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.
Our recommendation:
When looking to offset air travel, we recommend using a multiplier of 3 to account for all climate effects (Lee et al, 2020). Applying a multiplier should go hand in hand with purchasing high-quality offset credits.
Further, choosing low-carbon alternatives to air travel such as video conferencing or train travel should be prioritized over purchasing offset credits. The Stockholm Environment Institute’s TR2AIL project provides practical information on air travel and strategies for avoiding it.
If you still must fly, carbon offset purchases should be incorporated within organizational or company level strategies to achieve emission reductions. These strategies should prioritize internal reductions and supply chain or product emission reductions over carbon offset purchases.
Related pages:
Indirect Impacts from Nitrogen Oxide Emissions
Particulate Emissions from Aviation: Sulfates & Soot Aerosols