Technical Abstract: Bioenergy cropping systems could help offset GHG emissions, but quantifying that offset is complex. Adler et al. (2007, Ecol. Appl. 17:675-691) conducted a life cycle assessment of the net greenhouse gas flux from bioenergy cropping systems. Compared with the life cycle of gasoline and diesel, ethanol and biodiesel from corn rotations reduced GHG emissions in the near-term by 50-65%, reed canarygrass about 120%, and by about 145 and 165% for switchgrass and hybrid poplar, respectively. Based on the billion ton report, perennial crops could mitigate more than 300 Tg of CO2 annually.