Could the ocean help us fight climate change?

Until he realized it led nowhere.

"Getting more detailed information on the harms that we perpetrate on the ocean may not be very helpful to society other than paralyzing people on taking action," Riebesell told IE.

"At some point in my career, I felt that I needed to turn around and find options on how the ocean could be a solution to mitigate climate change. I didn't have to look too deep, as it already plays an enormous role by absorbing most of the heat — 93 percent. It is also taking in 25 percent of the yearly carbon dioxide emissions. So, in a way, the ocean is already part of a solution," said Riebesell.

Can this be used to our benefit, wherein we harness those natural processes, sequester, and store more CO2 as a way to mitigate climate change? This thought has spurred an approach referred to as marine Carbon Dioxide Removal (CDR) strategies. "There are several ways in which ocean could help fight climate change — the ideas aren't new, but the science fairly is," he said.

Adding the ocean to the larger picture

According to the Paris Agreement, countries should aim for greenhouse gas emissions to reach net-zero by mid-century, and measures should be adopted which limit the increase in global temperatures this century to 35.6°F (2° )above preindustrial levels, while pursuing the means to limit the increase to 34.7° F (1.5° C)degrees.

While this will require reducing existing emissions, in order to have the slightest chance of hitting the mark, we will also need to remove carbon dioxide which is already in the air. As per estimates, around 10-20 billion tons of CO2 must be removed each year through 2100, globally, which is a tremendous amount. With countries aiming to achieve a climate-neutral world by mid-century, ocean experts decided to look to the seas for an answer, only to consider CDR as a potential solution.

What is ocean-based CDR?

In its recent report, the Intergovernmental Panel on Climate Change (IPCC) has defined CDR as, "anthropogenic activities that deliberately remove CO2 from the atmosphere and durably store it in geological, terrestrial or ocean reservoirs, or in products. Carbon dioxide is removed from the atmosphere by enhancing biological or geochemical carbon sinks or by direct capture of CO2 from air".

The report notes that CDR could be implemented on a large scale in order to meet long-term climate stabilization goals.

The Aspen Institute, a global nonprofit organization headquartered in Washington, D.C., released a report titled 'Guidance for Ocean-Based Carbon Dioxide Removal Projects' which defines ocean-based CDR as a "range of intervention techniques that: (1) take place primarily in the ocean, including in coastal regions; (2) extract CO2 directly from the atmosphere, or from seawater leading to an additional reduction of atmospheric CO2; and (3) durably store the extracted CO2 for a significant period of time".

While conversations around land-based CDR involving air capture, reforestation or afforestation, and producing bioenergy using carbon capture and storage have been doing the rounds for a while, ocean-based CDR was an almost invisible solution.

"One of the several reasons for the delay in considering the ocean as a solution is that most people felt it wasn't ethically correct. At some point, it was even considered dirty science. That perspective has changed in the last few years, mostly because within the scientific community, there is a consensus that we'll have to look into options regarding active CO2 removal to achieve the goals of the Paris Agreement," says Riebesell.

Many methods, few answers

In 2021, The National Academies of Science, Engineering, and Medicine had released a new report titled 'A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration' which identifies pressing research questions about ocean-based CDR and develops a research program to answer them. The study, undertaken by the Committee on a Research Strategy for Ocean Carbon Dioxide Removal and Sequestration, was sponsored by sequestration specialists ClimateWorks Foundation.

Romany M Webb, an associate research scholar at Columbia Law School and Senior Fellow at the Sabin Center for Climate Change Law, and a member of the committee that worked on the report tells IE that while they're looking at several techniques, doing a deep dive into the existing legal frameworks for these international approaches is imperative. 

"We're also working with legal academies around the world to analyze domestic laws that would apply to the processes," she says.

The proposed approaches in the report include ecosystem restoration, large-scale seaweed cultivation, iron fertilization, ocean alkalinity enhancement, artificial upwelling, and electrochemical processes.

The first approach, restoring coastal carbon ecosystems, can not only increase the amount of carbon stored in coastal sediments but also benefit in improving water quality, reducing coastal erosion and flooding, and supporting livelihoods.

Meanwhile, there is already evidence that wild seaweed contributes to carbon removal -  research shows that using certain types of red seaweeds as feed supplements can reduce ruminant emissions by more than 50 percent. Seaweed cultivation may also reduce ocean acidification. However, large-scale seaweed cultivation requires a thorough examination of the potential ecosystem risks which can include changes to nutrient and oxygen availability and altered pH levels.

Iron fertilization, a controversial idea, involves adding trace amounts of iron to the ocean, increasing phytoplankton growth, which would absorb atmospheric CO2 as they grow. A portion will eventually sink to the ocean floor, resulting in permanent storage of that carbon in ocean sediments. "There are well-documented reports, both lab, and field studies, on its benefit but due to public perception and other risks, funding for iron fertilization has been stopped," Riebesell tells us.

