a trend that threatens to create vast dead zones and suffocate fish, according to a major new study.
Researchers at the Chinese Academy of Sciences used satellites and artificial intelligence to track oxygen levels in more than 21,000 rivers across every continent from 1985 to the present. Their analysis, published Friday in the journal Science Advances, found that dissolved oxygen concentrations have fallen by an average of 2.1% over that period — and the primary driver, accounting for roughly 63% of the loss, is rising water temperature.
“Warmer water holds less oxygen, and as climate change heats rivers, it becomes easier and easier for the same pollution problems to cause more severe, more long-lasting or more widespread hypoxia,” said Emily Bernhardt, a Duke University ecologist and biogeochemist who was not involved in the study.
The 2.1% global decline has occurred even as the world’s rivers have been burdened with nutrient pollution from fertilizer, urban runoff, and dam construction. But the study’s lead author, Qi Guan, an environmental scientist at the Nanjing-based institute, said temperature now outweighs those factors. “Deoxygenation is a very slow process,” Guan said. “If we have a long period, the negative impact will attack the river ecosystems. The low level of oxygen can cause a series of ecological crises such as biodiversity decline, water quality degradation and maybe some fish will die.”
If the rate of oxygen loss remains unchanged, the study projects that rivers will shed an additional 4% of their oxygen by the end of the century. Some hot spots will fare far worse. Under scenarios of continued moderate-to-high greenhouse gas emissions, waterways in the Eastern United States, India, the Arctic, and much of South America could lose roughly 10% of their oxygen — enough to push many into the danger zone.
“A small change can tip them into the danger zone,” said Karl Flessa, a University of Arizona geoscientist who was not part of the research. “If your favorite fishing hole gets too warm, oxygen levels will go down and there won’t be any fish to catch.”
Flessa added that the loss of oxygen portends “a future of more stinky dead zones, especially during heat waves.”
The accelerating trend is already visible in the Amazon, where days with dead-zone conditions have increased by nearly 16 days per decade since 1980, according to a 2025 study. Marc Bierkens, a hydrology professor at Utrecht University in the Netherlands, said his own research found that oxygen stress in rivers globally rose by 13 days per decade, while dead-zone occurrences climbed by nearly three days per decade over the same period. As the planet continues to warm, those numbers will climb higher, Bierkens said.
“Water pollution reduction is more important than ever and will be harder as rivers warm,” Bernhardt said.