Non-CO2 climate impact of flying could be cut significantly with small altitude changes to avoid contrail formation
Date added: 16 February, 2020
Some research by Imperial College, London, indicates that climate impact of aviation could be significantly reduced by making small changes to the altitudes at which planes fly. And more complete fuel burn. Contrails increase warming, due to a blanket effect, especially at night, preventing heat escaping out into space. This causes “radiative forcing.” Contrails form as water condenses around the tiny black carbon particles in the jet exhaust. They form more, and last for longer, in some weather conditions than others. While most contrails disappear within minutes, some spread and mix with other contrails and clouds, forming ‘contrail cirrus’ which can linger for as long as 18 hours. The study by Imperial indicated that flying around 2,000 feet lower or higher – avoiding the more humid air – can reduce contrail formation. Reducing the contrails of the planes having the most climate warming impact would help slightly. Unlike contrails, the impact of the CO2 produced lasts for hundreds of years. Flying higher or lower than normal cruise height could increase jet fuel burn and CO2 emissions. Aviation expects to grow fast between now and 2050, with contrail warming a big problem. However, CORSIA ignores this additional non-CO2 warming impact of aviation. . Tweet
Climate impact of flying could be cut significantly with small altitude changes, study suggests
‘This new method could very quickly reduce the overall climate impact of the aviation industry,’ scientist says
Andy Gregory (Independent) 14.2.2020
The climate cost of aviation could be significantly reduced by making small changes to the altitudes at which aircrafts fly, a new study suggests.
Contrails – the white fumes often spotted trailing behind planes – are thought to have more of a warming effect than the CO2 produced by planes.
But new research suggests changing the altitude of just 1.7 per cent of flights by a margin of 2,000ft could reduce contrails’ climate impact by nearly 60 per cent.
Furthermore, when combined with using cleaner engines, contrail-related harm could be slashed by up to 90 per cent, the study led by Imperial College London (ICL) scientists found.
“According to our study, changing the altitude of a small number of flights could significantly reduce the climate effects of aviation contrails,” said lead author Dr Marc Stettler, a senior lecturer at ICL’s faculty of engineering.
“This new method could very quickly reduce the overall climate impact of the aviation industry.”
Contrails are produced when hot exhaust gases meet colder, low-pressure air in the atmosphere. Moisture then condenses on the black carbon particles within the fumes, forming the white streaks noticeable from Earth.
While most contrails disappear within minutes, some spread and mix with other contrails and clouds, forming ‘contrail cirrus’ which can linger for as long as 18 hours.
These larger clouds produce an effect called “radiative forcing”, in which the balance between the radiation emitted from the Earth and that which it receives from the sun is disrupted, causing a change in climate.
Although studies suggest this process could play as large a role in heating the planet as CO2 – which remains in the atmosphere for hundreds of years – the impact of contrails is relatively short-lived, making it potentially easier to curtail.
Contrails only form and linger in thin layers of the atmosphere, which have a higher humidity – therefore small changes in the altitude at which aircrafts fly can in theory allow them to drastically limit the number of contrails they produce.
Using data from Japan’s airspace, the scientists in the ICL-led study, published in the journal Environmental Science & Technology, found that just two per cent of flights were responsible for 80 per cent of the “radiation forcing” within the surrounding airspace.
By simulating these planes to fly either 2,000 feet higher or lower than their actual flight paths, the scientists found they could cut this effect by 59 per cent.
While some extra fuel would be consumed by changing the flight path, researchers said the impact of the CO2 released as a result was more than offset by the reduced contrail formation.
Dr Stettler suggested that by only changing the flight path of a minority of aircrafts, large hikes in CO2 emissions could be avoided.
“We’re conscious that any additional CO2 released into the atmosphere will have a climate impact stretching centuries into the future, so we’ve also calculated that if we only target flights that wouldn’t emit extra CO2, we can still achieve a 20 per cent reduction in contrail forcing,” he said.
The researchers also suggested that up to a 90 per cent reduction in contrail-related harm could be avoided if this approach was used in conjunction with more efficient engine technologies, which spout less contrail-forming black carbon particles into the atmosphere.
Campaigners are increasingly warning of the importance of tackling non-CO2 aviation emissions.
Researchers from Germany’s Institute of Atmospheric Physics calculated last year that contrail-related warming was set to triple by 2050.
But the UN’s plan to offset any increase in global aviation emissions above 2020 levels, known as the “Corsia” agreement, does not take into account the industry’s non-CO2 impact on the climate.
