Biodiesel significantly reduces Greenhouse Gas (GHG) emissions compared to petroleum. The most comprehensive, accurate and up-to-date lifecycle analysis of U.S. biodiesel produced from soybean oil concludes that GHG emissions are reduced 66-72% relative to average U.S. petroleum
while not trading off food-for-fuel nor destroying forests.
All of this is carefully explained in the report Environmental Benefits of Biodiesel and the Renewable Fuel Standard.