Correct me if I'm wrong, but if for example we only used pure ethanol as fuel, carbon would still be emitted, but it would also be offset by the carbon absorbed by growing the next round of fuel crops. It seems like such a situation would be either carbon-neutral or close to it.
Especially if we develop (through GMOs or ordinary selective breeding) crops specifically for fuel purposes to make the whole process more efficient.
That's not correct. In a pure-ethanol economy, the total emissions will be CO2 from:
- Deforestation for the creation of corn fields. This includes the process of cutting down trees (gas powered machines, transport, etc.) and loss of trees that will no longer produce O2/absorb CO2
- Planting
- Fertilizers. This one is huge as creation of fertilizers is nasty business and fairly energy intensive.
- Harvesting
- Transport
- Fermentation
- Disposal of byproducts
- Transport again to the pump
- Combustion
- Other effects, such as having to manufacture more engines as they would wear out quicker and recycling of more worn out engines.
This would partially offset by the fact that the corn is absorbing CO2.
As I mentioned above (twice), fossil fuels have similarly complex sources of emissions (drilling/fracking, transport, refining, transport again, combustion).
The question to answer is this: if you sum up the totals for both methods, which is more efficient. I guarantee you, both still will produce net positive CO2 emissions. The only question is which will produce less net CO2 from all the sources.
Lastly, I should mention that theoretically, there is also a benefit of reducing other nasty emissions from fossil fuels such as sulfur. We could probably produce cleaner ethanol than gasoline. But none of that will matter if we spew so much CO2 into the air that we suffocate before we can measure these secondary pollutants properly.
I'm still kind of confused on this. In all the examples of energy expenditure, we assume that the energy comes from ethanol in the first place. The plants aren't magically creating more carbon, so if you grow the same amount as you burn, that carbon should be absorbed back.
That leaves deforestation for planting, which is a problem since human energy use isn't the only source of CO2. But if deforestation is avoided (this is where GMO crops would come in, grow more fuel in a given amount of space) or we implement other methods of CO2 sequestration (impractical now, but perhaps not in the future) we could still minimize that risk.
Especially if we develop (through GMOs or ordinary selective breeding) crops specifically for fuel purposes to make the whole process more efficient.