What Happens When Oceans Can't Breathe

“Every ocean basin in the modern ocean is losing oxygen.”

By now, it's no secret that climate change is seriously disrupting Earth's oceans. Rising temperatures, increased acidification, and habitat loss have all contributed to the grim outlook for future marine biodiversity.

Now, a team of scientists based out of the University of California, Davis have extensively outlined another looming, climate-driven threat: the loss of oxygen in huge swaths of ocean habitat. The team's results were published today in PLOS ONE.

"Oxygen is fundamental to life in the ocean, just as it's fundamental to life out of the ocean," lead author Sarah Moffitt told me over the phone. "Every ocean basin in the modern ocean is losing oxygen."

To anticipate the effect that this global deoxygenation might have on ocean ecology, Moffitt and her colleagues studied the most recent warming period, which lasted from about 17,000 to 10,000 years ago.

She emphasized that this prehistoric deglaciation period and our modern anthropogenic climate crisis are not perfectly analogous. But even so, broad assessments can be made regarding the effect of climate change on ocean oxygen levels.

"Modern climate change is so bizarre," said Moffitt. "It's so important to understand, and it's so critical right now that we as scientists are looking at these past events to understand what is the capacity for Earth's systems to change."

Ocean biodiversity would be adversely affected by deoxygenation. Image: NOAA

To get a clearer picture of the last warming period's effect on ocean oxygen levels, Moffitt's team studied 36 sedimentary samples, originating from wide range of geographic locations on the seafloor.

"These are these beautiful archives of environmental change," Moffitt said of the core samples. "You can use geochemistry, you can use paleoecology, and you can get records from these cores that are as good as an ice core record."

The team's comprehensive analysis of the samples revealed that Oxygen Minimum Zones (OMZs) were greatly expanded during the last warming period. While OMZs are normally a natural part of these ecosystems, formed by bacterial respiration in the intermediate layer of the oceans (among other processes), their climate-driven growth completely disrupted the late Pleistocene world.

"When the planet abruptly warmed at deglaciation, low oxygen zones expanded very rapidly," Moffitt told me. "They expanded across an enormous geographic footprint from the Subarctic Pacific to the California Current, to the Mexico margin, down into the southern hemisphere, into the Humboldt Current. In some places, they lost oxygen from almost 100 meters below the ocean surface down to 3000 meters in the abyssal ocean."

This literally breathtaking oxygen depletion severely impacted marine wildlife and biodiversity, and it will be just as devastating if anthropogenic climate change continues to warm the globe. As Moffitt points out, the animals that live in the upper, oxygenated part of the ocean are the ones feeding humankind, and that's without mentioning all of the endangered species that need oxygenated waters to stave off extinction.

A repeat of this cycle in our own time could completely destabilize global marine ecology, and may well already be on the way to doing so. Not only will existing species be edged out, but new top predators will emerge to fill the niches opened up by decreasing oxygen levels. Moffitt mentioned that this phenomena has already been observed with the opportunistic Humboldt squid, which has expanded its range significantly over the past decades.

Humboldt squid. Image: NOAA/MBARI 2006

"What I want to emphasize is that the scale of environmental disturbance that is possible with climate change completely dwarfs the way that we consider managing our environment right now," she told me. "We really have to open our eyes to see the scale, the capacity for these systems to really be abruptly disturbed, and we're talking a disturbance on the scale of Earth system change."

"From a human perspective," she added, "what I want is for my son to be able to grow up in a world that has as much abundance and beauty as I have known. I really think that our jobs as people is to care for the beautiful planet that we have in front of us and pass that richness and beauty off to the generations in front of us, and I am really deeply concerned that my son will not have the same kind of planet and environment to enjoy the rich experiences that I've had."