In terms of total impact, air travel results in atmospheric warming from CO2 and soot emissions, NOx induced O3 formation, and night-time contrail and cirrus cloud formation. To a lesser extent, air travel also results in cooling due to emission of sulfate aerosols, day-time contrail formation, and CH4 destruction due to NOx emissions.
| Climate Factors | CO2 | NOx -> O3 increase | NOx -> CH4 Decrease | NOx -> O3 Decrease | Sulfate Aerosols | Soot | Contrails and Cirrus Clouds |
|---|---|---|---|---|---|---|---|
| Climate Impact | Warming | Warming | Cooling | Cooling | Cooling | Warming | Warming |
| Duration | Centuries | Weeks to months | Decades | Decades | Days to weeks | Days to weeks | Contrails: hours Cirrus clouds: hours to days |
| Spatial Distribution | Global | Continental to global | Continental to global | Continental to global | Continental to global | Local to global | Local to global |
| Scientific Understanding | Good | Fair | Fair | Fair | Direct effects: good Indirect cloud effects: poor | Direct effects: good Indirect cloud effects: poor | Poor |
It is not easy to combine these different effects. Some influences are regional and only last for a few weeks. Others are global and last for centuries. Cloud formation (e.g., cirrus and contrail), which is poorly understood but may have a large impact, is particularly difficult to quantify. In addition, short-lived, regional effects can have enhanced impacts.
The total climate effects of flying are estimated to be 2 to 3 times larger than CO2 emissions alone (EU Commission 2019, UBA 2019). These estimates can vary broadly based upon how the effects are weighted, and the time horizon applied. This explains the wide range of results when assessing the overall climate impact of aviation and when using air travel calculators created by different organizations.
Digging into the science behind these models, the uncertainty exists because parameters used to determine effective radiative forcing (ERF) often do not account for the variability in aviation altitudes and latitudes. For example, the model parameters do not account for the strong regional sensitivity of ozone (which varies at different latitudes), or the ways that CH4 affects NOx, the strongest effects are at low latitudes (NOx in stratosphere vs. troposphere creates different chemical reactions).
Related pages:
Indirect Impacts from Nitrogen Oxide Emissions
Particulate Emissions from Aviation: Sulfates & Soot Aerosols