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Response: Marine CO2 removal & storage

 

Plankton bloom image: NASA Earth Observatory

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Marine CO₂ removal & storage (mCDR)


Summary

We think of trees and soil as carbon sinks, but the world’s oceans hold far larger carbon stocks and are more effective at storing carbon permanently. The total area of the Zealandia continent is about 6 million square kilometres, so the average rate of calcium carbonate storage [in the ocean floor] was about 120 million tonnes per year, which is equivalent to 53 million tonnes of carbon dioxide per year. – Sutherland and Alergret (2022)

  • Absorbing so much CO2 has changed the chemistry of the ocean, making it 30% more acidic than pre-Industrial levels. Any activity to store yet more CO2 in the ocean needs to take this into consideration.
  • Trawling the ocean floor for fish releases more carbon in a year than the pre-Covid global aviation industry. Aotearoa still continues to bottom trawl seamounts. Stopping an action that releases carbon is far more effective than trying to remove that carbon.
  • Several geoengineering strategies to remove CO2  from the ocean or enable the ocean to take up more CO₂ are under consideration or are currently in practice (Fig. 1 & Video 1). As with technology to capture CO2 from the air and store it underground, measuring how much CO2 is ‘permanently’ removed is key to the viability of any approach, and with monetlising it by selling carbon offsets.
  • These processes include:

  Response

Other sections

Home > Climate wiki > Response > marine co2 removal

Summary

We think of trees and soil as carbon sinks, but the world’s oceans hold far larger carbon stocks and are more effective at storing carbon permanently. The total area of the Zealandia continent is about 6 million square kilometres, so the average rate of calcium carbonate storage [in the ocean floor] was about 120 million tonnes per year, which is equivalent to 53 million tonnes of carbon dioxide per year. – Sutherland and Alergret (2022)

  • Absorbing so much CO2 has changed the chemistry of the ocean, making it 30% more acidic than pre-Industrial levels. Any activity to store yet more CO2 in the ocean needs to take this into consideration.
  • Trawling the ocean floor for fish releases more carbon in a year than the pre-Covid global aviation industry. Aotearoa still continues to bottom trawl seamounts. Stopping an action that releases carbon is far more effective than trying to remove that carbon.
  • Several geoengineering strategies to remove CO2  from the ocean or enable the ocean to take up more CO₂ are under consideration or are currently in practice (Fig. 1 & Video 1). As with technology to capture CO2 from the air and store it underground, measuring how much CO2 is ‘permanently’ removed is key to the viability of any approach, and with monetlising it by selling carbon offsets.
  • These processes include:

Marine Carbon Dioxide Removal (mCDR) processes

Fig. 1: Different types of marine carbon dioxide removal (mCDR) Image: National Academy of Sciences
Fig. 1: Different types of marine carbon dioxide removal (mCDR) Image: National Academy of Sciences
Video 1: Different types of marine carbon dioxide removal (mCDR)

Direct ocean capture (DOC)

As the ocean is the largest carbon sink in the world, the aim is to remove the excess from the ocean through well-understood electrochemcial processes, and then store the carbon underground or sell it, which risks putting it back into the atmosphere.

But this frenzied pace of development and the drive to commercialization are causing concern among some analysts who fear science is lagging behind industry ambitions.

Stripping seawater of carbon dioxide via electrochemical processes — thereby prompting oceans to draw down more greenhouse gas from the atmosphere — is a geoengineering approach under consideration for large scale CO2 removal. Several startups and existing companies are planning projects at various scales.

Once removed from seawater, captured carbon dioxide can be stored geologically or used commercially by industry.

