Study highlights the non-CO2 climate warming impact of aircraft contrails, that is currently ignored by governments

The issue of how much global warming is caused by the contrails from aircraft is complicated, and there is no firm agreement about how it should be added to the climate impact of aviation – in addition to the CO2 itself.  Contrails are ice crystals that form high altitudes, in certain conditions, around sooty particles from burnt fuel – they then often become large areas of cirrus cloud, that can last for hours – trapping heat, (like a blanket) which does not radiate back out into space. A new study sheds more light on this issue, and says the contrails are having a bigger impact on global warming than usually recognised. While the CO2 from flights will stay in the atmosphere for decades or centuries, the impact of the increased cloud cover from contrails is quite short term. With the huge expansion of the aviation industry, that it is hoping for in future decades, this will only get worse. Its effects might triple by 2050. That is the year in which Britain is committed to achieving net zero carbon emissions, though that includes only CO2 from domestic but not international flights. The UK currently takes NO account of the impact of contrails, which conveniently makes the overall climate breakdown impact of aviation appear smaller than it is, in reality. 


The UK used to use a multiplier for this – until 2011

Up till 2011, the DfT included a multiplier of 1.9 for the non-CO2 effects of air travel.In the DfT’s “UK Air Passenger Demand and CO2 Forecasts”, published in January 2009, a multiplier of 1.9 for “radiative forcing” was used. 31
[Note 31 . 31 UK Air Passenger Demand and CO2 Forecasts. January 2009. DfT ]
However, in the DfT Aviation Forecasts in August 2011, the multiplier is no longer used and in the DfT’s January 2013 Aviation Forecasts there is not even a mention of radiative forcing.

Aircraft leave a trail of climate destruction

Cloud trails formed by aircraft are having a bigger impact on global warming than all the CO2 emitted by aviation since the start of powered flight in 1903, scientists have said.

Researchers say the warming effects of clouds they create will triple by 2050, the year Britain is committed to achieving net zero carbon emissions.

The target includes CO2 from domestic but not international flights and does not include aircraft condensation trails, also known as contrails.

The ribbons of white cloud are created when ice crystals form at high altitudes around sooty particles from burnt fuel. Under certain conditions they become wider patches of cloud cover, known as contrail cirrus. These can linger up to 17 hours and have a net warming effect.

Their impact has been largely neglected in global schemes to offset aviation emissions, Germany’s national aeronautics research centre said. Its study in the journal Atmospheric Chemistry and Physicsforecast that the climate impact of contrail cirrus will triple by 2050 due to increased air traffic.

Researchers found that aviation accounted for about 5 per cent of climate warming caused by man in 2005, with contrail cirrus the largest single contributor.

Lisa Bock, a researcher at the German Aerospace Centre who co-authored the study, said: “It is important to recognise the significant impact of non-CO2 emissions, such as contrail cirrus, on climate and to take those effects into consideration when setting up emission trading systems or schemes like the Corsia agreement.”

Corsia, the UN’s scheme to offset air traffic carbon emissions from 2020, ignores the non-CO2 climate impacts of aviation.

Sir David King, the British government’s former chief scientist, who was not involved in the research, said: “This is an important study — we have taken our eye off the ball when it comes to aviation.”

Uncertainty remains over the scale of the impact of contrails. The scientific consensus is that it is about double that of the CO2produced by aviation, according to Keith Shine of the Department of Meteorology at the University of Reading, who was also not involved in the study.

“One thing to note is the longevity of the emissions. If aviation was halted today the contrails would disappear in a matter of hours. The CO2 emitted by aviation would persist for many decades.

The paper can be found at




27 June 2019

In the right conditions, airplane contrails can linger in the sky as contrail cirrus – ice clouds that can trap heat inside the Earth’s atmosphere. Their climate impact has been largely neglected in global schemes to offset aviation emissions, even though contrail cirrus have contributed more to warming the atmosphere than all CO2 emitted by aircraft since the start of aviation. A new study published in the European Geosciences Union (EGU) journal Atmospheric Chemistry and Physicshas found that, due to air traffic activity, the climate impact of contrail cirrus will be even more significant in the future, tripling by 2050.

