Liquid Hydrogen: A Mirage or Potent Solution for Aviation’s Climate Woes?

Abstract

The aviation industry faces a formidable challenge to cap its climate impact in the face of continued growth in passengers and freight. Liquid hydrogen (LH₂) is one of the alternative jet fuels under consideration as it does not produce carbon dioxide upon combustion. We conducted a well-to-wake life cycle assessment of CO₂ emissions and non-CO₂ climate change impacts per passenger-distance for 17 different hydrogen production routes, as well as conventional jet fuel and biofuels. Six other environmental and health impact categories were also considered. The Boeing 787-800 was used as the reference aircraft, and a range of flight distances were explored. Contrail cirrus contributes around 81 ± 31% of the combustion climate impacts for LH₂, compared to 32 ± 7% for conventional jet fuel, showing that research is needed to reduce uncertainty in the case of LH₂. The life cycle impacts of the two dominant commercial LH₂ pathways are on average 8 and 121% larger than conventional jet fuel. Some novel LH₂ pathways do show considerable potential for life cycle climate impact reductions versus conventional fuel (up to −205 ± 78%). LH₂ from renewable energy is not climate neutral, though, at best −67 ± 10% compared to conventional over the life cycle.

Short abstract

The potential climate benefit of liquid hydrogen jet fuel versus conventional jet fuel strongly depends on the hydrogen fuel pathway and is sensitive to the estimated difference in contrail cirrus impacts.

Introduction

Emissions and contrail cirrus from jet fuel combustion by civil aviation caused an impact of approximately 1800 Tg CO₂-eq in 2018 when evaluated with 100-year global warming potential (GWP).¹ Considering just the CO₂ emissions, a strong increase in revenue passenger km has outpaced efficiency gains, resulting in an annually averaged growth rate of 5% from 2013 to 2018, up from 2.2% growth p.a. from 1970 to 2012.¹ Aviation has contributed a growing fraction of global anthropogenic CO₂ emissions, reaching 2.4% in 2018.¹ Economists describe flying as a luxury good, whose demand is highly dependent on income, and there is no saturation apparent for the demand for aviation as people get richer. For the super-wealthy, air travel is a large source of greenhouse gas (GHG) emissions, second only to superyachts.²

The civil aviation industry faces considerable pressure to reduce its carbon footprint. In Montreal in 2019, a climate change rally led by Greta Thunberg turned its attention to the International Council on Aviation (ICAO) summit being held there; environmental activists continue to demand substantial improvements.³ Activists campaigned for taxes on private jets and bans on short-haul flights during the 27th Conference of the Parties (COP27) of the United Nations Framework Convention on Climate Change (UNFCCC) in November 2022.⁴ At the prior COP26, 23 nations participating in the inaugural meeting of the International Aviation Climate Ambition Coalition signed a declaration to “advance ambitious actions to reduce aviation CO₂ emissions at a rate consistent with efforts to limit the global average temperature increase to 1.5 °C”.⁵ This comes on top of the mandatory cap for global aviation emissions agreed to at the COP25.⁶

The industry is responding to the pressure through a variety of mechanisms, chiefly technological innovation and carbon offsets. Unlike land-based technologies, aircraft have the obvious but major constraint of needing to carry enough energy to power the entire trip. The volume and weight of the fuel impact the aircraft design and performance. One proposal that has resurged in popularity is the use of hydrogen as an aviation fuel. Research on hydrogen-powered aircraft engines began in 1937, with a variety of efforts occurring chiefly in the United States, Russia, Europe, and Canada in the decades since.⁷ Airbus invested in hydrogen research in the 1990s and 2000s but paused in 2010 citing the insufficient production of environmentally friendly hydrogen.⁸ In 2020, Airbus revealed three ZEROe (“zero-emission”) concept aircraft, intended to run on liquid hydrogen (LH₂) combustion supplemented with hydrogen fuel cells starting in 2035, and introduced the ZEROe demonstrator aircraft in 2022.^9,10 For smaller regional aircraft, Universal Hydrogen is making fuel cell kits to convert existing turboprop aircraft to hydrogen by 2025,¹¹ and ZeroAvia has demonstrated a gaseous hydrogen-electric six-seat aircraft;¹² in this study, we focus on jet aircraft capable of longer distances.