Altering the altitudes of less than two per cent of flights could reduce contrail-linked climate change by 59 per cent, says a new Imperial study.
This new method could very quickly help to reduce the overall climate impact of the aviation industry.Dr Marc Stettler, Department of Civil and Environmental Engineering
Aircraft contrails – the white streaks aircraft leave in the sky – could be as bad for the climate as their carbon dioxide (CO2) emissions. Now, Imperial College London-led research has found that flight altitude changes of just 2,000 feet could lessen their effect.
This, the researchers say, combined with using cleaner aircraft engines, could reduce contrail-caused harm to the climate by up to 90 per cent.
Lead author Dr Marc Stettler, of Imperial’s Department of Civil and Environmental Engineering, said: “According to our study, changing the altitude of a small number of flights could significantly reduce the climate effects of aviation contrails. This new method could very quickly reduce the overall climate impact of the aviation industry.”
The research is published in Environmental Science & Technology.
Contrail conundrum
When hot exhaust gases from aircraft meet the cold, low-pressure air of the atmosphere, they produce white streaks in the sky called ‘condensation trails’, or contrails.
The contrail fumes include black carbon particles, which provide surfaces on which moisture condenses to form ice particles. We see this condensation as fluffy white streaks.
Most contrails last only a few minutes, but some spread and mix with other contrails and cirrus clouds, forming ‘contrail cirrus’ that linger for up to eighteen hours.
Contrails and the clouds they help form might be just as bad for the environment as CO2 emissions
Previous research suggests that contrails and the clouds they help form have as much of a warming impact on the climate as aviation’s cumulative CO2 emissions, because of an effect known as ‘radiative forcing’. This is where the balance is disrupted between radiation coming to earth from the sun and heat emitted from the surface of the earth going out to space, forcing a change in the climate.
The key difference between CO2 and contrails, however, is that while CO2 will have an impact in the atmosphere for hundreds of years, the impact of contrails is short-lived and could therefore quickly be reduced.
Now, Dr Stettler and colleagues have used computer simulations to predict how altering aircraft altitudes might reduce the number of contrails and how long they linger, which would reduce their warming impact. This is because contrails only form and persist in thin layers of the atmosphere that have very high humidity. Because these layers are thin, small changes to flight altitudes would mean that aircraft could avoid these regions, leading to fewer contrails forming.
Using data from Japan’s airspace, they found that just two per cent of flights were responsible for 80 per cent of radiation forcing within the airspace. Dr Stettler said: “A really small proportion of flights are responsible for the vast majority of contrail climate impact, meaning we can focus our attention on them.”
Targeting the few flights that cause the most harmful contrails, as well as making only small altitude changes, could significantly reduce the effect of contrails on global warming.Roger TeohDepartment of Civil and Environmental Engineering
Taking into account the congestion in the airspace above Japan, the team simulated these planes to fly either 2,000 feet higher or lower than their actual flight paths and found that the contrail climate forcing could be cut by 59 per cent by altering the altitudes of 1.7 per cent of flights.
The diversion in flight paths caused less than a tenth of a per cent increase in fuel consumption – but, the researchers say, the reduced contrail formation more than offset the CO2 released by the extra fuel.
Dr Stettler suggests that their method of targeting only the few flights that cause the most climate forcing is the best way to avoid hikes in CO2 emissions. He said: “We’re conscious that any additional CO2 released into the atmosphere will have a climate impact stretching centuries into the future, so we’ve also calculated that if we only target flights that wouldn’t emit extra CO2, we can still achieve a 20 per cent reduction in contrail forcing.”
The study’s first author, Roger Teoh, also of Imperial’s Department of Civil and Environmental Engineering, said: “Our simulation shows that targeting the few flights that cause the most harmful contrails, as well as making only small altitude changes, could significantly reduce the effect of contrails on global warming.”
Industry impact
The researchers say aircraft engines themselves also play a part in how harmful contrails are. Black carbon particles are produced by incomplete fuel combustion, so new, more efficient engine combustion technology could help to reduce them by around 70 per cent.
This, combined with small altitude changes, could help reduce overall contrail harm by around 90 per cent.
Next, the researchers will refine their simulations to more accurately predict the characteristics and impact of contrails, and to evaluate the wider effects and practicalities of contrail mitigation strategies such as altering flight paths.
Altering aeroplane altitudes might help reduce contrail harm
