Climate change and ocean iron dumping
The idea of seeding the oceans with iron as a way to remove carbon from the atmosphere has received a boost from new analysis which finds the approach could be far more effective than was indicated by previous New Zealand-led research.
The new analysis, of an experiment eight years ago in which an area of the Southern Ocean was fertilised with iron sulphate, raises the possibility the technique could be considered as a way to mitigate climate change.
In February 2004, researchers involved in the European Iron Fertilisation Experiment (EIFEX) fertilised 167sq km of the Southern Ocean with several tonnes of iron sulphate.
Each atom of added iron pulled at least 13,000 atoms of carbon out of the atmosphere by encouraging algal growth which, through photosynthesis, captures carbon.
The experiment, involving the bloom and demise of single-cell algae, or phytoplankton, was monitored for 37 days by researchers on German boat Polarstern.
A paper in Nature reported that much of the captured carbon was transported to the deep ocean, where potentially it could be stored for centuries.
Study leader Victor Smetacek, a marine biologist at the Alfred Wegener Institute for Polar and Marine Research in Germany, said at least half the bloom dropped to depths greater than 1000 metres.
Despite the finding, Smetacek is discouraging any suggestions the technique could be used as a kind of geoengineering to counter climate change.
It was not known what would happen to such things as species composition if humans continuously added iron to the sea, he said.
Those issues could only be addressed by more experiments including longer term studies of natural blooms around some Antarctic islands.
Niwa principal scientist Dr Cliff Law, who was among the recipients of the 2011 Prime Minister's Science Prize for work on iron fertilisation, said the authors of the new analysis had been on site longer than were researchers in previous experiments, and had a measurement framework more focused on particle export.
As the carbon which fell into the deep ocean after being fixed by the iron-fuelled phytoplankton growth would be isolated from the atmosphere for decades to centuries, the results had implications for understanding of both past and present climate change, Law said.
The new work also showed that location and timing of an iron experiment were critical.
"The paper may re-open the debate around geoengineering, although the authors do not link their results to this," he said.
Observing and verifying the impacts of iron addition were huge technological challenges which would need significant financial investment to support major operations.
Until knock-on effects - oxygen depletion, production of greenhouse gas nitrous oxide, and loss of nutrients in the water - could be proven to be minor compared to the benefits of iron-induced carbon sequestration, then iron fertilisation could not be considered as a viable geoengineering approach.
Essex University marine sciences lecturer Dr Michael Steinke said Smetacek's experiment had fertilised "just the right part" of the ocean, but the logistics of finding such spots were difficult and costly.
He doubted the finding would open the gates to large-scale geoengineering to mitigate climate change.
Of 12 similar experiments since 1993, many showed the desired increase in carbon dioxide drawdown from the atmosphere, but the Smetacek group was the only example to demonstrate the all-important carbon burial in the deep sea sediments, away from the atmosphere.
The New Zealand-led fertilisation experiment found the increase in the ocean's ability to remove carbon dioxide was not as great as expected, would be costly and was fraught with complex side effects.
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