Study casts doubt on iron-induced carbon sequestration.
The theory that adding iron to the oceans can help suck up atmospheric carbon dioxide cheaply and efficiently has received a further blow. A study1 published in this week's issue of Nature (see page 577) finds that the potential of iron-induced carbon sequestration is far lower than previously estimated.
During the CROZEX experiment in 2004 and 2005, scientists on board the British vessel RSS Discovery observed the impact of natural iron fertilization on algal growth and carbon export near the Crozet Islands, an archipelago some 2,000 kilometres southeast of South Africa. The team found that, relative to one unit of added iron, the amount of carbon sequestered to 200 metres' depth, where it will stay for a couple of decades, was almost 80 times smaller than the amount that scientists had determined during a similar study in the nearby Kerguelen region2.
"Ecosystem response and carbon export seem to vary very substantially from region to region," says Ulrich Bathmann, a biological oceanographer at the Alfred Wegener Institute (AWI) for Polar and Marine Research in Bremerhaven, Germany, who was not involved in the new study. "And the closer you look, the more complex the story gets."
Separately, on 26 January, the German science ministry gave the green light to LOHAFEX, an Indo-German ocean-fertilization experiment that has been waiting to get going in the Southern Ocean. Responding to environmental and political concerns, the ministry had ruled on 13 January that an independent assessment be carried out before the experiment could start3. From aboard the German vessel RV Polarstern, the LOHAFEX team plans to dump 20 tonnes of iron sulphate into a 300-square-kilometre area between Argentina and the Antarctic Peninsula.
The CROZEX study, led by Raymond Pollard of the National Oceanography Centre in Southampton, UK, looked instead at natural ocean fertilization, in which iron-rich dust blowing off the Crozet Islands settles on the waters. The team observed that some 270 tonnes of iron triggered a two- to threefold increase in biological productivity over an area the size of Ireland. But sediment probes revealed that the export of carbon to the deep ocean was nowhere near as massive as the Kerguelen study, and lab experiments, had suggested.
Although the study design allowed for only a rough estimate of carbon sequestration, the findings are sobering. Previously, scientists had cautioned that removing 30% of the carbon released every year as a result of human activity would require treating an ocean area ten times larger than the entire Southern Ocean, the most fertile waters for iron seeding4. Scaled up, the CROZEX results imply that even if the world's total ocean surface had a craving for iron, satisfying that would not have a large effect on the levels of atmospheric CO2.
"You might get a different response if you shock the system by dumping a lot of iron all at once," says Pollard. "The effect will still be much smaller than some geoengineers would wish."
Some think it is game over. "Ocean iron fertilization is simply no longer to be taken as a viable option for mitigation of the CO2 problem," says Hein de Baar, an oceanographer at the Royal Netherlands Institute for Sea Research in Texel.
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Schiermeier, Q. Ocean fertilization: dead in the water?. Nature 457, 521 (2009). https://doi.org/10.1038/457520b