Slightly changing altitude could avoid some of the white trails behind aircraft and quickly reduce their effect on the climate.
These white lines are contrails. They appear when an aircraft passes through an area at altitude that is cold and humid enough. Water vapor from the engines then helps form tiny ice crystals. Many contrails disappear quickly, but some persist, spread out and become thin artificial clouds.
A Boeing 747 leaves several contrails in the sky.
Credit: Gunnar Ries / Wikimedia Commons — CC BY-SA 3.0.
The problem mainly comes from these persistent contrails. They can trap some of the heat that would normally escape into space. According to Google and Cathay Pacific, they could account for about one-third of aviation’s total climate impact.
Google is therefore trying to predict the areas where a persistent contrail is likely to form. Its system combines artificial intelligence models, satellite observations and weather data. The resulting information is sent to the teams responsible for the flight. The pilot can then slightly change altitude to avoid an area favorable to contrails.
This method has been tested with Cathay Pacific since the end of 2025. More than 100 flights had been selected for the trial, and more than 80 actually followed avoidance routes. According to Google’s analysis of satellite images, these flights reduced by about 40% the warming effect attributed to their contrails.
Contrails cross the sky above Lisbon, photographed from the space shuttle in February 2000.
Credit: NASA/JPL/UCSD/JSC.
Crews already regularly change altitude to account for weather, turbulence or air traffic. Here, the same principle is used to avoid layers of cold, humid air. Google says these adjustments are planned within the usual safety limits of flight operations.
The route between Hong Kong and Singapore provided a significant share of the results. Conditions favorable to persistent contrails are common there. Measures taken on this single route accounted for more than half of the estimated climate reduction during the trial. This also indicates that the method’s benefits can vary greatly depending on the region and weather conditions.
The system reaches the cockpit through Cathay Pacific’s digital flight-planning system. Pilots can therefore consult Google’s forecasts alongside their other operational data. However, not all proposed changes are implemented: airspace, payload and operational constraints remain the priority.
A second, larger phase will now cover Asian and transpacific routes, including very long-haul flights. The goal is to measure effectiveness in more real-world situations and determine how this method could be integrated into airlines’ routine operations.