In seeking to advance sustainability in the aviation industry, robust estimates of emissions are essential because they support informed decision-making. Previous ICCT research found that travelers on U.S. routes can reduce carbon dioxide (CO₂) emissions attributable to their ticket by 22% on average, and up to 63%, by choosing the least-emitting flight available.  

Consumers can access emissions estimates for future flights around the world from Google’s Travel Impact Model (TIM), and since 2023, the ICCT has worked with global experts to refine the model through an Advisory Committee that we lead. This partnership established seven core principles and a roadmap to ensure continuous improvement of the TIM for rigorous, transparent, and consistent emissions reporting.  

As the TIM continues to evolve, it holds promise to become the global standard in low-carbon travel search. Here I’ll highlight some of the work behind its development, including the model validation and model selection approaches that enhance its reliability for travelers, airlines, and policymakers.  

Estimating fuel burn is the first step in assessing emissions, and this is challenging due to the variability in flight operations. Emissions will differ based on aircraft technology, weather, and operational practices, and even for the same aircraft and route, fuel burn can vary significantly. The TIM uses the European Environment Agency (EEA) model, which bases estimates on aircraft type and route distance and creates simplified linear relationships. Using the operations of Brazilian airlines in 2019 and the four most commonly used aircraft types, Figure 1 compares the TIM (version 1.8.0) estimates (dashed lines) with real-world fuel burn at the flight level (dots). The large variability reflects the complexity and uncertainty of flight operations, where fuel burn depends on a range of interdependent and sometimes unpredictable factors such as weather and operational practices.
 

Figure 1. Fuel burn versus distance for each individual flight by the four most common aircraft types in the ANAC data in 2019

Model validation

Validation is a quantitative assessment of how well model prediction represents real-world data. For validating the TIM, we needed data from past flights, including fuel burn, ideally at the flight level or at least aggregated by route and aircraft type. The only public dataset identified that met the requirements was the Brazilian Civil Aviation Agency (ANAC) microdata, which provides historical flight data for Brazil since the year 2000 at the flight level. As it’s limited to Brazilian airlines, Google combined ANAC’s public data with private operational data shared by partner airlines worldwide. The aircraft types covered by this sample represent approximately 76% of global flights in 2019 and the validation sample now contains more than 3 million flights. The Google engineering team is continuously working with airlines to expand it to enhance model representativeness and reliability. There is a three-step process to promote reliability: 

1. Data cleaning: We remove irrelevant or incomplete data. 
2. Data aggregation: We group fuel burn data by route, aircraft type, and airline. This is necessary because some private airline data was shared in aggregated form; it contained fuel burn averages by route and aircraft type rather than at the flight level. By aligning our analysis with the level of granularity available in the shared data, we ensure consistency. 
3. Error analysis: We compare the TIM’s estimates with real-world fuel burn using metrics such as median absolute error and error distribution. “Error” is defined as the difference between actual and estimated fuel burn, expressed as a percentage. The actual fuel burn refers to the values in the validation dataset, and estimated fuel burn refers to the TIM estimates. Positive errors indicate overestimation and negative errors indicate underestimation. 

The TIM validation framework uses four key metrics for evaluation: median absolute error (the central tendency of errors), error threshold analysis (the percentage of estimates within different error bounds), distance-based metrics (error trends by route length), and the distance and aircraft error metric (error trends by route length and aircraft type). Details of the metrics are in this technical brief, and Figure 2 illustrates the error distribution curve for the TIM (version 1.8.0) estimates. As shown, the fuel burn is more often underestimated than overestimated by the model. The TIM underestimates the fuel burn for nearly 75% of the airline-aircraft type-route combinations in the validation sample.

Figure 2. Frequency (left) and cumulative (right) distributions of the error in the TIM’s fuel burn estimates when compared with the real-world fuel burn from the combination of ANAC 2019 and private airlines data

Model selection 

The TIM fuel burn estimation was originally based on the EEA 2019 model, which allows users to define only aircraft type and stage length; other significant factors like flight trajectory and payload are not included. Recognizing these limitations, the TIM Advisory Committee explored alternative fuel burn models. 

