Rivers, these vital arteries of our planet that provide water, food and livelihoods to millions of people, while at the same time supporting invaluable ecosystems, are gradually losing their oxygen. This is shown by a new international study published in Science Advances, which attributes the “suffocation” of the Earth’s rivers to the continuous warming imposed by the climate crisis, while also highlighting the role of extreme heat events, such as heatwaves, as well as human interventions. Based on the findings, the downward trend of river oxygen levels is expected to continue until the end of the 21st century, foreshadowing adverse consequences both for aquatic ecosystems and for humans, who depend on them.
An extensive environmental threat
According to researchers from the Institute of Geography and Limnology of Nanjing at the Chinese Academy of Sciences, the phenomenon of the gradual decline of dissolved oxygen in freshwater has now taken on global dimensions, disrupting the fragile ecological balance of river systems. The study is based on an extensive dataset from 21,439 rivers around the world, covering a period of nearly four decades, from 1985 to 2023. The results reveal that 78.8% of the rivers examined recorded a decline in dissolved oxygen levels in the water during this period.
Tropical regions appear particularly vulnerable, where the rates of decline are more intense and already coexist with lower baseline concentrations of dissolved oxygen compared with other geographical zones. These areas are emerging as the most sensitive points of a global water system that is changing quietly but steadily.
Studying oxygen from… space
To map global changes in the dissolved oxygen of rivers, the researchers relied on data from NASA’s Landsat system, one of the longest-running Earth satellite monitoring programs, which has systematically recorded the planet’s surface since 1972. The satellite images were processed through the Google Earth Engine platform, which enabled continuous monitoring of rivers.
The satellite data were then combined with chemical analysis data. The chemical data came from a multidimensional network involving field chemical analyses carried out over the past approximately four decades. The “marriage” of the data was undertaken by Artificial Intelligence, whose training used more than 10,000 field measurements of water quality from the international Global River Water Quality Archive (GRQA) database.
The database gathers data from major monitoring networks, such as the global GEMStat and GLORICH networks, the American WQP, the Canadian CESI and the European Waterbase, which also includes data from Greek rivers, such as the Evros, Nestos, Strymonas and Axios, within the framework of the national monitoring networks of EU member states.
Satellites record how water reflects light at different wavelengths, allowing the indirect estimation of parameters related to oxygen, such as turbidity or the presence of phytoplankton, organic matter and nutrients. This meticulous combined analysis led, for the first time, to the global mapping of changes in oxygen levels in the planet’s rivers.
A steady decline worldwide
As emerged from the study, the concentration of dissolved oxygen in rivers is decreasing at a rate of approximately 0.045 mg/L per decade, while the most intense declines are mainly found in tropical and subtropical regions, such as South America (especially the Amazon), India, Southeast Asia and parts of Africa.
In Europe and the Mediterranean, a general downward trend is also observed, although it is less extreme; nevertheless, European rivers are not unaffected, and continuous monitoring and careful management are therefore required, the researchers emphasize.
Climate models indicate that the decline of oxygen in rivers will continue its course until the end of the 21st century, even if pollutant emissions are drastically reduced. The total decrease is estimated to reach up to 4.7%, while the greatest losses are expected in tropical and mountainous regions.
Heatwaves and human intervention
Scientists emphasize that rising water temperatures are the main driver behind the reduction of dissolved oxygen, as they limit its ability to remain in water and are responsible for more than 60% of the observed change. At the same time, extreme thermal phenomena, such as heatwaves, play an important role, while changes in river flow and human interventions — including dams and reservoirs — also affect water temperature and dynamics.
The study is published at a time when the decline of oxygen in aquatic ecosystems is emerging as an increasingly concerning environmental challenge. Deoxygenation places a deep burden on aquatic organisms, disrupts their reproductive balance and, under extreme conditions, can even lead to massive losses of fish stocks.
Over time, this process erodes biodiversity and reshapes the structure and morphology of ecosystems, leaving its mark on humans as well, particularly in societies that draw their water supply, irrigation and fishing livelihoods from rivers.
The sources that have sustained civilizations for centuries now seem to be… losing their breath. And as the data show that this trend is difficult to reverse, the stakes extend beyond scientific understanding: they concern the very preservation of life within the ecosystems of the future.
