A press release recently reported research that claims biodiesel will cause more greenhouse gas effect than using petroleum oil. The gist is that agriculture with industrial fertilizers release some N2O unintentionally as part of nitrogen fertilization, and that nitrous oxide has a much stronger effect on global warming because it has a long lifetime in the atmosphere. The UN defines a "Global Warming Potential" for each greenhouse gas. Lifetime in the atmosphere is the primary factor (perhaps the only factor?), so nonreactive gases like CFCs, which live nearly forever, have incredible numbers (50000) for their "effect". CO2, though, is just defined as 1, even though it has a variable lifetime in the atmosphere of 5-200 years (page 38). I don't know if the spectral absorption of each gas was taken into effect for these--I see no information that it is. N2O lives for about 120 years in the atmosphere, a fairly nonreactive gas, and it is this multiplication factor, (296x), that drives the conclusion about how biodiesel via rapeseed is worse than petroleum in greenhouse gas emissions.
I would have liked to read the actual paper via my institutional subscription, but it's not available yet online or in paper.
The linked press release is actually from a tropical conservation group, and interestingly they happened to add a graph with yields of biodiesel per acre, showing rapeseed as middle of the pack in terms of yield. What would happen to the analysis with other crops? What if we didn't use so much nitrogen fertilizer or didn't exclusively produce rapeseed? (You can nitrogenate with legumes to avoid using some fertilizer). Was the absorption of CO2 from the atmosphere into the plant matter taken into consideration during the analysis? Questions I have, answers I can't find.
You might know rapeseed by another name: a special variety breed called canola (CANadian Oil Low Acid), which has much lower amounts of a naturally occurring toxic acid in it.