An additional point: construction-related CO2 emissions come mostly from the manufacture of cement and steel. There is research into a significant reduction of both. From what I remember, at least steel production is possible in a system closed from the viewpoint of the carbon cycle (i.e. without burning coal).

However, you can't make air travel carbon-free, however you tax it.

_{*Lunatic*, n.
One whose delusions are out of fashion.}