Carbon-based liquid fuels are highly valued for transportation; they are the world's largest form of commercial energy and second largest source of anthropogenic carbon dioxide (CO2) emissions. Strategies to address their CO2 emissions have been shaped by fuel cycle analysis (FCA), a version of lifecycle assessment that examines fuel products and their supply chains. FCA studies have diverse findings and large uncertainties. Disagreements are particularly sharp for biofuels, which are seen as key replacements for petroleum fuels.