The sentence “every second breath you take comes from the Ocean” is commonly used in Ocean Literacy and science communication to highlight the importance of Ocean oxygen. However, despite its widespread use, it is often not phrased correctly. When highlighting the role of the Ocean in the oxygen we breathe, we recommend to use the statements:
- The Ocean produces ~50% of Earth’s oxygen;
- Every second breath taken by all life on Earth comes from the Ocean; and
- Since the origin of life on Earth, the Ocean has provided most of the oxygen in the atmosphere and is responsible for six of seven breaths humans take.
In contrast, there is little awareness about the threat of global oxygen loss in the Ocean, called deoxygenation, particularly in comparison with other important stressors, such as Ocean acidification or increasing seawater temperatures. Deoxygenation is increasing in the coastal and open Ocean, primarily due to human-induced global warming and nutrient run-off from land, and projections show that the Ocean will continue losing oxygen as global warming continues. The consequences of oxygen loss in the Ocean are extensive and include decreased biodiversity, shifts in species distributions, displacement or reduction in fisheries resources, changes in biogeochemical cycling and mass mortalities. Low oxygen conditions also drive other chemical processes which produce greenhouse gases, toxic compounds and further degrade water quality. Degraded water quality directly affects marine ecosystems, but also indirectly impacts ecosystem services supporting local communities, regional economies and tourism. Although there are gaps in our knowledge (e.g. current and historic deoxygenation rates, processes controlling oxygen dynamics, interaction with multiple-stressors, impact of deoxygenation on marine life and vulnerability of ecosystem services, society and economy), we know enough to be very concerned about the consequences: the impacts might even be larger than from Ocean acidification or heat waves, and three out of the five global mass extinctions were linked to Ocean deoxygenation.
The sense of urgency to improve Ocean health is reflected in the UN Decade of Ocean Science for Sustainable Development (Ocean Decade) and the EU Mission: Restore our Ocean and Waters (Mission Ocean), and tackling the loss of oxygen in the Ocean is critical to achieving the aims of these two initiatives. Reducing and eventually reaching net zero greenhouse gas emissions, limiting run-off of nutrients and organic waste, reducing stressors, and increasing protection of marine ecosystems will all help to reduce Ocean deoxygenation and its impacts. Ocean deoxygenation needs to be recognised as one of the major threats to marine ecosystems and included in future projections of planetary health of intergovernmental bodies and high-level frameworks.