The ongoing absorption of excess carbon dioxide (CO2) from the atmosphere by the world's oceans is causing a significant shift in seawater chemistry known as ocean acidification. This process lowers the pH of seawater, making it more acidic, and poses a serious threat to marine ecosystems and the billions of people who depend on them. Effectively addressing this requires robust monitoring technologies and exploration of potential mitigation engineering approaches.
Monitoring Technologies
Consistent and accurate monitoring is crucial for understanding the rate and extent of ocean acidification and its impacts. Various technologies and methods are employed:
- Ship-Based Surveys: Traditional methods involve collecting water samples from research vessels and analyzing them in labs for parameters like pH, dissolved inorganic carbon (DIC), total alkalinity (TA), and the partial pressure of CO2 (pCO2). These surveys provide highly accurate data but are limited in spatial and temporal coverage due to cost and logistics. Conductivity, Temperature, and Depth (CTD) sensors are essential instruments on these cruises, providing fundamental water property data.
- Moored Buoys and Fixed Stations: Buoys equipped with sensors provide continuous, long-term data from specific locations. NOAA's Ocean Acidification Program supports buoys in coastal, open-ocean, and coral reef areas, measuring pCO2 and other parameters. Time-series data from these moorings are key to understanding long-term changes in ocean chemistry.
- Autonomous Platforms: Mobile platforms like Wave Gliders and Saildrones, along with profiling floats (like those used in the BGC-Argo program), significantly expand monitoring capabilities. These platforms can cover vast ocean areas and collect data, including pH and other biogeochemical variables, at various depths. They help fill gaps left by ships and buoys.
- Advanced Sensors: Innovations focus on developing smaller, cheaper, and more power-efficient sensors.
ISFET Sensors: Ion-Sensitive Field Effect Transistor (ISFET) technology enables near-continuous pH monitoring. Recent advancements include in situ calibration techniques (e.g., using tris buffer injections) that improve accuracy and repeatability compared to traditional bottle sampling. Automated calibration systems are also being integrated.
Optical Sensors: Novel optical pH sensors are being evaluated for accuracy, precision, and robustness in various marine environments. These sensors measure pH by shining light through seawater mixed with a pH-sensitive dye and measuring absorbance.
Miniaturized Sensors: Efforts are underway to develop miniature sensor systems that can measure multiple parameters (trace metals, nutrients, carbonate system variables, biotoxins) simultaneously, potentially deployable on fishing gear or smaller platforms for wider coverage.
- Remote Sensing and Modeling: Satellite data, combined with complex models, help estimate ocean acidification conditions over large areas where direct measurements are sparse. Machine learning techniques are being used to optimize global observation networks and improve the efficiency of monitoring efforts.
- Certified Reference Materials: Ensuring data quality across different monitoring platforms requires reliable standards. Efforts like building facilities to produce seawater certified reference materials are crucial for maintaining comparability and accuracy of measurements globally.
Potential Mitigation Engineering Approaches
While drastically reducing global CO2 emissions is the primary solution, several engineering approaches are being researched to mitigate ocean acidification directly or indirectly by removing CO2 from the atmosphere or oceans. These are often grouped under Carbon Dioxide Removal (CDR) or Marine CDR (mCDR) strategies:
- Ocean Alkalinity Enhancement (OAE): This approach aims to increase the ocean's natural capacity to store carbon by adding alkaline substances.
Mineral Weathering: Accelerating the natural weathering process by adding finely ground alkaline minerals (like olivine, limestone, or silicate rocks) to seawater increases its alkalinity and buffer capacity. This helps neutralize acidity and enhances the ocean's uptake of atmospheric CO2, storing it primarily as stable bicarbonate ions for potentially thousands of years. Research is ongoing to assess effectiveness, dissolution rates (especially in different environments like the Baltic Sea), scalability, and potential ecological impacts, such as effects on phytoplankton like coccolithophores.
Electrochemical Methods: These techniques use electricity (ideally from renewable sources) to alter seawater chemistry. One method involves splitting seawater into acidic and basic components, releasing the basic solution back into the ocean to increase alkalinity. Another, like Bipolar Membrane Electrodialysis (BMED), directly removes acidity from seawater without adding substances, requiring only seawater, electricity, and specialized membranes. Companies like Ebb Carbon are developing electrochemical processes to increase alkalinity.
- Direct Ocean Capture (DOC) / Direct Ocean Removal (DOR): Similar to direct air capture, these technologies aim to extract dissolved CO2 directly from seawater.
Electrochemical Acidification: Some methods use electrochemical processes (like electrolysis or electrodialysis) to temporarily acidify seawater, shifting the carbonate equilibrium to release CO2 gas, which is then captured. The treated, CO2-depleted water can then absorb more CO2 from the atmosphere. MIT researchers have developed a membrane-free electrochemical process that cycles between acidification (to release CO2) and alkalization (to return neutralized water to the sea).
- Enhancing Biological Processes:
Macroalgae Cultivation (Seaweed Farming): Large-scale cultivation of seaweeds like kelp can absorb CO2 from surface waters through photosynthesis. The captured carbon could potentially be sequestered long-term if the biomass is sunk to the deep ocean or used to create durable products like biochar. This may offer localized or seasonal mitigation benefits but requires investigation into potential ecological impacts and carbon storage permanence.
Microalgae Stimulation: Strategies like artificial upwelling (bringing nutrient-rich deep water to the surface) or ocean fertilization (adding nutrients like iron) aim to stimulate phytoplankton blooms, increasing CO2 uptake. However, these methods carry significant ecological risks and uncertainties regarding effectiveness and permanence.
* Biotechnology: Research includes developing genetically modified organisms (like algae) for more efficient carbon capture, using microbial communities to neutralize acidity or produce alkaline compounds, and developing chemical treatments (like antacids) for localized protection of vulnerable ecosystems like coral reefs.
- Coastal Ecosystem Restoration ("Blue Carbon"): Restoring and protecting coastal ecosystems like mangroves, salt marshes, and seagrass meadows enhances their natural ability to sequester carbon in biomass and sediments, while also providing local buffering against acidification.
All mitigation approaches face significant challenges regarding scalability, cost, energy requirements, long-term carbon storage permanence, monitoring and verification, governance, and potential unintended environmental side effects. Extensive research, including pilot projects and modeling studies (like those conducted by CSIRO, GEOMAR, and NOAA), is crucial to evaluate the feasibility, effectiveness, and safety of these techniques before any large-scale deployment. Many unknowns remain, and responsible development requires careful assessment of ecological and social impacts. Combining mitigation strategies with aggressive global emissions reductions remains essential.