International agencies and research efforts are getting involved as well. A European Commission (EC) study looked at economic considerations and tank-to-thrust environmental attributes of five hydrogen-powered aircraft designs.¹³ ICAO projects LH₂ fuel to comprise only 2% of aviation energy in 2050 but sees potential for growth if the fuel proves viable.¹⁴ Preliminary modeling of future global net-zero scenarios incorporating hydrogen suggests that a much larger share of aviation energy—up to 25%—would be supplied by hydrogen in 2050, representing 7% of the transportation sector’s overall hydrogen demand.¹⁵ These scenarios rely on aggressive decarbonization policies incorporating carbon pricing; otherwise, green LH₂ will likely not be cost-competitive with the dominant natural gas technologies.¹⁵

To realize any relative life cycle environmental benefits of using LH₂ as an aviation fuel, entirely new aircraft designs would need to be produced en masse, and a new fuel supply infrastructure would need to be scaled rapidly. Prior to society making such large-scale investments, a comprehensive comparison should be performed to indicate the conditions under which such a system would be both beneficial and feasible. Like electricity and synthetic fuels, and unlike fossil fuels, hydrogen carries energy generated by another means, with the potential of large emissions in the process of converting primary energy to an energy carrier, in addition to the emissions in primary energy production and fuel transport. Therefore, well-to-pump (WTP) production impacts must be considered carefully when comparing alternative fuel pathways. Note that “well” in this terminology refers to the extraction of fossil fuel resources via a well and is therefore not directly applicable to alternative energy sources; we propose the term “source” as a substitute in future work.

While there are several pathways to create hydrogen using renewable energy, currently, around ³/₄ of dedicated hydrogen production is from natural gas and nearly ¹/₄ from coal, with oil and electricity comprising a minute fraction.¹⁶ Donnelly et al. argue that producing hydrogen from natural gas or hydrocarbon-based electricity would release more CO₂ than burning a hydrocarbon jet fuel.¹⁷ Airbus insisted that hydrogen aviation fuel must be produced from renewable electricity during its introduction of the Zero-E aircraft, and Universal Hydrogen contends that powering single-aisle aircraft by green hydrogen is aviation’s only path to meet Paris Agreement emissions targets.¹¹ Some proponents assume hydrogen produced from renewable energy to be carbon-free,¹³ a position not supported by literature on the impacts of renewable hydrogen production.¹⁸

There are several well-to-wake (WTWa) life cycle assessment (LCA) case studies comparing aviation powered by LH₂ or conventional jet fuel, but none of these are sufficient for decision-making since they do not capture the full extent, uncertainty, and variability of the potential impacts. Koroneos et al. found all six LH₂ options studied to be preferable to conventional jet fuel in the deterministic results of a case study for an A319-100 aircraft.¹⁹ Pereira et al. explored four LH₂ production options in Portugal for six aircraft but only looked at five air pollutants that were converted into costs.²⁰ However, both studies have methods that lack in detail, exclude climate impacts of contrail cirrus, and have meanwhile become outdated. Bicer and Dincer performed an LCA that incorporates the production of the aircraft and airport as well but relied on default ecoinvent life cycle inventory (LCI) data, only included the hydrogen production from steam reforming of natural gas, and provided a single point estimate.²¹ Mukhopadhaya and Rutherford develop estimates of two evolutionary reference aircraft based on the Airbus Zero-E program that can run on green LH₂ (electrolysis from renewable electricity) or blue LH₂ (from fossil fuels with carbon sequestration).²² They do not capture parameter uncertainty nor do they include climate impacts from short-lived pollutants or contrail cirrus. Dray et al. consider green LH₂ as a potential net-zero aviation pathway and choose an optimistic scenario where future green LH₂ production has no climate impacts.²³

The non-CO₂ components of flight emissions could account for up to 90% of the sector’s future climate impact; neglecting them could compromise reaching climate targets.²⁴ Recent studies indicate that contrail cirrus is the largest component of radiative forcing (RF) from flight.^1,25 Depending on the atmospheric conditions along the flight path, the type of fuel combusted, and the engine characteristics, aircraft exhaust can form ice crystals that can create contrails and induce cirrus clouds, also known as aviation-induced cloudiness (AIC).²⁶ There is a balance between the cooling effect of contrail cirrus scattering incoming shortwave radiation from the sun back to space during the day and trapping outgoing longwave radiation from the Earth, overall creating a net warming effect.²⁶ Some individual contrails are warming while others are cooling, and therefore persistent warming contrails should be prioritized for mitigation.

