As others have mentioned, the propellant for three major upcoming rockets (Blue Origin's New Glenn, ULA's Vulcan, and SpaceX's Starship/Super Heavy) is methane

As others have mentioned, the propellant for three major upcoming rockets (Blue Origin's New Glenn, ULA's Vulcan, and SpaceX's Starship/Super Heavy) is methane (plus liquid oxygen). Currently that methane is coming from natural gas, and represents a tiny, tiny fraction of our current natural gas use. If we didn't want to use natural gas, methane can be made from hydrogen and carbon dioxide. This is critical to SpaceX's Mars plan, as they're planning on making their propellant for the return trip once they get there. If you can produce it on Mars, you can definitely produce it on Earth.