Will throwing iron in the ocean help stop global warming?

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Researchers have embarked on a test to see whether dumping iron into the ocean can help remove carbon dioxide (CO2) from the atmosphere, possibly alleviating global warming.

The controversial idea has been tested in small-scale projects before. But it has never been clear whether it would actually work, in part because it is difficult to track exactly what happens to the ecosystem after iron is added to the water. Now scientists intend to watch a large patch of ocean for a relatively long period of time in an attempt to find out.

The iron is expected to feed the growth of phytoplankton – single-celled algae that live in the sunlit upper layers of the sea – in areas where they are limited by little natural iron in the water.

As phytoplankton grow, they absorb CO2 from the atmosphere in order to photosynthesize. Phytoplankton are currently responsible for almost half of the overall photosynthetic activity on Earth. Some researchers think that increasing their activity would be a good way to reduce the concentration of CO2 in the atmosphere, helping to slow the rate of global warming.

But the phytoplankton will only remove CO2 from the air permanently if they die and sink to the bottom of the sea, says Victor Smetacek, a biological oceanographer at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany.

This might not happen. Instead, the phytoplankton could be eaten by zooplankton – miroscopic invertebrates that feed on algae. The zooplankton could in turn be eaten by larger sea creatures, which would release carbon dioxide back into the atmosphere by respiration. In that case, adding iron to the ocean would not decrease the amount of CO2 in the atmosphere.

Smetacek and 48 colleagues set sail last Wednesday on the German research vessel Polarstern to investigate what really happens.

The team plans to dissolve an iron sulphate solution in a in a 150-200 square-kilometre patch of the Southern Ocean, near Antarctica, where currents are expected to keep the iron within a limited area. The team will then monitor the growth of phytoplankton from a helicopter, and examine which kinds of algae and other creatures flourish for a period of eight to ten weeks.

“We need to find out whether the algae die after the bloom and sink down to the ocean floor,” says Smetacek. “Only then we can be sure that the carbon is permanently removed from the atmosphere.”

Researchers caution that even if the plan does prove capable of reducing CO2 levels in the atmosphere, many ecologists are concerned that interference with the marine food chain could have a dramatic and negative impact on ocean ecology1. Further studies will be needed to resolve those issues.