Given the urgency of climate change mitigation, airlines could incorporate strategies to mitigate persistent warming contrail cirrus into their flight management. One strategy slightly diverts flight paths expected to pass through ice supersaturated regions (ISSRs) that are prone to contrail cirrus formation. Accurate real-time modeling will be critical to avoid failed route diversion.²⁷ Teoh et al. estimate that just 12% of flights in the North Atlantic are responsible for 80% of the contrail impact, and modeling by Avila et al. indicates that elevating flight paths in the United States above ISSRs reduced the net RF of the contrail cirrus by 92%.^28,29 Formation flight involving two aircraft flying the same route in close proximity shows promise to reduce contrail cirrus impacts as well as improve fuel efficiency.³⁰

In this study, we focus our investigation on which fuel supply chains and conversion technologies can lower the WTWa climate impact of commercial aircraft. We do not, however, analyze issues of cost or scalability.

Methods

Goal

The goal of this study is therefore to assess whether and under what situations the WTWa climate, ecosystem, and human health impacts of aircraft powered by combustion of alternative jet fuels outperform those of aircraft powered by conventional jet fuel. We explore LH₂ produced by a variety of technologies and feedstocks and compare several biofuels, known as sustainable aviation fuels, since they are another popular mitigation strategy.

We incorporate uncertainty through a stochastic analysis, given the diversity of production pathways for alternative aviation fuels, unknown future conditions, and numerous judgment-based decisions to be made by the LCA practitioner. We provide exploratory comparative statistics and guidance for how to reduce uncertainty.³¹ We performed a Monte Carlo (MC) simulation with 2000 trials using Oracle Crystal Ball, varying 486 continuous variables along probability distributions and 48 discrete variables that determine production configurations and LCA choices. An earlier version of this research is contained in the Ph.D. thesis of Miller.³²

Scope

The scope of this study is informed by a Product Category Rule (PCR) for aircraft Environmental Product Declarations.³³ We deviate from the PCR to incorporate more life cycle stages and to accommodate different flight distances.

The fuel use and associated impacts vary with the aircraft type and size, engines, fuel type and production technologies, flight path, trip distance, and atmospheric conditions. We chose to model the long-range wide-body Boeing 787-800 (B788), both due to its two next-generation GEnx-1B64 engines that better reflect future technology and the availability of aircraft-specific global AIC estimates (see SI Table S3). With a maximum of 381 passengers and a first flight in 2011, Boeing delivered 386 B788s and had 30 unfilled orders as of February 2023.³⁴

The average flight distance from takeoff to landing in a single leg (stage length) for wide-body aircraft in the United States from 2010 to 2019 was 3435 nautical miles (1 nm = 1852 m).³⁵ We therefore used a 3500 nm stage length as a base case for the B788 rather than the PCR default 500 nm. We compare the sensitivity of the results across stage lengths from 125 to 7500 nm.

We extended the PCR’s system boundary to include airport infrastructure and operations to demonstrate its relative magnitude.³⁶ Critically, we included contrail cirrus from water vapor emissions to the upper atmosphere. Using a cutoff approach, we excluded the aircraft end-of-life processes as well as airport refueling infrastructure and fuel production facilities; these facilities are expected to have a small impact.^37,38 Since Airbus is considering launching LH₂-powered aircraft in the year 2035, we used projected production efficiencies and emissions for that year. The system boundary is illustrated in Figure 1.

Figure 1.

System boundary of WTWa assessment. Aircraft and airport end-of-life impacts are excluded, as is airport refueling infrastructure and hydrogen production infrastructure. Emissions considered are black carbon (BC), methane (CH₄), carbon monoxide (CO), carbon dioxide (CO₂), hydrogen boiloff (H₂), water vapor (H₂O), dinitrogen monoxide (N₂O), non-methane volatile organic carbon (NMVOC), nitrogen oxides (NO_x), organic carbon (OC), and sulfur oxides (SO_x). Effects considered are induced land use change (ILUC) and contrail cirrus.

