In general, anything mankind builds brings its own environmental burden; concrete, steel, plastic and glass all cost!

Specifically, most STEP schemes envisage locking up CO 2 in the place where the earth stores it; the deep ocean. Here CO 2 combines with water to form a hydrate 'snow' which is relatively inert, so not, as far as we know, reactive and harmful to marine life.

It is also a normal occurrence in the deep ocean; estimates are that there is already 10 thousand gigatons tons of various forms of hydrate on the seabed and 78 million gigatons of carbon in ocean sedimentation. A century of sequestration of the whole human CO 2 output would add 800 gigatons to this amount!

But the precautionary principle still applies; where you don't know for sure, you have to keep a watchful eye!