Global warming is causing rivers to slowly lose oxygen, threatening fish and other life in the waterways, a new study shows.Related video above: Why global calculations of rising sea levels are ... wrongResearchers in China used satellites and artificial intelligence to track and analyze oxygen levels in more than 21,000 rivers across the globe since 1985. They found oxygen levels have dropped an average of 2.1% since 1985, according to a study published Friday in Science Advances. That may not seem like much, but it adds up, and if it continues or accelerates, rivers in the Eastern United States, India and across the tropics could lose enough oxygen by the end of the century to suffocate some fish and create dead zones, the study said.Basic chemistry and physics dictate that warmer water holds less oxygen, scientists said. Warmer water, which happens with human-caused climate change, releases more oxygen into the atmosphere.If the oxygen loss rate continues at the current pace, the world's rivers on average will lose an additional 4% of their oxygen by the end of the century, and in some cases close to 5%, the study found. That is when oxygen loss — called deoxygenation — becomes problematic for fish and people who rely on rivers, according to the study's lead author, Qi Guan, an environmental scientist at the Chinese Academy of Sciences in Nanjing.More dead zones appearScientists worry that oxygen levels in rivers could fall so low that dead zones appear, as they have in the Gulf, Chesapeake Bay and Lake Erie. Those are areas where fish struggle to breathe and die.“Deoxygenation is a very slow process. If we have a long period, the negative impact will attack the river ecosystems,” Guan said. “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.”University of Arizona geoscientist Karl Flessa, who was not part of the study, said in an email that losing oxygen in rivers means “a future of more stinky dead zones (hypoxia), especially during heat waves.”Some rivers are in such bad shape that “a small change can tip them into the danger zone,” Flessa said. “If your favorite fishing hole gets too warm, oxygen levels will go down and there won't be any fish to catch.”India, Eastern US and the Amazon are hot spotsEarlier this century, India's heavily polluted Ganges River was losing oxygen more than 20 times faster than the global average, the study said. Even with moderate-to-high increases in global carbon dioxide emissions rates — not the implausible worst-case scenario — rivers in the Eastern United States, the Arctic, India and much of South America are projected to lose about 10% of their oxygen by the end of the century, the analysis showed.Guan said he worries about tropical rivers especially, such as the Amazon in Brazil. Since 1980, the number of days with dead zone spots in the Amazon rose by nearly 16 days per decade, a study last year found.Hydrology professor Marc Bierkens of Utrecht University in the Netherlands said a study he and colleagues did last year showed oxygen stress in the world's rivers increased by 13 days every decade, and dead zone occurrences increased by nearly three days a decade since 1980. As the world continues to warm, those numbers should jump even higher, said Bierkens, who did not participate in the Chinese study.Guan's study found several reasons for oxygen loss in the world's rivers, including nutrient pollution from fertilizer and urban runoff, along with dam construction, flow and wind issues. But nearly 63% of the problem is from warmer water, the study found.Duke University ecologist and biogeochemist Emily Bernhardt, who was not part of the study, said, “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.” Anoxia is the total loss of oxygen.“Water pollution reduction is more important than ever and will be harder as rivers warm,” she said.