Enter here all who be wonks. Others, you've been warned.
The popular argument goes that when calculating the emissions from biofuel powered vehicles you can ignore the emissions created by the vehicles burning them because the plants used to create biofuels already had sucked carbon out of the air and anything emitted by the car is just releasing it back into the atmosphere.It's a wash, so to speak, even if a crude one, in a writing on the back of the napkin with a really dull pencil, and maybe that napkin is a bit wet because you put it down in the sweat ring from your beer sort of way.
Well, a new piece in GCB Bioenergy strongly critiques this popular thinking.
Science Daily sums up the reasoning of Keith Smith and Timothy Searchinger:
[The conventional reasoning] double counts the carbon absorbed by plants when the bioenergy crops are grown on land already used for crop production or already growing other plants because the bioenergy does not necessarily result in additional carbon absorption. Biofuels can only reduce greenhouse gases if they result in additional plant growth, or if they in effect generate additional useable biomass by capturing waste material that would otherwise decompose anyway. The overestimation of bioenergy LCAs becomes increasingly magnified when the omission of CO2 is combined with the underestimation of nitrogen emissions from fertilizer application.
Smith says that the observed increase in nitrogen emissions from fertilizer is nearly double the value used in biofuel life-cycle assessments.
Here's the original piece: Crop-based biofuels and associated environmental concerns