As arguments rage over the legality of an attempt to fertilise algae in Antarctic seas, evidence is emerging that, legal or not, this kind of 鈥渙cean engineering鈥� may not suck enough carbon out of the atmosphere to reverse climate change.
A new study confirms that iron-enriched waters do, as hoped, encourage more carbon to be stored on the ocean floor. But the efficiency of artificial iron fertilisation could be as much as 50 times lower than previous estimates.
In some oceans, the Southern Ocean in particular, phytoplankton growth is limited by the amount of iron in the water. The hope is that by dumping iron into the water, we could stimulate plankton blooms that will absorb carbon dioxide from the atmosphere. When the plankton die and drop to the ocean floor, they should drag carbon down with them.
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The question is, how much carbon gets dragged down and stored on the sea floor and how much is recycled higher up in the water column?
To clarify this, of the National Oceanography Centre in Southampton, UK, and colleagues set up a number of sediment traps around the .
Natural experiments
Plankton bloom naturally every year north of Crozet, boosted by the iron swept off the island鈥檚 volcanic flanks. Pollard and colleagues monitored the fallout from one of these blooms 100 metres and 300 metres beneath the surface, and compared this with the carbon fallout in an area that did not bloom.
They found that three times as much carbon falls to the sea floor beneath a bloom as in 鈥渃lear鈥� waters. Sediment cores taken from the sea floor suggest the carbon stays there for thousands of years.
鈥淥ur carbon-to-iron ratio is 80 times less than one reported by another study a year ago,鈥� says Pollard. He believes the previous experiment underestimated the amount of iron that was seeping into the bloom area.
He says the more optimistic figure 鈥渨as so high it might have given geoengineers great hope that they could stick in not much iron and get huge quantities of carbon sequestered鈥�.
Limiting factors
Pollard accepts that artificial schemes might be more effective than natural blooms at boosting carbon stores. Suddenly dumping large amounts of iron overboard is quite different from the slow flow of iron into the ocean through natural processes, and it is possible that this would prompt different species of plankton to bloom, sequestering more carbon.
To test this possibility, Pollard welcomes open-ocean experiments such as the German-Indian project now under way in the Southern Ocean.
Another recent study has suggested that even widespread iron fertilisation could not significantly cool global temperatures. One problem with these schemes, says of the University of East Anglia, UK, is that once iron is dumped another nutrient is bound to become the new limiting factor.
Journal reference: (DOI: 10.1038/nature07716)