Nine models were assessed qualitatively (details in the technical brief), and five were shortlisted for detailed evaluation using the validation methodology: EEA 2023, OpenAP, Poll-Schumann, Piano 5, and ICAO ICEC. Because these models vary in structure and require different operational assumptions such as trajectory and payload, we standardized assumptions where possible to be able to compare them. The tested scenarios, based on real-world operations and described in the technical memo, reflect these simplifications. Figure 3 illustrates how the error distributions of these models compare with EEA 2019.  

Both EEA 2019 and EEA 2023 showed narrow error distributions, reflecting good accuracy. However, EEA 2023 consistently outperformed EEA 2019 across key metrics. In contrast, OpenAP demonstrated a wider error spread, indicating lower predictive accuracy for the data used. Intermediate performers, such as ICAO ICEC, Poll-Schumann, and Piano 5, showed moderate error variability. These evaluations showed EEA 2023 to be the most suitable model, and it was adopted in mid-2024. 

Figure 3. Comparison of the error distribution across alternative models

In January 2024, the Advisory Committee incorporated validation into the TIM workflow to evaluate fuel burn model updates. Then, in June 2024, in addition to adopting EEA 2023, they applied a distance correction factor that enhanced the TIM’s accuracy and alignment with real-world operations. The distance correction factor refines stage length inputs by replacing Great Circle Distance with an average route distance based on real-world flight paths. This adjustment reduced the median absolute error from 7.80% to 6.30%. Future Advisory Committee work on second-order fuel burn effects like payload, engine variants, and aircraft age is expected to further improve the accuracy of the TIM and thus further improve its value for a wide range of stakeholders, including the flying public.

Special thanks to Ana Beatriz Reboucas and Jayant Mukhopadhaya for their significant contributions to the research on the TIM website.

The most high-profile setback came when Airbus pushed back its ambitious goal to introduce hydrogen-powered commercial aircraft in 2035. Citing a mix of infrastructure challenges and slower-than-expected technology development, it made the announcement during the Airbus Summit 2025 and expects a 5-10-year delay. Airbus isn’t alone. Other manufacturers have recently dialed back plans or hit financial difficulties. Why this turbulence? Well, as with Airbus, the primary challenges are technology and infrastructure. 

Technologically, battery-electric aircraft remain limited to serving short routes (less than 500 km) due to severe battery weight limitations. Jet fuel is still roughly 50 times more energy dense than today’s most advanced lithium battery. While jet fuel contains about 43 MJ/kg of energy, the most advanced lithium-ion batteries today offer only around 0.9–1.0 MJ/kg, making them roughly 40 to 50 times less energy dense by weight. The additional battery weight increases the total mass the aircraft must lift and thus the energy to sustain flight—creating a compounding penalty. Hydrogen technologies such as fuel cells and combustion engines can offer greater range but face their own set of engineering hurdles, particularly concerning the need to develop lighter hydrogen storage tanks and more powerful fuel cells to minimize their significant mass and volume penalty. One of the major aircraft manufacturers, Embraer, recently pushed back its Energia project, a family of low-emission hybrid-electric aircraft, because both battery and hydrogen fuel cell advancements have been slower than hoped. This necessitated the delay of the project from its original intended entry-into-service year of 2035. 

Infrastructure is probably an even bigger barrier. Airbus noted that inadequate hydrogen infrastructure—including hydrogen production, distribution, and fueling facilities, airport expansion and modification, and new safety protocols—is one of the major roadblocks to its ZEROe program, the flagship hydrogen-powered aircraft initiative aimed at launching the world’s first commercial ZEP. Indeed, green hydrogen, which is produced using renewable electricity and water electrolysis, comprised less than 1% of global hydrogen production in 2024. And even if green hydrogen were being produced in large volumes, the infrastructure to reliably distribute hydrogen remains sparse and expensive to build. Regulatory frameworks are also lagging, and that’s creating uncertainty and slowing investment. For example, Airbus recently reduced the budget for ZEROe by 25%. 

