Micah is a podcaster for the Airplane Geeks podcast and he says that the Boom Supersonic zero-emissions aircraft is largely smoke and mirrors. It claims zero emissions because it's going to use biofuels instead of kerosene. It'll lower emissions, but the rest will have to be handled by buying Carbon offsets. So not strictly zero emissions. Micah says that Boom will likely be vaporware before it actually takes to the air.