The Azimuth Project
Iron fertilization



Iron fertilization is a proposed geoengineering scheme: the introduction of iron to the upper ocean to stimulate a phytoplankton bloom, which then may fall to the ocean floor, sequestering carbon.

Iron is a trace element necessary for photosynthesis in all plants. However it is highly insoluble in sea water, and is often the limiting nutrient for phytoplankton growth. Large phytoplankton blooms can be created by supplying iron to iron-deficient ocean waters. A number of ocean labs, scientists and businesses are exploring it as a means of sequestering carbon in the deep ocean, and also increasing marine biological productivity, which may decline as a result of climate change.

Since 1993, thirteen international research teams have completed ocean trials demonstrating that phytoplankton blooms can be stimulated by iron addition:

The work of John Martin, cited in this review, argues that iron levels in the ocean were much higher during glacial maxima than now. Martin estimated that this could have contributed perhaps 30% of the 80-ppm drawdown in atmospheric CO2 observed during these cold periods. However, the effectiveness of iron fertilization as a method of carbon sequestration remains controversial:

Which iron fertilization experiments have sequestered the most carbon so far—say, per ton of iron? All the experiments described below sound like flops. What did the best experiments “do right”, that these ones didn’t? Can iron fertilization ever be useful on a large scale?

Naqvi–Smetacek versus Coale

The following articles describe an iron fertilization experiment led by Wajih Naqvi and Victor Smetacek which led to relatively little carbon sequestration:

In an experiment done on 17 March 2009 in the south-western Atlantic off Chile, scientists applied 6 tons of dissolved iron to a 300 square kilometer patch of ocean inside the core of an eddy. The iron stimulated growth phytoplankton, which doubled their biomass within the first two weeks by taking up CO2 from the water. However, the increasing grazing pressure of copepods prevented further growth of the phytoplankton bloom, and as a result, only a modest amount of carbon sank out of the surface layer by the end of the experiment. The transfer of CO2 from the atmosphere to the ocean to compensate the deficit caused by the algal bloom was small compared to previous ocean iron fertilization experiments.

Another scientist, Kenneth Coale, said on the BBC:

“To date we’ve conducted experiments in what amounts to 0.04% of the ocean’s surface,” he told BBC News.

“All have indicated that iron is the key factor controlling phytoplankton growth, and most have indicated that there is carbon flux (towards the sea floor)—this is one that didn’t.”

Coale is talking about the experiment led by Naqvi and Smetacek. If you read Coale’s quote, you might think other experiments were getting significant amounts of carbon going to the sea floor. But Coale’s paper on his own experiment:

says that while iron stimulated the growth of phytoplankton, “the magnitude of the biological and geochemical response was much smaller than predicted”.

August 2008 volcano eruption

The eruption of a volcano in August 2008 can be seen as a test of iron fertilization. The results were disappointing:

  • Hamme, Roberta C., Peter W. Webley, William R. Crawford, Frank A. Whitney, Michael D. DeGrandpre, Steven R. Emerson, Charles C. Eriksen, Karina E. Giesbrecht, Jim F. R. Gower, Maria T. Kavanaugh, M. Angelica Peña, Christopher L. Sabine, Sonia D. Batten, Laurence A. Coogan, Damian S. Grundle, Deirdre Lockwood, Volcanic ash fuels anomalous plankton bloom in subarctic northeast Pacific, Geophysical Research Letters 37 (2010), L19604. Summary for the general public.

A popular account is here:

To quote:

Scientists investigated the August 2008 eruption of the Kasatochi volcano in the Aleutian Islands, which spewed iron-laden ash over a 620-mile (1,000-kilometer) swath of the North Pacific some 580,000 to 770,000 square miles (1.5 million to 2 million sq. km) in size.


Even if people wanted to artificially replicate this event to combat global warming, “it would be essentially impossible,” Hamme told OurAmazingPlanet. “What really allowed this ash to hit such a large area was the fact there was a storm system forming over the volcano when it erupted, and the ash got swirled around. Unless you could somehow put vast amounts of iron-containing particles into a storm going in the right direction for you, I think replicating this would be pretty hard to do.”

Another problem for replicating this event as a geoengineering scheme, Hamme and her colleagues estimated that this massive plume only absorbed about 4 million tons (37 billion kg) of carbon dioxide. While this might sound like a lot, the burning of fossil fuels releases nearly 7,000 times as much carbon dioxide annually at about 26.4 billion tons (24 trillion kg) a year, while the oceans naturally absorb about 8.1 billion tons (7.4 trillion kg) of carbon dioxide annually.

“Despite the huge area of iron addition and the optimal time of year when there was plenty of sunlight, the impact of this August 2008 event in terms of carbon dioxide absorption was quite small,” Hamme said. “This tells us that iron fertilization would have to be performed on a truly gigantic scale to have an impact on our climate.”


category: geoengineering