The oceans are the largest carbon reservoir on earth. So how can we harness them to remove even more carbon dioxide – but without damaging precious ecosystems? Maria Burke assesses the options
The IPCC published its third report on climate change at the beginning of April 2022. One key message was that carbon dioxide removal (CDR) strategies are now necessary if global temperatures are to stay below 1.5°C. It’s the first IPCC report to state clearly that CDR is needed to achieve climate targets.
While several land-based strategies – such as storing carbon in agricultural soil or changing forest management – are relatively advanced, less is known about ocean-based strategies, such as cultivating seaweed on a large scale or manipulating nutrients in seawater. Researchers say work is urgently needed to assess its potential.
In December 2021, a report by the US National Academies of Sciences, Engineering, and Medicine (NASEM)1 called for a new ten-year research programme to learn more about how ocean-based interventions could be used to mitigate the impacts of climate change. It estimated that this could cost up to $1.4-2.5bn.
‘If we want to make fully informed decisions about the future of our ocean and climate, we need to complete some very critical research in the next decade,’ says Scott Doney, Chair of the report committee, and Professor in environmental sciences at the University of Virginia, US.
‘There is no choice,’ says Shaun Fitzgerald, Director of the Centre for Climate Repair at Cambridge, UK. ‘The issue is that funding for this needs to ramp up significantly and quickly. We need to … seriously investigate the oceans.
Ocean-based CDR is the most likely able to actually scale up in sufficient time to help us get to net zero by 2050.’
The ocean is the largest carbon reservoir on Earth. Around 38,000Gt of carbon are stored in the ocean in the form of dissolved inorganic carbon, according to a report published in February 2022 by the ClimateWorks Foundation, a leading global platform for climate philanthropy.2 The oceans absorb 9Gt/year of CO2, about 23% of annual emissions.
The oceans store 50 times more carbon than the atmosphere and 20 times more carbon than soil and plants on land, so the capacity is there to make a big difference, explains Ken Buesseler, senior scientist at the Woods Hole Oceanographic Institution, US, and NASEM committee member. ‘And there is more space to do this in the ocean, so you are not competing with agricultural lands or living spaces.’
The NASEM report, A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration, assessed six approaches. Ocean alkalinity enhancement adds chemicals to increase alkalinity, which boosts reactions that take up atmospheric CO2. Nutrient fertilisation adds nutrients, such as phosphorus or nitrogen, to increase photosynthesis by phytoplankton, which increases the uptake of CO2 and the transfer of carbon to the deep ocean. Artificial upwelling and downwelling is where cooler, more nutrient-rich deep water is moved to the surface, stimulating phytoplankton growth (upwelling); and surface water and carbon is moved to the deep ocean (downwelling). Large-scale seaweed farming results in the transfer of carbon to the deep ocean or into sediments. Ecosystem recovery involves carbon removal and sequestration through protecting coastal ecosystems. Electrochemical processes involve passing an electric current through water to increase the alkalinity of seawater to enhance its ability to retain CO2.
The ocean is a major carbon sink - it has absorbed 25% of the carbon emitted since the Industrial Revolution – but that has come at the cost of a 30% increase in acidification, which makes it harder for marine organisms to live. Seaweed and other marine plants increase the ability of the oceans to absorb carbon, and do it in a way that decreases acidification, addressing two profound problems at once.
Megan Reilly Cayten senior advisor, Oceans 2050.
All approaches are lacking, to different degrees, the basic science needed to predict efficiencies for CO2 removal and any consequences, remarks Buessler. ‘The three that stand out [for me] are ocean fertilisation, seaweed cultivation and ocean alkalinity enhancement. But we also recommended parallel research paths on all six as being warranted, as no single ocean CDR approach could alone remove the 10Gt CO2 needed to reduce our impacts even with aggressive emissions reductions, which are needed in any scenario.’
There are many potential advantages, he adds. Costs could be less than land-based carbon capture and storage (CCS) methods, at least for the nature-based solutions that employ fertilisation, and seaweed or ecosystems restoration. ‘For these, sunlight is ultimately the energy source that drives photosynthesis converting dissolved CO2 gas into organic forms of carbon that then get transported to the deep ocean where they can be sequestered for 100-1000+ years.’
