As a member of Greenpeace, they emailed me asking me to support their campaign against the Heathrow airport expansion. I don't support the campaign, and here's why.
Let's assume that levels of CO2 are okay up to 450 ppm. Since there's an upper limit, it means we've got to use the CO2 emissions we're allowed, to their best effect. Perhaps UK air travel is the best way to use some of our permitted emissions? Or perhaps not.
The way to enforce a CO2 limit while making sure the emissions are used most wisely, is to raise the global cost of emissions through a carbon tax until the permitted level of CO2 is reached. The revenue from such a tax should be distributed equally amongst the world's population.
Then the climate change aspect of airport expansions is taken out of the equation, problem solved!