>High speed trains are a good alternative (for domestic travel) This is the only real answer. There's a hard limitation to how emissions friendly aviation can

>High speed trains are a good alternative (for domestic travel) This is the only real answer. There's a hard limitation to how emissions friendly aviation can ever be. The energy density requirements given current technology can only be met with hydrocarbon fuels. High speed trains are infinitely more efficient, and actually would have an outsized effect on emissions given that the type of short haul flights it would replace are the least efficient per mile travelled, since takeoff/climb uses far more fuel than cruise. We'll never be able to replace carbon intensive long haul international flights, but focusing on what can be replaced could go a long way.