As carbon dioxide (CO2) levels continue to rise in the Earth’s atmosphere, researchers globally have been seeking efficient methods to remove this gas from the air. Currently, the ocean acts as the planet’s primary carbon dioxide “sink,” absorbing about 30% to 40% of the CO2 generated by human activities.
Recently, a new approach has emerged: marine carbon dioxide removal (mCDR). This method shows promise for mitigating CO2 emissions and could potentially lead to net negative emissions in the future. However, similar to air carbon dioxide capture systems, this idea is still in its early stages and has not yet seen widespread adoption, although a few companies are exploring this field.
One promising method is Ocean Alkalinity Enhancement (OAE), which involves adding alkaline substances to seawater. This process accelerates the ocean’s natural ability to act as a carbon sink. By increasing the alkalinity, dissolved inorganic CO2 in the seawater is converted into bicarbonates and carbonates, stable forms of carbon that can last for approximately 10,000 years. The resulting CO2 deficit in surface waters is quickly balanced by the absorption of atmospheric CO2, helping to restore equilibrium.
The addition of carbonate minerals to stabilize seawater carbonate chemistry has been practiced in aquaculture for around a decade, especially after hatchery failures in the early 2010s due to ocean acidification in the US Pacific Northwest and British Columbia. For instance, the Chalk-Ex experiment in the early 2000s released crushed calcium carbonate over a 1.5 km² area in the Gulf of Maine, primarily to study the particles’ effects on upper ocean optical properties rather than carbon dioxide removal (CDR).
Ocean Alkalinity Enhancement involves depositing minerals into the ocean to enhance CO2 uptake, with effectiveness depending on the dissolution rate of the minerals compared to their sinking rate. The ocean’s capacity to hold large quantities of bicarbonate ions, combined with the vast availability of alkaline rocks, suggests that OAE could theoretically capture tens of gigatons of CO2 annually. However, this remains theoretical due to the current state of technology. Recent reports estimate the potential CDR from OAE as 1 – 15+ gigatons (Gt) CO2/year. Technical, economic, social, political, and governance factors may limit this potential, though the extent of these limitations is still uncertain. Cost estimates for OAE range from $25 to $160 per ton of CO2. OAE aims to sequester additional atmospheric CO2 in the ocean as bicarbonate ions, which do not exchange with the atmosphere, potentially achieving sequestration durations of over 20,000 years.
Despite its potential, several gaps need addressing to advance OAE development and implementation. While field trials are underway, a comprehensive understanding of OAE’s benefits, risks, and scalability remains elusive. Controlled trials across diverse ecosystems are necessary to assess impacts on marine chemistry and biology, requiring new modeling tools and methodologies. Moreover, verifying additional CO2 uptake is challenging due to the ocean’s dynamic CO2 flux, necessitating new observational techniques. To fully support field trials, current observational technologies (sensors, remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs)) and modeling tools must become more widespread and accessible to achieve the required spatial and temporal frequency of monitoring and sampling. Beerling and colleagues have noted that we need new technologies to minimize the cost and environmental impact of mining, grinding, and distributing alkaline rocks. In conclusion, marine carbon dioxide removal, particularly through ocean alkalinity enhancement, holds significant promise for addressing the increasing CO2 levels in our atmosphere. While the theoretical potential for capturing and sequestering CO2 is immense, practical implementation faces numerous challenges. Overcoming these hurdles will require extensive research, technological innovation, and collaboration across various sectors. With concerted efforts, mCDR could become a pivotal tool in our arsenal against climate change, contributing to a more sustainable and stable future for our planet.
