Jim Lovelock (he of the Gaia hypothesis) and Chris Rapley have proposed a means of geo-engineering our way out of the AGW mess:
The oceans, which cover more than 70% of the Earth’s surface, are a promising place to seek a regulating influence. One approach would be to use free-floating or tethered vertical pipes to increase the mixing of nutrient-rich waters below the thermocline with the relatively barren waters at the ocean surface. (We acknowledge advice from Armand Neukermans on engineering aspects of the pipes.) Water pumped up pipes — say, 100 to 200 metres long, 10 metres in diameter and with a one-way flap valve at the lower end for pumping by wave movement — would fertilize algae in the surface waters and encourage them to bloom. This would pump down carbon dioxide and produce dimethyl sulphide, the precursor of nuclei that form sunlight-reflecting clouds.
From an engineering perspective this is probably not feasible, but even if it was, the effect would be the exact opposite of that hoped for: more CO2 would be released into the atmosphere than would be absorbed by the algae blooms. The simpler of the two explanations of why this should be so is:
“The concept is flawed,” says Scott Doney, a marine chemist at WHOI. He says it neglects the fact that deeper waters with high nutrients also generally contain a lot of dissolved inorganic carbon, including dissolved CO2. Bringing these waters to the lower pressures of the surface would result in the CO2 bubbling out into the air. So contrary to the desired effect, the scheme could result in a net ‘outgassing’ of CO2...
Ah well, maybe we can live in polar cities, and in any case, nobody has shot down my Carbopult(TM) idea.