In June 2024, startup company Equatic (Santa Monica, CA, USA) announced that it had begun engineering work on the world’s largest and first commercial scale facility using the ocean to remove CO
2 from the atmosphere [
1]. The company claims its plant at a yet-to-be determined site in Quebec, Canada, will be operational by 2027, sequestering 300 t of CO
2 per day at full capacity. Equatic is already building a similar, but smaller, facility called Equatic-1 in Singapore (
Fig. 1) [
2]. That project, expected to be completed in 2025, will have the capacity to remove 10 t of CO
2 per day.
Equatic is part of a burgeoning band of scientists and companies developing technologies for marine carbon dioxide removal (CDR), joining others focused on land-based CDR. The work has been spurred by the increasingly accepted belief that limiting emissions from power stations, heavy industry, and other sources will not be enough to avert the worst impacts of climate change [
3], [
4].
In 2015, the Paris Agreement on climate change set a target to limit global warming to well below 2.0 °C above pre-industrial levels and pursue efforts to limit the temperature increase to 1.5 °C [
5]. The 2022 assessment of the United Nations’ Intergovernmental Panel on Climate Change states that, to meet these goals, dramatic reductions in CO
2 emission must be assisted by removing massive amounts of CO
2 from the atmosphere [
6]—up to 10 Gt per year by 2050, according to some estimates [
7]. Raising the stakes, many experts are seeing the need for action beyond emissions reductions becoming increasingly urgent as global warming appears to be accelerating [
8].
In response to this emerging reality, billions of dollars are now being invested to ramp up CDR technologies, although many argue that the investment is not near enough [
9]. “It is a massive undertaking,” said Gabby Kitch, marine CDR lead at the US National Oceanic and Atmospheric Administration (NOAA).
Land-based CDR projects have made some headway, with the largest including chemical plants engineered to capture CO
2 directly from the air [
3], [
9], [
10]. In May 2024, the Zurich-based, Swiss company Climeworks opened its second direct air CDR plant in Iceland, ten-times larger than its first plant completed in September 2021 [
10], [
11]. The new plant is designed to remove 36 000 t of CO
2 from the air per year and store it permanently underground. However, an analysis by the Institute for Sustainable Development and International Relations (Paris, France) published in March 2024 found that land-based CDR is unlikely to meet global CDR requirements without displacing needed agricultural land, reducing water availability, and harming biodiversity [
12].
That is where the ocean comes in. “The ocean has about 17 times more capacity to draw down carbon than land, biota, and soils combined,” said Kitch. “It is a great natural sink for CO2—it already absorbs about one-third of our carbon emissions every single year.”
Researchers and companies are now scrambling to exploit the ocean’s tremendous capacity to sequester carbon. Some efforts harness and accelerate natural biological processes that lock carbon into the sea, while others are using chemical technologies [
13]. But the methods are controversial. Like with direct air capture CDR [
10], big questions remain about whether marine CDR can operate at the scales that will be needed without using vast amounts of energy or incurring exorbitant costs; there are also concerns about detrimental impacts on coastal and aquatic ecosystems [
13].
Other approaches to make planet-scale climate interventions—broadly known as geoengineering—have faced stiff opposition from people worried that such interventions could have unforeseen and potentially catastrophic consequences, in addition to possibly jeopardizing hard-fought agreements to reduce emissions [
14], [
15]. One of the most high-profile geoengineering projects, the Stratospheric Controlled Perturbation Experiment (SCoPEx), would have used a high-altitude balloon in a demonstration study to deliver calcium carbonate particles into the stratosphere to reflect sunlight [
14]. But the controversial project, after many years of planning and negotiations, was finally cancelled in March 2024 [
16].
To avoid marine CDR facing similar pushback, researchers are busy investigating its efficacy and sustainability [
17]. Supporting these efforts, NOAA released a strategy for CDR research in 2023 and announced 24 million USD in funding for 17 research projects [
18], [
19]. “Marine CDR is an emerging field, so the foundational research is really critical for moving forward,” said Kitch.
Many marine CDR programs aim to accelerate natural processes (
Fig. 2). One of the simplest methods is to foster wetlands such as mangroves and salt marshes where plants store carbon in soils and sediments. But the potential to scale this up is probably too low to make a significant contribution to CDR requirements, Kitch said, although it could have important ancillary benefits with regards to moderating coastal damage from increasingly powerful storms and rising sea levels [
4], [
20].
Researchers have also tried adding nutrients such as iron to the ocean to fertilize the growth of photosynthesizing phytoplankton, which absorb CO
2 and then sink to the ocean floor when they die, trapping carbon there. But it is possible this marine CDR method may inadvertently induce toxic algal blooms and generate other potent greenhouse gases including methane and nitrous oxide [
21].
An additional natural strategy being assessed involves cultivating seaweed (also known as macroalgae), and then sinking it to the seafloor, but questions remain about how this would impact other marine biota [
18]. An alternative is to grow crops on land and then dump them in oxygen-poor regions of the ocean, where the biomass—and its carbon—would sink to the bottom and remain there for centuries without decaying; two companies pursuing these approaches are Carboniferous (Houston, TX, USA) and Rewind (Tel Aviv, Israel) [
22].
