The presence of variability in life cycle analysis (LCA) is inherent due to both inexact LCA procedures and variation of numerical inputs. Variability in LCA needs to be clearly distinguished from uncertainty. This paper uses specific examples from the production of diesel and jet fuels from 14 different feedstocks to demonstrate general trends in the types and magnitudes of variability present in life cycle greenhouse gas (LC-GHG) inventories of middle distillate fuels. Sources of variability have been categorized as pathway specific, coproduct usage and allocation, and land use change. The results of this research demonstrate that subjective choices such as coproduct usage and allocation methodology can be more important sources of variability in the LC-GHG inventory of a fuel option than the process and energy use of fuel production. Through the application of a consistent analysis methodology across all fuel options, the influence of these subjective biases is minimized, and the LC-GHG inventories for each feedstock-to-fuel option can be effectively compared and discussed. By considering the types and magnitudes of variability across multiple fuel pathways, it is evident that LCA results should be presented as a range instead of a point value. The policy implications of this are discussed.

Considerable research and development is underway to produce fuels from microalgae, one of several options being explored for increasing transportation fuel supplies and mitigating greenhouse gas emissions (GHG). This work models life-cycle GHG and on-site freshwater consumption for algal biofuels over a wide technology space, spanning both near- and long-term options. The environmental performance of algal biofuel production can vary considerably and is influenced by engineering, biological, siting, and land-use considerations. We have examined these considerations for open pond systems, to identify variables that have a strong influence on GHG and freshwater consumption. We conclude that algal biofuels can yield GHG reductions relative to fossil and other biobased fuels with the use of appropriate technology options. Further, freshwater consumption for algal biofuels produced using saline pond systems can be comparable to that of petroleum-derived fuels.

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Alternative fuels represent a potential option for reducing the climate impacts of the aviation sector. The climate impacts of alternatives fuel are traditionally considered as a ratio of life cycle greenhouse gas (GHG) emissions to those of the displaced petroleum product; however, this ignores the climate impacts of the non-CO(2) combustion effects from aircraft in the upper atmosphere. The results of this study show that including non-CO(2) combustion emissions and effects in the life cycle of a Synthetic Paraffinic Kerosene (SPK) fuel can lead to a decrease in the relative merit of the SPK fuel relative to conventional jet fuel. For example, an SPK fuel option with zero life cycle GHG emissions would offer a 100% reduction in GHG emissions but only a 48% reduction in actual climate impact using a 100-year time window and the nominal climate modeling assumption set outlined herein. Therefore, climate change mitigation policies for aviation that rely exclusively on relative well-to-wake life cycle GHG emissions as a proxy for aviation climate impact may overestimate the benefit of alternative fuel use on the global climate system.

All societies require energy services to meet basic human needs (e.g., lighting, cooking, space comfort, mobility, communication) and to serve productive processes. For development to be sustainable, delivery of energy services needs to be secure and have low environmental impacts. Sustainable social and economic development requires assured and affordable access to the energy resources necessary to provide essential and sustainable energy services. This may mean the application of different strategies at different stages of economic development. To be environmentally benign, energy services must be provided with low environmental impacts and low greenhouse gas (GHG) emissions. However, 85% of current primary energy driving global economies comes from the combustion of fossil fuels and consumption of fossil fuels accounts for 56.6% of all anthropogenic GHG emissions.Renewable energy sources play a role in providing energy services in a sustainable manner and, in particular, in mitigating climate change. This Special Report on Renewable Energy Sources and Climate Change Mitigation explores the current contribution and potential of renewable energy (RE) sources to provide energy services for a sustainable social and economic development path. It includes assessments of available RE resources and technologies, costs and co-benefi ts, barriers to up-scaling and integration requirements, future scenarios and policy options.GHG emissions associated with the provision of energy services are a major cause of climate change. The IPCC Fourth Assessment Report (AR4) concluded that "Most of the observed increase in global average temperature since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations." Concentrations of CO 2 have continued to grow and by the end of 2010 had reached 390 ppm CO 2 or 39% above preindustrial levels.The long-term baseline scenarios reviewed for the AR4 show that the expected decrease in the energy intensity will not be able to compensate for the effects of the projected increase in the global gross domestic product. As a result, most of the scenarios exhibit a strong increase in primary energy supply throughout this century. In the absence of any climate policy, the overwhelming majority of the baseline scenarios exhibit considerably higher emissions in 2100 compared to 2000, implying rising CO 2 concentrations and, in turn, enhanced global warming. Depending on the underlying socioeconomic scenarios and taking into account additional uncertainties, global mean temperature is expected to rise and to approach a level between 1.1°C and 6.4°C over the 1980 to 1999 average by the end of this century.To avoid adverse impacts of such climate change on water resources, ecosystems, food security, human health and coastal settlements with potentially irreversible abrupt changes in the climate system, the Cancun Agreements call for limiting global average temperature rises to no more than 2°C above preindustrial values, and agreed to consider limiti...

This paper analyzes alternative jet fuels in terms of how they could change emissions from military and civil aircraft and in terms of the challenges in meeting future energy goals. Estimations of the continental United States (CONUS) conventional jet fuel energy usage for the civil and military aviation fleets were used to inform the magnitude and logistics of where the fuels would be needed. To adequately meet military goals, the U.S. Air Force (USAF) and U.S. Navy (USN) would need to supply roughly 47,500 bpd and 18,800 barrels per day (bpd) of alternative jet fuels by 2016, respectively. The total amount of fuel for both military and civil goals would reach nearly 132,000 bpd within the next decade if tentative goals become actual policy. Quantifications of the emissions affecting surface air quality from CONUS civil, USAF and USN aircraft, as well as 50% and 100% synthetic paraffinic kerosene (SPK) combustion emissions normalized by conventional jet fuels were also provided. Although a 50% blend of SPK has been permitted, additional testing and analysis is needed for approval of higher blend percentages. It was found that NO X emissions from military aircraft tend be lower while primary PM 2.5 , CO and UHC emissions tend to be higher than their civilian aircraft counterparts. This is indicative of military aircraft being less efficient at lower power settings than civil aircraft during the LTO cycle. Emissions reductions with 50% and 100% SPK use could provide military and civil aviation planners with more options when locating aircraft in nonattainment areas within the CONUS. For some emissions, the introduction of SPK fuels could allow for additional aircraft for the same environmental impact or decreased overall air quality footprint for a particular location. SPK fuels from Fischer-Tropsch Biomass-to-Liquid (BTL) and Hydroprocessed Renewable Jet (HRJ) processes were examined for their ability to meet future alternative fuel and environmental goals. BTL facilities were found to have larger capital costs and HRJ required large land area. Lifecycle analysis (LCA) of greenhouse gas (GHG) emissions for select F-T BTL and HRJ were found to potentially meet or exceed organizational goals in the near term. High yield crops like algae could provide the energy and environmental goals, but additional constraints must be considered, such as water and CO 2 requirements; furthermore, these technologies need to be translated from the lab to commercial production. Additional research is required to provide an in-depth geographic analysis of the CONUS commercial and military demand centers and resource constraints to better understand the challenge in meeting future alternative fuel goals.