– Mongabay, Dec. 2024

Fig. 2 Direct ocean capture (DOC) uses an electrochemical process to split seawater (via bipolar membrane dialysis) to extract carbon dioxide from inorganic carbon held in the seawater. Lower alkaline seawater is then discharged back into the ocean and the captured carbon can either be stored (similar to direct air capture) or used. Image: Sarah Battle/NOAA
Fig. 2 Direct ocean capture (DOC) uses an electrochemical process to split seawater (via bipolar membrane dialysis) to extract carbon dioxide from inorganic carbon held in the seawater. Lower alkaline seawater is then discharged back into the ocean and the captured carbon can either be stored (similar to direct air capture) or used. Image: Sarah Battle/NOAA

Enhance ocean alkalinity to absorb more CO2

Another electrochemical method returns alkaline seawater to the oceans, causing increased carbon dioxide absorption over time.

In theory, these techniques could aid in carbon emission storage. But experts warn that as some companies rush to commercialize the tech and sell carbon credits, significant knowledge gaps remain, with potential ecological harm needing to be determined.

Achieving the scale required to make a dent in climate change would require deploying huge numbers of electrochemical plants globally — a costly and environmentally risky scenario deemed unfeasible by some. One problem: the harm posed by scale-up isn’t easy to assess with modeling and small-scale projects.        – Mongabay, Dec. 2024

Enhanced weathering by mining specific types of rocks on land (see ‘Enhanced weathering‘ this website), crushing them, and dumping them in the ocean comes at a carbon and financial cost of mining and transporting the material. The environmental impacts of mining the right rocks must be considered (see Explainer at the bottom of this page). Recent research shows that enhancing alkalinity is not necessarily going to have the desired effect:

We estimate that extreme alkalinity enhancement may promote the proliferation of coccolithophores, thereby reducing the CO2 removal potential of ocean alkalinity enhancement by 2–29% by 2100. However, less extreme alkalinity enhancement may only mitigate for adverse acidification effects on coccolithophores. – Lehmann & Bach, 2025
 

Ocean alkalinity can also be a secondary outcome of reducing acidification in rivers, which has been undertaken for some time to overcome the impacts of acid rain. Carbon Run in Nova Scotia, Canada, for example, has recently started this process using limestone, with rivers discharging into the ocean (Fig. 4):

Producing natural limestone powder involves just mining, crushing, and transport. This process results in only 5 tons of CO2 emissions for every 100 tons captured, compared to ‘lime,’ which generates significantly higher emissions. – Carbon Run
 
Fig. 3: Ocean alkalinity enhancement (OAE) removes carbon dioxide from the atmosphere by using electrical currents to split seawater into an acid and a base stream. The alkaline stream is then released back into the ocean, increasing pH in surface waters and allowing the ocean to absorb more atmospheric CO2. The acid stream can be stored or used by industry. Image: Sarah Battle/NOAA
Fig. 3: Ocean alkalinity enhancement (OAE) removes carbon dioxide from the atmosphere by using electrical currents to split seawater into an acid and a base stream. The alkaline stream is then released back into the ocean, increasing pH in surface waters and allowing the ocean to absorb more atmospheric CO2. The acid stream can be stored or used by industry. Image: Sarah Battle/NOAA
Fig. 4: Ocean Alkalinity Enhancement through enhanced weathering by spreading mined materials with high pH (alkaline) on the land or in the ocean. This process can increase the alkalinity of the ocean, enabling the ocean to absorb more carbon dioxide from the atmosphere. Material that dissolves easily can be spread in solid or dissolved form directly over the ocean’s surface, while material that does not dissolve as easily must be spread in areas where chemical conditions or wave action speeds up the dissolution process. Image: Sarah Battle/NOAA
Fig. 4: Ocean Alkalinity Enhancement through enhanced weathering by spreading mined materials with high pH (alkaline) on the land or in the ocean. This process can increase the alkalinity of the ocean, enabling the ocean to absorb more carbon dioxide from the atmosphere. Material that dissolves easily can be spread in solid or dissolved form directly over the ocean’s surface, while material that does not dissolve as easily must be spread in areas where chemical conditions or wave action speeds up the dissolution process. Image: Sarah Battle/NOAA

Nutrient fertilisation

In 1990, oceanographer John Martin hypothesised that iron dust swept from cold, dry landscapes played a crucial role in the ice age by fertilising massive blooms of diatoms and other phytoplankton, using theirs sugar along with copious amounts of carbon dioxide to fuel their growth (Fig. 5).

