CO2 emissions and the decarbonization of the aviation industry tend to get the biggest slice of the media attention pie, and quite understandably so. There is nothing quite like futuristic hydrogen planes and silent urban air mobility eVTOLs to remind us that the future we once imagined in sci-fi novels is right around the carbon-neutral corner. However, aviation impacts global warming in other ways than through its carbon dioxide contribution. Let's take a look at how, and some of the challenges it presents.

Beyond CO2

Aviation produces about 2.5% of all anthropogenic CO2 emissions. However, when non-CO2 impacts on climate are taken into account, the industry's effect on global warming increases to approximately 3.5%. Meanwhile, most non-CO2 emissions from aviation are not included in the Paris Agreement to limit global warming below two degrees Celsius. This means they could easily be overlooked. Particularly as international aviation is not included in the emission calculation for countries (domestic aviation emissions are).

In 1999, the International Panel on Climate Change (IPCC) published a special report on aviation's impact on the environment, called 'Aviation and the Global Atmosphere'. This estimated that aviation's impact on the climate had been two to four times higher than from CO2 emissions alone. However, pinning an actual number to just exactly how much - and even how, has been tricky, and research - and potential policies - are just beginning to properly catch up.

Britain COP 26 aviation impact
Photo: Getty Images

Dependent on time and place

In November 2020, the European Aviation Safety Agency (EASA) conducted a study that looked at the impact of oxides of nitrogen (NOx), soot particles, oxidised sulphur species, and water vapour in order to set the stage for forthcoming policy directives within the bloc.

The short atmospheric 'lifespan' of these pollutants make their impact on the climate highly dependent on the season, time of day, and location - unlike CO2 emissions which have a longlasting effect across the planet. However, the researchers confirmed that aviation's non-CO2 effect on climate change was at least as important in total as those of CO2 alone.

Speaking at a panel on the future of aviation during The Economist's Sustainability Week, Boeing's Vice President and Chief Engineer Sustainability and Future Mobility, Brian Yutko, acknowledged that,

"As we think about the decarbonization problem, we've started to think about eliminating the climate impact of aviation, which extends beyond CO2. (...) We need to do full lifecycle accounting of what and where all the emissions are."

Some heating, some cooling, but overall impact is detrimental

It is not as straightforward as saying that all of aviation's climate impact has a heating effect. For instance, NOx (nitrous oxide) lead to a higher concentration of O3 (ozone) in the atmopshere, but also lower amounts of CH4 (methane). The former has a short-lived warming effect, and the latter a longer-lived cooling effect.

Meanwhile, aircraft also emit sulphur dioxide, which leads to the formation of sulphate aerosols, soot, and water vapour. When taken altogether into account as measured by radiative forcing (a measurement of the influence a given climatic factor has on the amount of downward-directed radiant energy impinging upon the earth's surface) however, non-CO2 effects equal those of carbon dioxide emissions.

Aircraft-Contrails-getty
Photo: Getty Images

The contrail conundrum

Given certain meteorological conditions, jetliners also produce what is known as contrails. They arise as water vapour condenses onto soot particles in very cold, humid, and 'supersaturated air.' These can then evolve into cirrus clouds, trapping heat that would otherwise escape into space.

There is ongoing scientific debate and research concerning the exact impact of contrails on global warming. Some say they could account for as much as 57% of aviation's total climate impact. However, most agree that more research is needed to determine their precise effects.

Boeing has partnered with NASA Langley Research Center to test different kinds of sustainable fuels to test their emissions and understand their overall impact on contrail formation. Mr Yutko added,

"I think that this is an area of an urgent need for the scientific community - to do more research into contrail formation and contrail impact on the climate so that we can reduce the overall uncertainty of some of these estimates in the climate models and have more of a certain viewpoint on what pathways have benefit and which potentially have some significant drawbacks."

ZEROe demonstrator powered by the hydrogen combustion engine
The effects of hydrogen water vapor on contrails are yet to be determined. Photo: Airbus

Computational modeling of future technology

Rob Miller who is (among many other titles) Professor of Aerothermal Technology at Cambirdge University, says he could not agree more, particularly when addressing the uncertainty of the impact of future fuel and propulsion architecture solutions.

"...especially with hydrogen contrails, where there's more water vapour, but the contrails are of a different type. And we've had no test flights with hydrogen aircraft. So that is literally computational modelling, which is allowing us to get some first inkling of the effect. And the uncertainty on that is really high."

Etihad Boeing 787-9 Dreamliner
Photo: Getty Images

Balancing act between CO2 and other effects

Some airlines, such as Etihad, are using data driven approaches to alter flight paths to eliminate or at the very least reduce the formation of contrails. AI algorithms take into account parameters such as air pressure, temperature, and altitude weather patterns. They then calculate a plan adjusting accordingly, including changing flight levels.

Professor Miller is positive to these kinds of flight path adaptation solutions. He believes airfreight, with cargo planes being less sensitive to timetables, in particular, could adapt schedules to avoid causing condensation trails.

However, Cait Hewitt, Policy Director of the Aviation Environment Federation, the UK's leading NGO focused on policy change related to aviation’s impacts on the environment, says it is not so straight forward.

"One of the difficulties in terms of rerouting aircraft is that as soon as you extend the flight path, you're increasing the amount of CO2 emissions. So you can't simply try and fly around cold air masses in order to avoid contrails (...) The other thing is that airspace in the UK, and Europe, is very congested. So it's not a straightforward thing to sit in a lab and say 'well, you know, I've got a perfect flight path for this one aircraft' if you've got a whole load of other ones also competing for the same airspace."

Do you beleive policymakers need to wait until there is more clarity around the exact impact of non-CO2 aviation emissions, or should they act today with the information that is available? Leave a comment below and join the conversation.