Higher temperatures mean the oceans can hold less oxygen and fish are literally starting to drown. Last summer, more than 100 miles (161 km) of Florida’s coast around Tampa Bay became an oxygen-depleted zone littered with fish along the nearby shoreline. This summer the same thing happened in Greece, where thousands of dead fish were washed up on the shore after the water they live in ran out of oxygen.

Ocean deoxygenation is now affecting about 20% of the world's oceans, primarily in the North Pacific, according to scientists, bring with it a triple threat of acid, heat and deoxygenation.

Research indicates that the volume of oxygen-depleted "dead zones" has quadrupled since the 1960s, with areas completely devoid of oxygen becoming more common. For example, midwaters off the coast of Central California have seen 40% oxygen depletion in the last few decades.​

“Much of the conversation around our climate crisis highlights the emission of greenhouse gases [GHGs] and their effect on warming, precipitation, sea-level rise and ocean acidification. We hear little about the effect of climate change on oxygen levels, particularly in oceans and lakes. But water without adequate oxygen cannot support life, and for the three billion people who depend on coastal fisheries for income, declining ocean oxygen levels are catastrophic,” Nathalie Goodkin, a chemical oceanographer and an associate curator at the American Museum of Natural History, said in a recent opinion piece for Scientific American.

Global warming contributes to a reduction of oxygen in the oceans through a combination of rising sea temperatures, altered circulation patterns and increased nutrient runoff, which leads to a process called "ocean deoxygenation."

Warmer water holds less oxygen. As the ocean absorbs heat due to global warming, the surface waters, where most fish live, warm and hold less oxygen. This year has seen record high sea temperatures. August 2024 recorded an average global sea surface temperatures of 20.91°C, the second highest for that month on record, just slightly below August 2023 levels.

The stratification of ocean layers has also changed. When surface waters warm up, they become less dense, creating a stronger separation between surface and deeper water layers. This prevents oxygen-rich surface water sinking and so also asphyxiates fish in the deeper layers as well.

Climate change has also intensified rainfall and agricultural runoff, which carries more nutrients like nitrogen and phosphorus into the seas. This excess nutrient input leads to "eutrophication," where oxygen-eating algae bloom in large quantities leading to "dead zones" in the ocean where oxygen levels are so low that fish cannot survive.

“As ocean and atmospheric scientists focused on climate, we believe that oceanic oxygen levels are the next big casualty of global warming,” says Goodkin. “To stop the situation from worsening, we need to expand our attention to include the perilous state of oceanic oxygen levels – the life-support system of our planet.”

Oceans play a dominant role in the planets heat system. They take up just under 90% of the excess heat created by climate change during the Anthropocene, the period when human activity dominates the influence on the climate. Bodies of water can also help reduce emissions as they also absorb CO₂ and oxygen but only up to a limit. Another looming problem to affect the oceans is the potential collapse of the AMOC (Atlantic Meridional Overturning Circulation) that could come in the next few decades and will result in a new ice age in Northern Europe.

Large swathes of the oceans have already lost 10% to 40% of their oxygen, says Goodkin, and that loss is expected to accelerate as climate change progresses. This escalating problems for marine life are running in parallel with the accelerating sixth extinction event happening on land, as bne IntelliNews recently reported.

And these twin mechanisms are starting to compound each other in a positive feedback loop. Yet Goodkin says scientists still know little about how the maritime systems work, despite years of research.