Emissions on flights refer to the greenhouse gases and particles released when a jet engine burns fuel. When you see an emissions estimate on a booking site like Google Flights, it’s showing you how much carbon dioxide (CO2) that particular flight is expected to produce per passenger. But CO2 is only part of the story. Aircraft engines also release water vapor, nitrogen oxides, sulfur oxides, hydrocarbons, and soot particles, all of which affect the climate in different ways depending on the altitude at which they’re released.
What Comes Out of a Jet Engine
Jet fuel is a petroleum product, and burning it produces the same basic byproducts as burning gasoline in a car: CO2, water vapor, and smaller amounts of other pollutants. The key difference is where those byproducts end up. A car releases exhaust at ground level. A commercial aircraft releases it six to eight miles above the Earth’s surface, in the upper atmosphere, where even water vapor behaves differently than it does closer to the ground.
CO2 is the most talked-about emission because it’s easy to measure and lasts in the atmosphere for centuries. But nitrogen oxides, released at cruising altitude, trigger chemical reactions that create ozone (a greenhouse gas) in the upper atmosphere. Sulfur oxides and soot particles also interact with the surrounding air in ways that can either warm or cool the planet depending on conditions.
Why Contrails Matter More Than You’d Think
Those white lines you see trailing behind aircraft are contrails, formed when hot, humid exhaust meets extremely cold air at high altitude. Sometimes they evaporate within seconds. Other times they persist and spread into thin, wispy clouds that can linger for hours.
These persistent contrails act like a blanket. Their ice crystals trap heat radiating up from the Earth’s surface, contributing to warming. During the day, they also reflect some incoming sunlight back into space, which has a slight cooling effect. On balance, though, the net result is warming. Research published in Nature Communications found that the global warming effect of contrails is comparable in magnitude to that of aviation’s CO2 emissions, even though individual contrails last only hours while CO2 persists for centuries. Nighttime contrails are the biggest contributors, since there’s no sunlight for them to reflect.
This is why climate scientists use something called the Radiative Forcing Index (RFI) to capture aviation’s full climate impact. The RFI is essentially a multiplier: it compares the total warming effect of all aircraft emissions (including contrails and ozone formation) to the warming from CO2 alone. An RFI greater than 1 means the non-CO2 effects are significant. For aviation, they clearly are.
How Big Is Aviation’s Footprint
Aviation accounted for 2.5% of global energy-related CO2 emissions in 2023, according to the International Energy Agency. That may sound small, but aviation has been growing faster than rail, road, or shipping since 2000. And because that 2.5% figure counts only CO2, the true climate impact, including contrails and other high-altitude effects, is meaningfully larger.
For individual travelers, the comparison across transport modes is striking. Emissions are typically measured in grams of CO2-equivalent per passenger-kilometer. Trains produce 6 to 118 grams per passenger-kilometer, buses 22 to 92, and cars 57 to 322. Flights generally fall at the higher end of the spectrum, particularly for short routes where takeoff and climb (the most fuel-intensive phases) represent a larger share of the total journey.
What Those Numbers on Booking Sites Mean
When Google Flights shows an emissions estimate next to a flight, it’s drawing on data from the European Union Aviation Safety Agency and Google’s own Travel Impact Model. The calculation factors in the route distance, the specific aircraft type operating the flight, and how many passengers it typically carries. Each flight gets labeled as producing higher, typical, lower, or unknown emissions compared to other options on the same route. Flights with lower emissions get a green badge.
The International Civil Aviation Organization (ICAO) has its own carbon calculator that uses similar variables: aircraft type, route-specific data, passenger load factors, and cargo weight. Both tools give you a CO2 figure in kilograms, which represents your share of the total fuel burned on that flight. A half-empty plane means each passenger’s share is larger. A newer, more efficient aircraft brings the number down.
These estimates are useful for comparison shopping, but they typically capture only CO2. The contrail and nitrogen oxide effects aren’t included, which means the real climate cost of any given flight is higher than the number shown.
Why Some Flights Produce Less Than Others
Aircraft efficiency varies enormously by model. A study comparing actual flight data from the Airbus A350 and the older, much larger A380 found the A350 was roughly twice as fuel efficient when carrying the same payload. Newer narrow-body jets like the A321neo and Boeing 787 incorporate lighter materials, more aerodynamic designs, and improved engines that collectively burn significantly less fuel per seat than their predecessors.
Several factors determine any single flight’s emissions:
- Aircraft type: Newer planes burn less fuel per passenger. A route served by a 787 will typically show lower emissions than the same route on an older 767.
- Distance: Longer flights burn more total fuel, but short flights are less efficient per kilometer because takeoff and climb consume a disproportionate share.
- Load factor: A full plane spreads the fuel cost across more passengers, lowering each person’s share.
- Seat class: Business and first-class seats take up more floor space, so those passengers are assigned a larger share of the aircraft’s total emissions.
Sustainable Aviation Fuel and Reduction Efforts
Sustainable aviation fuel (SAF) is the industry’s primary tool for cutting CO2. SAF is a liquid fuel that works in existing jet engines without modification, but it’s made from non-petroleum sources like waste oils, agricultural residues, or synthesized from captured carbon. Over its full lifecycle, from production through combustion, SAF can reduce CO2 emissions by up to 80% compared to conventional jet fuel.
The catch is supply. SAF currently makes up a tiny fraction of total jet fuel consumption worldwide. Airlines are blending small percentages into their fuel supply, and mandates in the EU and other regions are pushing those percentages higher over the coming decade. For now, SAF’s impact on any individual flight you book is minimal, but it’s the clearest path the industry has toward meaningfully lower emissions per flight.
Some airlines also sell carbon offsets at checkout, which fund projects like reforestation or methane capture to compensate for a flight’s CO2. These vary widely in quality and don’t address the non-CO2 effects. If you’re comparing two flights on the same route, choosing the one with lower estimated emissions (newer aircraft, higher load factor) is a more direct way to reduce your impact than purchasing an offset after the fact.