The reference point is a total carbon budget of around 2 tCO2e per person per year, across all categories, compatible with +2°C: aviation has to fit alongside food, housing and other transport. A French person currently emits around 8 tCO2e per year. A Paris-New York return journey (around 2 tCO2e) therefore consumes this entire budget alone, Paris-Singapore (3.8 tCO2e) nearly 2 times and Paris-Sydney (6 tCO2e) 3 times: one long-haul journey is enough to reach or exceed the annual budget.
Read the source article: Flight carbon footprint: the figures by destination