[TrinaryWorksToo]

Wouldn't the CO2 be absorbed over a period of time, which the combustion would do it all at once, being a net positive as far as the environment is concerned?

Or perhaps I don't understand how CO2 harms the environment well.

[baddox]

Yes, but on a time scale of a year or more (disregarding corn reserves, which probably do exist but are probably too small to matter), we can only emit the CO2 from corn ethanol at the same speed we absorb it. This is unlike fossil fuels, which took millions of years to form but which can be used much faster.