The main point of the new runway is of course to increase the amount of flying and the Commission accept that this will increase CO2 emissions over the next 20 years. After that, it says, increasing fuel efficiency will reduce emissions so that by 2050 they will nearly, but not quite, be down to 2025 levels.
Now everyone (including the Committee on Climate Change and successive Labour, Coalition and Tory governments) says that UK emissions have to fall dramatically over the next 40 years so here is what the Commission's Sustainability Report says (para 16.8):
"... it is important to remember that an expanded Heathrow under a Heathrow Airport Northwest Runway scheme would still be producing .... a majority, of total UK carbon from aviation: in 2050 the carbon emissions from departing flights at Heathrow would represent 54.6% of the UK total."So that's official. All other activities must reduce their emissions more so that we can fly more.