With the continuous advancement of the global economy, the transport industry has undergone significant expansion, playing a crucial role in facilitating trade, connectivity, and socio-economic development. However, this growth has been accompanied by increasing fossil energy consumption. The transport sector accounted for 27.0% of final energy consumption in 2023 [1], making it one of the primary contributors to greenhouse gas emissions and resource depletion. As concerns over environmental sustainability intensify, there is a growing need for strategies that balance economic development with energy efficiency and emission reduction in the transport sector. The imbalance in energy use within the transport sector remains a critical issue. In 2022, fossil fuels dominated global road transport energy consumption, with crude oil and natural gas accounting for 95%, while renewable energy sources remained marginal [2]. This heavy reliance on fossil fuels underscores the urgent need for sustainable energy transitions in the transport sector. However, the transition requires a thorough evaluation of various alternatives to conventional fossil energy, particularly concerning resource consumption and environmental impacts.

Various emerging energy alternatives to conventional fossil fuels, such as bioenergy, hydrogen, and hydrogen-based fuels, are playing an increasingly prominent role in this transition [1]. As these alternatives continue to gain attention, understanding their relative effectiveness in reducing emissions becomes crucial. However, most studies remain focused on individual fuels or specific regional contexts, with insufficient emphasis on comprehensive, globally comparative life cycle assessment that evaluates the decarbonisation potential of multiple alternative fuels under unified scenarios. For instance, while some studies have compared three hydrogen production technologies, such as steam methane reforming, biomass gasification, and wind power electrolysis, using Global Warming Potential (GWP), Cumulative Non-renewable Energy Demand, and Acidification Potential (AP), their results are not always directly comparable due to inconsistent methodologies and the omission of the end-of-life (EoL) stage [3]. Even when using a broad scope that includes the full life cycle of fuel cell electric vehicles, all hydrogen types exhibited identical impacts during the vehicle cycle. However, the study remains limited in scope, as it focuses exclusively on hydrogen and considers only two renewable production technologies, potentially overlooking broader environmental trade-offs. Seven distinct renewable power-to-methane technologies, including wind- and solar-powered electrolysis, methanation with direct air-captured CO₂ and anaerobic digestion of sewage sludge, pig manure, dairy manure, food waste, and landfill gas upgrading, were evaluated alongside compression technology using a comprehensive set of human and environmental impact indicators [4], suggesting that the pig manure system outperforms the conventional system in 6 of the 11 indicators, with the majority of the renewable systems have a lower GWP than natural gas compression. While the approach provides valuable insights, its scope is limited by the exclusion of the EoL stage, which may hinder the comprehensiveness of the sustainability assessment.

Building on these evaluations, a broader perspective is needed to compare alternative fuel systems. A more extensive approach is evident in the comparison of the whole life cycles of battery electric, hydrogen, and methanol-powered internal combustion engine vehicles [5]. However, only one production pathway for hydrogen and methanol was considered, highlighting the need for a more inclusive life cycle assessment to facilitate sustainability in the transport sector. The fossil, nuclear, and renewable (wind/solar) hydrogen production pathways for Fischer-Tropsch (FT) and ethanol e-fuels were assessed by evaluating both standalone and integrated systems (within ethanol production) [6]. These findings underscore the importance of defining system boundaries carefully, as systems with standalone assessment scope exhibited significantly lower GHG emissions. While hydrogen may seem to be the sole viable option for decarbonising transportation due to its versatility and clean-burning properties, addressing the challenge of carbon-intensive transportation requires a systematic approach, including sociological considerations such as public acceptance and local employment impacts [7]. A variety of fuels and propulsion systems are essential for achieving long-term sustainability, as no single solution can solve such a complex issue.

Life Cycle Assessment (LCA) is a valuable method for evaluating the environmental impacts of various energy pathways across their entire life cycle, from raw material extraction to EoL disposal [8]. Unlike studies focused on specific technologies or fuel pathways, LCA enables the comparison of a wide range of alternative fuels under unified criteria, providing a clearer understanding of their environmental impacts. By covering all life cycle stages, LCA identifies potential trade-offs and environmental hotspots often overlooked in narrower studies. This comprehensive approach ensures reliable and consistent comparisons between fuel systems, especially when assessing the long-term sustainability of alternative fuels and technologies. While the studies mentioned above provide valuable insights into LCA methodology, they were limited by their focus on a small number of production technologies and alternative fuel types.

This study adopts a systematic LCA approach to assess the environmental impacts of six different alternative fuels alongside conventional fossil fuels and grid electricity in the transport sector. Taking a global perspective, it evaluates key indicators such as GWP and AP to identify trade-offs and environmental hotspots.