What about the innovative startups we heard about a few years ago? Universal Hydrogen, a U.S.-based company that aimed to decarbonize regional aviation by retrofitting planes to run on hydrogen fuel cells, achieved a remarkable goal when it flew the largest aircraft ever on hydrogen-electric power. However, Universal shut down a year later after failing to secure enough funding to continue research and development. Eviation, an electric-plane startup based in Washington state, paused its Alice program and laid off most of its employees. That these firms struggled to secure the substantial funding and regulatory support necessary to scale is a lesson for the aviation industry: Zero-emission flight requires sustained and coordinated investment and collaboration beyond the reach of individual companies.  

What does this all mean for aviation’s climate targets? Well, our recent study found that even with optimistic sustainable aviation fuel (SAF) and fuel efficiency assumptions, the net-zero carbon target could be exceeded as early as 2037. Therefore, to meet the 2050 net-zero target, accelerating efforts to develop ZEPs seems critical. But with Airbus and others suggesting that 2035 is infeasible, achieving the net-zero target by 2050 looks more difficult than ever.  

This puts more pressure on SAF, technical efficiency improvements, and operational optimization to fill the gap. Due in part to this reality, hybrid-electric solutions have emerged as intermediate alternatives because they can be integrated in the medium term and offer moderate emission reductions (even though they are short of climate goals, including when combined with SAFs).  

The upcoming update to our Vision 2050 report will reflect the latest industry realities and reassess the role that ZEPs, SAFs, and other mitigation measures must play to realistically meet global targets. It will also feature an updated version of the ICCT’s net-zero aviation roadmap, which for the first time will include the climate impact of non-CO₂ emissions. 

Ultimately, stakeholders have to ask: Are hydrogen tanks getting lighter? Are fuel cells getting more powerful? Is battery chemistry improving? The recent setbacks are a wake-up call and these indicators will tell us if ZEPs can move from prototypes to actionable climate solutions. Even if there is continued progress on the technology side, the business case will need to be addressed—both the capital and regulatory aspects—to make ZEPs into a reality. In the meantime, as important as ZEPs are to the long-term solution, to stay on track for 2050 it’s likely that policymakers and the aviation industry will have to focus on alternative measures that can deliver medium-term emission savings. 

There’s a famous koan, or philosophical question used in Zen meditation, that asks, “What is the sound of one hand clapping?” An aviation variant of the question is, “What is the boom from a supersonic aircraft that hasn’t been built yet?” It may surprise you to hear that this has real policy implications.

In late January, Boom Supersonic flew its XB-1 demonstrator aircraft faster than the speed of sound for the first time and then promptly retired it. CEO Blake Scholl subsequently said that XB-1 didn’t produce a sonic boom and called for the 50-year-old U.S. ban on overland supersonic flight to be lifted, based solely on this test flight. What’s going on here and why is it best for regulators to ask tough questions before undoing the overland flight ban?

Boom stated that XB-1 achieved “Boomless Cruise” and claims that its Overture aircraft, which it says it’ll bring into service in 2029, will do this, as well. Boomless Cruise was originally the goal of Aerion, a different supersonic startup that went bankrupt in 2021. It’s based on Mach cutoff, a theory which says that under the right atmospheric conditions a slow supersonic aircraft—typically flying between 110% and 130% the speed of sound—might not generate a sonic boom that propagates to the ground. Mach cutoff occurs when the sonic boom is reflected away from the ground due to a density difference in a warmer layer of air below the aircraft.

Given the novelty of the claim—Boom made no mention of a goal to achieve Boomless Cruise before the flight, and the company is literally named after the atmospheric disturbance—some scrutiny is in order. Modeling suggests that achieving Mach cutoff, which is sensitive to speed, altitude, temperature, humidity, and winds, will be tricky under real-world flight conditions.