Ocean alkalinity enhancement is one of several CDR approaches being explored by the ClimateWorks Foundation. It describes the strategy as the process of increasing the pH of the ocean very slightly to improve how the ocean absorbs and locks away CO2 from the atmosphere. There are two ways that carbon gets transferred naturally from the atmosphere to the ocean – rock weathering and direct dissolution.
In rock weathering, atmospheric CO2 dissolves in rainwater, forming a mild acid. When this lands on alkaline rock, it weathers or dissolves the surface of the rock, forming carbonates and bicarbonates in solution. This mildly alkaline water flows out through rivers and streams into the oceans. This process provides marine life with the minerals they need and keeps the oceans at the right pH. Over hundreds of thousands of years, the carbonate species end up at the bottom of the ocean where they form carbonate rock.
Direct dissolution of CO2 is much less beneficial. When the concentration of CO2 in the ocean is less than the concentration of CO2 in the air, the gas dissolves into the water. This forms carbonic acid and is responsible for acidification of the oceans.
Phytoplankton absorb CO2 as they grow and do this much more quickly than trees as they can double in mass every 24 hours.
Sunlight drives photosynthesis, converting dissolved CO2 gas into organic forms of carbon that then get transported to the deep ocean where they can be sequestered for 100-1000+ years.
Ocean alkalinity enhancement accelerates the natural process of rock weathering, explains Frances Wang, CDR Programme Manager at ClimateWorks. ‘Once CO2 is sequestered, it can remain stored for 100,000 years, making it one of the most durable storage options. Carefully and safely enhancing alkalinity also increases the local pH of seawater as a remedy for ocean acidification. This then helps to protect shellfish and other sensitive ecosystems like coral reefs from the impact of ocean acidification, which is a result of runaway climate change. There are many different ways to enhance ocean alkalinity, so there is a broad opportunity for research and innovation to identify methods that could remove significant amounts of atmospheric CO2 while also being environmentally friendly and socially acceptable.’
Due to the near limitless quantities of alkaline rocks, ocean alkalinity enhancement has the theoretical potential to capture 12Gt/year of CO2, according to the ClimateWorks report. However, real-world limitations such as shipping capacity to distribute materials and finding suitable ocean areas would reduce this figure to several gigatons CO2/year.
ClimateWorks is focusing on supporting small, controlled field trials to provide real-world evidence of specific approaches. It has funded, for example, a project to explore the implications of naturally occurring alkalinity flux from the Mississippi River into the Gulf of Mexico.
‘The Mississippi River has a very high alkalinity, and along with that alkalinity comes a lot of dissolved (stored) carbon,’ explains project co-leader Adam Subhas of the Woods Hole Oceanographic Institution. ‘Because the river discharges into the ocean, it delivers a large amount of stored carbon into the ocean that is permanently sequestered into seawater. The levels of alkalinity and dissolved carbon in the Mississippi River are uniquely high, and they change seasonally – in the fall [autumn], river alkalinity is around 20-30% higher than average seawater alkalinity.’ The research team is hoping to use this seasonal change to investigate how much carbon the Mississippi delivers to the ocean, and if any of that carbon is lost along the way, for example, by precipitating as calcium carbonate. In this way, they hope to get an idea of the potential effectiveness of large-scale alkalinity enhancement. They hope to report their findings in early 2023.
Oceans 2050, meanwhile, has chosen to focus on seaweed farming. The organisation, founded by Alexandra Cousteau – grand-daughter of underwater explorer Jacques Cousteau – and Carlos Duarte, a marine ecologist at the King Abdullah University of Science and Technology, Saudi Arabia, is based on Duarte’s research that demonstrated the role of seaweed aquaculture in helping the oceans and the climate recover.3
Seaweed captures carbon as it grows during photosynthesis. As seaweed matures, parts break off and sink to the bottom. Some of the carbon in that biomass is sequestered in sediment, both nearby and in deep ocean sinks.
