- The oxygen content of the ocean has declined by around 2% since the middle of the 20th century overall, while the volume of ocean waters completely depleted of oxygen has quadrupled since the 1960s.
- Ocean oxygen levels are expected to fall on average by 3–4% by 2100 overall due to climate change and increased nutrient discharges, though the scale of effect seen will vary regionally.
- Consequences of ocean oxygen decline include decreased biodiversity, shifts in species distributions, displacement or reduction in fishery resources and expanding algal blooms.
- Ocean deoxygenation threatens to disrupt the ocean’s food provisioning ecosystem services.
- To slow and reverse the loss of oxygen, humans must urgently mitigate climate change globally and nutrient pollution locally.
The ocean represents 97% of the physical habitable space on the planet and is central to sustaining life on Earth. Loss of oxygen from the ocean will have severe impacts on marine biodiversity and the functioning of ocean ecosystems. Much remains unknown as to the long-term human health, social and economic consequences of ocean oxygen loss.
Virtually all the ocean’s food provisioning ecosystem services for humans require oxygen to support organism growth and production. Oxygen is necessary to sustain the life of all the fishes and invertebrates, and deoxygenation is already causing measurable declines of fish species. Some of the ocean’s most productive biomes, supporting one fifth of the world’s wild marine fish harvest, are formed by ocean currents that bring nutrient-rich but oxygen-poor water to coasts that line the eastern edges of the world’s ocean basins. As naturally oxygen-poor systems, these areas are particularly vulnerable to ocean deoxygenation, and such effects may ripple out and affect hundreds of millions of people. Degraded habitats caused by climate change-driven hypoxia and increased algal blooms may lead to reduced catches for fisheries and possible collapse of regional stocks.
Ocean deoxygenation has started to alter the balance of marine life, favouring hypoxia-tolerant species (e.g. microbes, jellyfish and some squid) at the expense of hypoxia-sensitive ones (many marine species including most fish). Large species such as tuna, marlin, swordfish and sharks are especially sensitive to low ambient oxygen conditions because of their large size and are being forced into increasingly narrow surface layers of oxygen-rich waters. Changes in the distributions of fish species could have significant negative socio-economic impacts – where fish populations decrease in abundance or move away from traditional fishing grounds, fishers may struggle to adapt.
Ocean deoxygenation may also influence the movement of gases between the ocean and the atmosphere. Deoxygenated deeper ocean waters produce greenhouse gases such as nitrous oxide, carbon dioxide and methane that may reach the ocean surface and be released into the atmosphere, contributing to further warming.