As the world looks for ways to fight climate change, scientists are exploring new techniques to pull carbon dioxide (CO₂) out of the atmosphere.
One promising idea is to use the ocean—our planet’s largest carbon sink—to absorb more CO₂.
But a new study warns that some of these ocean-based methods could actually make things worse for marine life by reducing the amount of oxygen in the water.
The research, led by Professor Andreas Oschlies from the GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, shows that many marine carbon dioxide removal (mCDR) methods could harm the ocean’s delicate balance.
The findings were published in the journal Environmental Research Letters and highlight a surprising and important problem: what helps the climate doesn’t always help the ocean.
Over the past few decades, the ocean has already lost about 2% of its oxygen—a trend mainly driven by global warming. Less oxygen in the water can make life harder for fish and other sea creatures.
You might assume that reducing CO₂ and slowing climate change would help the ocean recover its oxygen. But this study suggests that some mCDR techniques could actually remove even more oxygen from the sea.
The most concerning methods involve biological processes that increase ocean plant growth, like ocean fertilization, large-scale seaweed farming where the plants are then sunk, or artificial upwelling, which brings nutrient-rich water to the surface.
These techniques boost the growth of ocean plants and algae, which absorb CO₂ through photosynthesis.
But once these plants die, they sink and decompose—a process that uses up oxygen in the deep ocean. In fact, the oxygen loss caused by these methods could be up to 40 times greater than any oxygen gains from slowing global warming.
“These methods are not harmless,” says Professor Oschlies. “They might reduce CO₂ in the atmosphere, but they could also seriously damage marine ecosystems.”
On the other hand, some non-biological approaches appear to be much safer. One example is ocean alkalinity enhancement, which involves adding substances like ground limestone to seawater. This helps the ocean absorb more CO₂ without causing major changes to oxygen levels.
Interestingly, the one method that showed a potential benefit for ocean oxygen was seaweed farming where the plants are harvested and removed, rather than left to decay in the ocean. This not only avoids oxygen loss but could even help restore lost oxygen—although it may also reduce the nutrients available for other marine life.
The researchers urge caution. They say all future studies and projects involving mCDR should include oxygen monitoring to track potential side effects. The ocean, they emphasize, is already under stress from climate change, pollution, and overfishing. Any large-scale efforts to remove carbon must be carefully planned to avoid causing further harm.
Even with strong climate policies in place, many countries will still produce greenhouse gases in the coming decades. That means carbon removal will likely play a role in reaching net-zero emissions. But if we turn to the ocean for help, we must be sure we’re not creating a new crisis in the process.
