The Startling Math of Ocean Iron Fertilization

Yes, we can afford to clean up our carbon mess

Imagine we gave ourselves an impossibly ambitious climate target. 

Imagine we decided to remove all the CO₂ humanity emits as we emit it. What would that really mean? Well, last year we emitted the equivalent of 53 billion tons of CO₂ worth of greenhouse gases. Removing planet-warming gasses on that scale would be a monumental challenge.

How would we go about it?

It’s tricky. There are hundreds of carbon removal companies, spanning a range of different techniques. It can be confusing. How do you prioritize? How do you separate the wheat from the chaff? 

If we can’t afford it, it won’t work

The first criteria has to be cost. To be scalable, a technique has to be affordable. Most of the techniques under consideration fail this test. For example, cutting-edge Direct Air Capture companies say they can store away atmospheric CO₂ for $300 a ton. At that price, capturing 53 billion tons’ worth would cost almost $16 trillion — an impossible sum, 16 times the budget of the Pentagon. 

To solve this problem quickly enough to make a difference in our grandchildren’s lives, we need something radically more affordable. 

Imagine a consortium of 53 generous climate funders pledged one billion dollars each to removing all those greenhouse gases each year. Could humanity remove CO₂ at scale for $1 a ton?

It sounds impossible. 

It isn’t. 

In 2022, the National Academies of Sciences, Engineering, and Medicine published a study finding it’s possible to capture a ton of CO₂ for less than 40 cents. Not dollars, mind you: cents. That’s three orders of magnitude cheaper than existing approaches. 

Follow Mother Nature: She removes CO2 on a grand scale for free

How? 

By mimicking the process nature has always used to cool the planet at scale: stimulating the growth of photosynthesizing ocean microorganisms, by feeding them iron, the missing nutrient that limits their growth.

Ocean Iron Fertilization (OIF) is the only technique available to us right now with the potential to remove carbon dioxide from the atmosphere at scale for a price humanity can afford. 

We have good reason to think it will work. Paleoclimatologists have found that rapid cooling periods in earth history usually correlate with sharp drops in atmospheric CO2, which follow an increase in ocean iron. We’ve seen this process play out live within my own lifetime: more than once, volcanic eruptions have deposited large quantities of iron in the ocean, leading to measurable growth in phytoplankton concentrations. It’s a well-established natural mechanism whose most visible side-effect is a sudden flourishing in marine life. 

Replicating what works—carefully

Ocean Iron Fertilization is not a magic wand, of course. A lively scientific debate is underway about how long the carbon that phytoplankton captures will stay out of the atmosphere. Historically, ice core records show that during ice ages, much of the captured carbon stays sequestered for thousands of years. We would expect that today, too, some of the biocarbon will sink and be stored away in the deep ocean for the long term. However most of it is consumed and metabolized by the ocean food chain, then released back to the atmosphere over days, months or years. Understanding the conditions that maximize permanent sequestration is an important research task. 

We are confident that, through a rapid engineering development approach, we can devise fertilization protocols that maximize the permanence of CO₂ sequestration, reproducing what nature does. The task calls for careful and sustained testing, including targeting fertilization on naturally occurring downwelling eddies: large ocean whirlpools that pull surface water gradually down towards the ocean depths. 

We have much to learn about how to optimize ocean fertilization to enable the growth of the types of microorganisms best able to biosequester carbon at scale, with our initial hypothesis centered on sustaining the growth of nitrogen-fixing bacteria (mainly Trichodesmium) crucial to sustaining long-term phytoplankton growth. 

Just as in farming, which uses the same iron fertilizer at 10 times the concentration needed in the ocean, ensuring the safety of ocean iron fertilization must remain a top priority for testing, scale-up, and long-term operation. On its face, a technique that feeds the top of the food chain looks environmentally benign: OIF’s main side-effect is a large expansion in ocean primary production. Everything in the ocean eats either phytoplankton, or things that eat phytoplankton: OIF has been observed to produce large expansions in overall ocean productivity, including for fisheries. Of course, this implies a rapid change in the local ecosystem, as after volcanic eruptions. A responsible OIF approach will need to carefully monitor the health of ocean microbiota and the wider ocean ecosystem. Again, as in farming, fertilization protocols will be adjusted in response to any unhealthy ecosystem changes. 

Scientists have a vast amount to learn about the way ocean ecosystems develop and change when small regions experience fertilization. After initial testing, the area needed for OIF at scale will be understood. CO2 data from 1992 indicates that it would be less than 1% of the ocean. There are many unanswered questions, including about the feedback mechanisms linking the atmosphere and the ocean, and how they impact the effectiveness of iron fertilization. 

The best time to start doing the research needed to bridge these knowledge gaps was 20 years ago, the second best time is now. 

We already know enough to act

The deeper story — which few yet appreciate— is that we know most of what we need to know to replicate how Nature removes vast amounts of carbon, and to do it affordably.

As we develop ocean fertilization techniques, we believe the already very low cost of carbon removal established in the 2022 NASEM report could be brought down farther still. It takes startlingly small amounts of iron to produce a large growth of CO₂-catching phytoplankton. According to the original studies in this field by the pioneering oceanographer John Martin, the ratio of iron fertilizer added to CO₂ sequestered could be as high as one million to one. That’s a thousand times more than the conservative value used in the NASEM report. Theoretically, a properly optimized approach to Ocean Iron Fertilization could bring the cost per ton of CO₂ sequestered as low as 2 to 3 cents: that would be four orders of magnitude cheaper than existing approaches. 

We’re not there yet. It will take a sustained research and development push to get us there. But Ocean Iron Fertilization begins with powerful demonstrations by nature and such a large cost advantage over competing approaches: it’s an obvious candidate for immediate and vastly expanded investment. 

Time’s a’wastin’. Let’s get to it.