I did my latest Science Progress column on this subject, in light of recent research suggesting "green" biofuels like ethanol might actually be bad for the environment. My conclusion?
...enough doubts have been raised that no one can reasonably postulate biofuels as an automatic solution to cut greenhouse gas emissions from the transportation sector. Depending on the details, they could actually cause dramatic setbacks. But it's also clear from the latest research that there will be more and less efficient-and more and less destructive-ways of generating and using biofuels. Despite huge hype, massive subsidies, and strong political support in the U.S., it seems corn-based ethanol runs near to last place in the competition.
The entire piece is here.