Where does Google get this information?
Google calculates emissions estimates using the latest version of the Travel Impact Model (TIM), a model administered by Google and overseen by an Advisory Committee of leading experts in sustainability and aviation. The TIM is a transparent and continuously improving emissions estimation model that is built from public and licensable external datasets and based on the latest science and internationally-recognized standards.
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Lifecycle emissions
The Travel Impact Model accounts for the full lifecycle of jet fuel emissions by estimating well-to-wake emissions. Well-to-wake emissions are the sum of emissions produced by making and transporting jet fuel, as well as the carbon emissions (CO2) from burning fuel during take-off, cruising, and landing.
In addition to CO2, the TIM converts non-CO2 emissions into their "CO2 equivalent" (CO2e) on the basis of their global-warming potential.
Typical emissions
Typical emissions are calculated following the Typical Flights Emissions methodology which estimates the median emissions for your searched route.
Emission estimates for each flight are compared to the route's Typical Flight Emissions. This is how Google identifies flights with higher, typical, or lower emissions.
For some searches, you may find no "lower emissions" flights. This happens when the flights on your searched dates aren’t less polluting than the route's Typical Flight Emissions. To find lower emission flights, try different dates or consider other airports nearby.
Unknown emissions
Factors impacting emissions
Actual emissions may vary and depend on factors such as:
- Aircraft model and configuration
- Speed and altitude of the aircraft
- Distance between origin and destination
- The number of passengers
To understand the emission estimates that we display, it's important to know:
- Non-stop flights aren't always less polluting, especially for long routes. It's possible for a multi-stop flight on fuel-efficient aircraft to emit less than the non-stop option.
- Aircraft with a similar capacity and range can have very different emissions. Contributing factors include the aircraft type, or the seating layout used by the airline.
- For flights to, from, and within the US, the model estimates passenger load factors using historical data from the US Department of Transportation. For flights outside the US, we use historical load factor data provided by ch-aviation when available. For all other flights, emission estimates consider a 2019 (pre-Coronavirus pandemic (COVID-19)) industry average load factor. More details on the data sources we use and how load factors are calculated can be found in our GitHub documentation.
- Our emission estimates don't yet consider factors such as direction of flight, the use of sustainable aviation fuel, or the weight of the plane's cargo.
Other warming effects of flying
In addition to releasing CO2 into the atmosphere, flying can cause other warming effects such as contrails.
In regions of high humidity, water vapor in the air condenses around particles of soot from an aircraft’s exhaust and freezes. This forms cloud-like trails of condensation, or contrails for short. Most contrails dissipate quickly, but for a small fraction of flights, atmospheric conditions align to produce contrails that persist and spread out, trapping heat in the atmosphere.
When taking contrails into account, the warming impact of flying may be up to 60% more than estimates based solely on fuel burn [Lee, 2021. CO2e/GWP100]. Even though we know that only roughly 10% of flights cause the majority of persistent contrails, predicting their formation and attributing the impact to individual flights is difficult — like predicting turbulence weeks or months in advance. Additionally, there is no scientific consensus on how the impact should be quantified for individual flights. For these reasons, it is not currently included in the model used to estimate emissions.
Google is working with scientists, academics, and industry experts to make reliable predictions about contrail impact per flight. Eventually, we plan to include these predictions in the TIM.
Train emission estimates
To calculate emissions for trains, Google uses a method that considers the kilometers traveled and the number of passengers in your search. Trains emit 19 grams CO2e lifecycle emissions per passenger kilometer on average, according to the IEA. Exact emissions depend on the train and operator. IEA’s data is updated annually and Google is working to source accurate information from train operators.