Atmospheric CO2 cannot be converted back into hydrocarbons without expending more energy than the fuel originally released in combustion. Atmospheric CO2 can be

Atmospheric CO2 cannot be converted back into hydrocarbons without expending more energy than the fuel originally released in combustion. Atmospheric CO2 can be converted into stable minerals that no longer drive global warming, at an energy cost less than the fuel released when it first burned. See this chapter from the IPCC Special Report on Carbon Dioxide Capture and Storage: https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapter7-1.pdf https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapte... In particular, section 7.2.2, "Chemistry of mineral carbonation." The carbonation of magnesium and calcium silicates is thermodynamically spontaneous but kinetically hindered. The kinetic hindrance is why an additional energy input is needed to draw down atmospheric CO2 in less than geological time: the mineral's accessible surface area must increase dramatically for fast silicate weathering. The thermodynamic spontaneity is why the additional energy input can be small compared to the original energy embodied in the fuels that generated the CO2.