We adopted a functional unit (FU) of 1000 revenue passenger nautical miles (pax-nm), which is similar to that of the PCR (pax-100 km). While the global average load factor in 2019 was 83%,³⁹ the PCR assumes 100% passenger load factor, meaning that all passenger seats are occupied. Here, we used a 60% load factor to align with the European Monitoring and Evaluation Programme/European Environment Agency (EMEP/EEA) modeled flight emissions dataset.⁴⁰ If the assumed load factor was varied, it would affect the absolute results per pax-nm but not the relative comparison across fuels. The PCR does not allocate any burdens to the freight or mail additionally carried by the flight, which aligns with the EMEP/EEA dataset that does not have freight or mail.

Models and Data Sources

The potential impacts for the complete life cycle WTWa results are the sum of the airport and aircraft (AA), WTP, and pump-to-wake (PTWa) phases. We slightly modified the discernability method to explore the WTWa comparison between the alternative fuels and conventional jet fuel; see the SI section Well-to-Wake (WTWa) Methods Details: Discernability for calculation details.⁴¹ Our choice of the comparison approach was guided by a decision tree of uncertainty statistics methods for MC simulations in LCA.³¹

The dissertation of Chester includes a detailed hybrid LCA of three aircraft: Embraer 145 (small), Boeing 737 (medium), and Boeing 747 (large).⁴² We assigned the B788 the large aircraft as a proxy based on similar seating. Identical AA impacts per pax-nm were applied for all fuel types and flight distances.

We selected GREET1 2020 from Argonne National Lab to estimate the WTP phase. GREET models the supply chains of a variety of hydrogen and aviation fuels through year 2050 and enables stochastic analyses; we adapted existing electrolysis calculations to additionally model hydrogen from renewable electricity. The aviation fuel pathways considered are shown in Table 1. Conventional petroleum jet fuel serves as a baseline for comparison. GREET enables estimation of carbon capture and storage (CCS) during the gaseous H₂ production phase for some fuels. While a variety of novel technologies exist at varying readiness levels, the vast majority of H₂ in the world is currently produced by steam methane reforming of natural gas without CCS (Nat. Gas – No CCS).

Table 1. Aviation Fuel Pathways Included in the Study^a.

^aColors reflect the type of the fuel pathway based on the commonly used H₂ terminology.

The LCI model for hydrogen and aviation fuels in GREET is well documented.⁴³⁻⁴⁶ While GREET is available as a standalone software, we used the spreadsheet-based version to explore calculations and more easily facilitate modifications. The spreadsheet provides probability distributions of hundreds of variables for MC simulations; we adopted all, except those centered on outdated values. We added additional continuous probability distributions to several key parameters for aviation fuels, deriving standard deviations from literature or applying 10 or 20% coefficient of variation to the provided mean value. We also added discrete probability distributions when choices are provided for key production parameters and LCA modeling decisions, such as co-product allocation options or electricity grid mix.

The bulk of the LCA model is driven by the amount of jet fuel burned. While GREET 2020 does include a PTWa module, there are shortcomings for this purpose: the data is for the United States in the year 2010, does not provide sufficient details on the aircraft considered, and does not include LH₂ as a fuel option. Following Cox et al., we modeled conventional fuel burn and emissions from the EMEP/EEA dataset, which is intended for national GHG inventory reporting.^40,47 Data is provided for the two subphases, landing and takeoff (LTO) and climb, cruise, and descent (CCD), over a range of feasible stage lengths.

Since the data from EMEP/EEA assumes conventional fuel, we estimated the fuel and emissions for the alternative aviation fuels. The fuel burn per pax-nm in LH₂-powered aircraft was increased by 9% for B788 to adjust for larger tanks causing fewer passengers based on the long-range hypothetical LH₂ aircraft developed in the CRYOPLANE project.⁴⁸ Some of the emissions indices (EI; kg pollutant/kg fuel) for the alternative fuels were derived independently (see Table S4). Other EI were estimated by adjusting the conventional fuel EI with estimates derived from literature on the relative amount of emissions released by combusting the alternative fuel. Based on the characterization approach, some items were only considered for either the LTO or CCD subphase since the effects of emissions differ between the ground level and the upper atmosphere.

