BEIJING, May 16 (Xinhua) -- A new study has found that climate warming is driving a widespread and sustained loss of oxygen in rivers around the world, putting freshwater ecosystems at risk.
The study, published in the journal Science Advances, was conducted by a research team led by researchers from the Nanjing Institute of Geography and Limnology under the Chinese Academy of Sciences (NIGLAS).
The researchers analyzed nearly 40 years of data from more than 21,000 river reaches across the globe. They used advanced machine learning to track changes in oxygen levels between 1985 and 2023.
The results are worrying. On average, rivers are losing oxygen at a rate of 0.045 milligrams per liter per decade. Nearly 80 percent of the studied rivers showed signs of deoxygenation.
Tropical rivers -- those located between 20 degrees south and 20 degrees north, including many in India -- are suffering the most. This goes against earlier expectations that high-latitude rivers would be hit hardest because of amplified climate warming in these high-latitude regions. The study found that tropical rivers already have low oxygen levels to begin with, and they are losing oxygen more quickly. This makes them highly vulnerable to extreme low-oxygen events, which can kill fish and other aquatic life.
The researchers found that climate-driven decline in oxygen solubility is the main cause, accounting for nearly 63 percent of the oxygen loss. Warmer water simply cannot hold as much oxygen as cooler water. Changes in ecosystem metabolism -- reflected in temperature, light and water flow -- accounted for another 12 percent of the decline.
Heatwaves also play a significant role. The study found that extreme heat events are responsible for nearly 23 percent of global river deoxygenation, speeding up the rate of oxygen loss by 0.01 milligrams per liter per decade compared to normal conditions.
The study also found that both very low and very high water flows can partly slow down deoxygenation. Low-flow conditions reduced the deoxygenation rate by about 19 percent compared to normal conditions, while high-flow conditions lowered it by 7 percent.
The researchers warned that declining oxygen levels threaten river biodiversity, as fish and other organisms struggle to survive in oxygen-depleted waters. They have urged policymakers to take action, especially in tropical regions, which are most in need of effective strategies to fight deoxygenation crises.
"This study provides a much-needed baseline for understanding and mitigating oxygen loss in rivers around the world. As climate warming continues, protecting the breath of our rivers may become one of the greatest environmental challenges of our time," said Shi Kun, a researcher at the NIGLAS. ■
