Flight emissions are the gases and particles released when aircraft burn jet fuel. Aviation accounts for 2.5% of global energy-related CO2 emissions, but its true climate impact is roughly three times larger than that number suggests, because planes release other warming agents besides carbon dioxide at high altitudes where they do the most damage.
What Comes Out of a Jet Engine
Jet fuel is a hydrocarbon, so burning it produces the same basic byproducts as burning gasoline or natural gas: carbon dioxide and water vapor. About 72% of combustion products (by mass, excluding the nitrogen and oxygen that pass through) are CO2, and roughly 28% are water vapor. The remaining fraction, less than 1%, includes nitrogen oxides, sulfur oxides, carbon monoxide, unburned hydrocarbons, and soot.
That sub-1% slice matters more than it sounds. For every kilogram of jet fuel burned, an engine produces about 3,160 grams of CO2 and 1,230 grams of water vapor. It also releases between 4 and 29 grams of nitrogen oxides depending on engine power, with takeoff producing the highest levels. Sulfur oxides add another 0.8 to 1.3 grams per kilogram of fuel, and soot particles range from 10 to 550 milligrams. These small quantities have outsized effects in the upper atmosphere.
Why CO2 Is Only Part of the Story
If you look only at carbon dioxide, aviation’s 2.5% share of global emissions seems modest. But planes cruise at altitudes above 8 kilometers, where several additional warming mechanisms kick in. A landmark 2020 study in Atmospheric Chemistry and Physics found that aviation’s total warming effect is approximately three times the rate associated with its CO2 alone. Non-CO2 effects account for about 66% of aviation’s total climate forcing.
The biggest single contributor isn’t CO2 at all. It’s contrail cirrus, the wispy clouds that form behind aircraft. When hot exhaust mixes with air colder than about minus 40°C, the water vapor and soot particles in the exhaust seed ice crystals. If the surrounding air is humid enough, these crystals spread into thin, high-altitude cloud sheets that can persist for hours. These clouds trap heat radiating up from Earth’s surface, creating a warming effect comparable in scale to natural variations in the sun’s energy output.
Nitrogen oxides add another layer of complexity. At cruising altitude, they trigger chemical reactions that both create ozone (a greenhouse gas) and destroy methane (another greenhouse gas). The net result is still warming, but the competing effects make nitrogen oxides one of the hardest components to quantify precisely. Non-CO2 factors contribute about eight times more uncertainty to aviation’s climate estimates than CO2 does.
Emissions on the Ground
About 21% of the soot mass and 12% of soot particle numbers from a flight are released during landing, taxiing, and takeoff, all within roughly 900 meters of the ground. This disproportionately affects communities near airports. Measurements taken near runways detect concentration spikes of over one million ultrafine particles per cubic centimeter, with most particles smaller than 25 nanometers in diameter.
Departing aircraft produce higher particle concentrations than arrivals, because takeoff requires significantly more engine thrust. These ultrafine particles are too small to register as much mass on standard air quality monitors, but their sheer number is what concerns health researchers. In terms of particle count rather than weight, airport emissions represent a significant local pollution source. As the hot exhaust cools and mixes with ambient air, volatile compounds also condense into additional particulate matter downwind.
How Flying Compares to Other Transport
Rail travel generally produces fewer emissions per passenger-mile than flying, but the gap depends on distance and technology. Electric trains emit about 0.134 pounds of CO2 per passenger-mile, while diesel trains average around 0.280 pounds. The mileage-weighted average for a mixed rail system like Amtrak’s northeastern corridor works out to roughly 0.227 pounds per passenger-mile.
Short flights on regional jets are the least efficient form of air travel per passenger-mile, because smaller planes burn more fuel relative to the number of seats, and a larger share of total fuel goes toward climbing to altitude. Single-aisle jets on longer routes perform better. At distances over 700 miles, single-aisle jets can actually match or beat diesel rail on a per-passenger-mile basis, partly because rail routes between distant cities cover more ground miles than a direct flight path. For shorter trips where rail is an option, the train wins clearly on emissions.
Efforts to Reduce Flight Emissions
The International Civil Aviation Organization adopted a goal of net-zero carbon emissions from international aviation by 2050, aligning with the Paris Agreement’s temperature targets. The goal is aspirational rather than binding. It does not assign specific reduction quotas to individual countries, instead allowing each nation to contribute based on its circumstances, development level, and aviation market maturity.
Sustainable aviation fuel, made from sources like waste oils, agricultural residues, or synthetic processes, is the most prominent near-term solution. The U.S. Sustainable Aviation Fuel Grand Challenge targets 3 billion gallons of domestic consumption by 2030 and 35 billion gallons by 2050, with a minimum 50% reduction in lifecycle emissions compared to conventional jet fuel. Current production remains a tiny fraction of those targets, and SAF typically costs several times more than standard kerosene.
Contrail avoidance is a newer strategy gaining attention. Because contrails only form under specific temperature and humidity conditions, rerouting flights to avoid those atmospheric zones could reduce a large portion of aviation’s non-CO2 warming. Small altitude changes of a few thousand feet can sometimes prevent contrail formation entirely, at the cost of slightly higher fuel burn. Whether the CO2 tradeoff is worth the contrail reduction depends on atmospheric conditions for each individual flight.