Per person, it is reliably the single most polluting thing you will do. The reason it accounts for a "small proportion" of emissions is that most people in the world do not fly at all. Indeed, a tiny (<1% IIRC) elite of the world's population account for most of the world's flight kilometers. Flying is an absolutely unscalable form of transport in a world of 8 billion people.

> Flying is an absolutely unscalable form of transport in a world of 8 billion people.

So is the private (ICE) motorcar. ~4000kg of CO2/y for typical US use.