The ocean absorbs up to a third of the world’s man-made carbon dioxide (CO2) emissions. However, this carbon absorption comes at a cost, as it leads to the acidification of seawater.
To combat this issue, scientists are exploring ways to increase the alkalinity of seawater, so it can absorb more CO2 without becoming increasingly acidic.
Professor Ulf Riebesell’s group at the GEOMAR Helmholtz Centre for Ocean Research, Kiel plays a critical role in this research.
Ocean alkalinity enhancement (OAE) involves adding alkaline substances to seawater to increase its capacity to absorb CO2. This process also helps counteract the acidification effects caused by anthropogenic CO2 emissions.
Alkaline substances, such as limestone, olivine, and various carbonates can dissolve in seawater and neutralize acidity, thereby enabling greater CO2 sequestration. The approach mimics natural processes where minerals are weathered and transported to oceans over geological timescales.
Different methods are being explored to implement OAE effectively. These include distributing finely ground minerals across large ocean areas or directly injecting alkaline solutions into seawater. The efficiency of OAE depends on factors like ocean currents, water temperature, and the types of minerals used.
Some forms of ocean alkalinity enhancement might also enhance the growth of marine organisms, such as phytoplankton, which play a role in the biological carbon pump by sequestering CO2 in deeper ocean layers.
The researchers tested the efficiency of CO2-equilibrated OAE, a moderate approach that adjusts seawater chemistry by using alkaline water that is pre-saturated with CO2.
This method ensures carbon dioxide removal (CDR) before releasing the treated water back into the ocean.
To simulate real-world conditions, the team used KOSMOS mesocosms (Kiel Off-Shore Mesocosms for Ocean Simulations). These large test tubes, which are submerged directly into the ocean, isolated eight cubic meters of water for this controlled study.
Different concentrations of sodium carbonate and bicarbonate were added to achieve varying levels of alkalinity, ranging from what occurs in natural conditions to a level that is double what is found in seawater.
Over a 33-day period, the team monitored the responses of zooplankton by examining metrics such as biomass, diversity, production, and fatty acid composition.
The results were encouraging. Plankton communities remained stable, and zooplankton tolerated the moderate chemical changes associated with CO2-equilibrated OAE.
While the nutritional quality of particulate matter – the primary food source for zooplankton – showed potential deterioration, it did not significantly impact these marine consumers.
The researchers attributed this resilience to the oligotrophic (nutrient-poor) conditions of the subtropical waters, which may have buffered the zooplankton’s responses.
“Our study shows that the increase in alkalinity has minor impacts on the zooplankton and that the food web as a whole remains stable,” said Nicolás Sánchez, Ph.D. student and first author of the study.
Ocean alkalinity enhancement holds promise as a powerful tool in the fight against climate change.
By enhancing the ocean’s capacity to absorb CO2 without increasing acidity, OAE could play a vital role in buffering global warming.
It could bridge the gap during the transition to renewable energy, mitigate emissions from hard-to-decarbonize industries, and facilitate the safe removal of historical carbon emissions.
However, significant gaps in knowledge remain. The study, which was conducted in the nutrient-poor subtropical waters, provides a promising start but its findings cannot be generalized to all marine ecosystems.
“Our experiment has shown that CO2-equilibrated OAE does not have a lasting impact on zooplankton and the food web in the nutrient-poor subtropical area we studied,” said Sánchez.
“But this does not say anything about how it will affect other marine environments, nor about the safety of other, technically more feasible forms of OAE that cause greater changes to seawater chemistry.”
The GEOMAR researchers stress the need for extensive investigations across diverse ecosystems and with various OAE approaches. These studies will be essential to develop a responsible framework for the application of alkalinity enhancement.
“Our study is a promising first step towards defining a responsible framework for the application of alkalinity enhancement,” Sánchez concludes.
While ocean alkalinity enhancement shows significant potential as a climate mitigation strategy, its implementation requires cautious, science-backed approaches.
This initial experiment demonstrates that moderate OAE interventions can be ecologically safe in specific conditions, but broader studies are crucial to address global marine variability.
By advancing research on OAE and understanding its implications, scientists can help ensure that this innovative approach becomes a safe and effective tool in the global effort to combat climate change.
The study is published in the journal Science Advances.
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