Discovering potential

Riebesell and his team at Geomar are currently focused on the other approaches - ocean alkalinity enhancement and artificial upwelling.

Alkalinization involves spreading large amounts of pulverized silicate or carbonate minerals onto the ocean surface (OAE) enhancing chemical weathering reactions through which atmospheric CO2 is consumed.

"Ocean alkalinity enhancement is considered as the option with the highest potential when it comes to scaling it up to gigatons of CO2 sequestration scales. So far, the ocean has only been alkalized in model studies, which gave promising results of possible permanent sequestration," he explains.

However, the process would require massive mining of alkaline minerals. "The effort in obtaining the minerals, grinding and dissolving them in the ocean is massive. Sequestering CO2 by reforestation on land will also be an enormous undertaking. Though the efforts for ocean alkalinity enhancement may seem ridiculously huge, in perspective, it is the same as the former," Riebesell adds.

Riebesell and the team have started studying the process under natural conditions in mesocosms, enclosures that can be lowered into the sea and manipulated with dissolved alkaline minerals to gauge how ocean life responds. "We've just completed a study on the process in the Canary Islands. We're now preparing for a similar study on the Norwegian coast. These places have two opposing ecosystem types in the ocean -- we can see how they respond," he says.

Riebesell says that artificial upwelling is more complex. "It involves biology -- the process relies on extra nutrients brought to the surface of the ocean which fertilizes the phytoplankton and CO2 uptake into organic matter -- which is complicated," he tells us. Artificial upwelling will also generate questions on the dissolution of the organic matter into the ocean, the depth it will be mineralized and its effect on the food web.

"One of our projects has been researching the potential of artificial upwelling for CO2 sequestration for the past five years. We figured that there needn't be a fixed ratio of carbon to nutrient uptake. However, among the two processes, not only is upwelling more complex, but the scalability is questionable. Also, bringing out enough upwelling pumps into the ocean to initiate the process will be way more demanding than dissolving alkaline minerals," says Riebesell.

The beneficial aspects of the processes range beyond solving reducing emissions. By alkalizing the ocean, the acidification levels in the ocean can be reverted and brought back to pre-industrialized pH levels, according to Reibesell. Similarly, artificial upwelling increases ocean productivity.

Rough seas ahead

Currently, ocean-based approaches are riddled with uncertainty, and legal and governance obstacles. "The biggest challenge is large parts of the oceans are in international waters, called the high seas, where no one country has exclusive jurisdiction. The areas are intended to be shared by all countries, and that inevitably creates challenges when you're thinking about these activities that do present risks and can have impacts in places that are very far from where the activity takes place," says Webb.

There have been attempts to establish rules around specific types of ocean-based CDR, in particular, ocean fertilization. Discussions under international agreements like the London Convention and London Protocol to establish a regulatory framework on ocean fertilization are ongoing, says Webb.

"The relevant instruments dealing specifically with ocean fertilization haven't entered into force at the international level. Even if they did, they need to be implemented through the country's domestic force to be enforced. It's not like there is no regime, but it only deals with that one approach and has gaps in its enforceability," she explains.

At the same time, the Convention on the Law of the Sea provides a framework for activities in ocean areas, including establishing offshore territorial waters and exclusive economic zones. Combined with domestic laws governing coastal areas, one could end up with multiple layers of laws and regulations which adds uncertainty for people who want to conduct research into these techniques.

"There are questions as to whether the international community will adopt a specific legal framework for these activities, which could be a long way off. Currently, there are discussions on developing a new agreement called Biodiversity Beyond National Jurisdiction, which deals with dealing with the protection of biodiversity in high seas, and whether ocean-based CDR could be integrated into it. It is difficult, time-consuming, and complex to negotiate international agreements," she says.

There are also a multitude of social science challenges, ranging from the social implications of different techniques to what it means for other users of the oceans, such as indigenous groups. "There are also more commercial uses of the techniques which could have implications. Then there are social licenses and public acceptance that are important when it comes to implementing them on a large scale," explains Webb.

What next?

The proposed methods aren't foolproof - unpredictabilities and potential risks definitely exist. But are they bigger than the risk of inaction?

"Potential risks are associated with each technique as they intervene into the ocean and climate change system. But we should be thinking of the risks in the context of other changes that are happening in the ocean system, brought by climate change, and take action," says Webb.

Responsible research into ocean-based CDR and other approaches must be facilitated and ensured that it happens in a responsibly, socially, and environmentally protective manner. "A government framework that balances those factors, investigates further, and figures out if the processes work in the fight against climate change, is required," she says.

Work on ocean governance must begin early on and in parallel to research, along with understanding public perception and answering questions on why it may become unavoidable to implement these approaches.

"As we know from the climate change models, any of those measures need to be implemented by 2030 at the earliest and wrapped up over the next decade or two. The divisions on which of these options must be implemented must be taken less than 10 years from now," adds Riebesell.