A recent study published in Environmental Research Letters delves into the potentially detrimental effects of specific marine carbon dioxide removal (mCDR) techniques, particularly their consequences on global ocean oxygen levels. Led by Prof. Dr. Andreas Oschlies from the GEOMAR Helmholtz Center for Ocean Research Kiel, this research underscores the necessity for a comprehensive assessment of mCDR approaches, given their potential to worsen ocean deoxygenation. While these methods may be positioned as solutions to mitigate climate change, they could unintentionally inflict harm on vital marine ecosystems.
Over recent decades, the ocean has witnessed a decline of approximately 2% in its oxygen inventory, a troubling trend that escalates with ongoing global warming. This depletion of oxygen has already prompted significant ecological repercussions, with marine life struggling to adapt to the changing conditions. As temperatures continue to rise, the loss of oxygen is anticipated to accelerate, further jeopardizing marine ecosystems. Although various climate mitigation strategies are often proposed to alleviate this loss, recent findings indicate that some methods aimed at reducing atmospheric CO2 might inadvertently contribute to greater oxygen depletion in the ocean.
Prof. Dr. Oschlies and his international research team utilized global modeling simulations to evaluate both the direct and indirect effects of various marine carbon dioxide removal approaches on ocean oxygen levels. Their findings reveal critical insights into the balance between combating climate change and protecting marine life.
Marine carbon dioxide removal (mCDR) methods consist of a range of strategies designed to enhance the ocean's ability to absorb carbon. While many of these techniques are considered essential for addressing climate change, they may also pose unintended risks to ocean oxygen levels. One of the most alarming discoveries from this research is the potential for certain biological methods—such as ocean fertilization, large-scale macroalgae farming, and artificial upwelling—to increase oxygen consumption in the ocean.
These techniques aim to stimulate photosynthetic biomass production, which, upon decomposition, consumes oxygen. Prof. Dr. Oschlies noted, “What helps the climate is not automatically good for the ocean,” emphasizing the need for a nuanced understanding of these interventions.
Several biotic mCDR methods investigated, including ocean fertilization and macroalgae farming, could exacerbate the ongoing oxygen loss in the oceans. An increase in biomass production leads to subsequent decomposition, a process that consumes oxygen and is particularly concerning given the existing challenges posed by global warming. The study estimates that the oxygen loss from these processes could be between 4 and 40 times greater than the potential oxygen gain anticipated from reducing CO2 emissions.
Prof. Dr. Oschlies cautions, “Methods that increase biomass production in the ocean, and subsequently lead to oxygen-consuming decomposition, cannot be considered harmless climate solutions.” This finding highlights the critical need for thorough assessments of climate interventions to ensure they do not exacerbate existing oceanic problems.
In contrast to biotic approaches, geochemical mCDR methods, such as enhancing ocean alkalinity through the addition of limestone-based alkaline substances, seem to have a lesser impact on ocean oxygen levels. These methods do not involve nutrient input, which may minimize their effects on marine oxygen concentrations. The research suggests that geochemical approaches are comparable to simply reducing CO2 emissions in terms of their effect on ocean oxygen levels. Unlike biological methods, which lead to oxygen-consuming decomposition, geochemical strategies could prove to be safer for marine environments, although their scalability and long-term effectiveness require additional scrutiny.
One mCDR method highlighted in the study as potentially beneficial is large-scale macroalgae farming combined with biomass harvesting. Unlike other methods that can increase oxygen consumption through decomposition, the harvesting and removal of macroalgae biomass may positively influence ocean oxygen levels. By extracting nutrients and limiting excess oxygen consumption elsewhere in the ocean, this strategy could even help reverse some of the historical oxygen losses attributed to climate change.
Model simulations indicated that this method could restore up to 10 times the oxygen lost over the past century due to climate-induced warming. However, the study also raises concerns that large-scale harvesting could disrupt biological productivity within the ocean, necessitating careful evaluation of its broader environmental impacts.
