It is a tricky question but if you drew a line in the sand, we think more than 700,000 flights a year from Heathrow alone would be quite a long way over it.
That is how many flights we are looking at if the proposed third runway at Heathrow gets the go-ahead.
It would make the airport the UK’s biggest source of carbon dioxide just as we are trying to slash our carbon emissions in the face of climate change.
This makes no sense at all, which is why the RSPB has today given its support to a legal challenge against the Government’s decision to allow a third runway.
We all like to fly. The explosion of cheap flights has opened up the world to many people. But flying comes at an environmental cost by adding to the build up of greenhouse gases in our atmosphere.
Left unchecked, climate change could destroy the very places we long to visit, together with thousands of species our children and grandchildren will never have the chance to see and enjoy as we have.
It isn't easy to reconcile the rich experience of air travel with its impact on the world we love. One way is to treat it as a precious gift, which we must use wisely and sparingly.
Expanding air travel at the speed and scale proposed by the Government is not wisdom. It is folly.
This is why the RSPB is opposed to the building of a third runway at Heathrow, and will continue to be so, until the Government can show airport expansion plans are genuinely compatible with its policies for protecting the climate.