In 1988 marine biologist John Martin made a bold claim during a seminar at the Woods Hole Oceanographic Institution in Massachusetts. “Give me half a tanker of iron,” he told the audience, “and I’ll give you an ice age.”
Martin had recognised that the growth (“blooms”) of marine phytoplankton (“plant plankton”) in parts of the world’s oceans is limited by the amount of iron – an essential nutrient – available to them. Add more iron to the ocean waters, and the bloom can be bigger.
The growth of these photosynthesising organisms removes carbon dioxide (CO2) from the surrounding air and “fixes” the carbon into the tissues of the cells. Boosting growth can potentially lower the concentration of atmospheric CO2 – the inexorable rise of which over the course of the 20th century was already by that time acknowledged as a potential cause of global warming.
Martin’s reference to an “ice age” alluded to the known correlation between low global average temperatures and low atmospheric CO2 levels in the distant past.
Laboratory experiments on “iron fertilisation” of plankton growth suggested that adding a ton of iron to the oceans – literally scattering the metal or its compounds into the surface waters – could remove up to 100,000 tons of carbon from the air. Doing this across the entire Southern Ocean, where huge blooms occur annually, could (in some estimations) eliminate as much as a quarter of the world’s CO2 emissions.
Martin’s iron-fertilisation hypothesis was one of the first proposals for a “geo-engineering” solution to climate change – a massive technological intervention in the Earth’s system that might prevent global temperatures rising to catastrophic levels. Other proposals include injecting small particles into the stratosphere that reflect some of the sun’s rays back into space; adding particles lower in the atmosphere to make clouds thicker, brighter, and more reflective; and building immense space mirrors or sunshades in Earth’s orbit.
Needless to say, such proposals are tremendously contentious. Some say that expensive, unproven and potentially hard-to-control techno fixes are not a realistic or even an ethical alternative to making the hard choices needed to mitigate global warming. Not only might they be dangerous in themselves, but they give governments and industry an excuse not to take actions that we already know would work.
Others argue that it is worth having geo-engineering options as an emergency back-up plan if things get really bad (see, for example, the Netflix hit Families Like Ours, about the evacuation of a flooded Denmark). Probably still the best overview of that complex debate is Oliver Morton’s 2015 book The Planet Remade.
Iron fertilisation is a good test case for geo-engineering. For one thing, it is amenable to experimental validation. From the early 1990s, several projects have added iron to the oceans from ships, on a scale too small to risk hazards but large enough to judge the feasibility. Such fertilisation did enhance phytoplankton blooms, but not as much as had been hoped and not with any lasting effect. To be effective, the carbon fixed in phytoplankton needs to sink to the ocean floor as dead cells or “fecal pellets” from the zooplankton that graze on them, there to be buried as sediments. But only a small percentage does; some is rapidly cycled up the food chain to large marine animals and then leaked back into the atmosphere.
And one can’t tamper with the bottom rung of the marine food chain without expecting knock-on effects. For example, bigger blooms more rapidly use up other nutrients like nitrates and phosphates, limiting further growth. A new study by a team in the US showed that iron in industrial emissions from East Asia can create inadvertent iron fertilisation in the north Pacific during springtime that depletes nitrate and depresses summer growth, leading eventually to a northward shift of the whole ecosystem.
By the mid-2000s there was general agreement that iron fertilisation would not be a viable means of ameliorating climate change. An international meeting at Woods Hole in 2007 was down on the idea, and that same year the London Convention, the international maritime organisation that oversees dumping of waste at sea, advised that large-scale iron-fertilisation schemes are currently not scientifically justified.
This has not, however, prevented some researchers from continuing to explore the idea. Geochemist Ken Buesseler of Woods Hole argued that “just because we don’t know all the answers, we shouldn’t say no to further research.” Last year Buesseler’s not-for-profit programme Exploring Ocean Iron Solutions (ExOIS) announced it was seeking to raise $160m for more trials in the Pacific in 2026. “We need to proceed with caution,” says the ExOIS website. They’re not wrong.
