Iron Fertilization

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Iron Fertilization


Iron fertilization, the stimulation of phytoplankton growth in the upper layers of the sea by the addition of iron, can occur either naturally or artificially. Phytoplankton are microscopic, single-celled green plants that float in the sea and are the base of the open-ocean food chain. Like other green plants, they obtain carbon to build their bodies by taking up CO2, keeping the carbon, and releasing the oxygen.

Small-scale experiments have shown that iron fertilization does produce blooms or sudden population explosions of phytoplankton. Advocates of iron fertilization as a way of mitigating climate change believe that it would be a cost-effective way of sequestering carbon: that is, removing carbon from the atmosphere and storing it in the deep ocean. Opponents argue that large-scale iron fertilization would probably be ineffective at sequestering carbon, according to experimental data, and, effective or not, might cause large-scale ecological harm.

Historical Background and Scientific Foundations

In the late 1980s, American oceanographer John Martin (1935–1993) was studying the role of iron as a micronutrient in phytoplankton. In 1990, Martin concluded that areas of ocean known as high-nutrient, low-chlorophyll regions or “ocean deserts,” which make up about a third of the world's ocean area, had low phytoplankton growth because of a limited supply of iron. (Chlorophyll is the chemical that green plants use to extract energy from sunlight; where phytoplankton are few, chlorophyll is low.) That is, all the nutrients needed for phytoplankton growth were present, including nitrate, phosphate, and silicic acid, but not iron.

An analogy can be drawn to the role of iron in human nutrition. Iron makes up only .004% of our body weight but is essential to life: humans could not live in an environment rich in fats, proteins, and carbohydrates but utterly devoid of iron. Such environments do not exist on land, however, because iron is a common ingredient of all soils; only at sea is iron ever truly rare.

Phytoplankton, like other green plants, take carbon from CO2 and turn it into solid biomass. (For land-dwelling plants, the CO2 is extracted directly from the air; for phytoplankton, it is first dissolved in the water.) When phytoplankton die, they sink, creating a rain of carbon particles that transports CO2 from the ocean surface to 650 ft (200 m) or greater depth. This process is called the biological pump. Once the dead phytoplankton reach these deep, cold waters, the CO2 is liberated from their tissues by decay and remains at that depth for a long time, because the deeper waters tend not to mix with the less-dense surface waters, from which CO2 can escape into the atmosphere.

By studying paleoclimate (ancient climate) data, scientists have found that there is a relationship between ancient climate changes and ocean iron levels. A major source of natural iron in the oceans is airborne or Aeolian dust. Increased natural iron fertilization of the oceans by airborne dust during glacial periods, according to a scientific concept called the iron hypothesis, accounts for lower atmospheric CO2 during those periods and so acts as a feedback for lower temperatures (less CO2 means less greenhouse effect and a cooler Earth). The importance of this effect on shaping paleoclimate is disputed, but scientists agree that iron does have an important role in the removal of carbon from the atmosphere.

Martin advocated the deliberate addition of iron to the ocean—especially the Southern Ocean, Earth's largest high-nutrient, low-chlorophyll area. He boasted humorously at one point, “Give me half a tanker of iron and I'll give you the next ice age.”

Impacts and Issues

Advocates claim that iron fertilization could sequester billions of tons of carbon. Since the European Union has established a market for carbon credits, carbon polluters in Europe could potentially hire companies to spread iron dust in the Southern Ocean at a net profit, gaining more value in carbon credits than the iron fertilization costs them.

Twelve experiments at sea from 1993 to 2007 showed that iron fertilization does cause phytoplankton blooms. However, there is scientific debate over how much carbon sequestration actually results from these blooms. Observations at sea revealed disappointingly low rates of carbon sequestration (i.e., carbon raining down from the upper levels of the sea to the deep waters). The carbon impact observed by the SOFeX voyage (Southern Ocean Iron [Fe] Experiments) in 2002 was so moderate that, according to scientists writing in the journal Science in 2003, to sequester a third of annual human carbon releases would require a million ships plying an area ten times greater than the Southern Ocean (the whole ocean south of 60° south latitude). Advocates argue that the experiments did not observe the effect of fertilization for a long enough time, and that it is too soon to conclude that iron fertilization will not work.

The possible environmental impact of iron fertilization is uncertain. A positive result might be enhanced fish populations, as phytoplankton blooms caused by fertilization provided a new source of food. However, it remains unknown what the actual effects on ocean ecology might be. Iron fertilization changes the species makeup of phytoplankton populations, with unknown ecological effects, and would reduce the down-current supply of nutrients. It might also cause oxygen depletion of the deeper waters, again with unknown effects on deep-sea life.

