So is everyone driving in the morning: a tiny fraction of the transport pollution. Add every tiny fraction and you get a significant fraction.

To add to staceymakano's point: in the transport industry, I feel very uncomfortable when a a new alternative doesn't perform better on the environmental side.

Not even remotely true in this case. Aviation accounts for about 2% of carbon emissions, by my cursory Google search. Boom aircraft will, in the absolute best of outcomes, never account for more than a very small fraction of that number. And a small fraction of 2% is still, no matter how one tries to spin it, a very small fraction.

In any event, power (P) required to increase velocity (V) is a cubic relationship (P=FV, where F is the drag force, which has as one of its components V^2), so doubling speed requires 8 times the power. There are ways to mitigate that increase -- fly higher to reduce air density, reduce drag by using specialized airfoils and reducing excrescence, take advantage of increasing engine efficiencies, etc. -- but I think it's safe to say that a supersonic aircraft will always require gobs more fuel than its subsonic cousins. By your logic then, one should probably never be manufactured.

Full disclosure: I once worked as an aerospace engineer.

Correct. My nerd side tells me supersonic planes are the coolest thing on earth, my hippie side is telling me we'll live better without it.

And I still disagree about the fact that a tiny fraction can be ignored.

The main reason why climate change is a thing is because we consistently and repeatedly failed to acknowledge our impact on the environment and justified it by comparing it to other people's impact (and I include myself in that statement).