Emission Reductions in the Aviation Sector: A Systematic Review of the Sustainability Impacts of Modal Shifts

Abstract

In the aviation industry, momentum for reducing emissions has rapidly increased in recent years. From international systems like the EU ETS and CORSIA, to the introduction of new fuels such as electricity and SAF as alternatives to conventional fuels, various approaches are being considered. Within this context, there is a further movement to reduce aviation emissions through a modal shift from air to high-speed rail. In this research, a Systematic Literature Review is undertaken to detail the nature of the modal shift from air to rail, uncovering energy policy and economic considerations. While research targeting China has increased recently, prior studies focus on Europe, leaving some regions understudied. From an emissions reduction perspective, the power source supplying rail is a critical factor. Capacity constraints on rail are also a key challenge to be addressed. Future research should address the need for additional regional studies. In the age of modal shift movements, the aviation industry is attempting to reduce emissions through the introduction of alternative low-carbon fuels. Policies to reduce emissions must consider this. Discontinuing flights could lead to unintended emissions. A synergistic approach combining modal shift and internal decarbonization is likely to be the most economically feasible and sustainable approach.

1. Introduction

The Paris Agreement states that global efforts are required to limit the increase in global temperatures to 1.5 °C compared to pre-industrial levels [1]. The International Civil Aviation Organization (ICAO) has set a goal of achieving net-zero GHG emissions in aviation by 2050 [2], and decarbonization efforts are being pursued across the entire aviation sector. Six strategies for reducing emissions from the aviation sector include (1) technology options for engine and airframe; (2) operational improvements for navigation; (3) alternative biofuels, synthetic fuels, and liquid hydrogen; (4) technological and operational trade-offs between CO₂ and non-CO₂ effects; (5) market-based offsetting measures; and (6) modal shift to high-speed rail (HSR) [3]. These strategies are identified as two discrete perspectives—Technology and Policy—as detailed in Figure 1.

Figure 1. Strategies for decarbonization in aviation sector (adapted from [3]).

Figure 1. Strategies for decarbonization in aviation sector (adapted from [3]).

As for Technology, innovations include the replacement of conventional fuels with low-emission fuels and the upgrading of engines and aircraft [2,4]. Improvements in operations and the introduction of new air traffic control methods are also cited as technological innovations [5]. Regarding alternative fuels, sustainable aviation fuel (SAF) is projected to contribute 53% of emission reductions in the aviation sector by 2050 and will have the greatest potential impact on emissions reductions overall [6]. Typical fuels that can replace conventional aviation fuels include electricity (batteries), liquid hydrogen, and SAF (via direct substitution). While SAF can directly replace aviation kerosene, electricity and hydrogen have barriers to their deployment. For hydrogen, aircraft design modifications to accommodate hydrogen tanks [7], and the establishment of hydrogen supply systems for hydrogen fuel are prominent barriers [8,9,10]. Battery weight and other deployment issues are also recognized. Unlike electricity and hydrogen, one advantage of SAF is that it can be supplied to aircraft using existing infrastructure facilities currently used for kerosene-based aviation fuel at airports [11]. SAF is also noted for its potential to mitigate price fluctuations and geopolitical risks associated with currently used fossil fuels [12].

On the policy side, there is the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) managed by ICAO and the European Union Emissions Trading System (EU ETS) implemented and operated within the European Union [13]. CORSIA is an international framework targeting international flights [13,14,15]. One criticism of these policies is that they do not target emissions other than CO₂ [10]. The EU ETS covers domestic and international flights within the European Economic Area (EEA), while CORSIA covers international flights outside the EEA [13]. In addition to the EU ETS and CORSIA, it has been pointed out that strengthening domestic policies is necessary to comprehensively reduce aviation-related emissions [15].

Meanwhile, modal shift to HSR represents a decarbonization movement within the transport sector, running parallel to the aviation industry’s own emission reduction efforts. Compared to aviation, rail emits a fraction of the carbon per passenger. Short-haul flights have the highest CO₂ emissions per passenger [16]. Therefore, the IEA’s Net Zero Emissions by 2050 scenario suggests that by maintaining long-haul flights at 2019 levels while shifting short-haul flights to HSR, emissions can be cut by 50% with only a 12% reduction in flight numbers [17]. Indeed, concrete actions include France’s move to ban air routes between cities connected by HSR within 2.5 h [18,19]. Similarly, Austria has committed to reducing 50% of domestic flight emissions by 2030 by eliminating domestic routes connected by HSR within 180 min [18]. However, the environmental performance of HSR depends on the source of its electricity [20], and the environmental advantage of HSR varies depending on the country where it operates. Nevertheless, many studies comparing the environmental performance of HSR and aviation focus on the operational phase, highlighting the importance of environmental assessment through life cycle assessment (LCA) [21].

This study conducts a scoping review focused on emission reductions in the aviation sector, with a systematic literature review specifically evaluating emission reductions achieved through modal shift from air to rail. While existing research extensively covers emission reductions and demand changes from modal shift on specific routes, systematic reviews remain scarce, leaving insights fragmented across individual case studies. Consequently, knowledge for policy formulation and long-term/new infrastructure investment is insufficiently organized. This study aims to systematically synthesize these findings to identify existing research gaps and challenges, thereby indicating concrete policy directions for the future.

Section 2 details the systematic literature review. Section 3 builds on the literature review, discussing carbon reduction activities within the aviation sector. Section 4 presents the Results and Discussion, and Section 5 provides the Conclusions.

