Earlier quoted context omitted.
The worst culprit for many people I know is flying. You can eat a lot of meat for the emissions of just one long-haul flight. (Which is sad because flying is great.)
Reference: http://www.kimnicholas.com/uploads/2/5/7/6/25766487/fig1full...
https://en.wikipedia.org/wiki/Environmental_impact_of_aviati...
I also read somewhere that propeller based airplanes flying lower in the atmosphere would be better option for short haul flights.