‘Plants in the ocean, including kelps and seaweed, seagrasses, and mangroves, represent just 0.05% of the plant biomass on land, but they grow so quickly that they cycle through a similar amount of carbon every day as all land-based plants,’ explains Megan Reilly Cayten, Oceans 2050 senior advisor. ‘The ocean is a major carbon sink – it has absorbed 25% of the carbon emitted since the Industrial Revolution – but that has come at the cost of a 30% increase in acidification, which makes it harder for marine organisms to live. Seaweed and other marine plants increase the ability of the oceans to absorb carbon, and do it in a way that decreases acidification, addressing two profound problems at once.’
The oceans absorb 9Gt/year of CO2 – about 23% of annual emissions – and store 50 times more carbon than the atmosphere and 20 times more carbon than soil and plants on land.
Seaweed farming today accounts for around 2000km2 of seabed and is growing at around 7%/year. It is estimated that if this growth rate was more like 20%, then by 2050, the sediment below farms would have sequestered 0.24Gt of carbon.
Duarte estimates that wild seaweed forests sequester as much carbon as the Amazon rainforest. But up till now, it’s not been known how much carbon is sequestered in sediment under seaweed farms. Oceans 2050 has monitored 21 seaweed farms across ﬁve continents. Duarte has calculated average sequestration rates of around 2t/ha/year, but in places were as high as 8t/ha/year.
‘The best predictor for sequestering the most carbon is the nature of the soils underneath,’ he said, speaking at Monaco Ocean Week in March 2022. ‘Muddy soils are much better than coarse or stony soils.’ He will publish his findings later in 2022.
Seaweed farming today occupies around 2000km2 of seabed and is growing at around 7%/year. Duarte estimates that if this growth rate was more like 20%, then by 2050, the sediment below farms would have sequestered 0.24Gt of carbon.
The study also aims to produce a verified carbon standard for approval by an international accrediting agency in 2022. Once approved, seaweed farms could issue credits to global buyers wanting to offset their carbon emissions.
Ocean-based Climate Solutions (OCS) was founded by Phil Kithil in 2005 when Hurricane Katrina struck New Orleans. Now its CEO, Kithil designed a wave-powered pump to transfer cold seawater to the ocean’s surface to reduce the intensity of hurricanes. At the same time, he discovered that it also encouraged the growth of phytoplankton, mimicking the natural process of ocean upwelling. Phytoplankton absorb CO2 as they grow and do this much more quickly than trees as they can double in mass every 24 hours.
In parts of the ocean where this natural process has slowed down because the surface ocean has absorbed heat from CO2 emissions, an upwelling pump could bring deep water with important nutrients to the surface and help phytoplankton grow again. When they die, they sink to the mid- and deep ocean where the carbon is stored for centuries or longer. Advantages of the technology include scalability, and no electricity, land or water requirements. Pumps are deployed far offshore and attract fish, making them popular with fishermen, the company adds.
In 2008, David Karl of the University of Hawaii and Ricardo Letelier of Oregon State University, US, investigated sequestration potential from pump-driven upwelling using data collected in the North Pacific near Hawaii.4 They calculated that upwelling of waters from 200m or deeper provides excess phosphate which triggers first nitrate, and then phosphate, uptake, which drives blooms of phytoplankton. The secondary phosphate bloom fixes more dissolved CO2 than is upwelled, resulting in net 2 sequestration.
OCS’ arrays of wave-powered pumps go further; they move high-nutrient seawater from 400m below the surface up to the surface where phytoplankton blooms can start. The company plans to measure how much CO2 is sequestered allowing customers to claim carbon credits. Demonstrations of its upwelling pump technology are planned near the Canary Islands in July 2022.
‘Our wave pump technology could play a key role in removing the billions of tons CO2 needed to restore a stable climate,’ says Kithil. ‘Corporations, including Microsoft and Stripe, have approached us to learn how our system can remove CO2 from the environment and thus directly subtract from their corporate CO2 emissions. This funding will accelerate our final testing and set the path for widespread adoption.’
1 A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration, The National Academies Press. https://nap.nationalacademies.org/read/26278/chapter/1
2 Ocean carbon dioxide removal: The need and the opportunity, ClimateWorks Foundation. https://www.climateworks.org/report/ocean-carbon-dioxide-removal-the-need-and-the-opportunity/
3 C. M. Duarte et al, Nature, 2020, 580, 39.
4 D. M. Karl and R. M. Letelier, Marine Ecology Progress Series, 2008, 364, 257.