2. Literature Review

This study employed the PRISMA methodology to identify prior research to undertake a systematic literature review (SLR). SCOPUS was used as the academic source search engine due to its robustness in journal standards and strong toolset to comprehensively test combinations of and/or search regimes and data export functionality. The potential for omission by using a single academic source search engine is recognized in this study. Search terms targeted “paper title, abstract and keywords” using the query “modal shift” AND (“air” OR “aviation”) AND (“HSR” OR ‘rail’ OR “train”). To focus on modal shift, “modal shift” was included as a required term. “air” OR “aviation” were used to identify the aviation industry, and “HSR” OR ‘rail’ OR “train” were used to identify rail-based transportation options. Based on this regime, 92 papers met the criteria. From these, the publication language was restricted to English, targeting two document types: Articles and Conference papers. To explore the latest trends, we further limited the search to papers published within the last 10 years (n = 35). This cohort was considered for full-text review, and papers mentioning both aviation and HSR were subsequently selected as the subject of review for this paper (n = 30). Snowball sampling was not employed. This review did not employ independent screening or an inter-rater process. It also did not utilize standard risk of bias assessment tools. Instead, papers were selected based on predefined criteria, relying on methodological clarity and data transparency. This point should be noted when interpreting the study’s findings. Following the SLR, a complementary environmental scan is undertaken on decarbonization efforts apart from modal shift, being undertaken within the aviation industry (Figure 2). Further, an included-studies master table with harmonized metrics is included in Appendix A.

2.1. Research Trends

A qualitative analysis of the 30 papers reviewed in this study is conducted. Figure 3 shows their distribution by publication year.

Six papers were published in 2024, followed by nine papers already published by September 2025. Since 2024, this topic has become a particular focus of research, with attention increasingly directed toward this field. The number of papers published prior to 2024 do not exceed 5 papers per year, demonstrating the recent uptick in research in this area.

Over the past decade, Europe has been the most frequently targeted region for quantitative analysis, with Finland being the most frequently studied country within Europe (Figure 4).

As can be seen in the figure, modal shift research in Europe dwarfs that of all other nations combined over the last decade, with China recently increasing focus in this area.

Among the papers included in this review, two papers are review [21,22] and one focuses on model analysis [23]. Empirical analyses, particularly those focusing on Europe, have been conducted consistently since 2019. Publications focusing on China have also increased in recent years, with China and Europe constituting most of the target areas in 2025 (including one paper covering both China and Europe [24];

Table 1. Heatmap of publication year and targeted region.

	Australia	Turkey	Europe	China	Total
2016	1				1
2017		2		1	3
2018	1				1
2019			1		1
2020			1		1
2021			3	1	4
2022			1		1
2023			2		2
2024			5	1	6
2025			5	4	9
Total	2	2	17	7	28

).

This table reflects the rapid development of China’s HSR network [25], which by 2023 contained the world’s longest HSR route length and highest operating speed [26], influencing publication numbers significantly after this date. Table 2 details the longest HSR networks around the world, and the maximum speed on these networks.

Table 2. The top 10 longest HSR network countries and maximum speed [26].

Country	Length (km)	Max Speed (km/h)
China	45,390	350
Spain	3993	300
Japan	3147	320
France	2760	320
Germany	1631	300
Türkiye	1232	300
Finland	1120	220
Italy	921	300
Sweden	895	200
Republic of Korea	874	305
USA	735	240

Table 2. The top 10 longest HSR network countries and maximum speed [26].

Country	Length (km)	Max Speed (km/h)
China	45,390	350
Spain	3993	300
Japan	3147	320
France	2760	320
Germany	1631	300
Türkiye	1232	300
Finland	1120	220
Italy	921	300
Sweden	895	200
Republic of Korea	874	305
USA	735	240

2.2. Economic Perspectives

Within the papers analyzed, regarding economic recommendations for railway companies, it was highlighted that currently, HSR tickets are sold at a fixed price and do not adequately reflect market supply and demand. Therefore, researchers propose introducing “market-driven dynamic pricing” [27]. Conversely, the modal shift to rail following the discontinuation of short-haul flights may reduce competition between airlines and railways, potentially leading to market concentration favoring rail. Therefore, policymakers should implement regulations and measures to promote competition among rail operators, preventing railway companies from raising fares or lowering quality in the market [18,28]. Simultaneously, regulatory deregulation for railways offers HSR network operators the potential to enhance profitability through low-cost, high-capacity operations. In such a scenario, airlines—which face fewer opportunities for regulatory innovation compared to railways—are expected to face significant pressure [24]. It is necessary to mitigate the risk of eliminating competition between intermodal options [29]. Measures are required to minimize adverse effects on consumers, such as implementing pricing mechanisms aligned with actual supply and demand [27]. Consideration must also be given to the massive investment and environmental costs associated with HSR construction. Construction should only be considered in areas where sufficient traffic volume is projected to absorb these costs [30]. Indeed, the time savings from the newly opened Berlin–Munich HSR line may have been offset by low-cost carrier fares, despite reducing air travel’s relative time advantage [31]. For some routes, night sleeper trains are mentioned as a potential alternative to air travel [30].

Economic policy instruments to encourage modal shift include carbon taxes, aviation taxes, and Tradable Mobility Credits (TMC) systems. Analyses of carbon tax implementation cover both passenger and freight sectors exist, and, for passengers, an analysis was conducted covering all domestic air and HSR routes in China. It was estimated that imposing a carbon tax of $35/tCO₂ could reduce aviation-related carbon emissions by 6 million tons per year [32]. A scenario analysis was conducted on the effectiveness of a government carbon tax on freight transport. The effectiveness of the carbon tax is said to depend on two factors: the sensitivity of shippers to the carbon tax and the strength of the carbon tax itself. Policy recommendations have also been made to invest carbon tax revenues in green transportation networks, etc., to lead to further emission reductions [33]. Regarding the introduction of a carbon tax, imposing a carbon tax of CNY 367.03/tCO₂ to curb new CO₂ emissions associated with a 3% increase in transport demand could promote a modal shift to HSR for freight transport and reduce air freight transport by 5.02% [34].

An aviation tax is proposed as a means to curb demand growth in the aviation sector [35]. However, imposing a £20 aviation tax on the London–Amsterdam route was found to only be able to reduce air passenger numbers by 4.5%, with only 8% of those switching to HSR, while the remainder would cancel their trips, resulting in only moderate effects [36]. The TMC system fixes total emissions by design, while the price varies based on travelers’ choices [37]. Unlike taxation, transactions occur at the market equilibrium price. If a TMC system were implemented in continental Europe, the credit price was estimated at 272 euros/tCO₂. This would result in a 9% modal shift to rail, with 8% of that shift originating from air travel [37].

