The world鈥檚 oceans have lost around 2 per cent of their oxygen, on average, over half a century, alarming scientists who have warned of the trend鈥檚 impact on fisheries and endangered species.
The number of low-oxygen sites along coasts globally has spiralled from 45 in the 1960s to around 700 now, according to by 67 researchers for the International Union for Conservation of Nature (IUCN).
Presented at the ongoing UN climate talks in Madrid, the research predicts the oceans will lose 3 to 4 per cent of their oxygen globally by the end of the century if carbon emissions continue unchecked. Climate change makes deoxygenation worse by warming the oceans, changing ocean circulation and reducing the mixing of waters vital for bringing oxygen from the atmosphere to deeper waters.
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鈥淚t鈥檚 almost insane that it鈥檚 2019, we鈥檝e heated the ocean and there鈥檚 hardly anyone talking about what this is doing as a combined stressor. We鈥檝e managed to successfully [raise awareness of] acidification and ocean heating, but deoxygenation is the ultimate wake-up call,鈥� says Dan Laffoley at the IUCN.
Species including tuna and marlins are both vulnerable when oxygen levels drop because of their high oxygen demand. Low-oxygen zones can also harm coral reefs 鈥� one event off the coast of Australia killed a million reef fish.
The 2 per cent average global decline between 1960 and 2010 masks big regional differences. While a handful of places may have gained oxygen, Laffoley says some areas off the coast of California have lost 30 to 40 per cent. The figures are almost certainly an underestimate due to poor monitoring in some regions, such as off the coast of China, he says.
Alongside ocean warming, the other big driver for deoxygenation is the run off of polluting nutrients from agriculture into rivers and then coastal waters. The nutrients fuel algae growth, and when bacteria eventually break down the algae they use up the oxygen in the water.
Nicolas Gruber at ETH Zurich, who wasn鈥檛 involved in the report, says the problem is oxygen loss is combining with other stresses on oceans and marine life. 鈥淚t鈥檚 a triple whammy. We have ocean warming, ocean acidification and deoyxgenation. Maybe one in itself may not be so bad, but then you add a second one and then a third one on top.鈥�
Laffoley says the good news is we know how to solve the problem of oxygen loss: radically reduce the carbon emissions released into the atmosphere and cut nutrient run-off.
