The future of our rivers: how climate change will drive a decrease in oxygen
Scientists have long warned that rising temperatures could reduce the amount of oxygen that water can hold. Until now, estimates of how this process will unfold have been limited to local studies. For the first time, this issue was tackled on a global scale, by PhD candidate Duncan Graham as part of the NWO Vidi project led by Prof. Michelle van Vliet. The results of their study show that rivers around the world will experience significant declines in dissolved oxygen levels over the coming decades due to climate change. The findings suggest that freshwater ecosystems will face longer periods of extremely low dissolved oxygen levels (hypoxia), with potentially severe consequences for aquatic life.
Graham and his colleagues designed a hybrid model and based on a scenario that closely resembles our current global emissions pathway. They found that the number of stress days will increase by around 20 days per decade from 2020 to 2100 – a rise 70% higher than that observed between 1980 and 2019. This dramatic change is largely driven by rising water temperatures caused by climate change.
Dissolved oxygen trends
The team calculated the average dissolved oxygen concentration in rivers across different regions using data collected between 1980 and 2019. They identified temperature and microbial oxygen consumption as the main factors influencing oxygen levels. Both have increased globally, leading to an overall decline in dissolved oxygen concentration.
Machine learning and process-based modeling
To conduct this research, Graham and the research team designed a hybrid model that accounts for natural processes as well as climate-driven variability. Process-based models are well suited for analyzing scenarios based on known physical and biological processes, but they struggle to represent extreme climate events such as droughts or heatwaves. Machine learning helps overcome this limitation by improving the model’s ability to better capture extremes, such as impacts of droughts and heatwaves on dissolved oxygen concentrations. By combining both approaches, the model can more accurately represent past conditions and future projections.
Consequences for wildlife
The drastic reductions in dissolved oxygen levels in rivers may have severe consequences for aquatic wildlife, notably increasing the frequency of fish kill events. However, the impacts will vary depending on region and species. Species living in cold and temperate regions are generally more sensitive to fluctuations in dissolved oxygen, while species in tropical regions tend to be more resistant. To fully assess the effects of declining oxygen levels, the authors call for further research into the adaptive capacities of freshwater fish.
Article
Duncan J. Graham, Marc F. P. Bierkens, Edward R. Jones, Edwin H. Sutanudjaja & Michelle T. H. van Vliet, ‘Climate change drives low dissolved oxygen and increased hypoxia rates in rivers worldwide’, Nature Climate Change, https://doi.org/10.1038/s41558-025-02483-y