Two aircraft windows onboard an Airbus A321 flying high over the Atlantic ocean. Aviation is tackling non-CO2 emissions such as the contrails produced by this and other aircraft

Spotlight on aviation’s climate impact shifts to non-CO2 effects

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Green Wing logo with white letters against a green backdrop, and leafs on either sideNon-CO2 emissions from aircraft account for as much as two-thirds of the aviation industry’s impact on climate change, yet their effect on global warming flies under the radar as the spotlight shines on cutting carbon dioxide. 

While the flight to decarbonization is long-haul and turbulent, a growing body of research into aviation’s non-CO2 effects suggests that this part of the industry’s climate conundrum could be much quicker and easier to resolve.

At an event hosted by the Royal Aeronautical Society (RAeS) in London this week, the spotlight shone brightly on the non-CO2 climate effects of aviation, which include the formation of contrails and the emission of nitrogen oxides (NOx). Contrails are formed in certain atmospheric conditions when water vapor from jet engines condenses and freezes around soot particles, leaving a trail that is visible from the ground. Over time, longer-lasting contrails can merge together to form heat-trapping contrail-cirrus clouds, which contribute to global warming.

Relative to the challenge of achieving net-zero carbon emissions by 2050 — which involves a massive ramp-up in sustainable aviation fuel (SAF) volumes and the development of new propulsion technologies such as hydrogen — reducing or eliminating the warming effects of contrails could be achieved by rerouting a small percentage of flights to avoid the atmospheric conditions that aid their formation.

“Contrail mitigation is the quickest way to reduce the climate impact of aviation,” Dr Marc Stettler, a senior lecturer in transport and the environment at Imperial College London, said during the RAeS event. Professor Steven Barrett, director of the laboratory for aviation and the environment at Massachusetts Institute of Technology (MIT), agrees, noting that “contrail avoidance is the cheapest and fastest way to mitigate and roll back the impact of aviation on climate”.

Dr Christiane Voigt, professor of atmospheric physics at the University of Mainz in Germany and head of the cloud physics department at the German Aerospace Center (DLR), adds that “the mitigation of non-CO2 effects provides a faster solution to the aviation climate challenge”. 

Voigt says that only a small percentage of flights at certain times of the day would need to be rerouted, with some contrails formed in the middle of the day even having a cooling effect because they block out some of the sun’s heat. In the North Atlantic region, for instance, she points out that 12% of flights produce 80% of contrails, “so only a few flights need to be rerouted for a significant climate gain”.

Dr Adam Durant, founder and chief executive of UK-based climate data intelligence provider Satavia, believes his company can help eliminate the non-CO2 effects of flying, which he says account for 60% of aviation’s climate impact. Durant says that “a small minority of flights” – around 5% – cause up to 90% of contrails, and “we think we can solve this quickly”. 

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Satavia’s DecisionX NetZero platform “enables aircraft operators to forecast, prevent, quantify and offset surface warming caused by contrails”, according to the company’s website. Satavia has been working with Etihad Airways to test contrail-prevention flights, and Durant says the company could deploy its technology “at scale today”, although this would require “close collaboration with ANSPs [air navigation service providers]”.

For its part, NATS believes the Shanwick North East Atlantic airspace it controls is “an ideal testbed for contrail-avoidance trials”, says Dr George Koudis, R&D technical lead at the UK-based ANSP. Four air traffic management strategies could be used – three of which “can be used immediately” – says Koudis, and all are “very much options for discussion”.

With ANSPs seemingly on board, technology providers offering possible solutions and academics stressing that aviation’s non-CO2 climate effects are quick and easy to resolve, the obvious question is: Why is this not being addressed with the utmost urgency? The answer appears to be that the aviation industry is waiting for more scientific clarity before taking action.

Addressing the conference, Eric Maury, head of environment and energy at Airbus Engineering, said that while non-CO2 effects are “a very important subject for aviation”, there is “a high level of uncertainty” around the issue, and “the impact of contrail-cirrus is difficult to assess”. 

David Lee, a professor of atmospheric science at Manchester Metropolitan University, says that “uncertainties remain large”, while Robert Sausen of Germany’s DLR admits that “we need better measurement of humidity at cruise level and we need better contrail detection from satellite- and ground-based cameras”. 

But Matt Finch, UK policy manager at NGO Transport & Environment, argues that the non-CO2 effects of aviation are “somewhere between huge and absolutely massive”, and postponing action due to scientific uncertainty is “not a good look for airlines at the moment”.

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