Whereas pollutants are emitted whenever aviation fuel is combusted, contrail cirrus only occur during a fraction of flights depending on the conditions. Here, we derive average contrail cirrus RF for the B788 powered by conventional jet fuel from a study that used it as the reference aircraft in a global fleet model⁴⁹ and adjusted the RF based on uncertainty ranges in literature (see the SI section Pump-to-Wake Climate Impact Characterization). The LH₂ RF was adjusted with a triangular distribution²³ (min = −90%, mode = −15%, and max = +60%), and the biofuel RF was adjusted −4 to +18%.⁵⁰ While updated modeling and experiments are needed to more accurately represent the contrail cirrus RF from LH₂ combustion, existing work suggests that it is lower on average from conventional jet fuel combustion. Some of the complex factors considered include that much more water vapor is emitted during LH₂ combustion, which could lead to more frequent contrail formation, but that effect could be offset by lack of particle emissions that facilitate ice nucleation.²³

Lee et al. estimated the global effective radiative forcing (ERF) of aviation considering CO₂ and non-CO₂ effects.¹ The ERF metric was introduced as a “better indicator of the eventual global mean temperature response” than RF as it includes modeled effects of rapid atmospheric adjustments to a forcing agent.⁵¹ Therefore, we convert the RF of non-CO₂ impacts to ERF using values from their ensemble study, except for contrail cirrus since the global fleet model had already made necessary adjustments to their RF.

Impact Characterization

Combining different climate change characterization factors across the phases presented a challenge. See Table S1 for a list of emissions and corresponding characterization approaches across the study. We adopted GWP with a 100-year time horizon (GWP₁₀₀) as it is the most commonly used CO₂ equivalency metric, acknowledging that there are arguments for why alternate metrics better represent non-CO₂ emissions’ influence on future global temperatures.⁵² For the CCD phase, we used updated absolute GWP of CO₂ (AGWP_CO2) values for different time horizons found in Lee et al. to convert from climate change impacts in terms of ERF to GWP₁₀₀.¹ For the WTP and LTO phases, we used IPCC AR5 GWP₁₀₀ factors for well-mixed GHGs: CO₂, CH₄, and N₂O. We apply GWP₁₀₀ factors from IPCC AR5 to the short-lived climate forcers, BC, CO, NO_x, NMVOC, and OC, in half of the trials and found their influence to be relatively unimportant. GWP estimates used for the aircraft and airport (AA) stage were previously calculated using 1996 IPCC GWP₁₀₀ factors, introducing some error.⁴² Biofuel indirect land use change (ILUC) emissions were previously calculated, seemingly using GWP₁₀₀ factors from IPCC AR5 as well.⁵³

We explore six additional impact categories related to ecosystem quality and human health to observe trade-offs across the metrics (see SI Table S2). Midpoint characterization factors from ILCD 2018 are used for ground-level emissions to estimate potential acidification, marine and terrestrial eutrophication, respiratory effects, and smog impacts. Premature mortalities induced by species-specific LTO and CCD aviation emissions were assessed with a discounted global weighted-average value of a statistical life.⁵⁴

Results

The modeling indicates that hydrogen as an aviation fuel only can achieve zero or negative WTWa climate impact if it includes a carbon removal mechanism using biomass as a feedstock and CCS (Figure 2). Using renewable hydroelectric and wind electricity, which are not carbon-neutral options, for green LH₂ production reduces the potential climate WTWa impacts of conventional aviation fuel (67 and 64% on average, respectively).

Figure 2.

Boxplot of the WTWa climate impact of different fuels analyzed for the B788 aircraft at a 3500 nm stage length. (A) WTWa impact, kg CO₂-eq per 1000 pax-nm. (B) Percent difference of alternative fuels from conventional fuels, where a negative value indicates a reduction. The β values along the bottom represent the likelihood that the alternative fuel is less impactful than the conventional fuel. Sorted by fuel type and median value. Colors reflect the fuel pathway. Extreme outliers not shown.

In Figure 2A, we show the WTWa climate change impact estimates for the fuels listed in Table 1, combusted by the B788 aircraft at a 3500 nm stage length. The median estimate and confidence interval for conventional fuel are provided as reference values for comparison to the alternative fuels. Here, we consider the WTWa impact including the contrail cirrus impacts. Note that the contrail cirrus impacts are based on global weighted averages and represent a mix of flights that do and do not create persistent warming contrails. Since airlines can now opt to use innovative software to divert flights likely to cause persistent warming contrails, the results without contrail cirrus impacts are provided in Figure S4.

