In his letter, Alec Dunn suggests that pumping nutrient-rich deep ocean water to the surface would stimulate plankton growth and hence capture atmospheric carbon (18 August, p 32). This could be complemented by ocean thermal energy conversion (OTEC) (22 November 2008, p 28).
OTEC is a relatively marginal alternative energy source that uses cold deep water and warm surface water to run the equivalent of a reverse fridge cycle. Cold water is pumped to the surface to liquify a working fluid and then discarded.
If value is given for the indirect carbon dioxide captured in this way through carbon credits or subsidies like those given to wind and nuclear energy, the overall economics may be supportable. Alternatively, especially when located far from land, the OTEC cycle can be used simply to power the water pumping.
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