Tyndall Centre for Climate Change Research, NOC, Southampton, UK

An oceanographer sees potential in accelerating rock weathering to soak up carbon dioxide from the air.

With CO2 emissions increasing by more than 2% per year, rather than decreasing by the 3% or so needed to effectively mitigate climate change, I am not surprised that many scientists are seeking alternative solutions to simply cutting greenhouse-gas outputs.

Various geoengineering schemes have been proposed — such as fertilizing the oceans with iron, a limiting resource for planktonic algae that take CO2 from the atmosphere — but these are unlikely to sequester large amounts of carbon in the long-term and may have serious ecological side effects. The thermodynamics of enhancing geochemical weathering look feasible, but the reactions are too slow to be really practicable.

Geochemists and engineers at Harvard University in Massachusetts and Pennsylvania State University recently suggested a kinetically preferable idea. They propose using the electrolysis of sea water to produce sodium hydroxide and hydrochloric acid, in a variant of the well-known industrial 'chloralkali' process, (K. Z. House et al. Environ. Sci. Technol. 41, 8464–8470; doi:10.1021/es0701816 2007).

Sodium hydroxide could either be used to scrub CO2 directly from the air, producing sodium bicarbonate, which is neutral and could be discharged into the sea, or be pumped directly into the ocean, increasing sea water's alkalinity and so its ability to absorb CO2. The hydrochloric acid could be neutralized fairly easily, because it reacts rapidly with both carbonate and silicate rocks.

The scheme House et al. outline looks promising if it were operated using a solar or geothermal electricity source near a supply of basic rocks. A mid-ocean volcanic island would be good. And the environmental consequences of the scheme's discharges should be less severe than those of the ocean acidification that humans are already causing.