As of 2007, a private San Francisco company, Plank-tos, was planning to create a large phytoplankton bloom near the Galapagos Islands using 90 metric tons of hematite, an iron-containing mineral. The plan was controversial, and the parties of the London Convention, an international treaty governing ocean dumping, released an official statement of concern about the project.

Primary Source Connection

Phytoplankton—microscopic, photosynthetic plants that live in water—produce half of the world's oxygen. Phytoplankton also capture carbon dioxide from the atmosphere during photosynthesis. Some of this carbon dioxide is permanently removed from the air when dead phytoplankton sink into the ocean along with trapped carbon dioxide. This article tells how one company hopes to mix iron particles—an essential element for phytoplankton—into the ocean in order to encourage phytoplankton growth. The company hopes that the phytoplankton would capture carbon, which would allow the company to sell carbon credits on the open market. This plan has many critics, though, who argue that increasing iron levels in the ocean could have unintended consequences on the ecosystem.

Steven Mufson is a Washington Post staff writer.


A small California company is planning to mix up to 80 tons of iron particles into the Pacific Ocean 350 miles west of the Galapagos islands to see whether it can make a splash in the markets where people seek to offset their greenhouse gas emissions.

Planktos—with 24 employees, a Web site and virtually no revenue—has raised money to send a 115-foot boat called the Weatherbird II on a voyage to stimulate the growth of plankton that could boost the ocean's ability to absorb carbon dioxide from the air. The company plans to estimate the amount of carbon dioxide captured and sell it on the nascent carbon-trading markets.

The boat is still in Florida, but the plan has already stirred the waters in Washington. Environmental groups say the Planktos project could have unforeseen side effects, and the Environmental Protection Agency has warned that the action may be subject to regulation under the Ocean Dumping Act.

Disputes like the one over Planktos may be the wave of the future in the new carbon-conscious era. As countries and companies seek to slow climate change, taking carbon dioxide out of the atmosphere can be financially rewarding.


BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.

CARBON CREDITS: Units of permission or value, similar to monetary units (e.g., dollars, euros) that entitle their owner to emit one metric ton of carbon dioxide into the atmosphere per credit.

CARBON SEQUESTRATION: The uptake and storage of carbon. Trees and plants, for example, absorb carbon dioxide, release the oxygen, and store the carbon. Fossil fuels were at one time biomass and continue to store the carbon until burned.

PALEOCLIMATE: The climate of a given period of time in the geologic past.

PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.

In a bid for attention for another of its projects, Planktos said earlier this month it would offset the Vatican's carbon emissions by donating credits from trees being planted in a Hungarian national forest. The company said it would make the Holy See “the world's first carbon-neutral sovereign state.” It released a video that panned across St. Peter's Square to music from Johann Sebastian Bach's “St. Matthew Passion” and then cut to Cardinal Paul Poupard, who thanked Planktos chief executive Russ George.

Other groups have looked on the company with less indulgence. The Surface Ocean Lower Atmosphere Study, an international research group, said last month that “ocean fertilization will be ineffective and potentially deleterious, and should not be used as a strategy for offsetting CO2 emissions.” The International Maritime Organization scientific group, the Friends of the Earth and the World Wildlife Fund have condemned it. And a group called the Sea Shepherd Conservation Society said its own ship would monitor the Planktos vessel and possibly “intercept” it.

On Wednesday, George appeared before the House Select Committee on Energy Independence and Global Warming and lashed back at his critics. The EPA was working with “radical environmental groups,” he said. In written submissions, he said his firm's work had been “falsely portrayed” to “generate public alarm.”

Planktos's Web site boasts that it “offers investors the single most powerful, profitable, and planet-friendly tool in the worldwide battle against global warming.” Though its revenue amounts to a few thousand dollars, raised by selling “a few thousand tons” of credits to individuals and small businesses largely through its online “store,” the Foster City, Calif., company has a market value of $91.4 million.

Serious science is involved in the company's ocean concept. Planktos plans to suck water from the ocean, insert the equivalent of a teaspoon of iron into a volume equal to an Olympic-size pool, and pump the water back into the ocean as the ship makes a grid about 62 miles by 62 miles— “like plowing a field,” one Planktos official said. Iron is a nutrient for phytoplankton, which absorb carbon dioxide from the air and convert it to carbon and oxygen through photosynthesis. The plankton blooms form within a day or two and last six months. The tropical Pacific Ocean is widely regarded as a good spot to experiment because there is relatively little iron-rich dust carried from land, but there are other nutrients. George said “it's the clearest ocean on Earth because it's lifeless, and it's not supposed to be that way.”

