Elon Musk plans on refueling his rockets on Mars using locally produced methane. Using solar power, he'll create methane and liquid oxygen from CO2 and water. If he then burns the methane and liquid oxygen in a rocket engine you get your CO2 and water back. Thus the process is carbon neutral.

Before he does it on Mars he'll have to perfect it on Earth first. I hope he does more than that and starts using carbon neutral methane for all his Starship flights on Earth.