Climate change drives low dissolved oxygen and increased hypoxia rates in rivers worldwide

Increased water temperatures under climate change will probably cause decreases in dissolved oxygen and an associated increase in the number of days with hypoxia. This could have major consequences for freshwater ecosystems, but the extent of this threat remains unclear. Here we analyse trends in dissolved oxygen concentrations and days with stress and hypoxia in rivers worldwide between the periods 1980–2019 and 2020–2100 under global change. To achieve this, we train a hybrid process-based and machine learning model on approximately 2.6 million observations of dissolved oxygen, and we apply this model under both past and future conditions globally. The model projects significant decreasing trends in dissolved oxygen in most of the world’s rivers, resulting in on average 8.8 ± 2.3 more hypoxia days per decade globally between the years 2020 and 2100, and indicating a potentially major threat to freshwater ecosystems worldwide. Dissolved oxygen concentrations are expected to decline with rising water temperatures under climate change. This study projects declining oxygen levels for most rivers globally and an increase in hypoxic days by the end of the century, with implications for ecosystem and fish health.