George asserts that the potential is enormous. He said that the annual drop in ocean plant life was like losing all the rain forests every year. “If we succeed, we'll have created an industry,” he told the House committee. “If we don't succeed, we'll have created a lot of great science.”

But leading ocean and climate experts have poured cold water on the Planktos plan by saying that the company can't accurately measure how much additional carbon would be stored in the ocean or for how long. One reason: Some organisms sink and store carbon deep below the surface. But the overwhelming majority are eaten by fish or other organisms that convert the carbon back into carbon dioxide.

“Actually knowing how much carbon stays down there is a really hard thing,” says Daniel Schrag, director of the Harvard University Center for the Environment.

Schrag said the Planktos project could also generate new algae, which could reduce the amount of oxygen at depths that would endanger other ocean life. “Doing a large-scale ecological experiment before you understand the system is a dangerous thing,” he said.

Others doubt the benefits. “I think iron fertilization in the ocean is not going to make a significant difference to the CO2” problem, said Wally Broecker, a professor of earth and environmental sciences at Columbia University.

There are other issues. The area is in international waters, so some critics ask why Planktos or any company should be able to reap profits there. And if the company started selling large amounts of ocean-based carbon credits, it could flood the market, reducing incentives for more reliable and measurable projects aimed at reducing greenhouse gases in the atmosphere.

In addition, the benefits of reforestation projects are almost as hard to measure as ocean plankton, and people at funds that trade carbon credits are raising questions about Planktos's Hungarian forest project. Although the project is in Europe, it remains unclear whether any forest projects will meet the strict standards for credits that can be sold in the European Union's cap-and-trade system, where credits currently sell for $26.85 per ton of carbon dioxide.

So how is Planktos going to offset the Vatican's emissions? The Vatican doesn't emit much—about as much as 500 U.S. households, says David Kubiak of Planktos. To offset the Vatican's current emissions, Planktos is using credits that it expects to receive from its Hungarian tree-planting venture—in the future. Those new seedlings won't produce carbon benefits for eight years, Kubiak said.

The Vatican isn't part of the E.U. cap-and-trade system, so Planktos can use the credits even if they do not meet E.U. standards. These are called voluntary credits because the buyers, like those in the United States, are not required to offset emissions. The voluntary credits trade at a fraction of the price that E.U.-certified credits do.

Many companies are calling for Congress to set standards for voluntary credits if it does not establish a U.S. version of Europe's more rigorous cap-and-trade rules.

“The global market for voluntary carbon offsets is currently unregulated,” said Derik Broekhoff, senior associate at the World Resources Institute, “which has led to growing concerns about whether buyers are really getting what they are paying for.”

Steven Mufson

mufson, steven. “iron to plankton to carbon credits: firm's emission plans have critics aplenty,” washington post, july 20, 2007: D01.

See Also Carbon Sequestration Issues.



Blaine, Stéphane, et al. “Effect of Natural Iron Fertilization on Carbon Sequestration in the Southern Ocean.” Nature 446 (April 26, 2007): 1070–1074.

Boyd, P. W., et al. “Mesoscale Iron Enrichment Experiments 1993–2005: Synthesis and Future Directions.” Science 315 (February 2, 2007): 612–617.

Buesseler, Ken O., and Philip W. Boyd. “Will Iron Fertilization Work?” Science 300 (April 4, 2003): 67–68.

Mufson, Steven. “Iron to Plankton to Carbon Credits: Firm's Emission Plans Have Critics Aplenty.” Washington Post (July 20, 2007).

Schiermeier, Quirin. “The Oresmen.” Nature 421 (January 9, 2003): 109–110.

Schrope, Mark. “Treaty Caution on Plankton Plans.” Nature 447 (June 28, 2007): 1039.

Web Sites

Allsopp, Michelle, et al. “A Scientific Critique of Iron Fertilization as Climate Change Mitigation Strategy.” Woods Hole Oceanographic Institute, September 2007. <> (accessed October 20, 2007).

“News Release: Effects of Ocean Fertilization with Iron to Remove Carbon Dioxide from the Atmosphere Reported.” Woods Hole Oceanographic Institute, April 16, 2004. <> (accessed October 20, 2007).

Larry Gilman