Abstract:This paper presents a detailed lifecycle analysis of emissions of greenhouse gases and urban air pollutants from a number of alternative fuel and feedstock combinations for highway vehicles, forklifts, and residential and commercial heating. I find that among the major near-term energy sources, natural gas and LPG have relatively low lifecycle emissions of urban air pollutants, and relatively low lifecycle emissions of greenhouse gases. Ultimately, using natural gas instead of oil as a primary resource will reduce greenhouse-gas emissions if the lower carbon/GJ content of NG is not offset by higher fuelcycle energy requirements or higher fuelcycle emissions of methane. It is important to look beyond end-use emissions and consider the entire lifecycle, because emissions from "upstream" fuelcycle activities, and from the lifecycle of materials, can be significant relative to end-use emissions. Findings for one sector, such as highway vehicles, do not necessarily apply to other sectors, such as offroad engines or residential heating, on account of differences in end-use energy efficiency and emissions.