Marine carbon dioxide removal methods are gaining attention as tools to fight climate change. But what if some of these solutions actually worsen another critical problem—the loss of oxygen in our oceans? A new study led by Prof. Dr. Andreas Oschlies from the GEOMAR Helmholtz Center for Ocean Research Kiel raises urgent concerns that certain marine carbon dioxide removal (mCDR) techniques could inadvertently trigger a dangerous decline in ocean oxygen levels, threatening marine ecosystems worldwide.
The Growing Threat of Ocean Oxygen Depletion
Over the past several decades, our oceans have already lost nearly 2% of their oxygen inventory—a number that might sound small but carries enormous consequences. Oxygen is essential for marine life, and its decline has led to shrinking habitats, stressed species, and altered ocean chemistry. Scientists warn that accelerated global warming will only make this problem worse, deepening the crisis. At the same time, efforts to reduce atmospheric carbon dioxide—a critical part of combating climate change—may have unforeseen side effects on marine oxygen.
Prof. Dr. Oschlies’ team used advanced global ocean models to explore how different marine carbon dioxide removal methods might impact oxygen levels beneath the surface. Their findings suggest that while these techniques can help reduce CO2 in the atmosphere, some risk intensifying ocean oxygen depletion instead of alleviating it.
The Double-Edged Sword of Biotic Carbon Removal Methods
Marine carbon dioxide removal methods come in various forms, broadly categorized into biotic and geochemical approaches. Biotic methods like ocean fertilization, large-scale macroalgae farming, and artificial upwelling aim to boost photosynthesis—the process plants and algae use to convert CO2 into organic matter. At first glance, these strategies seem promising for capturing atmospheric CO2.
However, the study highlights a critical paradox: when biomass created through enhanced photosynthesis eventually decomposes, the breakdown process consumes oxygen. This means that stimulating biological activity in the ocean can lead to increased oxygen consumption. According to the research, oxygen loss caused by these biotic methods could exceed oxygen gains by a factor of 4 to 40, potentially worsening oceanic hypoxia—a dangerous condition where oxygen levels become too low to support most marine life.
Prof. Dr. Oschlies cautioned, “What helps the climate is not automatically good for the ocean.” This stark reminder underscores the complexity of marine ecosystems and the need for careful evaluation before deploying widespread mCDR technologies.
“Methods that increase biomass production in the ocean…cannot be considered harmless climate solutions.” – Prof. Dr. Andreas Oschlies, GEOMAR Helmholtz Center
To visualize these processes, Environmental Research Letters published a detailed schematic illustrating how biotic mCDR methods may reduce dissolved oxygen through enhanced respiration after photosynthesis-driven biomass growth. (DOI: 10.1088/1748-9326/ade0d4)
Geochemical Methods: Safer but Not Without Questions
In contrast, geochemical carbon dioxide removal techniques, such as ocean alkalinity enhancement involving the addition of limestone-based substances, appear to have minimal direct effects on ocean oxygen levels. These approaches work by chemically capturing CO2 rather than promoting biological growth, thereby avoiding the oxygen-consuming decomposition phase.
While geochemical methods seem to offer a safer alternative regarding ocean oxygen balance, their long-term effectiveness, ecological impacts, and scalability remain areas requiring further scientific investigation. The study recommends continued research to fully understand these nuances before large-scale implementation.
Macroalgae Farming: A Potentially Balanced Solution
One particularly interesting finding from the research concerns large-scale macroalgae (seaweed) farming combined with biomass harvesting. Unlike other biotic methods, harvesting the cultivated algae before it decomposes can help remove carbon and nutrients from the ocean, potentially reversing some of the existing oxygen losses caused by climate change.
Model simulations showed that this approach could restore up to 10 times the oxygen lost in the ocean over the last century due to warming. However, researchers caution that such industrial-scale farming of macroalgae might disrupt natural marine food webs and biological productivity if not carefully managed.
Including this method in a suite of balanced carbon removal strategies could hold promise, but its ecological trade-offs need thorough assessment.
Expert Insights and the Way Forward
This study, recently published in Environmental Research Letters, represents one of the first comprehensive evaluations of mCDR methods’ impact on ocean oxygen levels. It highlights the delicate balance between reducing atmospheric CO2 and preserving marine health, emphasizing that climate interventions do not exist in isolation.
Dr. Hannah Jones, a marine biogeochemist not involved in the study, explained in a recent interview: “This research adds vital understanding that carbon removal strategies must be holistic. The ocean’s oxygen cycle and its diverse ecosystems are intricately linked, so we can’t afford to trade one crisis for another.”
The Environmental Research Letters publication invites policymakers and scientists alike to develop carefully vetted, adaptive mCDR frameworks that incorporate ecological risk assessments alongside climate benefits.
Ocean deoxygenation is a less visible but equally urgent threat alongside climate change. As this study reveals, some carbon dioxide removal approaches currently under consideration could unintentionally deepen this crisis by accelerating oxygen loss.
For those passionate about marine conservation and climate solutions, the take-home message is clear: We need strategies that mitigate climate change without sacrificing ocean health. Integrating scientific expertise, ecosystem awareness, and precaution can help guide innovations that protect both our planet’s atmosphere and the life beneath the waves.
How do you feel about using ocean-based carbon removal technologies, given their potential risks? Share your thoughts and engage with our community below—together, we can push for smarter climate actions that consider the ocean’s vital breath.