The US uses approximately 140 billion gallons/yr of gasoline.
This converts to ~1.4 billion tons of CO2 per year, second only to electric power production with coal (~2.5 billion tons).
To see this more clearly, imagine a 10,000 gal tanktruck of gasoline going down the highway. Then imagine 1,204 tanktrucks following it.
Those 1,204 TT's represent the amount of CO2 emitted from that single
TT of gasoline.
If we replaced half the gasoline with ethanol, we would save 700 million tons of CO2 emissions and eliminate all imported oil.
This would also create a 70 billion gallon market for domestic ethanol producers. We would easily meet the Renwable Fuel Standards
mandate of 36 billion gallons of renewable fuel by the year 2022.
In terms of net CO2 emissions, it doesn't really matter whether we use corn, cellulose, or algae to make the ethanol. Using all those materials is using carbon that's already in the atmosphere.
What we need to do to reduce global warming is to stop digging up new carbon (oil and coal) and burning it. We need to make our fuel from carbon already above ground.