An interesting contribution from the comments thread on Lovelock and Rapley propose cure for global warming by Peter Williams which I thought I’d hoist up here for wider circulation. — Oliver

Lovelock and Rapley (Nature, 449,403, 2007) put forward the idea that by pumping up nutrient rich deep oceanic water, the subsequent stimulation of planktonic photosynthetic production would give rise to a very significant drawdown atmospheric CO2. The concept is flawed scientifically on two accounts. Planktonic photosynthesis results in the assimilation of inorganic nitrogen and CO2 in a ratio which has a modal value in the region of 6.6 – the so-called Redfield ratio. A fraction of the organic particles that arise as a consequence of photosynthetic production, sink into the deeper parts of the ocean. The C/N ratio of these particles is somewhat higher than the Redfield ratio, as there is some fast decomposition of the nitrogen (and phosphorus) rich organic components before the particles reach deep water. The particles are eventually decomposed in the deeps, with the production in inorganic nutrients, along with CO2. If this water, now enriched in inorganic nitrogen (and phosphorus), were brought to the surface, it would indeed stimulate planktonic photosynthesis and result in the assimilation of CO2. However, the upwelled water is not only enriched in inorganic nitrogen but also CO2 produced at the same time, the latter being slightly in excess of the Redfield requirement due to the elevated C/N ratio of the settling particles. Thus, rather than drawing down atmospheric CO2 from the atmosphere, there would be export of CO2. The situation in fact would be worse, as the upwelled water would need to warm up (otherwise it would simply sink back again) this would reduce the solubility of CO2, resulting in further export of oceanic CO2 into the atmosphere.

Further, from the engineering point of view the concept is infeasible – to lift up a 10m diameter column of dense (cold) to the surface would require a net lift of a number of tonnes and would almost certainly collapse a flexible tube or would cause a ribbed tube to concertina.

Even if the engineering problems could be solved, and the system made cost effective, both of which seem very doubtful, the proposal would have the reverse effect of that claimed.