If we're to be at all serious about this being an existential threat to humanity, then personal air travel is an easy win in reducing emissions.
Additionally, its not really a massive win. Air traffic only makes up 8% of less than 30% of American emissions. Cars, on the other hand, make up more than 75% of that same 30%[0]. Switching to aircraft might actually make more sense for the environment.
[0] https://www.epa.gov/greenvehicles/fast-facts-transportation-...
But here in Canada our carbon tax is facing strong opposition from populists, and I expect that the next Government is likely to axe it. The current Government is already putting in place boutique exclusions; for instance, home heating fuel is excluded for the next three years. As an alternative, to discourage a major common form of driving, we should consider ways in which to make businesses find operating office space impossibly expensive.
Maybe we could have some kind of a permitting system where you could apply for permission to travel to see, for instance, your dying parent. But you would need to submit a letter from a doctor stating they are likely to die in the next 7-14 days, of course.
How about a ban on recreational activities like hiking and skiing? If we’re serious about this being an existential threat, this should be an easy decision.
Maybe a ban on human existence altogether.