Leading marine scientists from across the world have issued a warning that it is too early to sell carbon offsets from ocean iron fertilisation.
Published on Friday in the journal Science, signatories include scientists from the US, Japan, Hawaii, New Zealand, The Netherlands, India, Germany and the UK. The UK is represented by Prof Andrew Watson of the University of East Anglia and Dr Richard Lampitt of Southampton University's National Oceanography Centre.
Prof Watson said: "While we do envision the possibility of iron fertilisation as an effective form of carbon offsetting, we believe larger scale experiments are needed to assess the efficiency of this method and to address possible side effects.
"There remain many unknowns and potential negative impacts."
Ocean iron fertilisation (OIF) is one of several marine-based methods proposed for mitigating rising atmospheric CO2. Research since 1993 has shown that releasing iron onto the ocean surface can stimulate the growth of plankton.
However, the efficiency with which OIF sequesters carbon from the atmosphere and retains it in the deep ocean is still uncertain and unintended ecological impacts are not yet fully understood.
Despite the scientific uncertainties, private companies are currently planning larger-scale iron releases to generate the sale of carbon credits.
The joint letter concludes: "This group feels it is premature to sell carbon offsets from the first generation of commercial-scale OIF experiments unless there is better demonstration that OIF effectively removes CO2, retains that carbon in the ocean for a quantifiable amount of time, and has acceptable and predictable environmental impacts."