2.3. Environmental Perspectives

The origin of the concept of a modal shift is also a critical factor in calculating carbon reduction effects. Scenario analysis undertaken in Turkey indicated that if a significant modal shift occurs from air travel to HSR, the carbon reduction effect from this modal shift increases substantially [38]. The importance of environmental assessment in LCA is also highlighted in a review [21], which advocates for the advancement of LCA analysis in environmental evaluation. Two studies analyzing the modal shift from short-haul air travel between Sydney and Melbourne to HSR, which builds upon and further develops the work of [39], evaluated CO₂ emission reductions based on the life-cycle perspective [39,40]. While CO₂ emissions over the life cycle depend on infrastructure requirements, CO₂ emissions from HSR are equivalent to or lower than those from aviation, demonstrating the potential for emission reductions [39]. Specifically, the introduction of renewable energy and achieving a 60% modal shift could result in significant emission reductions according to LCA analyses [40]. In these analyses, functional units included emission rates of transport mode per passenger kilometer [21], grams of CO₂ emitted by passenger kilometers traveled (GCO₂/PKT) [38], and passenger kilometers traveled multiplied by trip distance to provide CO₂ load for city pair journeys [39,40]. Electricity mixes are based on national electricity grids in each case, and system boundaries consider construction, production, operation, maintenance, and end-of-life treatments [21], and cradle to grave [39,40] scopes.

A study using Chinese data from 1993 to 2012 revealed a negative correlation between railway speed improvements and air travel demand. However, this relationship between railway speed and air travel demand is only significant for city pairs within 1100 km [41]. Meanwhile, examining the relationship between China’s HSR and aviation over the 10-year period from 2009 to 2019, HSR introduction led to a reduction in airline seats in the medium-distance (600–1400 km) aviation market. Short-distance routes under 600 km and long-distance routes over 1400 km were unaffected by HSR entry in terms of airline seat capacity [42]. Furthermore, modeling of Europe’s high-speed rail network indicates that achieving high-frequency operations even on cross-border routes of 1000–2000 km could double HSR’s mode share for long-distance travel exceeding 200 km [43]. These findings contrast somewhat with analyses focused on Finland, where studies based on door-to-door travel time set the advantage of HSR at −400 km [16,30].

While modal shift is widely discussed as one means of reducing aviation emissions, there is relatively little research that also considers reducing emissions from the aircraft fuel perspective. The production of low carbon fuels is key to aviation decarbonization, and switching fuels away from fossil fuels is one factor that can advance decarbonization significantly [35]. Research acknowledges that SAFs are not yet widely adopted and mentions the potential for HSR to reduce emissions before SAFs become widely available [42]. Scenario analyses in Australia also employed scenarios considering SAF blending [40]. Studies focusing on Finland, particularly those using the first generation electric aircraft (FGEA) [44,45], are analyses based on the assumption of electrically powered aircraft. While they require modifications to existing aircraft designs, they hold potential for reducing emissions.

Limitations to emission reductions through modal shift have also been pointed out. Modal shift itself faces limitations in achieving further energy savings and CO₂ emission reductions due to the capacity constraints of HSR [27]. Therefore, to enhance rail transport capacity, railway companies require organizational measures and investments from businesses [18], along with complementary measures for rail infrastructure [29]. Furthermore, with the discontinuation of short-haul flights, an increase in flights to major hub airports, as noted in [46], and a rise in transfer passengers, accurately measuring emissions from origin to destination becomes difficult. A similar reduction was observed in Austria, where approximately 25% of transfer passengers rerouted via Frankfurt following the suspension of flights between Linz and Vienna [47].

2.4. Social Perspectives

When discussing modal shifts from air travel to HSR, analyzing user attributes is also a crucial aspect. Analysis focused on Germany indicated that a 1% increase in rail speed reduces air travel demand, with business travelers experiencing a decline exceeding 2% [48]. This pattern is also observed between London and Paris, where business travelers place high value on service frequency, travel time, and punctuality [28]. Furthermore, while limited to academic travel, analysis suggests the greatest reduction potential lies in combining modal shift with restrictions on air travel distance. This is because intercontinental flights account for over 64% of GHG emissions, whereas the reduction effect from modal shifts to rail for intra-continental and domestic flights is limited [49]. On the other hand, leisure travelers exhibit high price sensitivity [28,36], making the analysis of usage attributes an essential component of demand analysis when discussing modal shift. Furthermore, face-to-face surveys in Turkey revealed that most HSR users prioritize travel time, safety, and punctuality, with the majority preferring economy class or having no preference [50].

Furthermore, policy decisions should consider door-to-door travel time when analyzing the required time for different transportation modes [30]. When comparing using real travel time (RTT) after introducing FGEA, rail maintains an advantage over non high-speed rail (non-HSR) up to 170 km and over HSR up to 400 km [30,45]. However, the FGEA considered here is a small aircraft accommodating only 9–19 passengers. While not as large as a commercial aircraft, it has the potential to improve accessibility and traveler flexibility through new route development [44], but its passenger capacity per flight is limited. Meanwhile, the new HSR line between Berlin and Munich opened in 2017, reducing travel time from 6 to 4 h. Although this did not affect the total number of air passengers between these cities, the proportion of low-cost carrier (LCC) users increased significantly [31].

Routes targeted for the elimination of short-haul flights, a topic under discussion in Europe, should be carefully selected, considering factors like reduced regional accessibility for the affected areas. The impact on airlines is expected to be greater for full-service carriers (FSCs) than for LCCs. This is because the routes under discussion are concentrated in regions already enjoying high levels of accessibility, and FSCs have a stronger presence at major airports [18]. However, it is also pointed out that since rail infrastructure capacity is also limited, the discontinuation of short-haul flights should be aligned with rail infrastructure capacity [29]. In contrast to policy-driven discontinuation of short-haul flights based on travel time considerations, analyses avoiding market intervention also reveal that HSR introduction can reduce demand for short-haul flights by allowing the market to autonomously regulate and identify redundant services [23].

