I proposed addition of limestone to the ocean in a patent filed in 2007 and now published GB2447513A. The patent contains details of the ocean chemistry model, calculations etc. that I used. Cost is equivalent to about an extra 12p/litre of petrol. Maximum scope is currently about 3 billion tonnes per year of carbon dioxide removal (10% of global emissions) but this will treble when limestone solubility in the deep ocean increases as the more acidic water currently downwelling at the poles spreads further.

Essentially limestone addition allows the ocean to continue removing atmospheric carbon dioxide on the scale it does today indefinitely. Even if all the fossil fuel still in the ground (some 13 times that used to date) were burnt, atmospheric carbon dioxide concentration could be contained or returned to today’s level.

The only problem is that it simply cannot remove carbon dioxide at the rate we are making it today. No additional limestone can be dissolved in the upper ocean layers and the deep ocean does not recirculate quickly enough to carry sufficient dissolved limestone up to where it can contact the atmosphere.

I do not believe there are any adverse environmental effects; New Scientist made no specific suggestions as to what problem they envisaged. I think they were confusing limestone (calcium carbonate) with quicklime (calcium oxide) which is lethal because it is readily soluble causing dramatic local pH increases.

Limestone is only very sparingly soluble and takes years to dissolve, so pH increase even locally is very small. Moreover the process is completely natural; limestone rains down naturally as the skeletal remains of dead ocean creatures and the shallow ocean floor is already completely covered in the stuff in most areas.