What is already happening?
  • Dissolved oxygen concentrations in the global ocean have declined by 2% since the 1960s because of a decrease in the solubility of oxygen and increase in stratification due to increasing ocean temperature. 
  • Some areas of UK marine waters have been identified as oxygen deficient in late summer. However, hypoxia has not been detected in UK marine waters unlike other regions in the North-west European shelf sea. 
  • Observations from 1990 to 2010 indicate decreased oxygen concentrations and saturation in the North Sea. Reduced oxygen solubility due to ocean warming and an increase in oxygen utilisation are contributing factors to the trend. 

Low evidence, medium agreement

On a global scale, there is high confidence that the oceans are losing oxygen due to ocean warming. Both observations and models agree that the UK coastal and shelf seas are losing oxygen, but the magnitude and causes of this decline are still being unravelled. As well, the North Sea is much better sampled than other UK waters.

What could happen in the future?
  • Dissolved oxygen concentrations will decrease as ocean temperature increases, due to a reduction in the solubility of dissolved oxygen alongside a predicted increase in the strength and duration of stratification. 
  • Models predict that by the end of the century, dissolved oxygen concentrations in the global ocean may decline by up to 4%, whilst concentrations in UK regional seas, such as the North Sea could decline by up to 11.5%. 

Medium evidence, medium agreement

At the global scale, there is high confidence that an increase in temperature will continue to lose oxygen. For coastal and shelf seas, the magnitude of loss and attribution remains uncertain due to strong seasonality in nutrient supply in a shallow water column and interaction with the sediment.

Key Challenges and Emerging Issues
  • Identifying when and where dissolved oxygen changes are being affected by human stressors (e.g., ocean warming or nutrient enrichment) rather than just natural variability. 
  • Establishing long-term datasets (outside the North Sea) to record the occurrence, frequency, and spatial extent of oxygen deficiency in UK coastal and shelf waters. 
  • Improving the resolution of dissolved oxygen data to provide more confidence in testing coastal and shelf sea models. 
  • Reducing model uncertainty in the individual and coupled processes that control dissolved oxygen dynamics (especially for coastal and shelf sediments).  
  • Establishing long-term time–series data to test coupled physical-ecosystem models, and variability in functioning between sites with different conditions.