This is because two-thirds of the greenhouse gas emissions occur while growing the crops used to produce biodiesel, according to a recent study. Thus, when you view the entire cycle of producing it, it’s no improvement over gasoline.
The best answer, as always, is to cut consumption.
I didn’t know that crops contribute to global warming. It’s counterintuitive, since trees do not. However, further reading suggests that cropland’s effect is due to N2O released as a result of overfertilization. This occurs from both pretroleum-derived and natural (manure) fertilizers. But cropland also sequesters carbon, effectively reducing CO2 in the atmosphere, and if N2O is controlled through improved farming practices, cropland can have a significant beneficial impact on greenhouse gases.
The theme seems to be that overfertilization is common practice, but doesn’t need to be. (Why doesn’t this surprise me?) IF (and I emphasize IF) N2O production were minimized in biodiesel production, one wonders whether this would tip the scale toward a net benefit with respect to greenhouse gases.
It’s also harvesting of corn say, in the Midwest, then transporting that adds to the problem.