In Figure 2B, the WTWa estimates for alternative jet fuels are compared to conventional fuel. The chart shows the percent difference between the alternative jet fuel and the conventional jet fuel for each MC trial. The numbers along the bottom reflect the likelihood β that the alternative fuel is preferable to the conventional fuel, as described by eqs 6 and 7 in the SI. The β values are quite similar between the WTWa estimates with and without contrail cirrus included (see Figure S4), which is expected since the contrail cirrus impacts of the LH₂ are modeled to typically be just 15% lower than as the conventional fuel’s impacts.

It is clear from Figure 2 that the globally predominant method of producing H₂, steam methane reforming of natural gas without CCS, does not consistently reduce the climate change impact over the life cycle when compared to conventional jet fuel (+8% mean percent difference ± 40% standard deviation). The second most common method, coal gasification without CCS, is consistently worse (+121 ± 62%).

While LH₂ from biomass gasification with CCS (−205 ± 78%) and fermentation with CCS (−129 ± 52%) are modeled to be consistently carbon neutral or negative, they are still being demonstrated and their impacts are worse than conventional fuel across all other impact categories (see the SI section Comparison across All Impact Categories). A few other alternative fuels have low climate impacts relative to conventional fuel: green LH₂ produced by hydroelectricity (−67 ± 10%) or wind (−64 ± 10%) and pink LH₂ from nuclear with a high-temperature gas-cooled reactor (HTGR, −65 ± 10%). Some other alternative fuel options provide consistent, though less substantial, reductions. These are thermochemical cracking of water with heat from nuclear fission (−48 ± 23%), natural gas with CCS (−43 ± 36%), electrolysis with PV electricity (−40 ± 10%), biomass gasification without CCS (−33 ± 26%), and electrolysis with geothermal electricity (−23 ± 10%).

Biofuels from the Fischer–Tropsch process with CCS (−137 ± 19%) and without CCS (−52 ± 13%) are the only categories of biofuels modeled to offer a consistent climate benefit. However, like the biomass-based LH₂ fuels, all of the biofuels perform worse on other environmental impact categories (see Figure S7). Recognizing the many discrete modeling assumptions needed to estimate the climate impact of biofuels, the results of an exploration into the influence of these assumptions are provided in the SI Biofuel Allocation Sensitivity section. Aside from the presence of CCS, we observe limited variation due to allocation selection and various other production assumptions such as feedstock choice and farming practices.

Figure 3 illustrates the sensitivity of comparative results for green LH₂ fuels to assumptions about the contrail cirrus climate impact. The regression line shows that if the contrail cirrus impact of LH₂ is assumed to be 10% less than that of conventional jet fuel, the comparative life cycle impact of the green LH₂ fuel improves by 2.7%. This is because the contrail climate cirrus impact for LH₂ combustion is modeled as a product of the estimated impact of conventional fuel combustion and the impact adjustment assumption for the combustion of LH₂. The contrail climate cirrus impact is therefore independent of the LH₂ fuel production pathway, and the difference in each panel of Figure 3 is based on the WTP impact of producing the fuels. The comparative results are rather insensitive to the stage length; see Figure S5 for WTWa sensitivity to stage length and Figure S31 for variation in fuel burn with aircraft stage length.

Figure 3.

Sensitivity analysis of the relative WTWa climate impact of the four green LH₂ fuels to the modeled climate impact of contrail cirrus for the B788 aircraft at a 3500 nm stage length. The y-axis is akin to that of Figure 2B, the percent difference of WTWa impacts between LH₂ fuels and conventional fuels. The x-axis presents the percent difference of contrail cirrus climate impacts between LH₂ fuels and conventional fuels; this is a model input with an uncertainty distribution. Along each axis, a negative value indicates that LH₂ combustion has a lower impact than conventional fuel combustion. The color scale indicates the estimated LH₂ contrail cirrus climate impacts.

