Use nukes to pulverize a massive amount of basalt. The exposed magnesium and calcium oxides will react with CO2 and sequester it.
In the future, all of the extra CO2 in the atmosphere will react with minerals like those found in basalt. That will lock the CO2 up for a very, very long time-- until enough layers of sediment build up on top of it that the CO2-containing rock eventually makes its way to the bottom of the earth's crust. Down there, the heat from the earth's mantle will release the CO2 again.
So eventually, the CO2 problem will be taken care of. Unfortunately, that time scale is much too slow to help us.
The reason the natural process is so slow is that the minerals involved occur in the form of consolidated deposits. Very little surface area results in a very slow reaction.
To emphasize, the surface area is the problem. The kinetics of the underlying chemical reaction are not the problem. With enough surface area, the process will run fast enough for our purposes. Pulverized rock has enormously more surface area than a consolidated mass. That's where the nuke comes in. Nukes are very, very good at pulverizing very, very large quantities of rock.
The Kerguelen Plateau is an enormous deposit of the type of mineral we need. Picture a triangle formed by South Africa, Australia, and Antarctica. The Kerguelen Plateau is a mass the size of a mini-continent at the bottom of the sea at the center of that triangle.
Save civilization by setting off nukes at the bottom of the sea in one of the remotest places on the planet.
Category of the action
What actions do you propose?
Get nuke. Bring nuke to remote location. Place nuke at bottom of the sea. Boom.
Who will take these actions?
Someone with nukes and a boat. And a warm, waterproof outfit, since we'd be doing this at sea near Antarctica, probably in winter.
Where will these actions be taken?
The Kerguelen Plateau.
What are other key benefits?
That many less nukes pointed at people?
What are the proposal’s costs?
Cost of nukes. And a boat. You could rent the boat instead of buying it.
Could probably be done in just a few weeks, but would want to wait until wintertime in the Southern hemisphere. Go "boom" when the whales aren't there to feed. Also give the radioactivity time to disperse while most living things are either absent or hibernating.