Ocean climate fixes could worsen the oxygen crisis

In the race to slow global warming, scientists have turned to the ocean. As the planet’s largest carbon sink, the ocean absorbs around a quarter of human-made CO₂ emissions. This vast system offers potential solutions to reduce atmospheric carbon and possibly restore declining oxygen levels.

Yet, as new research shows, not all solutions help the ocean itself. In fact, some of the most widely discussed marine carbon dioxide removal (mCDR) techniques could worsen one of the ocean’s most urgent problems: deoxygenation.

Oceans are losing oxygen at a startling rate. Since the 1960s, global oceans have shed about two percent of their oxygen inventory. This trend is not just statistical. It plays out in rising fish mortality, biodiversity loss, and shrinking habitats for marine life.

Global warming, which causes warmer water and stronger layering, reduces the ocean’s capacity to circulate oxygen to deeper layers. But what if the very methods designed to slow warming make this loss worse?

Unintended impacts of carbon removal

A new study published in the journal Environmental Research Letters confronts this question directly. Led by Dr. Andreas Oschlies of GEOMAR Helmholtz Centre for Ocean Research, the research team analyzed several mCDR methods.

These included ocean fertilization, macroalgae farming, artificial upwelling, ocean alkalinity enhancement, and others. Using idealized global simulations, they examined both the direct and indirect effects of these techniques on ocean oxygen.

“What helps the climate is not automatically good for the ocean,” said Oschlies. His team collaborated with the UNESCO Global Ocean Oxygen Network (GO2NE), comparing how each method balances the goal of carbon removal against its unintended oxygen impacts.

The study shows that many biotic mCDR methods, especially those involving biomass production and sinking, can cause severe oxygen losses.

Ocean fertilization’s deep drawbacks

Among the oldest proposed mCDR methods is ocean fertilization. It involves adding nutrients, usually iron, to stimulate phytoplankton growth in the ocean surface.

These microscopic plants absorb CO₂ and then sink after death, carrying carbon to deeper waters. But as the phytoplankton decay, they also consume oxygen.

Model simulations show that 100 years of continuous Southern Ocean fertilization would deplete the region’s oxygen by about three percent globally. That is more than double the oxygen loss caused by warming alone.

In some areas beneath the fertilized zones, oxygen levels could drop by over 50 micromoles. These changes mirror or exceed the deoxygenation seen under high-emission climate scenarios.

Simulations also reveal that any oxygen gains from reduced global warming, thanks to this carbon removal, are dwarfed by the losses from biomass decomposition.

“Our model simulations show that such approaches could cause a decrease in dissolved oxygen that is four to 40 times greater than the oxygen gain expected from reduced global warming,” said Oschlies.

Seaweed solution and its limits

Macroalgae farming has gained popularity as a promising mCDR strategy. Seaweeds grow quickly, require no freshwater, and absorb large amounts of carbon. But once again, what happens after growth determines its environmental impact.

If macroalgae is sunk into the deep ocean, it will eventually decompose, just like fertilized plankton. Complete remineralization of sunk macroalgae could reduce global ocean oxygen by more than ten percent within a century.

Even if only a portion of it breaks down, the oxygen impact is still severe. These results raise red flags for projects proposing large-scale open-ocean kelp sinking as a climate fix.

Yet, there is one version of this method that does not harm oxygen levels. When seaweed is harvested and removed, either used in products or stored safely, oxygen is spared. No decomposition occurs in the ocean, and the removal of nutrients even reduces oxygen use elsewhere.

Model results show that harvesting could boost global ocean oxygen by over two percent. In fact, this gain is ten times larger than the benefit from reduced warming alone.

Cooler surface, deeper damage

Artificial upwelling is another mCDR idea. It involves pumping cold, nutrient-rich deep water to the surface to stimulate photosynthesis.

While this can help absorb CO₂, it also increases organic matter production. That biomass, once it sinks, consumes oxygen as it breaks down.

Simulations show that artificial upwelling can lower oxygen by around 1.5 Pmol, less than some other methods, but still significant. The impact is concentrated in the upper few hundred meters, especially in tropical and subtropical zones already vulnerable to oxygen decline.

Some areas may see oxygen levels fall by ten percent. Worse, if used alongside macroalgae farming, it could intensify the oxygen drain even further.

Cooling effects from upwelling do offer minor gains in oxygen solubility. But these benefits are offset by biomass decay. As the study notes, upwelling could even lead to more warming in the long term if not managed carefully.

Daily oxygen cycles in the ocean

Some proposals involve sinking land-based biomass like wood or crop waste into the ocean. In this case, the ocean acts as a storage site, not a carbon producer. Depending on the composition of the biomass, oxygen consumption could be slower than with marine material.

But if nutrients from this decay reach the surface, they might fertilize surface waters and trigger the same oxygen-consuming cycle seen in ocean fertilization.

Blue Carbon Ecosystems, such as mangroves, salt marshes, and seagrass beds, also receive attention in mCDR strategies. They absorb CO₂ and store it in sediments. Oxygen impacts in these zones are complex. Photosynthesis boosts oxygen during the day, but respiration reduces it at night.

These systems can develop intense daily oxygen cycles, which may stress marine life in both oxygen-rich and oxygen-poor phases. While small in global area, their local oxygen dynamics deserve careful monitoring.

Geochemical option with fewer risks

Not all mCDR methods worsen oxygen loss. Ocean alkalinity enhancement, or OAE, stands out for its minimal oxygen impact. This method involves adding alkaline substances like ground limestone to seawater. It changes ocean chemistry to absorb more CO₂ without stimulating biomass production.

Model results show that large-scale OAE could slightly increase global ocean oxygen. The rise is modest, about 0.5 Pmol, but consistent.

Unlike biotic methods, OAE does not add nutrients that might fuel oxygen-consuming decay. For now, this makes it one of the safer marine carbon strategies from an oxygen standpoint.

Oxygen tracking and ocean carbon removal

The research concludes with a strong recommendation. Any mCDR project, whether a small-scale trial or a full deployment, must measure oxygen from start to finish.

“The ocean is a complex system which is already heavily under pressure,” said Oschlies. “If we intervene with large-scale measures, we must ensure that, no matter how good our intentions are, we are not further threatening marine environmental conditions that marine life depends on.”

Oxygen is one of six Essential Climate Variables already tracked for ocean health. It links directly to biodiversity, ecosystem services, and carbon storage. Yet, it is often overlooked in carbon removal proposals.

Monitoring oxygen is not only about detecting harm. In some cases, oxygen increases, like those from macroalgae harvesting, could signal co-benefits. In others, it could reveal unintended risks in vulnerable areas.

Either way, oxygen provides a critical feedback on whether our climate fixes help or harm the sea.

Carbon removal must protect ocean oxygen

Carbon dioxide removal is no longer optional. Even the most ambitious emission cuts will leave behind some unavoidable emissions. To meet net-zero goals, we must pull carbon from the air. The ocean, with its scale and buffering power, seems like an obvious partner.

But solutions must be judged on more than their climate impact. They must protect marine life, support biodiversity, and avoid causing further damage.

This study makes clear that biotic mCDR strategies must be handled with care. Without oxygen, marine ecosystems collapse. And a climate solution that empties the ocean of life is no solution at all.

The study is published in the journal Environmental Research Letters.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–