Reducing the net global warming potential (GWP) of energy use is a major factor driving interest in biofuels. Bioenergy cropping systems vary in contribution to the GWP due to the crop yield and resulting quantity of fossil fuels displaced, quantity and quality of C added to the soil, feedstock conversion efficiency, N₂O emissions, N use efficiency, and inputs required for crop production and operation of farm machinery. The objective of the study was to use DAYCENT to model the impact of climate change on net greenhouse gas (GHG) emissions of bioenergy cropping systems (corn, soybeans, alfalfa, switchgrass, reed canarygrass, and hybrid poplar) in Pennsylvania for inclusion in a full C cycle analysis. Weather data driving climate change scenarios were from VEMAP, Canadian Climate Model (CGCM1), and Hadley Center Model (HADCM2) assuming CO₂ doubled from 2004 - 2100. The quantity of displaced fossil fuel was the largest GHG sink. Soil C sequestration was the second largest GHG sink. Although crops with higher soil C inputs, such as switchgrass and hybrid poplar, will have higher equilibrium soil C levels, the change in system C will approach zero in the long term. N₂O emissions were the largest GHG source. When the credit for the amount of fossil fuel displaced was not taken into account and soil C storage was assumed to have reached its maximum capacity, switchgrass and hybrid poplar were the only cropping systems to remain a sink for GHGs. Therefore, use of switchgrass and hybrid poplar for production of biofuels has the potential to be GHG neutral and may even be a long-term sink for GHGs.