Probably the cheapest mechanical method to remove mega tons of
CO2 from the atmosphere is to harvest fast growing plants in the tropical rain forests, barge the harvest out to the deep ocean, and sink it into the abyssal depths. There the high pressures will compress the plant material to specific gravity well below 1, and the carbon will stay permanently interred in the depths. (At depths of 5,000 feet hydrostatic pressures approximate 2,000 PSI. Wood that floats on the surface compresses and sinks at depths exceeding 2,000 feet.) Rough calculations show cellulose carbon can be harvested in the Amazon basin and interred in the Atlantic Ocean at costs of $10 to $20 per ton. CO2 diffusion rates are high enough that CO2 gas emitted anywhere on Earth will soon show up in Brazil.

An alternative variation on this scheme would be giant nuclear
powered ships that scoop up sargassum seaweed, rupture the flotation bladders, and allow the lower density mass to sink into the abyssal depths. The ocean depths presently hold over 50 times as much CO2 as exists in the Earth’s atmosphere. This huge amount of oceanic CO2 can only be released if pressures are reduced, and to a much smaller extent, if the water temperature increase. It’s hard to imagine how the abyssal pressures might be significantly reduced or the temperatures increased.

One of the major problems of land based CO2 capture and sequestration schemes is the impracticality of permanently maintaining pressurized storage in geological strata. It’s a settled fact that deep ocean waters have been doing this naturally for hundreds of millions of years.

Elimination of the costs of high-pressure compressor machinery and drilled injection wells through ocean interment of plant material is the cheapest possible means of permanent sequestration.

I do not see either of these least bad schemes as practical solutions, but if environmentalists want to feel good about their futile efforts they will get more bang per wasted buck with abyssal interment of plant material.