Where does Google get this information?
Typical emissions are the median carbon emissions for your searched route. The median is calculated as the middle value amongst all the possible carbon emissions per route, and considers all available dates and flights.
Carbon emission estimates for each flight are compared to the route's median. 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 median. To find lower emission flights, try different dates.
Factors impacting carbon emissions
Actual carbon 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
The most commonly used aircraft types are supported. If we don't have estimates for a new aircraft, we'll use the closest estimate we have available. As new aircraft enter the market and the scientific community advances their calculation methods, we'll continue to update our algorithms to the latest scientific insights and standards.
Actual carbon emissions between route options may vary and depend on a number of factors that we consider. To understand the carbon emission estimates that we display, it's important to know a few things.
- 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 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.
We continue to improve the accuracy of these estimates through ongoing refinements to the Travel Impact Model, which Google has published to document exactly how emissions estimates are calculated.
Other warming effects of flying
In addition to carbon dioxide (CO2), aircraft release nitrogen oxides (NOx) and soot into the atmosphere, both of which contribute to the warming effects of flying.
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 true impact of flying may be up to 60% more than what is currently estimated [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 model used to estimate emissions.
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.