Understanding Carbon Offsets

Total Climate Impacts from Aviation

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 FactorsCO2NOx -> O3 increaseNOx -> CH4 DecreaseNOx -> O3 DecreaseSulfate AerosolsSootContrails and Cirrus Clouds
Climate ImpactWarmingWarmingCoolingCoolingCoolingWarmingWarming
DurationCenturiesWeeks to monthsDecadesDecadesDays to weeksDays to weeksContrails: hours
Cirrus clouds: hours to days
Spatial DistributionGlobalContinental to globalContinental to globalContinental to globalContinental to globalLocal to globalLocal to global
Scientific UnderstandingGoodFairFairFairDirect effects: good
Indirect cloud effects: poor
Direct effects: good
Indirect cloud effects: 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, 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).  How the effects are weighted and which time horizon is used influences the results considerably. This explains the wide range of results when assessing the overall climate impact of aviation and when using air travel calculators.

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:

CO2 Emissions

Indirect Impacts from Nitrogen Oxide Emissions

Particulate Emissions from Aviation: Sulfates & Soot Aerosols

Contrails and Cirrus Clouds from Aviation