The Principle Carbon sequestration is the process of mineralizing or storing carbon dioxide underground to reduce atmospheric levels more rapidly than occurs via natural processes.
Why bother? Most of us are familiar with the carbon cycle. I recall learning about it in elementary school. Plants take up carbon dioxide through photosynthesis and convert it to basic six-carbon sugars. Respiration returns some to the atmosphere within minutes or hours. Some is converted to cellulose and lignin--wood, which is stored for the life of the plant. Once dead, fungi and other saprophytes begin the process of breaking the woody material down and returning the carbon dioxide back into the atmosphere.
The biogenic storage of carbon can be extended under certain conditions. Peat build-up in ponds or tundra leads to longer-term storage. Arctic permafrost in Siberia has been dated to 650,000 BCE.
Longer term burial of organic matter can result in the familiar fossils coal (terrestrial plant material) and oil (marine deposits of plankton and algae). The oldest known coal deposits date to the Carboniferous Period, some 300 million years ago. The oldest oil deposits date to 1.4 billion years ago, though most formations date to the age of the dinosaurs or more recent.
There are other carbon cycles many of us are less familiar with. For example, when carbon dioxide is absorbed into seawater, it can be mineralized into calcium carbonate. A process harnessed by marine creatures early in their evolution. Mollusks use this mineral for their shells. Bony fishes use it in their bones.
Lastly, there is inorganic mineralization of carbon dioxide into carbonates. This occurs in soils, in rock and in the ocean.
On average, a carbon dioxide molecule remains in the air 300 to 1,000 years. Atmospheric levels have been rising since the beginning of the industrial age in the mid-1700s, when it stood at about 280 ppm. By 2000, the concentration of atmospheric carbon dioxide was 370ppm. By 2020 it has risen to 412 ppm.
World leaders have set goals for their countries to achieve carbon neutrality (the amount added to the atmosphere is balanced by the amount taken out) typically by mid-century. However, the global warming attributed to anthropogenic atmospheric carbon dioxide and methane(i.e., greenhouse gases) may not necessarily stop or reverse.
Scientists have identified feedback loops that will continue atmospheric warming after human-caused carbon dioxide emissions stabilize. The loss of arctic sea ice decreases albedo. The sunlight that normally reflects off the pack ice back into space is absorbed by the waves in open water, raising surface temperatures and the atmosphere above it.
Thawing of arctic permafrost has already released vast quantities of carbon dioxide and methane, which are not accounted for by present reduction goals.
Longer, more frequent heat events and drought conditions have resulted in increased number and individual acreage of wildfires. Increased wildfire releases otherwise-stored carbon dioxide.
The results of these carbon dioxide feedback loops? Additional warming, deeper droughts, more intense and longer storm seasons, and rapid sea level rise that inundates low coastal development from Miami, Florida to Dhaka, Bangladesh.
The options If we rely on natural processes to reduce carbon dioxide concentrations (consideration of feedback loops aside) in about a thousand years, 20% to 30% of human-emitted carbon dioxide will still remain in the atmosphere. Global temperatures will stabilize at their reached levels(again, discounting any feedback loops) and stay high for hundreds of years.
Clearly, simple carbon-neutrality may not be sufficient to stave off the serious side-effects of climate change, given the feedback loops mentioned above. Two technologies have emerged to sequester carbon dioxide pulled from the atmosphere or out of fossil fuel emissions. Both rely on direct capture to collect it. A process known as "incumbent amines" is the most mature carbon-capture technology in use today. It pumps emissions from industrial processes such as steel and cement production and the burning of fossil fuels for power generation through a solution that absorbs carbon dioxide but allows other atmospheric gasses to pass through. The carbon dioxide -rich solvent then flows into a boiler where heat drives the pure gas out of solution. The carbon dioxide can be collected for containerized transport or transported via pipes to sequestration sites.
Currently, the most common sequestration sites are oil or natural gas fields, where carbon dioxide is injected to drive out more product. While this results in a net increase of atmospheric carbon dioxide, it still lowers total emissions by the amount trapped underground. Some of it is mineralized to carbonates, some remains as free gas within the pores and voids of the deposit rock formations.
The Orka carbon sequestration project in Iceland is the world's largest. It sequesters 10,000 tons of carbon dioxide annually, a natural by-product from the Hellisheidi geothermal power plant. The underlying basalt is high in magnesium and calcium, which bind it as carbonate minerals.
An even more attractive option for carbon dioxide mineralization is mantle rock sequestration. Mantle rocks can mineralize 500kg of carbon dioxide per ton of rock compared to 170 kg of carbon dioxide per ton of basalt. Where mantle rock has been exposed at the Earth's surface, peridotite reacts with airborne carbon dioxide dissolved in rainwater that percolates through cracks, forming white veins of carbonate minerals. Mantle deposits in Oman, Alaska, Canada, California, New Zealand and Japan could potentially mineralize and store 60 to 600 trillion tons of carbon dioxide. A single injection well under study in Oman could capture up to 50,000metric tons of per year. While direct capture of atmospheric carbon dioxide is not financially feasible today, gas captured from industrial processes could be shipped to injection wells positioned above such mantle deposits.
This could become important in the production of blue hydrogen if demand for green hydrogen outstrips demand while electrolyzer capacity ramps up, assuming direct reinjection of carbon dioxide back into methane wells is impractical.
To actually reduce atmospheric carbon dioxide levels to mitigate for feedback loops will require carbon negative processes. Hydrogen production from biogenerated methane or emissions from cogeneration powerplants would pull carbon dioxide out of its short-term cycle, reducing atmospheric carbon dioxide over shorter time spans.
Seeding the oceans with iron to "fertilize" plankton growth would also remove additional carbon dioxide out of the atmosphere where it would settle to the sediments under the world's oceans. However, this type of ecosystem tampering carries risks, possibly to human food chains.
An industrial process developed to capture atmospheric carbon dioxide directly would require enormous facilities to trap appreciable quantities of gas. They could be co-sited with extensive photoelectric facilities, which would power the carbon capture processes.
How prevalent will carbon sequestration be by 2035? For the near-future, efforts (and funding) will prioritize green power generation, electrification of transportation and green/blue hydrogen production. But, once the low hanging fruit is picked ten years from now, more funding will be directed to carbon sequestration technologies. This tech may be especially important to industries where de-carbonizing is problematic, like commercial aviation. Carbon sequestration should allow such economic sectors to achieve carbon neutrality sooner, allowing their conversion to hydrogen-based thrust generation systems over a longer time span.
For further reading
https://climate.nasa.gov/news/2915/the-atmosphere-getting-a-handle-on-carbon-dioxide/https://en.wikipedia.org/wiki/Greenhouse_gashttps://en.wikipedia.org/wiki/Effects_of_climate_change#Amazon_rainforesthttps://cen.acs.org/environment/greenhouse-gases/capture-flue-gas-co2-emissions/99/i26
https://www.scientificamerican.com/article/rare-mantle-rocks-in-oman-could-sequester-massive-amounts-of-co2/https://www.hydrogenfuelnews.com/green-hydrogen-production-emerson/8550898/?mc_cid=6c14c8d0bc&mc_eid=45b5618dca 
