Study Rules Out Iron-seeding to Fight Warming

Seeding oceans with iron to encourage plankton blooms shown as impractical over long-term.

Published: 20-Mar-2004

lass=wallacepara>A once-promising theory that seeding oceans with iron to create plankton blooms to extract greenhouse gases from the atmosphere has turned out to be impractical in the long term, a study released says.

A study led by New Zealand's University of Otago of Dunedin found that while a ton of iron would fertilise a phytoplankton bloom, it would also require at least 5,000 tons of silicate to sustain it.

"It's just not practical," Otago oceanographer Philip Boyd said in a paper published in the science journal Nature and issued on Friday by the university.

Phytoplankton harvest sunlight to fix carbon that is then either re-mineralised to form carbon dioxide in the surface waters of the ocean and released back into the atmosphere, or "pumped" down to the deep ocean layer as the plankton sinks.

In 1999, Mr Boyd and a team of other New Zealand and international scientists distributed around 8,000 kilograms of an iron compound in solution over a patch of the Southern Ocean eight kilometres in diameter.

The result was a five-fold increase in phytoplankton stocks during the developing bloom and it was believed that simply adding iron might be the answer to increasing the amount of atmospheric carbon dioxide locked up in the ocean.

"What we found, however, is that adding iron to the ocean produces a very different picture in the longer-term," Mr Boyd said.

After 18 days of a similar experiment in the Gulf of Alaska, the iron-induced bloom declined and satellite pictures show merely a ghost of the plankton-rich patch that had blossomed initially.

"We think the decline was initiated by the drop-off in iron levels, but the secondary factor is the removal of all of the silicate by phytoplankton," Mr Boyd said.

"Until now, we had not realised the importance of silicate in causing the bloom's decline and while it might be feasible for us to add iron to the ocean to stimulate blooms, for every ton of it we throw overboard, we'd need to add at least 5,000 tons of silicate to enable the blooms to persist for long enough to impact on atmospheric carbon dioxide levels," he said.

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