3. Decarbonization Efforts in the Aviation Sector

Cognizant of modal shift issues and the potential for HSR to reduce carbon emissions compared to short-haul travel. This section details the decarbonization efforts being undertaken within the aviation sector, focusing on both macro and micro level policy factors, and technological issues associated with carbon neutral fuels.

3.1. Macro Level

At the macro level, toward decarbonization, CORSIA and EU ETS are taking a major role. However, despite their implementation, the EU ETS and CORSIA face multiple challenges. First, CORSIA does not cover domestic flights [13,15] and the EU ETS only covers aircraft operators that fly to and from the EU [51]. It was suggested that the EU ETS only contributed to domestic aviation emissions and had no impact on emissions from international aviation [52]. As detailed in [13], there are some cases in which routes are not subject to CORSIA, meaning that although overall volumes of air travel are high, the effect of the scheme may be diluted.

Moreover, it has also been highlighted that CORSIA and the EU ETS do not cover emissions other than CO₂, such as NOx and SOx. Based on this shortcoming, the EU decided to introduce a monitoring, reporting and verification system for non-CO₂ emissions [53]. Second, it has been pointed out that the impact of the EU ETS on airlines is reduced in two ways: the first issue is the proportion of emission allowances distributed to airlines free of charge, and the second is the airlines’ abatement effort cost. The former is likely to reduce the incentive to trade emission allowances on the market, while the latter will require airlines to pay higher costs, which could reduce their profits. It is also important to note that airlines are likely aware that policymakers will take measures to modify the proportion of emission allowances traded in the future [54].

In addition, the EU ETS has failed to suppress demand for passenger traffic, and it was found that alternative approaches such as higher aviation taxes imposed in Germany and Austria have reduced demand more effectively than the EU ETS [55]. It has been shown that the EU ETS has had a political effect on short-haul flight cost competitiveness between low-cost carriers and trains. Furthermore, it was also suggested that investing in rail-based travel is a key complementary measure for the EU ETS scheme [56].

Lastly, it was also identified that changing aircraft size according to demand is one way to reduce CO₂, and this has had the impact of directly encouraging the expansion of aircraft sizes for airline operators under the EU ETS [56].

Given these limitations, it has also been argued that CORSIA or the EU ETS alone are not sufficient as reduction mechanisms, and that there is a need to introduce complementary policies that will bring about more rapid results (e.g., a tax on jet fuels or quota obligations for biofuels, etc.) [10].

Although there are some challenges in implementation, the overall approach toward reducing aviation sector emissions is currently housed in the combination of the EU ETS and CORSIA [13] with further policy measures being proposed such as distance-based tariffs and bio-fuel blend ratios to strengthen these measures over time [10].

3.2. Micro Level

Following the macro-level analysis, an investigation of micro-level aspects is undertaken. The micro-level decarbonization efforts in the aviation industry are underpinned by primarily centered in Europe, such as the social phenomenon of “flight shame”.

Since 2019, the concept of “Flight Shame” has emerged as a global trend, referring to the anxiety about the impact of air travel on climate change, indicating a change in social norms, where flying has been seen as a symbol of social status [57]. Also, according to [57], there was no significant change in travel behavior in spite of the decline in domestic air transport demand. It was also shown that there is support for policies that impose emission reductions on airlines and subsequently increase the cost of using air travel through market principles. In contrast, [58] points out the disconnect between the decarbonization and low-cost airfares and the norm of air travel shaped by airline and aviation stakeholders. While it is noted that the flight shame argument should be seen as a part of supporting a change in social norms, people overestimate the carbon footprint of air transport and underestimate the efforts to reduce these carbon footprints, as noted in [59]. Flight shame can be considered as a linked concept with, and a catalyst for, modal shift efforts at the personal and systemic level.

3.3. Fuels

The three fuel types currently under consideration in the aviation industry include electricity, hydrogen and SAF. The benefits and limitations of each are discussed here.

3.3.1. Electricity

First generation narrow-body aircraft powered by electricity were found to be uncompetitive with jet engines in terms of cost. Electric aircraft will only realize a cost advantage if the price of jet fuel increases or the price of electricity decreases to below 4–6 cents per kilowatt hour (kWh) [60]. Moreover, electricity production is required to increase by 26% to electrify commercial aviation using ground power [61]. In order to replace existing energy sources with electricity and achieve zero emissions, it is necessary to generate all of this new electricity exclusively from RE sources [44]. Some research exists, proposing a power source change for aircraft from liquid fuels to batteries, but due to the low energy density of current battery technologies, it may be difficult for battery-based aircraft to replace existing large commercial aircraft in the near future [8]. Even if energy density were to improve by several hundred percent, the CO₂ reduction achievable through this fuel source conversion would amount to only 5% overall [61]. It is estimated that it will take decades for technology to develop, such as new aircraft designs, technological advances, decarbonization of conventional power generation [60], and the price of batteries [62] toward electric aircraft having an impact on the aviation industry. From an economic perspective, there is a need to create a model of the operating costs of electric aircraft and to quantify the impacts of factors such as the operational reliability and maintenance costs of these aircraft compared to conventional and other proposed approaches [62]. In addition, Ref. [63] compares the effects of jet fuel tax, electricity subsidies, seat tax, and fleet restriction policies on the adoption rate of electric aircraft. For seat taxes, electric aircraft require a tax rate equivalent to 30% of the average unit cost to be competitive.

From a global aviation perspective, more than 90% of GHG emissions come from commercial aircraft carrying 100 or more passengers. Focusing on technologies that can be applied to these aircraft is likely to be the most effective approach [64]. Electric propulsion may offer emission reduction benefits, but its implementation is most likely to occur first in urban air taxis, regional jets, and business jets [64]. It is envisioned that emissions can be reduced by transitioning small aircraft to fully electric power and large aircraft to a hybrid system combining SAF and clean electricity [6].

3.3.2. Hydrogen

Compared to conventional aircraft fuel, aircraft that use hydrogen (H₂) were found to be better for the environment [65], with specific regional studies confirming this finding [66].

