tl;dr:

Global warming, though not ocean acidification, is quickly and cheaply reversed by ejecting calcite nanoparticles (with an average radius in the ~90nm range) into the stratosphere, using a propeller-based system to prevent particle clumping. The particles should be carried up by hydrogen balloons, and very preferably released over the tropics. The total amount needed will be on the order of several hundred kilotons yearly, and the total cost should be somewhere between $1B and $5B yearly.

https://nephewjonathan.substack.com/p/diy-geoengineering-the-whitepaper