To gain a better understanding of the relative contribution of each life cycle phase, Figure 4 presents waterfall plots for key fuel pathways; a full set of such plots is available in the SI Additional Results section. The AA stage on average contributes a small fraction (1 to 9%) to the overall WTWa impacts across the non-neutral LH₂ fuels. For conventional jet fuel, the CCD phase dominates due to the CO₂ emissions during combustion, which does not occur during combustion of LH₂. In the most preferred pathway, LH₂ from biomass gasification with CCS, the benefits arise chiefly from the CO₂ capture during plant growth and removal via CCS in the fuel production phase. Looking at the green LH₂ pathways most often discussed, wind has on average a 58% lower WTP impact than conventional but the WTP impact of PV is modeled to be three times as large as conventional due to the embodied impacts of creating solar panels. New PV technologies, more efficient manufacturing, and a decarbonization of the electricity production will help mitigate these PV emissions.⁵⁵ LH₂ from natural gas or coal without CCS have very large WTP impacts, 8 and 20 times that of conventional, respectively, which make them unfavorable from a life cycle perspective.

Figure 4.

Waterfall plot of the WTWa climate impact (kg CO₂-eq per 1000 pax-nm) across the phases for selected fuel pathways for the B788 aircraft at a 3500 nm stage length. Bar totals represent the mean, and whiskers represent the 5th and 95th percentiles. Note: Reference lines in the WTWa phase reflect conventional fuel WTWa. Waterfall plots of all fuel pathways are available in the SI.

To validate our WTP results, our modeled climate impact estimates are compared against literature for several fuel pathways in Figure S2 of the SI. Since published estimates of the impacts of LH₂ production are rare, the related impacts of GH₂ production are compared instead. We observe an agreement for biomass gasification without CCS. Our estimates are slightly higher than literature for fermentation and thermochemical cracking of water with nuclear and somewhat lower than literature for Nat. Gas – No CCS and coal gasification pathways. As such, our results may unintentionally give a slim advantage to fossil-based hydrogen fuels, which in light of their poor performance against conventional jet fuels in this study suggests that modifying underlying assumptions about fossil-based hydrogen is unlikely to result in different conclusions.

Taking a closer look at the combustion-related climate impacts during CCD in Figure 5, CO₂ stands out as the largest contributor to the climate impact of conventional jet fuel, causing 67 ± 8% of the CCD impact at GWP100, followed by contrail cirrus at 32 ± 7% (see Table S15). The contrail cirrus impacts sharply increase to 81 ± 31% for LH₂ and 88 ± 16% for biofuels. While the contrail cirrus effect factor of LH₂ combustion is modeled to be on average 15% lower than that of conventional fuel, the reduction is partially offset by the larger emissions of water vapor.

Figure 5.

Comparison of components of the climate impact assessed at three time horizons (20, 50, and 100 years) during CCD across three fuel types for the B788 aircraft at a 3500 nm stage length. Components ordered by lowest to highest global net RF in Lee et al.¹

It is important to consider alternate and scientifically based time horizons to meet climate temperature goals. For instance, Abernethy and Jackson propose using 24- and 58-year time horizons to meet the 1.5 and 2.0 °C targets, respectively.⁵⁶ The climate impact of CO₂ is unaffected by time horizon since the units are in CO₂-eq. The shorter-lived non-CO₂ impacts have a larger contribution when the GWP is based on shorter time horizons, 20 and 50 years, as compared to the common but somewhat arbitrary 100 years. Because of this, the relative climate impact of alternative jet fuels increases and is closer to that of conventional jet fuel on the shorter time horizons.

Discussion

In public discussions, hydrogen can appear as the silver bullet to address the climate liability of the aviation industry. A WTWa analysis of all climate impacts of flying shows a more nuanced picture:

• For many LH₂ pathways, GHG emissions from fuel production are not only larger than those of conventional fuel production but in some cases may even be larger than the combined impacts of production and combustion of conventional jet fuel.

• Several LH₂ pathways offer reductions in the WTWa climate impact relative to conventional jet fuel, but these are not currently the commonly used technologies. Currently, low-carbon hydrogen only represents less than 1% of global hydrogen production.⁵⁷ The aviation industry will need to ensure that there is a sufficient dedicated supply of LH₂ from electrolysis powered by renewables or new carbon-negative technologies for a transition to LH₂ to reliably provide a climate benefit.

• The combustion of LH₂ causes higher emissions of water vapor in the stratosphere than that of conventional jet fuel, causing climate impacts. The water vapor also raises concerns about the potential contrail cirrus climate impacts. Further research is needed to better estimate the contrail cirrus impact from LH₂, and at least partial mitigation through flight diversion is possible.

