Since we can make the very same ethanol fuel from cultivated grains with standard processes and from uncultivated biomass using bacteria why does one way produce less net carbon in the atmosphere?

Answer 1

First of all, you have to understand that the whole grain-ethanol thing is more of a back-door subsidy for corn farmers than an environmental initiative. That said, I'm not sure that either method of making ethanol results in a net reduction of carbon dioxide in the atmosphere. In the case of grain ethanol, it takes fossil fuels (in the form of diesel) to grow the corn. And some estimate that it takes more diesel fuel just to grow the corn than will be replaced by the resulting ethanol. So, you would be better off, in terms of fossil fuel usage, to just burn the diesel in cars and trucks. But fossil fuels are not the only fuels that put carbon dioxide into the atmosphere. Burning ethanol also releases carbon dioxide. Maybe not as much as fossil fuels, but whatever it produces would have to be added to the carbon dioxide that was produced in growing the corn. And that is most certainly more than the carbon dioxide that would have been produced by burning the fossil fuel directly in automobiles. In the case of uncultivated biomass, you don't have tractors and other fossil-fuel-burning equipment contributing to the carbon dioxide in the atmosphere. However, the process of converting uncultivated biomass to ethanol is much more energy-intensive than converting grain to ethanol. So you still have quite a bit of fossil fuel burning required to produce the ethanol. And you still have all the carbon dioxide released when the ethanol itself is burned.