The World’s Oceans Are Being Starved of Oxygen

An alarming report found that there are 700 marine sites impacted by low oxygen levels—up from 45 in the 1960s

Algae in a bay in Qingdao, China's eastern Shandong province.
Loss of oxygen allows algae to thrive, which in turn has cascading effects on marine ecosystems. -/AFP via Getty Images

For years, scientists have been ringing alarm bells over the dire state of the world’s oceans, which are becoming warmer, more acidic, and increasingly filled with plastic. Now, a report by the International Union for Conservation of Nature has revealed the alarming scope of yet another affliction: oceans are losing oxygen, quickly.

According to the Guardian’s Fiona Harvey, the new report was issued at COP25, the United Nations climate conference in Madrid. A collaboration between 67 scientific experts from 17 countries, the peer-reviewed study is “the largest ... so far into the causes, impacts and possible solutions to ocean deoxygenation,” the IUCN states.

Researchers have long known about dead zones, where oxygen concentrations are so low that most marine life dies or leaves. But according to the report, the deoxygenation problem has been rippling across the planet’s oceans. There are now 700 marine sites around the world that are impacted by low oxygen levels—up from 45 in the 1960s, the report found. The volume of anoxic waters, which are completely deprived of oxygen, has quadrupled. Between 1960 and 2010, oxygen concentrations declined worldwide by about two percent.

That might not seem like a significant loss, but we need to put the number into perspective, Dan Laffoley, an editor of the report, tells Kendra Pierre-Louis of the New York Times. “[I]f we were to try and go up Mount Everest without oxygen, there would come a point where a two percent loss of oxygen in our surroundings would become very significant,” Laffoley says. If nothing is done to reverse the current trend, we can expect oceans to lose between three and four percent of their oxygen by the year 2100, according to the researchers.

The report identifies two main causes of ocean deoxygenation. The first is climate change. Warmer waters hold less oxygen, as a general rule, and increasing temperatures also interfere with the ocean’s ability to properly distribute its oxygen concentrations. Typically, oxygen-rich waters at the surface of the ocean mix with deeper layers that are not as abundant in oxygen; but warm waters that hold less oxygen are more buoyant, leading to the stratification of the water column.

Another driver of deoxygenation is nutrient pollution, which seeps into the oceans in the form of fertilizer, sewage and animal waste from the meat industry. Excess nutrients can stimulate an overgrowth of algae, which sucks oxygen out of the water as it decomposes.

The impacts of oxygen-depleted waters are far-reaching. According to the IUCN, species like tuna, marlin and sharks are particularly susceptible because they are large and have high energy demands; when oxygen in their habitats is reduced, these animals are driven to shallow waters, where they become vulnerable to overfishing. Deoxygenation also allows certain species to thrive at the expense of many other marine mammals, throwing ocean ecosystems off balance. Jellyfish, for instance, need little oxygen to survive, and their populations are booming—which in turn disrupts human industries and harms fish.

What’s more, oxygen losses threaten to disrupt fundamental processes like the cycling of nitrogen and phosphorous—elements that are essential to life on Earth. “We lower these oxygen levels at our peril,” Laffoley tells Pierre-Louis.

Steps can be taken to save the ocean from further oxygen deprivation—steps like reducing atmospheric greenhouse gas emissions, curbing nutrient runoff and protecting vulnerable species from overfishing. “Ending overfishing is a quick, deliverable action which will restore fish populations, create more resilient ocean ecosystems, decrease CO2 pollution and increase carbon capture, and deliver more profitable fisheries and thriving coastal communities,” Monica Verbeek, executive director of the group Seas at Risk, tells the Guardian’s Harvey.

But the will to take action needs to exist. According to the report’s authors, “near-monumental efforts … will be needed by governments and society” to reverse years of damage and ensure that our oceans are able to continue breathing.