Before humans came along, everything that died in the ocean fell to the ocean floor, locking away much of this carbon (see the carbon cycle). Martin proposed that using iron to trigger diatom blooms might help combat global warming. “Give me half a tanker of iron and I’ll give you an ice age.”

While there is a correlation between the amount of iron in the ocean and the amount of CO2 in the atmosphere, and hence, global temperatures, it’s not a simple cause and effect. Ocean currents play a critical role in the biological carbon pump, something that should not be overlooked because the most important of these currents, the AMOC and ACC are rapidly slowing down, with concern that one or both will cease before 2100. Moreover, ocean acidification, rapidly increasing ocean temperatures, the historic slaughter of the biggest players in the biological pump, and a staggering quantity of microplastics in the ocean* is having a negative impact on the biological pump.

Nevertheless, the idea led to several large geoengineering iron fertilization experiments in the ocean. Some  experiments demonstrated that carbon could be removed to the ocean floor this way. But iron also fertilises highly toxic algae, leading to oceanic dead zones that devastate marine ecosystems, a problem that’s already getting worse due to ocean acidification and increasing ocean temperatures. Moreover the process is costly and far too slow given the speed at which the climate is changing.

The United Nations Convention on Biological Diversity has subsequently placed a moratorium on all large-scale ocean fertilization experiments. This moratorium does not apply to small scale experiments.
 
*Microplastics attract certain microbes and algae that form part of marine snow, causing it to float longer. This means there is more time for the marine snow to decompose at the surface, releasing carbon dioxide and the more potent greenhouse gas, methane, into the atmosphere, before (if) the snow eventually sinks to the bottom.

The biological carbon pump (BCP) is a critical part of Earth’s carbon cycle, removing CO2 – the main greenhouse gas responsible for global warming – from the atmosphere and locking it away in the deep ocean. This occurs when microscopic organisms called phytoplankton take in CO2 during photosynthesis or by creating calcium carbonate shells, then die and sink through the water column, carrying the CO2 with them. 

Known as “marine snow”, this shower of biogenic particulate material transfers an estimated 10 billion tonnes of carbon every year into the ocean’s interior, roughly the same amount as that emitted annually by fossil fuel burning. As a result, it plays an important role in regulating the amount of CO2 in the atmosphere and hence Earth’s climate.

How efficiently the ocean’s BCP transfers carbon-rich marine snow to the deep ocean – known as transfer efficiency – is a fundamental research question which scientists have grappled with for decades, and which has acquired new urgency due to climate change.

Fig. 5: The correlation between iron in the ocean (red line) and atmospheric CO2 content (blue line) over the past 120,000 years. The relationship is not straighforward as it also involves how ocean currents behave.
Fig. 5: The correlation between iron in the ocean (red line) and atmospheric CO2 content (blue line) over the past 120,000 years. The relationship is not straighforward as it also involves how ocean currents behave.

Artificial upwelling and downwelling

Fig. 6: "Technological transport of cold, nutrient-rich water to the surface to stimulate primary production and increased export of carbon to depth (Artificial Upwelling, left) and CO2-rich water from the surface to depth where it can be sequestered (Artificial Downwelling, right). Note that artificial upwelling can bring naturally high-CO2, low-O2 waters to shallower depths where they may impact surface biological systems, or outgas CO2 back to the atmosphere prior to the onset of high primary productivity resulting from nutrient additions. Additionally, these methods can be energy intensive, and are therefore often recommended to be deployed in conjunction with renewable marine energy sources." Image: Sarah Battle/NOAA
Fig. 6: “Technological transport of cold, nutrient-rich water to the surface to stimulate primary production and increased export of carbon to depth (Artificial Upwelling, left) and CO2-rich water from the surface to depth where it can be sequestered (Artificial Downwelling, right). Note that artificial upwelling can bring naturally high-CO2, low-O2 waters to shallower depths where they may impact surface biological systems, or outgas CO2 back to the atmosphere prior to the onset of high primary productivity resulting from nutrient additions. Additionally, these methods can be energy intensive, and are therefore often recommended to be deployed in conjunction with renewable marine energy sources.” Image: Sarah Battle/NOAA

