How emissions are estimated

Google Flights shows lifecycle greenhouse emissions estimates for the number of selected passengers next to each flight. Flights are labelled as having higher, typical, lower or unknown emissions.

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 recognised standards. 

For more information, view the:

• Frequently asked questions
• Detailed specification

Typical emissions are the median emissions for your searched route. The median is calculated as the middle value amongst all the possible emissions per route and considers flights over the next year.

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 emission' 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.

For some flights, we don't have emissions data available, nor are we able to make a close estimate. This might happen for a very specific aircraft type. In these cases, we won't show any emission estimates and the flight will be labelled with 'unknown emissions'.

Actual emissions may vary and depend on factors such as:

• The aircraft model and configuration
• The speed and altitude of the aircraft
• The distance between origin and destination
• The number of passengers

To understand the emission estimates that we display, it's important to know that:

• Non-stop flights aren't always less polluting, especially for long routes. It is 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 that 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 the 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 vapour 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.

To calculate emissions for trains, Google uses a method that considers the distance travelled and the number of passengers in your search. Trains emit 19 grams of CO2e lifecycle emissions per passenger kilometre 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.