Penn State researchers found that for certain windspeeds and headings, a 1% change in cruise speed can disrupt Mach cutoff and lead to a focused boom being concentrated down on the ground. That’s because a precise combination of speed, aerodynamics, aircraft mass, and atmospheric conditions is needed to produce Mach cutoff, and that needs to be supported by precise avionics and accurate atmospheric tracking. So, without first being demonstrated under a variety of flight conditions, “boomless” aircraft might prove to be anything but.

Boom did not release any observational data about the XB-1 test flight and such data is needed to help understand how representative the conditions were that XB-1 was operating under. For example, Georgia Tech research found that conditions of low humidity and low temperature lead to lower perceived boom on the ground. XB-1 was flown over the Mojave Desert in late January, which might be ideal for Mach cutoff.

To understand why Scholl may be interested in raising this issue, you need to know a bit about how the economics of supersonic flight intersect with the challenge of sonic boom. How large the market will be for Overture is hotly debated. Boom estimates that it can sell more than 1,000 aircraft to service routes over water. It’s a bold claim given that only 14 Concorde—which had famously bad economics—ever entered commercial service.

How likely are those 1,000 planes? Well, back in 2022, we analyzed the potential market for supersonic aircraft in 2035 using a model that MIT’s Laboratory for Aviation and the Environment developed for NASA. The model used consumer’s willingness to pay for time savings to project the potential supersonic market. We estimated that there could be a market for about 235 Overture-sized supersonic aircraft in 2035, with about 70% of the potential demand in the Middle East and North America (map).

But there’s a catch. That’s an unconstrained forecast that assumes no overland flight ban. If supersonic aircraft can only operate over water, the modeled market falls by 95% to around 12 aircraft. So, right back to Concorde levels—and Concorde was tried on a variety of routes before settling into its core, profitable markets of Paris to New York and London to New York.

This result suggests that there just aren’t enough profitable routes over water to justify the more than $20 billion needed to build a new supersonic aircraft. That’s a problem for a cash-strapped startup that has raised about 4% of that capital to date. Prior to the test flight, Boom said it had identified 600 to-be-announced routes over water on which Overture could operate profitably. Now, Scholl is arguing that Overture will be so quiet that the United States should repeal its ban on overland supersonic flight outright. Conveniently, that would translate to a much larger market for Overture, assuming that the public can handle any sonic boom.

Due to the powerful engines needed to achieve sustained supersonic flight, supersonic aircraft will always be noisier than subsonic designs. According to an upcoming UN report, supersonic aircraft are, on average, expected to emit 25% more noise per flight and about 6 times more noise per seat during takeoff and landing. Still, the Achilles’ heel of supersonic aircraft is sonic boom: An aircraft operating supersonically propagates a shock wave continuously along its flight path. Sonic boom has been found to be deeply unpopular with the public and supersonic flight is banned overland not just in the United States but in most countries. The current ban on overland supersonic flight is enforced via a speed limit for aircraft.

Back to Boomless Cruise. If achieved, this would increase the market for Overture but wouldn’t be a panacea. Flying at 110% the speed of sound will save some time but also increase fuel burn, thereby increasing both fuel costs and greenhouse gas emissions. The German Aerospace Center estimated that achieving Mach cutoff speeds by a commercial supersonic airliner would increase fuel burn about 25% compared with operating at Mach 0.95.

Unfortunately, with XB-1’s retirement, we’re unlikely to get additional data on Boomless Cruise until at least 2028, the target date for Overture’s first flight. To complicate things, we won’t know if Overture’s full-scale, four engine turbofan design, which bears little resemblance to the smaller turbojet XB-1, can achieve Boomless Cruise until after it’s built.

What’s a careful regulator to do? A prudent approach would be to continue to support an international standard for sonic boom intensity, which the UN is aiming to deliver in 2031—not much later than Overture’s target entry-into-service date of 2029. That would supplant the overland speed limit with a robust and objective noise standard for supersonic aircraft. A bit of patience here to project the American public from unwanted noise pollution is preferable to hasty regulatory changes to support a product that may or may not be built, and that may or may not boom.

