“Dead zones” are rapidly appearing in the world’s oceans as they lose oxygen at an unprecedented rate due to climate change, sewage pollution and farming practices, presenting an existential threat to marine life and ecosystems.

The overall level of oxygen in the oceans has dropped by roughly 2 per cent, while the number of known hypoxic “dead zones” – where oxygen levels are dangerously low – has skyrocketed from 45 known sites in the 1960s to at least 700 areas now dangerously devoid of the life-giving compound, some encompassing thousands of square miles.

“This is perhaps the ultimate wake-up call from the uncontrolled experiment humanity is unleashing on the world’s oceans as carbon emissions continue to increase,” said Dan Laffoley, co-editor of the International Union for the Conservation of Nature’s (IUCN) study, the largest ever analysis of the causes and impacts of ocean deoxygenation, which the organisation describes as “one of the most pernicious, yet under-reported side-effects of human-induced climate change”.