The end of a utopian idea: iron-seeding the oceans to capture carbon won't work

For a while, some scientists thought they had found a simple 'quick fix' to remove carbon dioxide from the atmosphere via a biological process. The idea was to seed iron into the oceans to stimulate the growth of algae. The phytoplankton trap carbon dioxide via photosynthesis, and store it in their cells. After a while, the algae drop down to the ocean floor and take the carbon with them, where it remains for a long time. However, earlier experiments and economic studies showed that this 'geo-engineering' strategy doesn't pay off; large quantities of iron are needed to result in a small effect, making the technique too expensive.

By comparing the natural process with the artificial iron-seeding technique, scientists from France's leading research institute, the CNRS, have now been able to show exactly why the mimicked process is not efficient. At the same time, they found clues to a question that has been debated for a long time amongst paleoclimatologists: that of the role of iron circulation in the oceans during the climate changes observed in the period between glacial and interglacial eras.

A 47 strong research team of French, Belgian, Dutch and Australian oceanographers and biogeochemists from the international oceanographic mission KEOPS ('KErguelen Ocean and Plateau compared Study') set out in 2005 to analyse the process as it occurs near the Îles Kerguelen in the Southern Ocean. They published their results in the April 26 issue of the journal Nature and found that a much more complex stream of nutrients released according to a specific timing pattern is needed to trigger algae bloom formation that effectively captures and transfers carbon to the ocean floor, than merely adding iron. They conclude that geo-engineering the oceans won't work.

Two ocean pumps
Oceans are the most important carbon sinks on the planet. Two major mechanisms allow these vast reservoirs to extract carbon from the atmosphere: the 'physical pump' and the 'biological pump'. The 'physical pump' is a mechanism that, because of the natural ocean circulation, gradually forces carbon-rich surface waters to the deep, where the carbon remains locked. In the 'biological pump' (image, click to enlarge), carbon gas is taken up via photosynthesis into the cells of micro-organisms or in the calcium carbonate shells of sea creatures, which sink to the ocean floor as waste or when they die.

For more than a century already, a third of the anthropogenic carbon emitted into the atmosphere has been taken back by the oceans. Surprisingly, this work is done exclusively by the 'physical pump'. The 'biological pump' does not contribute to this process and simply continues its old cycle as it existed before the industrial age. However, the biological system is not operating at its maximum capacity. In vast parts of the world's oceans, the biological pump even works in slow motion, because of a lack of micro-organisms. The Southern Ocean in particular is poor in phytoplankton, despite the fact that the waters there consist of very nutrient-rich salts. So what exactly is holding the micro-organisms back from proliferating there? This was the crucial question for the research team:
bioenergy :: biofuels :: energy :: sustainability :: algae :: oceans :: carbon cycle :: iron-seeding :: nutrients :: geo-engineering ::

It is a question of major importance, because if we could tap the potential of these oceans to store more carbon via the biological cycle, then we could help fight global warming.

Between 1993 and 2005, around 12 oceanographic expeditions have allowed scientists to ascertain that phytoplankton in the great seas, and particularly in the Southern Ocean, lacked iron and started blooming when more iron was artificially added to their environment. However, the hypothesis that there is such a thing as a top-down transfer of carbon from the micro-organisms that thrive on the surface and that sink towards the bottom of the ocean, has so far not been proved.

Mother nature knows best
The KEOPS mission sailed out to find out. Contrary to the previous campaigns, this mission focused on the natural processes that drive phytoplankton blooms in the nutrient-rich waters near the Îles Kerguelen. This location was not chosen randomly: satellite images revealed that each year in the summer a very localised algae bloom emerges, a phenomenon that can only be explained by the presence of iron. Would this region of the Southern Ocean be a privileged zone for the 'biological pump' to operate?

The KEOPS researchers found that, indeed, the occurence of these blooms is the result of a continuous and natural flow of iron in the surface waters: via a series of complex steps, this iron is pumped up from deep water layers to the surface. This natural fertilisation process was then compared to artificial fertilisation campaigns. The result: the carbon transfer from the surface to the bottom of the ocean was found to be twice as large in the natural process. The total efficiency of the fertilisation - defined as the relation between the quantity of carbon transferred to the bottom of the ocean versus the amount of added iron - was at least ten times higher than artificial iron seeding.

Answers to old questions
The researchers found that this huge difference is due to the fact that a far wider range of natural nutrients are involved in creating algae blooms and carbon transfers than was previously found. It also answers the question asked by other scientists as to why iron seeding is not cost-efficient. For the time being, we cannot mimic the complex flow of nutrients needed to drive the process.

These discoveries have important repercussions in the quest to validate a paleoclimatic scenario which says that a part of the variations in the concentration of carbon dioxide in the atmosphere between the Ice Age and interglacial periods was caused by modifications in the processes by which iron circulates in the oceans.

The findings also shed new light on the impacts of climate change on the important 'biological pump' that continuously traps and transfers carbon gas.

The end of the iron-seeding idea?
Finally, the researchers think their results mean the end of the 'geo-engineering' utopia that consists of artificially seeding the oceans with iron: the very intricate and slow but highly regulated process of iron addition as it occurs in the natural process, combined with the complexity of the composition of nutrients, make it almost unreplicable. For the process to occur, each location has its own interaction of different nutrient flows and a finetuned timing, which make it impossible and even unwarranted to try to replicate it in a standard way elsewhere.

The effectiveness of artificial iron-seeding as a way to induce carbon trapping algae blooms is put into question. But more importantly, the secondary effects of such a geo-engineering strategy on other marine creatures are not yet known.

The iron seeding idea is made impossible by a catch-22: on the one hand, the results from small-scale experiments cannot be extrapolated to proposed large scale efforts, precisely because of the intricacies and complexity of very localised circumstances that determine the effectiveness of the effort, whereas on the other hand, skipping small scale tests and immediately implementing large scale campaigns poses the risk of unwanted secondary effects on the biodiversity of vast swathes of the oceans.

The KEOPS mission was supported by the Institut national des sciences de l'Univers (INSU/CNRS), with the logistical support of the Institut polaire français Paul-Émile Victor (IPEV). Working on board of the Marion Dufresne, the science team was headed by Stéphane Blain, researcher at the Laboratoire d'océanographie et de biogéochimie de Marseille (LOB/COM, CNRS / Université Aix-Marseille 2).

Translated and adapted by JVDB from CNRS: Fertiliser les océans : la fin d'une utopie? - April 26, 2007.

More information:
Stéphane Blain et al., "Effect of natural iron fertilization on carbon sequestration in the Southern Ocean", [*abstract], Nature, 446, 1070-1074 (26 April 2007) | doi:10.1038/nature05700

Nature: "Only mother nature knows how to fertilize the ocean - Natural input of nutrients works ten times better than manmade injections" - April 23, 2007.

The Scientist: "Iron Seeding Just Doesn't Pay" [*abstract], The Scientist, 5 July, 2004, 18(13):26

Sixteen laboratories from across the world participated in the KEOPS mission, which has its own website.