The novelty of this work lies in its comprehensive comparison of multiple alternative fuels within a global, life cycle framework, accounting for various production pathways and real-world production shares. This approach ensures a more practical, inclusive, and accurate assessment compared to previous studies focused on individual fuels or production pathways and relying on unrealistic scopes and scenarios. The findings offer valuable insights for policymakers and the development of sustainable fuel strategies in the transport sector.

The remainder of this work is structured as follows: the methods section describes the approach and data sources used in the study, the results and discussion section presents and interprets the findings, and the conclusion offers final remarks and recommendations for future research.

To effectively evaluate the viability of alternative fuels and technologies, critically assessing their potential in different global contexts is crucial. Fuel diversity is essential to accommodate regional variations in resource availability, infrastructure, and policy incentives. For example, compressed natural gas (CNG) is widely adopted in regions such as India [9], Pakistan [10], and Iran [11] due to local natural gas availability and supportive policies. At the same time, Italy has led the adoption of CNG in Europe [12]. Methanol is a strategic alternative in China to reduce oil dependency [13], whereas Iceland explores its renewable methanol production using geothermal energy [14]. Strong government initiatives in Japan drive hydrogen fuel adoption [15], South Korea [16], and Germany [17], while Brazil dominates sugarcane-based bioethanol production, primarily for E100 and flex-fuel vehicles [18]. The U.S. relies on corn-based ethanol blends like E15 and E85 [19], like most of Europe, with the E10 gasoline blend being the norm [20]. Synthetic fuels (e-fuels) are emerging in Saudi Arabia, leveraging renewable hydrogen for sustainable fuel production [21]. These regional variations underscore the importance of regionally tailored policies and infrastructure investments to align fuel production with local resource availability and emission reduction targets, which will be considered in this work when comparing various fuel solutions for the transport sector.

The main goal of this study is to assess and identify the optimal short- and long-term alternative fuel options for the transport sector using a comprehensive LCA approach. The fuels evaluated include gaseous hydrogen, CNG, methanol, ethanol, and FT gasoline and diesel, representing gaseous, alcohol-based, and synthetic fuel types with distinct but occasionally interconnected production pathways. Conventional gasoline, diesel, and grid electricity serve as benchmark fuels to contextualise their competitiveness. This comparative framework enables an in-depth evaluation of the relative performance and viability of alternative fuels in both present and future contexts. The subsequent sections detail the methodology, including data sources and specific evaluation criteria.

All emission data utilised for calculating the two indicators presented in this section are generated using the GREET 2024 Excel model. Subsequent calculations, including data conversion into indicators and determination of numerical results for additional analyses, are performed within the same Excel framework to maintain consistency in data processing. The results for the 2025 and 2050 scenarios, along with a comparative study of both, are provided in the following subsections.

Environmental impacts in the 2025 scenario

The environmental impacts of various fuel options in the 2025 scenario are evaluated and presented in this section, focusing on GWP and AP. The analysis compares six alternative fuels with conventional gasoline, diesel, and grid electricity to assess their relative environmental performance. The results, presented in Figure 1, illustrate the differences in emissions across fuel pathways, highlighting key trade-offs and potential benefits associated with each option.

In both Figure 1a and Figure 1b, the environmental impacts at the feedstock stage (acquisition, preparation and transportation) are represented by grey bars. In contrast, those at the fuel production stage are shown as blue bars. The impacts of the operation stage (EoL phase for the fuel product) are depicted using amber-coloured bars. Benchmark fuels are presented with darker colours. The benchmark values for comparison are represented by different line styles: a solid grey line for gasoline, a black dashed line for diesel, and a black dotted line for electricity. Black diamond markers indicate the total impact for each fuel.

Considering GWP, Figure 1a demonstrates that CNG (65.27 gCO_{2 eq}) and E85 EtOH (54.81 gCO_{2 eq}) emerge as the most sustainable options, exhibiting the lowest net and gross emissions, respectively. Notably, these are the only two alternative fuels with total GWP lower than those of the three benchmark fuels. CNG exhibits a particularly low carbon intensity, primarily due to its simple feedstock acquisition and production processes [42], resulting in a combined feedstock and production phase emission of only 9.04 gCO_{2 eq}. In contrast, bioethanol (e.g., E85 EtOH, the fourth bar in Figure 1a) offsets a significant portion of its emissions through biological feedstocks, such as corn and sugarcane, which absorb substantial amounts of CO₂ during their growth cycles [43], leading to a carbon offset of −32.42 gCO_{2 eq} in the current scenario.

Figure 1.

