One of the lowest emitters of CO2 at an airport are the aircraft. The guilty parties are building heating, lighting and air-conditioning then we have the tugs, GPU's, Busses, Airport lighting systems - but remember, the really naughty boys are the cars that people drive to the airport in.
To reduce the carbon footprint of aircraft, remove speed and descent constraints which means we'll fly economy speeds and profiles at all times, allow visual approaches which means we'll spend less time in the approach phase and if you provide quality fixed electrical power and air-conditioning at a sensible price, airlines will use that instead of using an APU when parked.
Unfortunately, Green Ringpieces and rapacious bean-counters tend to get inside airport management which results in pointless rules, restrictions and procedures giving rise to huge carbon footprints but fat bonuses for those driving policy.
PM