Flight emissions and climate change: it’s not only about CO2

The true environmental cost of flying is far greater than just its carbon footprint – approximately two-thirds of its warming impact result from emissions that are not carbon dioxide.
Written by Team Oncarbon
Carbon management platform for air travel, API-first
December 13, 2022

Discussions of aviation’s impact on climate tend to focus on decarbonization, i.e. reducing the CO2 emissions that result from flying. However, aviation also affects global warming in ways other than through its carbon dioxide emissions.

Because these non-CO2 emissions were not initially included in the Paris Agreement to limit global warming*, they are often overlooked when calculating the climate impact of flying. However, according to researchers, their overall impact on climate change may be up to three times bigger than the warming caused by aviation’s CO2 emissions alone1.

Let us take a closer look at these non-CO2 emissions.

Flight emissions and radiative forcing index

Like other engines that burn fossil fuels, aircraft emit gasses, noise, and particulate matter.

The gasses include:

  • Carbon dioxide (CO2)

  • Nitrogen oxides (NOx)

  • Water vapor

  • Carbon monoxide (that bonds with oxygen to form CO2).

Other emissions include atmospheric particulates such as soot and sulfate aerosols.

When emitted at high altitudes, these emissions affect atmospheric physical and chemical properties, resulting in an increase in greenhouse gases and the potential formation of persistent contrail cirrus. An aircraft flying at a high altitude causes more warming from these gasses, because higher altitudes affect fuel combustion and emission characteristics.

To describe the rate at which aviation emissions at high altitudes warm the atmosphere compared to associated CO2 emissions alone, we use the radiative forcing index (RFI).

In 1999, the Intergovernmental Panel on Climate Change (IPCC) estimated that the radiative forcing of aviation in 1992 was 2.7 times (2 to 4) that of CO2 emissions alone2. More recent studies by David S. Lee and others, published in the journal Atmospheric Environment, conclude that a realistic RFI for today’s flights is 3.03.

Radiative forcing index discrepancies

Because there is no universal agreement on RFI, different flight emissions calculators provide different estimates for the total footprint of a given flight. Some follow the 2013 guidelines from the U.K. Department for Environment, Food and Rural Affairs, which set the RFI at 1.9. Others use the IPCC’s 1999 estimate of 2.7.

IATA (International Air Transport Association) and ICAO (International Civil Aviation Organization) do not apply the RFI at all in their calculations. In its recently published “Recommended Practice of Passenger CO2 Calculation Methodology”4, IATA states that “the use of the Radiative Forcing Index (RFI) is not recommended as long as scientific uncertainties exist about the exact effects and as long as no consensus has been reached in the global scientific community on the value(s) to be applied.” ICAO stated that it “will only adopt a multiplier if the scientific community reaches general agreement on this issue.”

As Oncarbon, we would like to emphasize that the lack of agreement on radiative forcing does not mean that there is no additional warming effect. We have chosen to use the value from the most recent research on the subject – the RFI of 3.0.

Contrails – what are they?

Non-CO2 effects from aircraft emissions include water vapor produced during fuel combustion. Water vapor condenses at high altitudes in cold and humid conditions, forming visible line clouds called contrails (shorthand for condensation trails).

Depending on the temperature and moisture content of the air at the aircraft’s altitude, contrails evaporate quickly (in low humidity conditions) or persist and grow (in high humidity conditions).

These persistent contrails are of particular interest to scientists – firstly, because they would not have formed in the atmosphere without the flyby of an aircraft, and secondly, because persistent contrails often evolve and spread into extended cirrus clouds that are indistinguishable from naturally occurring cloud cover. And because changes in cloud cover affect the temperature of the atmosphere, cloud cover caused by human activities can contribute to long-term climate changes.

Contrails are line-shaped clouds composed of ice particles, visible behind jet aircraft engines.

The exact impact of contrails on global warming is the subject of scientific debate and study. Some say they may account for as much as 57% of the total climate impact of aviation. However, most agree that more research is needed to determine their exact impact.

Emissions from aviation and the environment

Despite scientific advancement many flight emissions calculators do not account for non-CO2 effects, resulting in a gross underestimation of the total climate footprint of a flight. We believe that more discussion of non-CO2 effects is needed, as well as more transparency in communicating emissions overall – especially as the environmental impact of flying becomes more important to airline customers.

According to the Oncarbon Sustainable Travel Report, 67% of respondents would like to receive environmental information about their holiday and 63% of respondents believe booking websites, travel agents and tour operators should be required to disclose the carbon footprint of the flights and other travel products they sell. 72% of travelers would be willing to book a more sustainable flight option if they had such an option.

As Oncarbon, we offer rigorous emissions calculations and include non-CO2 emissions in our calculation model. To learn more about this – and what options you can offer your passengers to actually reduce these emissions – check out our demo and learn more about carbon withdrawal credits and SAF credits.

Cover photo: USGS / Unsplash.

Team Oncarbon
We’re on the mission to show you that we need – and we can – travel more consciously. We bring closer to you the topics of carbon footprint, sustainable travel and aviation, and transparency in emissions reporting.