However, in order to use hydrogen as an aviation fuel, it is necessary to design and build new aircraft [6,7,15] and to establish a hydrogen production and distribution system. It has been estimated that it will take several decades to develop these complementary aspects of aviation infrastructure [10]. With regard to changes in aircraft design, the energy per unit mass of LH₂ is three times that of conventional kerosene; however, the low volume energy density will heavily influence aircraft shape [6]. This would lead to smaller cabin sizes and a decrease in the number of passengers that can be carried (see Figure 5) [67].

From the supply side, according to [6,68], AIRBUS is working on developing engines and liquid hydrogen (LH₂) tanks, etc., while also investigating the requirements for hydrogen ground facilities, under a project called the “Hydrogen Hub At Airports”, and working with the aviation industry to develop H₂ infrastructure. According to [69], at Hamburg Airport, 60% of the aircraft taking off from the airport in 2050 could be powered by hydrogen, reducing CO₂ emissions by 0.5 million tons. However, according to [70], it would take several hours to refuel (supply LH₂) for long-haul flights using current technology, which is not comparable to current refueling facilities’ ability to refuel aircraft. In addition, other externalities, including production costs also need to be taken in to account when considering the supply of H₂ [9]. This point was also raised in [8], which mentioned the cost of developing H₂ infrastructure as a major stumbling block. Although the efficiency of hydrogen fuel aircraft has improved, the development of hydrogen fuel aircraft has been delayed due to the high cost of hydrogen production using renewable energy, insufficient hydrogen fuel infrastructure, and the need for hydrogen storage and aircraft configuration changes [71]. To this end, it is necessary to build a hydrogen supply chain and propose the need for policymakers and stakeholders to engage in the development of green hydrogen production technologies.

According to [72], Japan Air Lines (JAL) which has signed an agreement with a hydrogen-electric aircraft and engine manufacturer, is hoping to promote research into hydrogen-electric aircraft and raise awareness of hydrogen-electric aircraft in Japan. However, at present, the performance of hydrogen-electric aircraft is only at the level of an ~80 passenger class aircraft and requires further development before reaching the size of current aircraft or replacing major routes. Currently, all-electric aircraft are 25–50% heavier than conventional kerosene-powered aircraft. By improving both fuel cell technology and storage capacity, it may be possible to achieve the goal of no weight change compared to conventional aircraft. In the medium term, it is possible that aircraft will be designed with hybrid engines combining current engines and hydrogen engines, but at present, the realization of hydrogen based all-electric aircraft appears difficult [73].

On the other hand, the energy efficiency of aircraft that use hydrogen as a fuel varies depending on the cruising range. While energy efficiency improves by 12% for long distance flights, for small aircraft in the short distance (<4000 km) class, the reduction in fuel weight due to the switch to hydrogen is small, so overall energy efficiency is poor [74]. The cross-over range, where the energy efficiency of an aircraft using hydrogen as a fuel is equivalent to that of conventional kerosene, is 5000–7000 nautical miles (NM) [74]. In addition to emitting only water vapor, another advantage is that there is less weight change during flight compared to conventional aircraft. On the other hand, weight increase during landing is a concern compared to conventional aircraft [75].

Finally, hydrogen significantly reduces pollutants in exhaust gases, but the method of hydrogen fuel production is also important, and if hydrogen produced by fossil fuels is used, emissions would be higher than for current kerosene-based aircraft, creating a situation that does not mitigate climate change [65]. There are also some issues with hydrogen storage and combustion when used as a fuel. Designing and constructing a new aircraft is a particularly significant issue. From the economic perspective, it was identified that for H₂ to be prioritized as an aircraft fuel, the carbon trading price would need to be more than 12 times the EU ETS price in 2022, and the price difference with existing fuels and the carbon trading price are important factors in the adoption of alternative fuels [66].

3.3.3. Sustainable Aviation Fuel (SAF)

SAF is expected to play a major role in reaching net zero emissions in the aviation industry by 2050 [76], and it was identified that the overwhelming benefit of SAF is that it can be supplied to aircraft using the infrastructure facilities currently used for conventional aircraft fuel [11]. AIRBUS, for example, set a target of mixing 15% SAF into the fuel used for flight operations and customer deliveries by 2024 [77].

Several studies have considered the life cycle impacts of SAF [78,79] and, compared to traditional aviation fuels, have identified the potential for a reduction in emissions of between 76 and 97 percent depending on the origin of the SAF, with cooking oil-derived SAF showing the best performance [79]. Combining LCA and techno-economic analysis, it was demonstrated that the cost of SAF could be superior to fossil derived fuels via the processes of pyrolysis and hydroprocessed esters and fatty acids (HEFA), with HEFA showing the potential for environmental and economic co-benefits [80]. Although technically distinct from biofuels, e-fuels are also a candidate for future aviation fuels as shown in Figure 6.

Figure 6. The pathway to SAF (adapted from [81]).

Figure 6. The pathway to SAF (adapted from [81]).

The Power to Liquid (PtL) pathway holds potential as a long-term solution. A key advantage is that the CO₂ used in fuel production via PtL offsets the CO₂ emitted during combustion [82]. However, this requires increasing the production of renewable hydrogen, which necessitates further research and development [83]. Additionally, future challenges for PtL include its higher cost compared to other biofuels [80] and the potential for competition with renewable energy sources for electricity consumption [82]. Furthermore, the low efficiency of the PtL conversion process [84,85] also poses a challenge for the future expansion of PtL. Table 3 details the merits and challenges of each technological approach.

Table 3. Aviation fuel technological merits, sources and anticipated future challenges.

Technology	Merit	Sources	Future Challenges	References
HEFA	High energy content Less CO2 Mature production process	Used cooking oil Waste animal fats	Production cost Limitation of material Low percentage of hydrocarbon	[82,86,87,88]
FT process	Mature production process Cost intensive Low conversion related emission	Biomass Coal Natural gas	Technology and running cost conversion efficiency	[8,86,87,88]
AtJ	Less CO2 Similar energy density with conventional fuel	Agricultural waste products	production cost conversion efficiency	[78,86,88,89]
PtL	Closed carbon cycle	Electricity Water CO2	Higher cost Competition with the electricity consumption Low conversion efficiency	[80,82,83,84,90]

Table 3. Aviation fuel technological merits, sources and anticipated future challenges.

