Comparing greenhouse gas footprints and payback times of crop-based biofuel production worldwide

Two metrics are typically used to quantify the full greenhouse gas (GHG) balance of biofuel production: the GHG footprint (in kg CO2-eq/MJ bioenergy produced) and the GHG payback time (in years). The goal of the present study was to quantify and compare spatially explicit GHG footprints and payback times of crop-based bioethanol and biodiesel production, focusing on locations where the four main feedstocks - corn, sugarcane, soybean and rapeseed - are currently cultivated. The largest GHG footprints and longest payback times were found for sugarcane in the tropics, owing to large original carbon stocks in the tropical ecoregions. The lowest GHG footprints and shortest payback times were found in the temperate ecoregions of the USA and Europe, with relatively low initial carbon stocks. For corn, GHG footprints were below the fossil reference and payback times were shorter than 30 years in 71-74% of cumulative production (for sugarcane, in 29-30% of the cumulative production). Soybean and rapeseed have intermediate results, at 49-62% of the cumulative production. The two metrics were highly correlated, except for locations where an infinite payback time was calculated. Overall, GHG footprints and payback times yield similar results when it comes to quantifying GHG balances of biofuels.