Now JetA isn't service station gas so its slightly unfair to compare the 15 pMPG of a Concorde to a 12 MPG single passenger pickup truck. But it does give you a general idea that everything being equal (size, technology level, risk?) the supersonic plane will burn about twice the carbon footprint of a similar subsonic plane.

Note that with a full modern subsonic plane its quite easy to get a carbon footprint staggeringly better than a single driver in an obese modern SUV or truck. The problem is if you're fair and let the aircraft people use any thing they want such as 2016 technology and engineering, you have to let the car people use any thing they want, and my wife's Prius is trivially capable of providing reasonable comfort at 300 passenger miles per gallon.

Supposedly the largest 777 model does better than 70 pMPG per my commercial pilot buddy (who teased me quite a bit about my wife's Prius only getting 60 pMPG when there's only one passenger, so he burns less gas flying than she does driving). So everything being equal if stereotypical subsonic planes have roughly doubled in pMPG then its not ridiculous to expect over 30 pMPG on a modern supersonic plane. My Yaris commuter car struggles to get 30 pMPG with a single passenger (me) so to a first approximation a modern supersonic aircraft damages the environment about as much as driving alone in my subcompact commuter car. Not great, not awful. Eh.

Obviously my car's performance in transatlantic and transpacific service is somewhat inferior to the performance of the proposed plane.