Scanning the sky for lite flights

Consumers concerned about greenhouse gas emissions from air travel have begun to reduce their emissions from flying by purchasing offsets, or simply avoiding trips. While this increased awareness is great news, consumers have largely ignored another effective approach: booking less-emitting flights.

Emissions of different flights on the same route vary greatly. ICCT analysis shows that on average, the least-emitting itinerary on a US route releases 63% less carbon than the most-emitting option, and 22% less than the average option. Therefore, making emissions data available at the point of ticket purchase is important in helping consumers make informed choices.

To our knowledge, four travel search engines currently display emissions information: Google Flights, Lite Flights, Kayak.com, and Skyscanner. Among these, Lite Flights is specifically designed to search for “greener” flights; users cannot book tickets on the site. In this blog, I will compare my experience using the four platforms and explain why standardization across platforms is greatly needed.

The four sites are accessible and fairly easy to use. Google Flights, Lite Flights, and Kayak all display absolute emissions in kilograms of CO₂-equivalent. Skyscanner compares a flight’s emission values to the route’s average emissions: “This flight emits 16% less CO₂ than the average for your search” is an example.

Google Flights and Kayak enable users to sort search results by CO₂ emissions. Google Flights displays emissions with every flight returned in a search, while Kayak requires consumers to take the extra step of sorting by CO₂ values before they are displayed. Lite Flights automatically sorts results from least to most emissions, while Skyscanner provides a filter for showing “greener flights” only (the top X percent of lowest-emission flights; the share is unspecified). All in all, consumers seeking information on flight emissions can find it on the four sites.

Table 1. Comparison of four analyzed platforms (based on publicly available information).

Lite Flights and Kayak rely on Atmosfair’s emissions calculator to generate emission estimates by route, while Google Flights uses the European Environmental Agency’s (EEA) emissions model. Because of the different methods used, these platforms report rather different emission estimates. Take for example a search for an LAX-JFK roundtrip in October 2021. Table 2 presents the least-emitting flight by each carrier according to each platform.

Two important realities are apparent: first, the absolute as well as the relative emission estimates differ across platforms. Beyond creating confusion for shoppers, the discrepancies indicate fundamental differences in the underlying models and the need for greater standardization. Second, emission estimates vary even across platforms that use the same model. For example, Lite Flights and Kayak both use Atmosfair’s model, but the CO₂e estimates for United’s and Delta’s least-emitting flights are about 8 percent different.

Table 2. Ranking of airlines by emissions per passenger on an LAX-JFK roundtrip, in ascending order.

Moreover, the figure below shows that, for the same flight, Google Flights’ estimate can be 17% to 41% lower than the average of Lite Flights’ and Kayak’s estimates, depending on the itinerary. The absolute emission values matter for emissions accounting purposes, especially for the increasing number of companies calculating their Scope 3 emissions from business travel.

Figure 1. Google Flights’ estimates of CO₂ emissions per passenger, versus estimates from Lite Flight and Kayak, for five roundtrip itineraries (boxed) between LAX and JFK.

I tried a few other searches and found varying results across platforms for other routes as well. Why do the estimates differ? There are a few likely reasons:

1. Non-CO₂ emissions: while the radiative forcing (i.e., the magnitude of climate warming or cooling effects of a pollutant) of CO₂ is well studied, scientific uncertainty remains around the radiative forcing (RF) of non-CO₂ aviation emissions, such as nitrogen oxides, black carbon, and especially the impact of contrails/cirrus clouds. The uncertainties have to do with the complexity of modeling atmospheric reactions. The magnitude of RF also depends on the altitude at which pollution occurs and the background emissions at that time. Both Atmosfair and Google Flights reference existing scientific literature for non-CO₂ RF. Specifically, Atmosfair multiplies CO₂ emissions above 9,000 m by 3 to account for non-CO₂ impacts (based on an IPCC report and Lee et al. [2010]), while Google Flights used the lower bounds found in the literature but did not give a complete list of references. It is worth noting that the Lee et al. paper that Atmosfair references has been updated; the new findings should be incorporated by the search platforms.
2. Aircraft specification: engine and especially aircraft type are key factors affecting a flight’s fuel burn. A new generation of aircraft (e.g., A320neo and 737 MAX) is about 15% more fuel efficient than the previous generation (e.g., A320ceo and 737NG). However, aircraft and engine information is not always accurate at the level of individual flights, especially when it comes to seating configuration, whether or not winglets are used, and aircraft variants (e.g., 767-300 vs. 767-300ER). Some fuel burn models are also limited in their coverage of aircraft and engine types. For instance, for aircraft not included in their estimation model, Google Flights uses the closest estimate for new aircraft types, and they do not provide estimates at all for certain aircraft types, such as the A220.
3. Great circle distance (GCD) adjustment: flight fuel burn is directly correlated to flight distance. Flight paths normally deviate from the great circle distance (the shortest distance between two points on a sphere) for various reasons, which can be corrected for in various ways. Atmosfair adds 50 km to all flights to correct for detours. The EEA model underlying Google Flights’ CO₂ feature does not explicitly correct for the extra distance.

Moreover, emission estimates are after all not as accurate as data on actual fuel burn. Ideally, governments would mandate that airlines disclose fuel burn data, which the platforms could use to generate more accurate flight emissions information. However, fuel burn modeling remains relevant for validating data disclosed by airlines and generating estimates for very new aircraft and/or routes.

Providing sustainability information is tricky. Without robust standards, a dozen different flights might claim to be the “greenest”. Nevertheless, search engine efforts to reveal emissions information are valuable and worth applauding. If aviation stakeholders can work together to improve the quality and consistency of these data, the tools could become even more valuable and informative.

The ICCT is playing a role in this standardization effort. Recently, the UK government included emissions disclosure as part of its “jet-zero” strategies (a policy package to achieve net zero aviation emissions by 2050), and ICCT made recommendations for designing such a mandate. Initiatives for promoting sustainable tourism like Travalyst may also be important. We are optimistic that consumer preferences for low-emitting flights can shape the market for air travel going forward. Airlines operating less carbon-intensive flights stand to benefit.

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