The Starship is worst-case carbon neutral (burning methane generated from CO2 in the atmosphere) and best-case carbon negative. It can be carbon negative because methane is a potent greenhouse gas that is expensive to recover, with little economic value, and largely brought up to the surface from underground oil wells. Thus, this methane is often vented right to atmosphere. Actually burning it in Starship converts it into CO2, which has about 1/20 the long-term climate effect as would the otherwise-vented methane.