‘Dead zones’ expanding rapidly in oceans as climate emergency causes unprecedented oxygen loss

“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, according to a vast new study.

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.

Many larger and more active sea creatures, like sharks, marlins and tuna, are unable to survive in these areas, risking mass extinction in the long term unless current trends are reversed.

“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 report.

The International Union for the Conservation of Nature’s (IUCN) study is 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”.

The study was presented at the UN’s climate conference (COP25) in Madrid, which has been described as signatories’ last chance to ensure the Paris agreement’s aim to limit global warming to 1.5C above pre-industrial levels remains achievable.

World leaders and delegates are trying to establish new international rules for emissions trading and broker systems of compensation for poorer countries bearing the brunt of climate breakdown, with many countries needing to hash out the details of targets set for 2030 and 2050 before the five-year grace period ends in 2020.

“To curb ocean oxygen loss alongside the other disastrous impacts of climate change, world leaders must commit to immediate and substantial emission cuts,” said IUCN acting director general, Dr Grethel Aguilar.

“The potentially dire effects on fisheries and vulnerable coastal communities mean that the decisions made at [COP25] are even more crucial.”

While an overall drop of 2 per cent may sound insignificant, some of the world’s most productive areas of flora and fauna are formed by ocean currents, rich in nutrients but low in oxygen – meaning these vital hubs of life are particularly vulnerable to even small changes in ocean oxygen levels.

Impacts to these currents “will ultimately ripple out and affect hundreds of millions of people”, the IUCN said in a statement, adding that the loss of oxygen “is starting to progressively alter the balance of life” as species incapable of surviving such conditions die out.

The level of oxygen loss already recorded is significant enough to affect the planetary cycling of elements such as nitrogen and phosphorous, which are “essential for life on Earth”, Dr Laffoley told The New York Times.

While mainstream attention has focused on plastic pollution and overfishing, the report states “there is no environmental variable of such ecological importance to marine ecosystems that has changed so drastically in such a short period of time as a result of human activities as dissolved oxygen”.

While the introduction of too many nutrients that enter the ocean via farming and pollution are contributing to the oceans’ deoxygenation, the most significant factor is global warming.

The temperature of the oceans has broken records nearly every year over the past two decades, while the rate of ocean warming is equivalent to five Hiroshima-size atomic bombs exploding every second, Dr John Abraham, a professor of thermal sciences, wrote in January.

This has drastic implications for the rate of global warming as a whole.

So far, oceans have acted as a buffer, absorbing more than 90 per cent of the heat associated with greenhouse gas emissions.

If the heat absorbed by the oceans since 1955 had gone into the lower levels of the atmosphere instead, land temperatures would be 36C warmer, Dr Laffoley said.

As the oceans heat up, they expand, meaning oceanic heating bears more responsibility for sea level rise than melting ice caps.​

In addition, the oceans absorb nearly a third of greenhouse gas emissions, but studies have shown this process slows as they grow hotter, leaving more heat-inducing emissions in the atmosphere.

While seemingly fortunate for life on land, this absorption has resulted in the acidification of the ocean, which is now thought to be 26 per cent more acidic than in pre-industrial times.

The report finds that oceans are now expected to lose between 3 and 4 per cent of their oxygen by 2100. But the kilometre closest to the water’s surface, where many species are concentrated, will be more negatively impacted than deeper areas, which are less rich in life.

“Urgent global action to overcome and reverse the effects of ocean deoxygenation is needed,” said IUCN global marine and polar programme director Minna Epps.

“Decisions taken at the ongoing climate conference will determine whether our ocean continues to sustain a rich variety of life, or whether habitable, oxygen-rich marine areas are increasingly, progressively and irrevocably lost.”