Although the requirements in the FuelEU Maritime and ReFuelEU aviation regulations already send a strong signal about future demand for low-greenhouse gas (GHG) emission fuels, many renewable fuel projects still haven’t reached final investment decision (FID). Costs are the primary challenge, but now there’s new hope for progress. The European Commission’s recently announced Clean Industrial Deal (CID) is generating lots of well-deserved buzz, and as we’ll outline here, the Commission’s commitments could help support low-GHG emission advanced fuels in a few important ways. 

We define advanced fuels as (1) second-generation biofuels produced from hard-to-break-down cellulosic materials that require nascent technologies to convert them to biofuel and (2) renewable hydrogen and its derivatives, also known as renewable fuels of non-biological origin (this includes e-fuels like e-kerosene). Advanced fuels are critical for the long-term decarbonization of maritime shipping and commercial aviation because first-generation fuels like low-GHG hydroprocessed vegetable oils, although already produced at commercial scale, are only available in limited quantities and carry fraud risk. It’s second-generation advanced fuels made from cellulosic wastes and residues or renewable electricity that could be produced at a much greater scale in the European Union.  

While advanced fuels offer much larger GHG savings than many of the existing alternative fuels on the market, they’re also more expensive and challenging to finance. Previous analyses highlighted how the high upfront capital costs of second-generation fuel pathways make it challenging to attract investment due to uncertain market prices and project timelines. Many industry stakeholders have called on the European Union to implement policies to de-risk these investments and foster market certainty. The CID’s intention to centralize and expand the funding available from the EU Emissions Trading System (ETS) and the Innovation Fund can respond to this call by providing funding for capital support and assisting early stage projects. The CID also calls for adoption of a new State Aid Framework and this, too, can help Member States better provide the needed capital support. 

The EU ETS currently reserves 20 million allowances for a sustainable aviation fuel (SAF). As detailed in a previous ICCT blog post, the funds from EU ETS reinvestment by themselves could only cover a portion of the projected cost gap between fossil jet and e-kerosene, in order to meet the European Union’s e-kerosene targets. More importantly, the reinvestment subsidy is given to airlines, not fuel producers, and none of the allowances are earmarked for advanced fuels; for this reason, it does little to reduce market uncertainty for producers of more challenging advanced fuels. Additional grant funding or price support mechanisms stemming from the CID could instead directly improve the fuel producers’ balance sheets, which would support them in reaching FID and allow them to get the needed investment to start building production facilities.  

Member States are also urged in the CID to finalize revisions of the Energy Taxation Directive (ETD). If Member States accept the 2021 proposal for fossil jet taxation and roll back the aviation sector’s traditional exemption from fuel taxes, it would be a critical component of bridging the cost gap between SAFs and conventional fossil jet and be a potential additional revenue source for renewable fuel. The abovementioned ICCT blog post also highlighted how the proposed ETD tax rate on jet fuel could significantly contribute to closing the cost gap for e-kerosene.   

There’s more good news for SAFs in the CID’s proposal to expand the Hydrogen Bank program with an additional auction and to create a new Hydrogen Mechanism in Q2 2025; the mechanism would de-risk investments in aviation and marine fuels by connecting producers and offtakers and providing them with financing instruments. Such support for renewable hydrogen is likely to be critical for e-kerosene production to scale up in Europe.  

The Commission’s commitment in the CID to reducing permitting requirements for electricity, grid expansion, and storage may not only impact the power sector, but could also speed up the rollout of domestic e-fuel production and vehicle fast charging infrastructure. Bottlenecks in upgrading the grid raise the cost of electricity, and that’s one of the most important factors influencing e-fuel cost.  

The FuelEU Maritime and ReFuelEU aviation policies are setting a global precedent for how to decarbonize the maritime and aviation sectors by pairing rigorous sustainability criteria with binding long-term targets. However, those policies are at risk if emerging advanced fuel technologies fail to scale up. The world is watching to see if the European Union succeeds in implementing these regulations, and it’s important that advanced fuel pathways get the support they need to reach FID and start production. The upcoming Sustainable Transport Investment Plan will outline further measures to support renewable and low-carbon fuels for the aviation and marine sectors, and the ICCT’s research and analysis will inform this process.