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Adding iron to the oceans might not be an effective way of curbing climate change, according to new research.

A study published in this week's Science confirms previous findings that adding iron to oceans increases the uptake of carbon dioxide by algae in the water. But it also shows that the amounts do not appear to be large enough to make this a viable way of tackling rising levels of atmospheric carbon dioxide.

The results come from one of a series of studies carried out as part of the Southern Ocean Iron Fertilisation Experiment, which aimed to probe the effectiveness of 'iron fertilisation' — adding iron to the ocean surface to encourage the growth of phytoplankton. These algae absorb carbon dioxide from the ocean, which in turn causes more of the gas to migrate from the atmosphere into the water.

The proposed technique is controversial: some US organisations are already experimenting with iron fertilisation in the hope that one day companies and governments wishing to cut greenhouse-gas emissions will pay them to fertilise the oceans with iron. But marine researchers argue that marine life could suffer side effects.

As part of their study, a team of scientists from the US-based Woods Hole Oceanographic Institution measured the amount of carbon taken up and transported below the ocean surface after iron was added. The uptake of carbon dioxide did increase, but "when compared to the rates of carbon released globally due to human activities, this [carbon uptake] is indeed small," they write in Science.

They add that "it is difficult to see how ocean iron fertilisation with such a low efficiency [of carbon uptake compared to the amount of iron added] could easily scale up to solve our larger global carbon imbalance problems".

Link to research paper by Ken Buesseler et al in Science

Reference: Science 304, 414 (2004)