The method used to estimate the impact of additional capacity at Heathrow on UK aviation CO2 emissions in the 'Adding Capacity at Heathrow Airport' consultation document is referenced at paragraph 2.23 of the impact assessment (annex B). Full details of the forecasting method can be found in chapter 3 of 'UK Air Passenger Demand and CO2 Forecasts', available at:
There is no internationally agreed way to allocate to individual countries the CO2 emissions from international flights. For reporting purposes, the UN framework convention on climate change requires each nation to report emissions based on aviation bunker fuel consumption, which is closely related to fuel used on departing flights. To ensure consistency with historic reported totals, and to avoid double-counting CO2 emissions, our forecasts include international aviation emissions on a departing flight only basis.