Growing seaweed (algae / giant kelp) and dropping it into deep waters

Fig. 7: "The large-scale farming of seaweed, also known as macroalgal cultivation or aquaculture, draws down carbon dioxide from the atmosphere as part of the process of photosynthesis that occurs as the seaweed grows. The carbon is then stored when the biomass (vegetation) sinks to the ocean floor naturally or through intentional sinking, and becomes part of the sediment. Alternatively, seaweed can be used by harvesting and processing for food, fuel, fertilizer, or other uses. These activities would ultimately release carbon dioxide back to the atmosphere on a short timescale, and are therefore considered to be a form of carbon dioxide utilization, not carbon dioxide removal. Some of the carbon may also be released back into the atmosphere during farming." Image: Sarah Battle/NOAA
Fig. 7: “The large-scale farming of seaweed, also known as macroalgal cultivation or aquaculture, draws down carbon dioxide from the atmosphere as part of the process of photosynthesis that occurs as the seaweed grows. The carbon is then stored when the biomass (vegetation) sinks to the ocean floor naturally or through intentional sinking, and becomes part of the sediment. Alternatively, seaweed can be used by harvesting and processing for food, fuel, fertilizer, or other uses. These activities would ultimately release carbon dioxide back to the atmosphere on a short timescale, and are therefore considered to be a form of carbon dioxide utilization, not carbon dioxide removal. Some of the carbon may also be released back into the atmosphere during farming.” Image: Sarah Battle/NOAA

See also ‘Blue carbon/seaweed‘ on this website for more detailed information on kelp and how it is being used in multi-trophic aquaculture in Aotearoa. Note that biomass must fall below 1,000m before it’s considered to be ‘permanently’ stored and CO2 and CH4 (methane) can’t escape.

Dumping plant and forestry waste into the ocean

Sunk terrestrial biomass doesn’t steal nutrients from marine life, removing it from land could deplete soil of nutrients. ‘Over time we’re going to also be losing some of the fertility that crops and forests need.’ – Science News December, 2023

Ignoring the CO2 costs of collecting and transporting forestry slash and other agricultural waste, the idea is to ship the material offshore and dump it into oceanic dead zones (Fig. 8). There is little to no oxygen in these dead zones and hence very little if any life can exists, so the material won’t rot, and therefore won’t release CO2. 

As we are creating an ever-growing number and scale of lifeless oceanic dead zones in part due to toxic runoff, increasing ocean heating and acidification, several companies in the US are now viewing dead zones as resources to dump plant waste and sell the carbon credits.

Fig. 8: The little red dot off Invercargill may be the only official 'dead zone' around Aotearoa, but other areas are fast catching up.
Fig. 8: The little red dot off Invercargill may be the only official ‘dead zone’ around Aotearoa, but other areas are fast catching up.

More information

  • Explainer: What rocks are suitable?
     
    Olivine and other mafic rocks – igneous rocks rich in magnesium and iron – are suitable.
     

    Our preliminary investigations have shown Mg can also be extracted from basalt; however, we will primarily focus our     discussion on two enriched and accessible olivine deposits: the Semail ophiolite (Oman) and the Red Hills Ultramafic Complex (New Zealand) which conservatively contain 1.4 × 105 and 871 billion tonnes of olivine, respectively.  –  Scott et al (2021)

    But Maungakura Red Hills near St Arnaud is part of the Mt Richmond Forest Reserve; it’s surrounded by native ecosystems that already store massive quantities of carbon dioxide (images). The whitish area in the centre of the second image is an ephemeral riverbed, not a road. How much carbon-absorbing and life-supporting ecosystem services provided by biodiversity are we prepared to destroy through carbon-emitting mining and processing activities?

    (Images: Cody Whitelaw)