Such biological approaches tend to be low-tech and low-cost, but they are also protracted because they depend on the relatively slow growth of biomass. This has led other companies to focus their efforts on faster chemical methods with greater potential for rapid scale-up. Planetary Technologies (Halifax, Canada) has been adding magnesium hydroxide to the ocean to accelerate CO
2 absorption from the air [
23], mimicking natural rock weathering that washes alkaline minerals into the ocean. Raising the pH of the water in this way helps to lock up CO
2 as soluble bicarbonate anions (HCO
3−) and may also potentially benefit marine ecosystems by counteracting ocean acidification. Alkalinity enhancement has previously been used to rejuvenate lakes affected by acid rain, and to protect oyster beds from acidic waters that can thin the bivalves’ shells [
24]. But researchers are still assessing how large-scale alkalinity enhancement might affect other ocean organisms like marine microbes and plankton [
15].
Other companies are using electricity to accelerate these chemical reactions even further. Captura (Pasadena, CA, USA) has developed an electrochemical process, for example, which splits seawater into positive hydrogen ions and negative hydroxide ions, producing two separate streams of acidic and alkaline water (
Fig. 3) [
25]. The acidic stream is then mixed with fresh seawater to turn dissolved bicarbonate into CO
2, which is captured and can be stored in geological formations. The alkaline stream is then used to return all the water in the plant to its natural pH before it is poured back into the ocean. In principle, this CO
2-depleted water should then be able to absorb more CO
2 from the atmosphere. Captura is now working with Norwegian energy company Equinor (Stavanger, Norway) to open a pilot plant on Norway’s coast later this year (2024) engineered to capture 1000 t of CO
2 per year [
26].
Meanwhile, Ebb Carbon (San Carlos, CA, USA) is testing its own alkalinity enhancement system at the Pacific Northwest National Laboratory (Richland, WA, USA) and hopes to establish a larger facility to remove at least 500 t of CO
2 per year from the atmosphere [
27]. Their electrochemical plant also generates hydrogen gas as a by-product, which is increasingly viewed as a key non-polluting source of energy with the potential to replace some use of fossil fuels [
28], [
29].
In Singapore, Equatic is arguably the furthest along in commercializing its electrochemical CDR technology at the island’s Tuas Desalination Plant. Equatic-1 will use renewable electricity to electrolyze seawater to produce separate acid and alkaline water streams, as well as hydrogen gas [
30]. It then passes atmospheric air through the alkaline stream, which traps CO
2 as bicarbonate and reacts with calcium ions in the water to form solid calcium carbonate [
31]. The company says this mixture can be safely discharged into the ocean to store the CO
2 for tens of thousands of years, but the solids can also be removed for use as building materials. Finally, alkaline rocks are used to neutralise the acidic stream so that it too can be poured back into the ocean.
To achieve 1 t of CDR, the plant will process 220 m3 of sea water and use about 1 t of alkaline rock, while producing 30 kg of hydrogen and 250 kg of calcium carbonate, said Edward Sanders, chief operating officer of Equatic. Sanders added that the combination of CDR and hydrogen production makes the technology economically viable. “You have two really valuable products that cross-subsidise each other,” he said.
For all these electrochemical methods, the electricity required to power their electrolyzers is the most significant cost. At its Quebec plant, Equatic plans to use hydroelectric power, and also run off the supply of hydrogen it generates. Sanders said the company expects to offer CDR for less than 100 USD·t
−1 of CO
2 by 2030, meeting a crucial cost milestone set by the US Department of Energy [
32].
Companies paying for marine CDR will want to be sure they are getting value for money. One of the principal mechanisms will involve buying “carbon credits”—purchasing one tonne of carbon sequestration enables a company to emit an equivalent amount of CO
2 [
33]. But these purchased carbon offsets are not always carefully verified and monitored; for example, some forests planted to generate carbon credits have been burned in wildfires [
34].
“The interesting thing with marine CDR is that there are lots of efforts to create very rigorous standards,” said Kitch. In May 2024, for example, Equatic published a comprehensive outline of its carbon accounting system [
35]. In addition, in 2023 the company signed a 50 million USD deal with aerospace company Boeing (Crystal City, VI, USA) to supply it with carbon credits and green hydrogen [
36]. Meanwhile, the European Union has outlined plans for a more-rigorous certification framework for carbon credits from CDR technologies [
37].
All those involved in CDR, both land-based and marine, stress that the technology will not decrease the need to reduce CO
2 emissions, and for now their projects can only remove relatively tiny amounts of CO
2 from the atmosphere [
38]. Whether the technology will become big enough, soon enough, to make a meaningful difference in mitigating climate change remains unclear. Unfortunately, however, it seems time may be short. “Unless we do the scaling up now, we will not be able to deploy it,” said Sanders. “We need the technology to exist at scale in 5 to 10 years.”
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