The synthetic route uses oil as the source of ethene feedstock.
This means that combustion will add carbon dioxide to the atmosphere,
and is one reason that interest in ethanol-powered transport is
lower in the UK than in the US.
In contrast, fermenting and burning ethanol has the potential
to be carbon neutral, as carbon dioxide is fixed through photosynthesis
by the plants used as feedstock for fermentation. Energy is needed,
however, for distillation and if this is from carbon-based fuel
it will generate some additional carbon dioxide.