NANJING, June 26 (Xinhua) -- A little more than a month after scientists warned that climate change is causing widespread oxygen loss in the world's rivers, a new study delivers a hopeful message: targeted interventions can reverse this trend, even as global temperatures climb.
Published on Friday in the journal Nature Geoscience, the research found that oxygen levels in China's inland waters actually increased between 2005 and 2022, despite surface waters warming at a rate of 1.2 degrees Celsius per decade. The main factor behind this improvement wasn't nature, but substantial investment in wastewater management.
The study, led by researchers from the Nanjing Institute of Geography and Limnology under the Chinese Academy of Sciences (NIGLAS), analyzed 18 years of monthly data from 972 rivers and 354 lake sites across China. The findings directly challenge the prevailing narrative that aquatic deoxygenation -- the steady decline of dissolved oxygen in water -- is an unstoppable consequence of global warming and human development.
According to the study, on average, dissolved oxygen levels rose by 0.93 milligrams per liter per decade in rivers and 0.38 milligrams per liter per decade in lakes. The improvement was so significant that hypoxic, or low-oxygen, events in rivers plummeted from 170 occurrences between 2005 and 2010 to just 25 between 2017 and 2022.
The researchers used advanced machine learning and variance partitioning to determine what was driving this unexpected recovery. While warmer water naturally holds less oxygen, they found that reductions in organic pollution had more than compensated for the oxygen loss expected from warming. The strongest predictors of rising oxygen levels were decreases in biochemical oxygen demand, ammonium, and chemical oxygen demand -- all key indicators of organic waste.
The team credits China's massive environmental push for these gains. Between 2000 and 2022, wastewater treatment coverage expanded from 34.3 percent to 98.1 percent of the population, leading to nationwide declines in organic pollutants and nutrients like nitrogen and phosphorus.
"The correlations between provincial investment in sewer infrastructure, the volume of wastewater treated, and the magnitude of dissolved oxygen recovery are exceptionally strong," said Zhou Yongqiang, a professor at NIGLAS, adding that the recovery was most pronounced in small headwater streams and the warm-temperate zones of central China.
Earlier research published in the journal Science Advances in May found that nearly 80 percent of rivers globally were losing oxygen, with tropical rivers being hit the hardest. That study, also led by researchers from the NIGLAS, highlighted the threat to biodiversity and the urgent need for action.
"The new findings do not contradict that earlier warning," Zhou said. "Rather, they provide a clear and hopeful message: with effective water quality management, societies can protect aquatic life and reduce the risk of deoxygenation even as the climate continues to warm."
"These results provide clear optimism for global restoration efforts," Zhou said. ■
