> Modern jet turbines are running within a few % of maximum thermodynamic efficiency. Indeed they are marvels of engineering. But that doesn't make them "susta

> Modern jet turbines are running within a few % of maximum thermodynamic efficiency. Indeed they are marvels of engineering. But that doesn't make them "sustainable". It just takes a lot of energy to push something through the air at high speed, no matter how efficient the engines are. Apart from the CO2 emissions, there's also the issue of the GHG forcing effect of high altitude water vapor emissions. Current estimates are that flight at typical airliner altitudes of ~10 km results in a GHG forcing (over some decades) of roughly 3 times more than burning the same fuel on the ground. > And for that matter, how would biofuels possibly be worse fossil fuels, given that the whole point is to not emit net CO2 into the atmosphere. Obviously it's "efficient" because all the energy we put in has to come from the sun, rather then obtained for free but that's the point. The major issue is due to land use changes. Biodiversity is already in a sharp decline (arguably this is an as big, if not bigger issue than climate change), largely driven by converting nature into cropland. Secondly, converting nature into crops, whether for fuel or food, tends to result in large CO2 emissions due to the carbon that has been stored into the soils being released. And finally, biofuels just don't scale. There isn't enough arable land on the planet to replace the amount of fossil fuels we use today.