How fertilising the oceans with iron could help fight the climate crisis

01 August 2025

As levels of atmospheric carbon dioxide continue to increase, scientists are exploring how to better utilise our seas

There’s an attention-grabbing moment in Sir David Attenborough's 2025 movie Ocean where he explains how whales fertilise the seas: their faeces is a rich source of iron, a necessary nutrient for phytoplankton, which themselves are the building block of life in the ocean. The idea hit me: could we take inspiration from the whales and harness this process to increase the capacity of phytoplankton to capture atmospheric carbon dioxide (CO2)? Could we ‘fertilise’ the ocean with iron to fight the climate crisis?

I’m not the first person to have had this idea, I have to confess. It’s an issue that’s been discussed in academic circles since the 1980s, with some major studies having taken place in 1993. The late American oceanographer John Martin was known for his research on the role of iron as a phytoplankton micronutrient, and its significance for so-called "high-nutrient, low chlorophyll" regions of the oceans. He advocated the use of iron fertilisation to enhance oceanic primary production and act as a sink for fossil fuel carbon dioxide. “Give me a half tanker of iron, and I will give you an ice age,” he joked, during a seminar at Woods Hole Oceanographic Institution (WHOI) in 1988. 

Now, as levels of CO2 rise to ever more concerning levels as a consequence of the burning of fossil fuels, better utilising the potential of our oceans is very much back on the agenda. At the 2025 UN Ocean Conference, which took place in Nice, France, this past June, in fact, there were multiple committees for action including one entitled “Leveraging ocean, climate and biodiversity interlinkages”.

Figure 1: Atmospheric levels of CO2 have risen at an alarming rate over the past 100 years triggering climate change, extreme weather and droughts.

Biochemistry

When phytoplankton – also known as microalgae – die, some of them sink to the ocean depths, carrying the carbon they have absorbed through photosynthesis with them. This process, known as the biological pump, can help remove CO2 from the atmosphere and store it in the deep ocean for extended periods. Alternatively, phytoplankton get metabolised by various sea creatures that capture the metabolised CO2 in their own bodies as biomass. Whales not only fertilise the oceans with their poo but also capture CO2 in their massive bodies, which become part of the ecosystem of the ocean floor after their deaths.

Iron plays a crucial role in phytoplankton's growth: it’s involved in energy production, biochemical catalysis and the synthesis of chlorophyll. While iron is abundant on Earth and coastal regions, it's less available in the ocean due to reactions with oxygen and formation of poorly soluble minerals. Many parts of the ocean experience iron deficiency, particularly in open ocean areas and this deficiency can limit phytoplankton growth and ocean productivity, impacting the marine food web and carbon sequestration.

Several experiments, like LOHAFEX (2009), EIFEX in 2004 (The European Iron Fertilization Experiment) and SEEDS in 2001 (The Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study), have successfully demonstrated that introducing iron (often in the form of iron sulphate) to ocean areas can trigger significant phytoplankton blooms. These blooms are visually evident through satellite imagery and measurable through changes in chlorophyll concentrations.

In many experiments, diatoms, which are larger and faster-growing phytoplankton with silica-based shells, become dominant in the bloom. Diatoms are more likely to sink to deeper waters when they die, potentially sequestering carbon for longer periods.

Figure 2: Phytoplankton Can Help Capture Atmospheric CO2 Through the Process of Photosynthesis

Estimates by Bigelow Laboratory for Ocean Sciences, an independent, nonprofit research institute based in Maine, USA suggest that iron fertilisation could potentially remove many gigatonnes per year of atmospheric CO2. Current annual anthropogenic CO2 emissions are around 37.4 gigatons from fossil fuels and land-use change. This process is scalable: the more ocean iron fertilisation (OIF) the more carbon dioxide removed.

We can add iron to the oceans in several ways, adding ferric salts using tankers or installing iron rigs that can corrode slowly and release iron into the ocean. We can also accelerate this process through electrolysis, using floating solar-powered rigs that generate an electrical current which can dissolve the iron electrodes attached to them in minutes. While natural corrosion takes a long time, electrolysis can produce ferric ions instantaneously.

Using electrolysis to produce ferric in the ocean has an additional benefit: it produces alkaline hydroxide ions, which increase the alkalinity of the water. This can help offset the increasing ocean acidification associated with higher CO2 levels. Acidification is an issue of great concern to biologists of marine systems partly because of its consequences for crustacean shell formation: a pH of around 7.7 is often cited as a point where shell formation becomes problematic. At this level, shells may become thinner, growth rates may decrease and dissolution may occur. We are currently at an average pH of around 8.0, down from 8.2 a hundred years ago.

Figure 3: Ocean pH has been decreasing over the years due to increased levels of CO2 in the atmosphere that dissolve in the ocean.

Environmental concerns

While iron fertilisation holds promise for carbon capture, there are environmental concerns associated with the technology. One worry is the potential impacts on other marine ecosystems and the possibility of nutrient robbing, where iron fertilisation could lead to so-called ‘dead zones’ where algal blooms consume all the oxygen in the water, killing other marine life. Blooms of phytoplankton could also consume other nutrients, making them unavailable for other organisms elsewhere.

Ocean iron fertilisation was in fact banned internationally for commercial purposes in 2013 under the London Protocol – a global ocean pollution treaty – after public backlash. However, research is still taking place to work out how much CO2 the technique could capture and what impact it might have on marine ecosystems. A not-for-profit consortium of experts called Exploring Ocean Iron Solutions (ExOIS) are hoping to start trials across up to 10,000 square kilometres of ocean in the northeast Pacific as early as 2026.

ExOIS believe that enhancing the ocean’s natural biological carbon pump may be a responsible way to help control increases in atmospheric carbon dioxide. They are encouraged by analyses of natural and deliberate ocean iron fertilisation (OIF) field experiments that have demonstrated low deployment costs and high global capacity for natural CO2 removal.

As world atmospheric temperatures rise due to greenhouse gas emissions, robust solutions for capturing CO2 become more and more vital for the survival of many species, including us humans. Direct air capture methods for CO2 removal have proved too costly and inefficient due to the large volumes of atmospheric air that need to be processed.

Trees capture CO2 but with the global population increasing, it seems likely that we’ll see more deforestation, not less, as the need for agricultural food production ramps up. All this has created a space where ocean CO2 capture could be a viable solution.

--

Stay up-to-date with our free monthly 'The Environment' newsletter – subscribe here.

Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM is managing director of Kremesti Environmental Consulting Ltd