Environmental impacts of six alternative fuels and three conventional fuels in the 2025 scenario: Global Warming Potential – GWP (a) and Acidification Potential – AP (b)

Although FT fuels (the fifth and sixth bars in Figure 1a) also exhibit high negative offsets (i.e., −46.65 and −51.12 gCO_{2 eq}, respectively), their pre-use phases, including a hydrogen production in the feedstock phase and fuel synthesis in the production phase, are highly carbon-intensive [44] resulting in total emissions that exceed those of the benchmark fuels. Methanol, while demonstrating relatively low pre-use CO₂ emissions (34.72 gCO_{2 eq} from both phases combined), remains more carbon-intensive than conventional fuels due to its comparable tailpipe emissions. The current hydrogen production mix is dominated by grey hydrogen, which significantly increases its carbon emissions, placing hydrogen at a disadvantage in terms of overall emissions [45].

Figure 1b shows that half of the alternative fuels have nearly double the AP compared to conventional fossil fuels. However, the AP levels of today’s electricity far exceed those of all alternative fuels, primarily due to the emissions associated with battery production for energy storage [46], resulting in a total AP of 0.129 gSO_{2 eq}. Hydrogen, CNG, and M85 MeOH demonstrate relatively minor differences in SO₂-equivalent emissions, with values of 0.0575, 0.0549 and 0.0548 gSO_{2 eq}, respectively. These fuels have lower AP due to their relatively cleaner production processes. For instance, AP of the hydrogen is relatively low due to its production from low-emission sources like electrolysis, even though this is contingent on the electricity source [47]. CNG has a low AP because its feedstock acquisition and production processes are less polluting compared to other fuels [42], leading to reduced sulphur and nitrogen oxide emissions. At the same time, M85, being a blend of methanol and gasoline, also benefits from a cleaner production process compared to pure gasoline.

Environmental impacts in the 2050 scenario

This section evaluates the environmental impacts of various fuel options in the 2050 scenario. Figure 2a and Figure 2b present the emission differences across six alternative fuels and three conventional fuels, with a focus on GWP and AP.

Figure 2.

Environmental impacts of six alternative fuels and three conventional fuels in the 2050 scenario: Global Warming Potential – GWP (a) and Acidification Potential – AP (b)

In the 2050 scenario, M85 exhibits a carbon footprint comparable to conventional fossil fuels, with an emissions value of 70.16 gCO_{2 eq}, making it the least favourable option among the alternatives. It is mainly due to the emissions from methanol production and combustion. Both CNG and E85 EtOH continue to exhibit lower carbon intensity, benefiting from carbon sequestration during feedstock growth [48]. Hydrogen (33.54 gCO_{2 eq}) and FT e-fuels, including FT gasoline (33.24 gCO_{2 eq}) and FT diesel (29.15 gCO_{2 eq}), are predicted to have a lower carbon footprint than the projected electricity benchmark (38.74 gCO_{2 eq}), as illustrated in Figure 2a, due to increased use of renewable energy in hydrogen production and carbon capture technologies in FT fuel synthesis [49].

In Figure 2b, the AP for these fuels remains similar to the 2025 scenario. EtOH and FT fuels show higher SO₂-equivalent emissions, mainly due to fertiliser use in ethanol production and emissions from the FT synthesis process [50]. The least acidifying options in the 2050 scenario are gaseous hydrogen (0.0381 gSO_{2 eq}), CNG (0.0392 gSO_{2 eq}) and M85 MeOH (0.0449 gSO_{2 eq}). Overall, hydrogen demonstrates the best long-term potential among the alternatives, based on both impact indicators evaluated in this study.

The GWP and AP of six alternative fuels in the transport sector are evaluated and compared with three conventional fuels across both 2025 and 2050 scenarios to identify the optimal options for the short- and long-term decision-making. In the 2025 scenario, CNG comes as the most favourable choice due to its simple production technology and cleaner combustion properties with a GWP of 65.27 gCO_{2 eq} and an AP of 0.0549 gSO_{2 eq}. By 2050, gaseous hydrogen appears to be the least polluting alternative fuel, achieving a significantly lower GWP of 33.54 gCO_{2 eq} and an AP of 0.0381 gSO_{2 eq}. In both scenarios, bioethanol, particularly with the advancement of second-generation technologies, and synthetic e-fuels are strong contenders for the short- and long-term sustainability, respectively.

While this study identifies promising low-emission alternatives, it does not deeply analyse conventional fossil fuels in the future scenario or include the full life cycle of vehicles using these fuels. Future research should expand the scope by incorporating a broader range of alternative fuels (e.g. liquid petroleum gas, dimethyl-ether or biobutanol), assessing vehicle life cycles, and integrating additional environmental and economic indicators. Further investigation into the techno-economic feasibility of emerging fuel technologies and the development of comprehensive life cycle inventories would provide valuable insights for policymakers and industry stakeholders, supporting the transition to a more sustainable transportation sector.