Potential Solutions

In the 1990s and early 2000s, a series of field studies deliberately added iron to the surface ocean to stimulate phytoplankton growth. These tests demonstrated that adding iron led to phytoplankton blooms, but the extent to which that carbon sank to depths that would lead to it being reliably sequestered from the atmosphere wasn’t a focus of those studies. In addition, the phytoplankton were not able to use all of the iron for growth before the mineral sank, unused. As a result, we still do not know how much iron would be needed to durably remove carbon from the atmosphere at scales that would make it a useful CDR strategy .

Researchers did document that diatoms became more abundant than many other types of phytoplankton. These algae can be up to 1,000 times larger than cyanobacteria, allowing them to take up more carbon dioxide via photosynthesis. Diatoms are also unique in that they form a silica-based, glass-like shells that add weight, increasing the likelihood that they will sink faster than other, less-dense phytoplankton when they die. Their fast growth rates and loss to the deep sea would, in natural systems, make them more able to remove carbon from the atmosphere and sequester it deeper in the ocean.

Early iron fertilization experiments faced resistance. This can largely be tied to three factors. First, about half of the early experiments created blooms that included rapid growth of Pseudo-nitzschia, a diatom genus that, in coastal waters, led to harmful algal blooms (HABS) that can be detrimental to marine ecosystems and to humans. When tested, none of the diatoms had more domic acid, the toxin responsible for HAB impacts, per cell than naturally occurring cells found in other open-ocean areas, and none of the experiments resulted in a bloom that caused human or environmental damage. That being said, this type of unintended consequence needs to be carefully monitored.

The second factor of concern was production of other greenhouse gasses, such as nitrous oxide and the overall reduction of deep-water oxygen as the organic material sank and decomposed. Nitrous oxide changes were measured in prior experiments and the heating effects of nitrous oxide were predicted to be about 6-12% of the cooling effects resulting from carbon dioxide removal. Oxygen depletions are not worrisome at the scales of the next generation of field experiments, though careful prediction and monitoring are needed as field studies become larger and longer. Interestingly, nine of the earlier field experiments produced dimethyl sulfide (DMS) which can lead to the formation of aerosol particles that enhance cloud formation over the ocean and, hence, have an additional cooling effect on the atmosphere and surface ocean. Again, this was a small and variable impact relative to the intended removal of carbon.

The final factor that was harder to quantify, was the public perception of “rogue” geoengineers, who, without open accountability and permissions, conducted open-ocean iron additions to generate marketable carbon credits. Ill-conceived, poorly documented studies, and un-sanctioned studies such as these are one of the reasons why ExOIS is pushing for the adoption of a common code of conduct for iron fertilization and all forms of CDR research. Field experiments must be designed and conducted in a transparent manner to promote the collective benefit of humankind, with a goal to study not just carbon removal, but the intended and unintended ecological consequences that will result. Open and cooperative research frameworks and clear mechanisms for governance and accountability are needed, all with public involvement, as we consider new ways to enhance the ocean’s capacity for carbon removal in this fight against climate change.

Because iron is a micronutrient, phytoplankton need only trace amounts for it to have a massive impact. Ash from the 2008 eruption of Kasatochi in the Aleutian Islands created an algal bloom that is estimated to have removed 10 million tons of carbon from the surface ocean. Soot from the 2019-2020 Australian wildfires caused a bloom between New Zealand and South America that may have removed as much as 150 to 300 million tons of carbon. Studies of remote islands in the Southern Ocean also show efficient carbon removal and healthy ecosystems that are supported by a steady supply of iron from the seafloor sediments around these hot spots of phytoplankton productivity.

If relatively small amounts of iron can be added to the ocean’s surface in key, iron-poor regions, it might help remove large amounts of carbon dioxide from the atmosphere and set us on a path to “net negative” carbon emissions—removing more carbon from the atmosphere than human activity adds to it. But it will only work if the carbon removed from the atmosphere sinks to the ocean depths, where it will be locked away for at least a century. This would buy time for a full transition from fossil-fuels to renewable sources of energy.

To achieve this level of durability requires getting the carbon to sink below ocean depths of 500-1000 meters (1500-3000 feet) depending upon the location. We know these depths from sophisticated ocean models that tell us the time it takes for deep waters to return to the surface ocean as part of the ocean’s thermohaline circulation, or what is commonly called the “great conveyor belt.”

It's important to remember that CDR strategies such as iron fertilization do not replace the need for immediate and major reductions in the use of fossil fuels that produce carbon dioxide in the first place. In fact, even with all the carbon the ocean removes from the atmosphere, our planet is on track to exceed a 1.5°C increase in average global temperature within the next 20 years. As a result, a 2021 report from the Intergovernmental Panel on Climate Change concluded that we need to do more than just cut our reliance on fossil fuels—we need to find new ways to remove, and safely store, more carbon than we add to the atmosphere each year, something known as “net negative emissions.”

The goal of ocean iron fertilization is to stimulate photosynthesis, which enables phytoplankton to remove dissolved carbon dioxide from surface waters and convert it to other forms of carbon, mostly organic carbon, a fraction of which is then durably removed when it sinks into the deep ocean. The stimulation of phytoplankton and subsequent storage of carbon at depth is an essential first step but does not directly result in a draw-down of atmospheric carbon dioxide several reasons. First, much of the organic carbon that enters the ocean’s mid-waters, also known as the twilight zone, provides food for organisms that live there. As a result, these mid-water animals, from microbes to fish, return carbon to its dissolved forms before it has a chance to sink into the deep ocean through respiration. Up to 90 percent of the sinking organic carbon, often called “marine snow” can be lost in this manner and returned to the surface and possibly the atmosphere. However, these losses vary widely in the ocean and in some diatom blooms, as much as to 50% of the organic carbon contained in marine snow can make its way to the deep ocean. A challenge the scientific community faces is to find better ways to measure and enhance the efficiency of this process if it is going to become a viable CDR technique.

A second issue is the response time for the ocean to absorb more carbon dioxide from the atmosphere in order to replace what has been removed via photosynthesis. This happens naturally and already makes the ocean sink for about one-third of society’s atmospheric carbon dioxide emissions. The question is, is how fast does this air-sea gas exchange process happen relative to the rate at which surface waters are mixed vertically, and so, removed from contact with the atmosphere. This rate of gas exchange versus the vertical mixing of waters will vary widely across the ocean and depending on local conditions, as well as large-scale phenomena such as El Nino and monsoons. These processes can be estimated with better measurements of carbon dioxide drawdown and more sophisticated ocean models that can predict the rate of replacement of surface waters by vertical mixing.