Technology	Merit	Sources	Future Challenges	References
HEFA	High energy content Less CO2 Mature production process	Used cooking oil Waste animal fats	Production cost Limitation of material Low percentage of hydrocarbon	[82,86,87,88]
FT process	Mature production process Cost intensive Low conversion related emission	Biomass Coal Natural gas	Technology and running cost conversion efficiency	[8,86,87,88]
AtJ	Less CO2 Similar energy density with conventional fuel	Agricultural waste products	production cost conversion efficiency	[78,86,88,89]
PtL	Closed carbon cycle	Electricity Water CO2	Higher cost Competition with the electricity consumption Low conversion efficiency	[80,82,83,84,90]

In the short term, the primary factor influencing SAF adoption by airlines is the supply cap [91]. In terms of the potential supply availability of SAF, a study addressed the use of specific crops (Pennycress, Camelina and Carinata) in the US, finding that SAF could be produced at competitive price levels by the year 2048; however, this contribution would likely only account for about 3% of the US SAF target [92]. The high manufacturing costs of SAF, coupled with the larger price gap between SAF and fossil fuels in the aviation sector compared to the price gap between road transport fuels and low-carbon/renewable fuels, pose challenges for the adoption of SAF as jet fuel [93]. Unlike road fuels, aviation fuels receive tax incentives, leading to intensified competition between conventional fuels and SAF [93]. Many airlines procure aviation fuels at prices lower than market prices [94]. Further, it was shown that if measures to narrow the price gap between SAF and conventional fuels, such as the injection of public funds, are insufficient, the price will rise, and consumers will incur these price increases. It was also pointed out that in order to promote the optimization of SAF-related technology as the use of SAF continues to expand, cooperation between governments and the aviation industry is essential in terms of efficiency and toward reducing production costs [88].

On the demand side, it was confirmed through simulations undertaken in the United States that implementing a policy similar to CORSIA could stimulate demand for SAF and reduce CO₂ emissions [95]. Therefore, to promote the spread of SAF, it is necessary to introduce incentives for adopting SAF and to establish penalties for GHG emissions through a carbon tax system [96]. A study on the interplay between government policy, carbon trading mechanisms and the development of SAF has shown that dynamic subsidy policies may be effective. They may promote the development of SAF via the incorporation of both subsidies and penalties in the aviation industry [97].

4. Results and Discussion

4.1. Overview of Current Research Landscape

The systematic literature review identified a growing global focus on the modal shift from aviation to HSR as a decarbonization strategy, with the number of studies in this field rising markedly after 2019. The research landscape is geographically concentrated in Europe and China, reflecting their leadership in both HSR infrastructure and transport decarbonization policies [27,28]. Europe’s long history of integrated transport policy and China’s rapid HSR network expansion have underpinned these regions’ focus on modal shift analysis.

Beyond Europe and China, comparative evidence from other Asian regions remains notably scarce. Japan, despite having one of the world’s most mature high-speed rail systems, has received limited quantitative assessment of its potential to substitute short-haul aviation [18,44]. Similarly, Republic of Korea’s integrated transport network and India’s emerging HSR corridors offer valuable contexts for testing the transferability of European and Chinese modal shift findings. Expanding empirical research to these regions would provide essential insights into how institutional structures and travel cultures influence decarbonization potential across Asia [18,29].

The literature also reveals a strong emphasis on empirical and scenario-based modeling studies, reflecting the maturity of modal shift research as a quantitative discipline. However, most of these studies evaluate idealized conditions rather than ex-post assessments of actual policy interventions [46]. This highlights a methodological gap: few studies assess the real-world performance of modal shift policies or capture the behavioral and institutional complexities influencing their effectiveness [22,30].

4.2. Economic Implications of Modal Shift and Aviation Decarbonization

From an economic perspective, modal shift and aviation decarbonization are both strongly shaped by pricing dynamics, regulatory frameworks, and market structures [14,27]. The literature advocates for market-driven dynamic pricing in HSR to improve competitiveness and reflect supply-demand balance [28]. However, this must be balanced against risks of monopolization if short-haul flight bans or capacity restrictions are imposed without ensuring fair intermodal competition [27].

Complementary policy instruments such as carbon taxes, aviation levies, emissions trading schemes (ETS), and TMCs emerge as critical yet unevenly effective tools for influencing travel behavior [18,29,46]. Scenario analyses indicate that a carbon tax of approximately $35/tCO₂ could reduce aviation emissions by several million tons annually, while a TMC system priced at €272/tCO₂ could shift up to 9% of passengers from air to rail [18,29]. Nonetheless, aviation taxes alone often result in trip cancelations rather than substitution, suggesting that multimodal integration is essential to achieve sustained behavioral change [57].

This finding aligns with broader macro-level decarbonization frameworks identified in the environmental scan, particularly the EU ETS and CORSIA [14,56]. Both represent market-based instruments for internalizing the cost of carbon yet suffer from incomplete coverage. CORSIA applies only to international flights, leaving domestic routes untreated, alongside issues of free allowance allocation and exclusion of non-CO₂ emissions [56,57]. Consequently, the EU ETS has had only marginal effects on aviation demand and carbon intensity [14]. These limitations underscore that carbon pricing mechanisms, while foundational, are insufficient alone to deliver deep decarbonization or substantial modal shift.

The literature also highlights that policy efforts to discontinue specific routes based on travel time or distance to promote modal shift can be inconsistent [27,28]. In several cases, the opening of new HSR lines failed to generate the expected modal shift, illustrating that price, convenience, and user attributes such as time valuation and trip purpose are decisive [28,43]. Understanding these user characteristics is central to designing effective economic instruments, including carbon taxes or TMC systems [18,29].

Investment allocation is another key challenge. Rail capacity constraints limit the feasibility of absorbing passengers displaced from short-haul air routes. Increasing train frequency can mitigate these effects, but only if the necessary infrastructure exists [30,46]. The literature emphasizes the importance of removing bottlenecks in the short term while pursuing long-term investments in HSR and cross-border interoperability [43,44]. Investments must prioritize high-demand, high-capacity routes, ensuring that carbon revenues, whether from taxes, ETS, or offset schemes, are reinvested in low-carbon transport infrastructure such as electrified rail lines and airport-rail integration [38,40,42].

