Ocean-Based Carbon Dioxide Removal (CDR)

Microalgae Cultivation

Our planet is being transformed by climate disruption, with some of the worst impacts occurring in the ocean. Currently, most efforts to address climate change are focused on reducing emissions of greenhouse gases. While those efforts remain vital, they’re no longer enough. We must also clean up the “legacy” greenhouse gas pollution already in our atmosphere.

 

Ocean Visions believes that we may be able to harness the power of the ocean to restore the climate and the ocean itself. The ocean already holds more carbon than any other part of the biosphere and has the potential to contribute even more. The sheer scale of the ocean means that any ocean-based carbon dioxide removal (CDR) solutions proven to be viable and safe would have the potential to clean up billions of tons of CO2.

 

A number of ocean-based CDR approaches are being explored—including microalgae cultivation.

nasem-ocean-cdr-graphic_microalgae-cultivation-3
Graphic © National Academies of Sciences, Engineering, and Medicine. 2021. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. https://doi.org/10.17226/26278. Adapted and reproduced with permission from the National Academy of Sciences, Courtesy of the National Academies Press, Washington, D.C.

Microalgae Cultivation Overview

Primary productivity in the ocean is the creation of new organic matter (biomass) via photosynthesis, which also results in the fixation of carbon dioxide (CO2) into that organic matter. By far the greatest amount of primary productivity in the ocean comes from microscopic algae, also known as phytoplankton.

This production is only limited by sunlight and the inorganic compounds (nutrients) in the water needed to interact with the sunlight. Fertilizing surface waters with nutrients like iron, phosphorus, and nitrogen, can stimulate photosynthesis and cause phytoplankton (microalgae) to grow, which absorb CO2 as they do.

Increasing net primary productivity in the ocean could, under the right circumstances, lead to increased CO2 fixation from the water, and ultimately transfer of that CO2 into the deep ocean where it might be sequestered for long periods of time.

There are various ways net primary productivity might be increased. One would be to use vessels to distribute nutrients into nutrient-limited areas of the sea. Artificial upwelling, which replicates natural upwelling through the use of pumps, is another potential means to fertilize surface waters. 

Technical Potential

Stimulating primary productivity via adding iron, nitrogen, and/or phosphorus to ocean waters has been tested in the field and shown to create phytoplankton blooms. However, it is still unclear how much ocean fertilization increases carbon export to the deep ocean. While iron fertilization has been under investigation for over two decades, tracking carbon to the deep ocean remains a technological challenge, and more research, direct testing and new monitoring tools are necessary to improve quantification. This quantification is critical to know how effective of a tool fertilization will be for carbon removal. Similarly, field studies of artificially upwelled deep ocean water have demonstrated a boost in phytoplankton blooms, but it remains unclear whether as to how much increased carbon sequestration is achieved.

Environmental Co-Benefits and Concerns

Unintended ecosystem impacts of ocean fertilization are largely unknown but could include alterations to the marine food web structure and a depleted supply of nutrients for other organisms. For example, iron enrichment in the Southern Ocean could absorb nitrate which would then later not be available for phytoplankton in other areas as surface waters travel north to the Mid-Atlantic and the North Sea. Finding the right nutrient concentrations is also an area needing further testing. Too high a concentration, especially of nitrogen and phosphorous, can cause oxygen depletion, resulting in deoxygenation (or creation of dead zones)—such as what happens in areas with too much nutrient addition via agricultural runoff.

The introduction of artificial upwelling may also have impacts. Deep ocean waters tend to be cooler, more acidic, and more nutrient-rich than surface waters. It is conceivable that through artificial upwelling nutrient-poor waters could be transformed into more productive systems supporting more marine life, including fisheries. It is also conceivable that upwelling close to coral reefs could have a beneficial cooling effect, but potentially interfere with these delicate ecosystems by increasing nutrient levels. By bring up more acidic seawater, artificial upwelling could also have a negative effect by outgassing CO2.

Cost Considerations

Costs of ocean fertilization may include mining, manufacturing and transport of nutrients to fertilization sites. Development of necessary monitoring equipment and tracking organic carbon export to the deep has research and development costs. Costs of artificial upwelling depend on the pumping method (whether powered by wave, wind, or solar electric energy), as well as pumping depth. So far, no large-scale upwelling system has been installed and maintained, so cost estimates remain uncertain.

Dive Deeper

Join the Ocean-Based CDR Community

Ocean Visions’ CDR Community brings together stakeholders to advance the state of knowledge, build bridges across disciplines, and help the community move towards safe and equitable testing and piloting of the most promising ocean-based CDR approaches.

Explore Ocean-Based CDR Road Maps

Ocean Visions’ ocean-based CDR road maps provide overviews of potential technologies, obstacles they face, and first-order priorities needing attention to advance the field. The road maps are intended to catalyze global collaboration and engagement and will be updated and refined as advances emerge in science, technology, governance, and policy.