How does flying affect my CO2 emmissions

Using very rounded numbers for simplicity and a not very scientific approach, it does still make you think.
In 2000 the world's greenhouse emissions were 34 billion tons of CO2-equlivalent per year and the approximate population of the world is 6 billion.
So roughly we can say that the average emissions per person is 5 or 6 tons of CO2-equivalent per year.

Average person's emissions = 6 tons

A typical round-trip intercontinental flight (whatever that is) emits about 2 tons of CO2 per passenger.

Average round trip = 2 tons

So one trip per year is something like a third of the average person's carbon emissions.

Please read the excellent (and free) book Sustainable Energy without the hot air from David JC Mackay 2009