Overall, economic policy must evolve from isolated instruments toward integrated multimodal carbon pricing frameworks that encourage behavioral shifts across the transport sector and align infrastructure financing with decarbonization objectives [18,29].

4.3. Environmental Synergies and Limitations

From an environmental standpoint, both the literature review and environmental scan confirmed that modal shift provides clear CO₂ reductions for routes under approximately 1100 km, where rail offers a practical substitute for air travel [32,38]. When powered by renewable electricity, HSR demonstrates substantially lower life-cycle emissions than aviation, with benefits amplified under high utilization scenarios [35,40]. These results reinforce the environmental rationale for promoting modal shift as part of near-term decarbonization strategies [42].

Across the reviewed literature, the substitution potential of HSR over aviation converges within specific distance and travel-time ranges. Empirical studies in Europe, China, and Finland consistently identify a break-even threshold of roughly 400–600 km (≈2–3 h door-to-door) for short-haul routes, beyond which aviation regains a time advantage. Medium-distance corridors of 600–1100 km (≈3–5 h) exhibit the greatest elasticity in demand, where both carbon and time savings favor rail when supported by high service frequency. Long-haul segments above 1400 km (≈5 h or more) show minimal substitution potential under current infrastructure. Synthesizing these results suggests a mean crossover point near 800 ± 300 km (≈4 h travel time) at which HSR’s environmental benefits and competitive journey time intersect, as inferred from [30,32,38,41,42,44,45]. Quantitatively, this range encompasses about 65–70% of intra-European air routes, implying substantial decarbonization potential if modal shift policies target this distance band. A quantitative summary of these thresholds is shown in Table 4.

Table 4. Summary of Distance and Travel Time Thresholds for Modal Shift Competitiveness between Aviation and Rail.

Distance Band	Approx. Travel Time	HSR Advantage Status	References
<170 km	<1.5 h	Non-HSR > FGEA	[30,44]
170–400 km	≈2 h	HSR > FGEA	[44,45]
400–1100 km	≈2–5 h	HSR > Air	[32,38,41]
>1400 km	>5 h	Air > HSR	[42]

Table 4. Summary of Distance and Travel Time Thresholds for Modal Shift Competitiveness between Aviation and Rail.

Distance Band	Approx. Travel Time	HSR Advantage Status	References
<170 km	<1.5 h	Non-HSR > FGEA	[30,44]
170–400 km	≈2 h	HSR > FGEA	[44,45]
400–1100 km	≈2–5 h	HSR > Air	[32,38,41]
>1400 km	>5 h	Air > HSR	[42]

However, structural limitations remain. Capacity constraints, infrastructure construction emissions, and potential rebound effects such as increased hub-and-spoke flight operations following short-haul bans may offset overall gains [75,76]. Eliminating short-haul flights without controlling airport slot allocation may unintentionally free capacity for long-haul routes, thereby increasing total emissions [18,47]. Thus, decarbonization frameworks must incorporate measures such as total emissions caps at airports or sectoral TMC systems to prevent unintended increases [18,76]. As any additional travel induced by decreases in prices incurs additional emissions (separate to those reduced by the modal shift itself), pricing regimes need to be monitored as one key performance indicator among total emission reductions (minus those from additional demand), speed, and reliability for all considered travel corridors. In a study by CE Delft considering the shift from road transport (including freight) from road to rail, this rebound effect was found to have an impact in the order of approximately one-quarter of avoided emissions [98].

Comparative analyses also indicate that LCA methodologies are essential for accurate environmental evaluation [35,69]. Yet, data availability challenges often result in studies focusing solely on operational emissions, omitting upstream impacts of energy generation or infrastructure [40]. As electricity supply decarbonizes at different rates across countries, the true comparative advantage of HSR depends on national energy mixes [32,38].

In parallel, technological innovations in aviation including electric aircraft, hydrogen propulsion, and SAF present complementary pathways for emission reduction [23,48,65,94,97]. SAFs offer the advantage of compatibility with existing infrastructure and can be deployed in the near term, though they face feedstock constraints, high production costs, and limited supply chains [11,46]. Hydrogen and electric aircraft promise deep long-term reductions but require extensive redesign of aircraft, large-scale industrial investment, and sustained political support [65,94,97].

Therefore, modal shift and SAF adoption should be seen as sequential, synergistic measures: modal shift delivers immediate reductions in short-haul segments, while SAF, hydrogen, and electric aircraft target medium- and long-haul emissions over longer horizons [11,48]. Governments should adopt integrated decarbonization roadmaps combining short-term modal shift incentives with long-term fuel and technology transitions, supported by harmonized LCA-based accounting [23,42].

4.4. Social and Behavioral Dimensions

Social and behavioral factors also play a decisive role in determining the success of modal shift and aviation decarbonization efforts. The literature identifies significant variation in responsiveness by traveler type: business travelers prioritize punctuality and total travel time, while leisure travelers are more price sensitive [27,43]. This segmentation implies that dynamic pricing, service reliability, and travel time reduction are key to maximizing carbon-saving modal shift [28].

The rise of the “flight shame” movement reflects a growing social norm of climate-conscious travel behavior, originating in Europe but increasingly recognized globally. While awareness of aviation’s environmental impact has increased, studies emphasize that awareness alone is insufficient to produce behavioral change unless viable, affordable, and convenient alternatives exist [27,38,40,42].

At the micro level, emissions from frequent flyers who account for a disproportionate share of aviation emissions remain largely unaddressed by current climate measures. Targeted interventions, such as differentiated carbon pricing or loyalty program reform, could more effectively influence this small but impactful group [57].

Policymakers can amplify behavioral shifts through coordinated public campaigns, transparent carbon labeling of travel modes, and incentives for sustainable business travel. Door-to-door intermodal integration (e.g., rail-air ticketing, synchronized schedules) is equally critical to enabling passengers to act on environmental preferences [38,44]. Together, social norms and behavioral policies must be designed to reinforce structural decarbonization measures rather than relying on moral appeal and emerging social norms alone [6,7,15,60,67].

