At several points in the earth’s past the oceans have given up their oxygen and borne witness to massive die-offs of marine creatures--extinction-level events--that forever changed the composition of marine life. These episodes of oxygen depletion seem to have been caused by molten magma being released into the seas, which belched massive quantities of iron into the ocean, prompting iron-loving algae to bloom in huge fields and robbing the seas of oxygen. Or, these eruptions sent carbon dioxide into the air, beginning a greenhouse effect that warmed seawater, reducing its ability to hold oxygen. Either way, when the ocean loses oxygen on a wide scale, it is very bad.

And now, it’s happening again.

Last week, an article in the journal Science showed that since about 1950, the zones of the world’s oceans that have lower-than-normal oxygen concentrations have increased dramatically in size. Big sections of the seas have such low quantities of oxygen that fundamental processes of life for marine organisms have been rendered impossible.

A warming planet is one of the culprits this time around. As the climate warms, the seas warm, and as the seas warm they release oxygen during evaporation, and have a harder time absorbing oxygen from the atmosphere.

Hand-in-hand with climate change, the other big driver of deoxygenation, according the Science report, is the massive amounts of fertilizers and other sources of nutrients dumped into rivers and estuaries that find their way into the oceans. More nutrients means more algae, and as the algae bloom and die, they provide food for bacteria, which rapidly consume oxygen.

What does all this mean? Massive, massive reductions in sea life. Throughout the Earth’s history, when the seas lose oxygen content, it seems to trigger the sorts of cataclysmic levels of extinction that essentially forces life to reboot once oxygen levels stabilize. For those worried about the near future, that could mean massive displacement and die-offs of fish and shellfish that billions of people on the planet depend on for food and income.

According to the Atlantic, this may already be happening. Off the Oregon coast, rockfish have moved on from their traditional territories along the shallow continental shelf, as oxygen-depleted water has begun to creep in.

While in the past deoxygenation events correlated with volcanic activity, the same effects today are being driven today by processes that produce similar outcomes: fossil fuel consumption and pumping excess nutrients in the form of runoff into the sea.

What can be done about it is unclear. According to the report: “Numerical models project further oxygen declines during the 21st century, even with ambitious emission reductions.”

Nobody really knows what the future of our oceans looks like, and unlike volcanic activity millions of years ago, the causes of deoxygenation today can be halted. But that itself may prove to be as hard as plugging an erupting volcano.