Over dinner last week a friend asked me about different options for carbon removal. When I mentioned iron fertilization, he had no idea what I was talking about, so when I got home I started to put together a small explainer. This isn’t a deep dive, and please don’t scream at me over the math. These are just some notes, numbers and ethical considerations appropriate for a holiday dinner or an 8th grade science project.
As the climate crisis intensifies, scientists and policymakers are exploring innovative strategies to remove carbon dioxide (CO₂) from the atmosphere. One approach, iron fertilization, proposes to enhance the ocean’s natural ability to sequester carbon by stimulating the growth of phytoplankton. While the concept holds promise, it also raises significant ecological and ethical questions. Why would we consider this method? Is it a quick fix, a long-term solution, or an experiment with unknown risks? By examining how iron fertilization works, its costs, benefits, and risks, we can better evaluate whether it deserves a place among carbon removal strategies or if alternative approaches, like planting mangroves or seagrasses, are the safer, more sustainable path forward.
What is Iron Fertilization?
Iron fertilization involves adding iron, a micronutrient often lacking in certain ocean regions, to stimulate phytoplankton growth. Phytoplankton, microscopic plants at the base of the ocean food web, use CO₂ during photosynthesis. When phytoplankton blooms occur, they absorb large amounts of carbon from the atmosphere. Some of this carbon sinks to the ocean floor when phytoplankton die, where it can remain sequestered for centuries or longer.
This method specifically targets high-nutrient, low-chlorophyll (HNLC) regions, such as the Southern Ocean and parts of the Pacific, where iron is the primary limiting factor for phytoplankton growth.
How Does It Work?
- Introduction of Iron: Iron sulfate, a soluble and inexpensive compound, is spread over the ocean surface in iron-deficient areas.
- Phytoplankton Bloom: The added iron triggers a rapid increase in phytoplankton populations, driving photosynthesis and carbon uptake.
- Carbon Sequestration: As phytoplankton die, some sink into the deep ocean, taking absorbed carbon with them.
- Long-Term Storage: The carbon becomes buried in sediments, where it remains out of the atmosphere for centuries or more.
Projects like LOHAFEX, a joint Indian-German experiment in the Southern Ocean, demonstrated the ability of iron fertilization to stimulate large phytoplankton blooms (source: https://doi.org/10.1016/j.jmarsys.2019.02.002). It was controversial and it also revealed complexities, such as unpredictable ecological effects, that highlight the need for caution.
Benefits of Iron Fertilization
- Carbon Sequestration Potential: Studies suggest iron fertilization could remove between 10–100 million metric tons of CO₂ annually if scaled globally (source: https://www.frontiersin.org/articles/10.3389/fmars.2019.00022/full).
- Cost-Effectiveness: Iron fertilization is estimated to cost $5–50 per ton of CO₂ sequestered, significantly cheaper than most terrestrial carbon capture methods (source: https://www.edf.org/sites/default/files/documents/Ocean%20Fertilization.pdf).
- Boosting Productivity: In some regions, increased phytoplankton blooms could temporarily enhance food availability for marine life, benefiting local fisheries.
Environmental Costs and Risks
Despite its potential, iron fertilization is not without serious risks:
- Hypoxia and Dead Zones: Decomposing phytoplankton can deplete oxygen in deep waters, creating dead zones harmful to marine life.
- Food Web Disruption: Artificial blooms can favor certain phytoplankton species, altering food webs and displacing species reliant on natural conditions.
- Harmful Algal Blooms: Some phytoplankton species release toxins, which can harm marine life and humans.
- Ocean Acidification: While carbon is stored in deep waters, increased CO₂ flux could exacerbate acidification in certain areas.
- Worst-Case Scenarios: Poorly managed projects could lead to large-scale hypoxia, harmful blooms, or long-term damage to marine ecosystems.
Additionally, iron mining, processing, and transporting iron sulfate for fertilization projects itself carries a carbon cost. I pooled info from a few papers but I estimated 1–5% of the total carbon that would be sequestered by the resulting phytoplankton bloom is actually produces in iron production process. So for every 30,000 tons of CO₂ sequestered, approximately 300–1,500 tons of CO₂ could be emitted.
Iron Fertilization vs. Terrestrial Solutions
Just so you get an idea, let’s compare the cost of iron fertilization with alternative carbon sequestration methods like mangrove planting:
Iron Fertilization
- Iron Needed: 10 tons of iron sulfate.
- CO₂ Sequestered: 30,000 tons.
- Carbon Cost: 1–5% (300–1,500 tons of CO₂ emissions).
- Area Treated: 100 square kilometers.
- Cost: ~$5–50 per ton of CO₂.
Mangrove Planting
- Area Needed: 1 hectare sequesters ~3.7 tons of CO₂ annually
- To Match 30,000 Tons: Approximately 8,000 hectares of mangroves.
- Carbon Cost: Minimal, primarily from planting and maintenance.
- Cost: ~$200–1,000 per ton of CO₂.
While iron fertilization appears cheaper and quicker, mangrove planting provides additional benefits, such as coastal protection, biodiversity support, and long-term sustainability. The long-term effects of planting mangroves are practically incalculable, as they provide immense ecological value, from stabilizing shorelines to serving as critical habitats for countless species.
Best Practices to Mitigate Risks
If iron fertilization is pursued, strict guidelines are essential to minimize harm:
- Target Open Oceans: Focus on remote HNLC regions with minimal human or ecological interaction.
- Monitor Ecosystems Closely: Assess phytoplankton species composition, oxygen levels, and long-term carbon flux.
- Avoid Sensitive Areas: Stay away from biodiversity hotspots, upwelling zones, and regions prone to hypoxia.
- Limit Scale: Start with small, controlled projects to monitor effects before scaling up.
- Optimize Conditions: Favor areas with natural downwelling zones, which transport carbon to the deep ocean more efficiently.
So where does this leave us?
Iron fertilization presents an intriguing opportunity for carbon sequestration, but it is not without risks. Its relatively low cost and high carbon capture potential make it appealing, yet the environmental uncertainties demand caution. While it may offer short-term results, we must ask ourselves, Why are we pursuing this method? Is it a quick sink to buy time, or part of a larger, long-term solution?
Compared to sustainable carbon solutions like planting mangroves, seagrass beds, or terrestrial forests, iron fertilization lacks co-benefits such as biodiversity restoration or ecosystem resilience. If explored, it must be done carefully, with constant monitoring and safeguards to prevent unintended harm. We must keep a close eye on ecological indicators and prioritize safety to ensure that this experimental approach remains just that. An experiment. Until its true potential and risks are fully understood.
Our solutions for climate change must be as sustainable and thoughtful as the future we hope to create. I’d love to hear your thoughts.
By Giacomo Abrusci