4.5. Integration of Modal Shift and Aviation Decarbonization Strategies

Synthesizing the findings from both the literature review and environmental scan reveals that modal shift and aviation decarbonization are complementary, not competing, pathways toward net-zero transport. Modal shift reduces emissions in short-haul aviation through substitution by electrified rail, while SAF, hydrogen, and electric aircraft address medium- and long-haul routes where rail is not viable. Both require strong policy coordination, carbon pricing, and cross-sectoral governance [14,18,56].

An optimal policy trajectory can be conceived as a phased, multimodal decarbonization roadmap:

➢

Short term (2025–2035): Promote modal shift through short-haul flight restrictions, dynamic HSR pricing, integrated ticketing systems, and reinvestment of aviation carbon revenues into HSR capacity and electrification.

➢

Medium term (2035–2045): Scale up SAF production via blending mandates, lifecycle-based emission standards, and harmonized international certification schemes. Strengthen cross-border rail interoperability and expand renewable electricity supply for rail systems.

➢

Long term (beyond 2045): Deploy hydrogen and electric aircraft where technologically and economically viable, supported by a fully renewable-powered HSR network. Integrate TMC systems and airport-level emission ceilings to ensure system-level net-zero alignment.

Finally, future research and policy efforts should prioritize comprehensive, regionally diverse analyses including underrepresented Asian contexts and adopt a lifecycle perspective that considers evolving technological and social dynamics. Coordinated multimodal policy frameworks that align aviation and rail strategies will be indispensable in achieving a just and effective transition to carbon-neutral mobility.

5. Conclusions

This study conducted a systematic review of modal shifts from aviation to rail, focusing on emissions reduction, based on a literature survey grounded in the SLR approach, followed by an environmental scan of complementary aviation carbon reduction initiatives. At the macro level, it clarified the current status and limitations of international frameworks such as ICAO’s CORSIA and the EU ETS. At the micro level, individual behavior and social norms toward aviation, such as flight shame, as well as initiatives undertaken by airlines themselves have received increasing social attention. These micro level factors and the underlying social norms driving decarbonization also influence the aviation industry’s decarbonization efforts.

Regarding fuels, new aviation fuel candidates like hydrogen and electricity each have their advantages and future challenges. Among these potential future fuels, it is expected that SAF will play a major role in achieving decarbonization in the aviation sector in the future [76]. Further research is needed on the construction of a supply chain for SAF production and distribution, as well as a continuation of the work on the LCA of SAF considering a broad range of processes and feedstocks.

If SAF and other alternative fuels reduce emissions compared to current fossil-based aviation fuels from an LCA perspective, the prevailing perception that aircraft are harmful to the environment may be resolved. Technological advances could provide solutions to micro-level challenges. This gap needs to be thoroughly addressed through future research, such that the issues of flight shame and environmental aspects can be considered comprehensively.

Currently, SAF production is expensive and not yet competitive with existing jet fuels, but the growth of the SAF industry depends on appropriate incentives, policies and technological initiatives [12]. It is necessary to encourage both the supply and demand sides of SAF through incentives, and it will be necessary to expand and create domestic policies and international frameworks led by international organizations. This remains a crucial perspective for the aviation industry to develop with achieving decarbonization. In the same way, long-term challenges which hamper the penetration of electric and hydrogen-based solutions need to be addressed.

While modal shift from air to HSR has gained increasing attention in recent years, the degree of this shift varies significantly by region. Notably, much existing research focuses on Europe. Although attention is beginning to turn to China, where rail network development is substantial, the need for analysis in Asia, including Japan, Republic of Korea and India, where high speed rail networks exist or are being rapidly developed, is evident. Each of these nations presents distinct policy environments, infrastructure maturity levels, and energy mixes that could meaningfully shape the outcomes of future modal-shift and aviation decarbonization research. Long-term infrastructure development for the rail network is necessary due to capacity constraints. If short-haul flights are discontinued solely based on time factors such as between cities connected by HSR at specific times, existing HSR services are highly unlikely to fully meet that demand. Decisions on which routes to discontinue should be made after comparing door-to-door travel times and understanding the characteristics of air and HSR users. Policies to promote modal shift are needed, based on this understanding.

The emissions advantage over air travel fluctuates depending on the power source mix for HSR electricity. However, there is limited mention of reducing emissions from aviation fuel itself (specifically, replacing conventional aviation fuel with new aviation fuels like SAF). Moving forward, discussions on modal shift should further develop to consider emission reductions through fuel changes in aviation alongside HSR development. Building on this understanding, advancing modal shift discussions and implementing appropriate policies will enable emission reductions to be achieved in both aviation and rail sectors. Furthermore, in environmental assessments, evaluation based on LCA standards is crucial, and conducting comprehensive emissions assessments not limited to the operational phase remains a key challenge for the future.

Key policy recommendations include integrated carbon pricing, linking aviation and rail within unified carbon markets (e.g., cross-sector ETS) to ensure consistent decarbonization incentives; reinvestment mechanisms, directing revenues from aviation taxes or ETS allowances to expand HSR infrastructure and renewable energy capacity; dynamic intermodal regulation to maintain competition between air and rail while preventing monopolistic outcomes post-flight restrictions; infrastructure planning which aligns short-haul flight reduction policies with verified HSR capacity and door-to-door travel time considerations; technological coordination such that SAF, electric and hydrogen-based fuel research and development are supported in parallel with rail electrification, ensuring balanced progress across transport modes; and behavioral incentives which promote societal awareness of travel-related emissions through carbon labeling and corporate travel policies. Finally, lifecycle accountability is also critical, through the mandating of LCA-based emissions accounting for both aviation fuels and HSR construction to ensure holistic carbon neutrality assessments.

One limitation of this paper is that while the literature review section selected studies partially adhering to PRISMA guidelines to ensure transparency in the scoping review, it did not employ methods such as dual independent screening. This limitation should be noted when interpreting the results of this study.