Cracking CO₂ into Carbon and Oxygen
A mole of CO₂ is 44 grams. CO₂ enthalpy of formation is 393.5 kJ/mole, or 8.943 MJ/kg. The atmosphere weighs 5.15e18 kilograms. 1 ppm by weight of that is 5.15e12 kg. So, converting 1 ppm of atmospheric CO₂ into carbon and oxygen requires at least 4.61e19 Joules (at 100% efficiency). 4.61e19 J = 1.459 terawatt-years. 100 Terawatts could (at 100% efficiency) remove 68.5 ppm of atmospheric CO₂ per year.
I have no clue how to actually do this, or how efficient a real CO₂ cracking machine could be. If the process occured high in the stratosphere, the heat involved could radiate into space, not to the surface.