Global warming is slowly stripping oxygen from rivers around the world, and scientists warn that the trend could intensify into conditions that harm fish ecosystems and worsen water quality. The new findings, reported in a study published in Science Advances, drew on satellite observations and artificial intelligence to analyze oxygen levels across more than 21,000 rivers over multiple decades. The researchers also projected where the pattern could worsen most by the end of the century.
The study’s analysis shows that oxygen levels have fallen on average by 2.1% since 1985. While the percentage drop appears modest, the researchers said the cumulative effect matters because oxygen is essential for aquatic life, and continued warming can tilt rivers toward lower-oxygen states. In their description of the underlying physics, scientists said warmer water holds less oxygen as human-caused climate change raises water temperatures.
If the oxygen-loss rate continues as measured, the study found the world’s rivers would lose an additional 4% of oxygen by the end of the century, with some cases nearing 5%. The researchers said that level of oxygen loss—called deoxygenation—can become a problem for fish and for people who rely on rivers. Qi Guan, an environmental scientist at the Chinese Academy of Sciences in Nanjing and lead author of the study, said deoxygenation is slow, but that a long period of low oxygen can stress river ecosystems.
Guan warned that sustained low oxygen can set off multiple ecological problems. He said, “Deoxygenation is a very slow process. If we have a long period, the negative impact will attack the river ecosystems,” adding that low oxygen can cause “a series of ecological crises such as biodiversity decline, water quality degradation and maybe some fish will die.” Another geoscientist who was not involved in the study, Karl Flessa of the University of Arizona, said in an email that oxygen loss points toward “a future of more stinky dead zones (hypoxia), especially during heat waves.”
Flessa described how quickly systems can cross into danger in river places already under stress. He said that a “small change can tip them into the danger zone,” and noted that higher temperatures reduce oxygen in ways that can affect fishing. He said, “if your favorite fishing hole gets too warm, oxygen levels will go down and there won’t be any fish to catch.”
The study also identified regions that face heightened risk. It said India’s heavily polluted Ganges River was losing oxygen more than 20 times faster than the global average earlier this century. The analysis projected that rivers in the Eastern United States, the Arctic, India and much of South America could lose about 10% of their oxygen by the end of the century, even under scenarios that include moderate-to-high increases in global carbon dioxide emissions rather than an extreme worst-case pathway.
Tropical rivers were singled out as a concern as well. Guan said he worries about tropical rivers such as the Amazon in Brazil, and the study cited prior research suggesting that since 1980 the number of days with dead-zone spots in the Amazon has risen by nearly 16 days per decade. Another outside expert, hydrology professor Marc Bierkens of Utrecht University, said a study he and colleagues published last year found oxygen stress in the world’s rivers increased by 13 days every decade and that dead zone occurrences increased by nearly three days a decade since 1980. Bierkens said those numbers should rise as the world warms.
To explain why oxygen is declining, Guan’s team pointed to multiple contributing factors. The study cited nutrient pollution from fertilizer and urban runoff, along with dam construction and other local issues involving flow and wind patterns. Still, it found that nearly 63% of the oxygen-loss problem comes from warmer water, aligning with the basic chemistry argument that warmer water supports less dissolved oxygen. Duke University ecologist Emily Bernhardt, also not part of the study, said the warming trend can worsen how existing pollution problems play out, telling AP that “as rivers warm it becomes easier and easier for the same pollution problems as before to cause more severe, more long lasting or more widespread hypoxia and anoxia.” Bernhardt said that “Water pollution reduction is more important than ever and will be harder as rivers warm,” underscoring the role of both climate and pollution in determining how low-oxygen conditions evolve.