Several LH₂ Fuel Pathways Offer a Significant Mitigation of Climate Impacts from Aviation

The following categories of LH₂ pathways are estimated to have lower life cycle climate impacts on average than conventional jet fuel: LH₂ from biomass with CCS (−205 ± 78 and −129 ± 52%), green LH₂ from renewable electricity (−67 ± 10 to −23 ± 10%), pink LH₂ from nuclear energy (−65 ± 10 and −48 ± 23%), and one type of blue LH₂ from fossil fuel with CCS (natural gas with CCS, −43 ± 36%). If airlines strategically mitigate contrail cirrus formation, the life cycle impacts of flight with green and pink LH₂ pathways are further reduced and estimates become more certain. Future use of novel biomass-based LH₂ produced with CCS could offer negative emissions.

Importance of Contrail Cirrus Mitigation

Our results demonstrated the sensitivity of the WTWa climate impact of LH₂ to the contrail cirrus impacts. In recent years, modeling and empirical analysis of contrail cirrus have improved the science and helped reduce uncertainty in the climate impacts from aviation. However, since the interest in hydrogen-powered aircraft faded for a period of time, research on contrail cirrus forcing from hydrogen combustion has stalled since around 2006, though Gierens recently modeled relevant temperature and humidity conditions for hydrogen-based contrail formation.^58,59 Given the large mass of water vapor emitted, there is potential for increased contrail cirrus formation, especially from flights in the tropics and subtropics.⁴⁹ The lack of recent studies led Aviation Week to question whether it would be the Achilles’ heel of hydrogen aviation.⁶⁰ Therefore, we strongly support investigation in this area to refine WTWa estimates and guide decision-making around hydrogen aviation investments. For instance, the DLR German Aerospace Center has an H2CONTRAIL project underway, but the results are still pending.

Aside from just hydrogen-powered aviation, contrail cirrus is a formidable challenge for the aviation sector to tackle in general. Contrail cirrus mitigation has been de-prioritized, perhaps due to prior impact estimates with large uncertainties (which have since been reduced) combined with shorter atmospheric residence times of contrail cirrus than GHGs. However, in late 2022, a promising Contrail Impact Task Force was formed with several airline and aircraft companies working in partnership with research institutions.⁶¹ Still, aviation organizations have yet to commit to mitigation plans, and many flight carbon footprint calculators and offsetting schemes opt to neglect them or include a rough multiplier.

Fuel Pathway Design and Contrail Cirrus Mitigation Are Key

Many but not all hydrogen production pathways increase WTP emissions compared to conventional jet fuel, while hydrogen as a fuel could cause more frequent contrail cirrus formation than conventional jet fuel. These impacts can be mitigated through fuel pathway design and flight pathway selection, respectively. Only by diligently paying attention to these issues can the aviation industry move toward zero climate impact.

Key Study Limitations

There are bound to be uncertainties in estimates when performing an LCA of a technology not yet commercially available and with climate impacts that vary with the particular circumstances of each flight; these estimates should become more accurate and precise as LH₂ aviation advances and new data are released (see the SI section Additional Model Limitations and Future Improvement). The WTP LCI can be updated with newer versions of the GREET model and further modified to reflect new trends in commercial hydrogen production. Regarding the climate impact assessment, Dahlmann et al. demonstrate the sensitivity of non-CO₂ emissions to the initial cruise altitude, whereas in this study, we apply CFs that do not vary with altitude.⁶² We have not included potential impacts due to leakage of H₂ from the aircraft due to data limitations. Also, while LCA typically assumes that impacts scale linearly with consumption, Bickel et al. show that there is a nonlinear relationship between contrail cirrus impacts and increase in air traffic since the saturation effect reduces the impact of individual contrails in dense air traffic.²⁵ Finally, since most of the ERF conversion factors here are derived from an ensemble of model simulations reflecting previous years’ patterns,¹ those could be updated with estimates from models projecting ERF from future aviation.

Data Availability Statement

The underlying data and code are publicly available: https://github.com/reedmiller17/lh2-jet-lca.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.2c06286.

• Review of relevant literature including well-to-pump gaseous hydrogen comparison; additional methods details including model limitations and future improvements, aircraft and airport methods details; additional results including Tables S10–S19 which provide summary statistics for Figures 2, 3, and 5 (PDF)

The authors declare no competing financial interest.