Contrail cirrus change global cloudiness, which creates an imbalance in the Earth’s radiation budget – called ‘radiative forcing’ – that results in warming of the planet. The larger this radiative forcing, the more significant the climate impact. In 2005, air traffic made up about 5% of all anthropogenic radiative forcing, with contrail cirrus being the largest contributor to aviation’s climate impact.

“It is important to recognise the significant impact of non-CO2 emissions, such as contrail cirrus, on climate and to take those effects into consideration when setting up emission trading systems or schemes like the Corsia agreement,” says Lisa Bock, a researcher at DLR, the German Aerospace Center, and lead-author of the new study. Corsia, the UN’s scheme to offset air traffic carbon emissions from 2020, ignores the non-CO2 climate impacts of aviation.

But the new Atmospheric Chemistry and Physics study shows these non-CO2 climate impacts cannot be neglected. Bock and her colleague Ulrike Burkhardt estimate that contrail cirrus radiative forcing will be 3 times larger in 2050 than in 2006. This increase is predicted to be faster than the rise in CO2 radiative forcing since expected fuel efficiency measures will reduce CO2emissions.

The increase in contrail cirrus radiative forcing is due to air traffic growth, expected to be 4 times larger in 2050 compared to 2006 levels, and a slight shift of flight routes to higher altitudes, which favours the formation of contrails in the tropics. The impact on climate due to contrail cirrus will be stronger over Northern America and Europe, the busiest air traffic areas on the globe, but will also significantly increase in Asia.

“Contrail cirrus’ main impact is that of warming the higher atmosphere at air traffic levels and changing natural cloudiness. How large their impact is on surface temperature and possibly on precipitation due to the cloud modifications is unclear,” says Burkhardt. Bock adds: “There are still some uncertainties regarding the overall climate impact of contrail cirrus and in particular their impact on surface temperatures because contrail cirrus themselves and their effects on the surface are ongoing topics of research. But it’s clear they warm the atmosphere.”

Cleaner aircraft emissions would solve part of the problem highlighted in the study. Reducing the number of soot particles emitted by aircraft engines decreases the number of ice crystals in contrails, which in turn reduces the climate impact of contrail cirrus. However, “larger reductions than the projected 50% decrease in soot number emissions are needed,” says Burkhardt. She adds that even 90% reductions would likely not be enough to limit the climate impact of contrail cirrus to 2006 levels.

Another often discussed mitigation method is rerouting flights to avoid regions particularly sensitive to the effects of contrail formation. But Bock and Burkhardt caution about applying measures to reduce the climate impact of short-lived contrail cirrus that could result in increases in long-lived CO2 emissions, in particular given the uncertainties in estimating the climate impact of contrail cirrus. They say that measures to reduce soot emissions would be preferable to minimise the overall radiative forcing of future air traffic since they do not involve an increase of CO2 emissions.

“This would enable international aviation to effectively support measures to achieve the Paris climate goals,” Burkhardt concludes.

Please mention the name of the publication (Atmospheric Chemistry and Physics) if reporting on this story and, if reporting online, include a link to the paper ( or to the journal website (

More information

This research is presented in the paper ‘Contrail cirrus radiative forcing for future air traffic’ published in the EGU open access journal Atmospheric Chemistry and Physics on 27 June 2019.

The study was conducted by Lisa Bock and Ulrike Burkhardt (Institute of Atmospheric Physics, DLR).

Citation: Bock, L. and Burkhardt, U.: Contrail cirrus radiative forcing for future air traffic, Atmos. Chem. Phys., 19, 8163-8174,, 2019



Expert reaction to vapour trails and climate change


Science Media Centre

Research published in Atmospheric Chemistry and Physics shows that Contrail cirrus radiative forcing is expected to increase significantly over time and will have a significant climate impact by 2050.

Prof David Lee, Professor of Atmospheric Science, Manchester Metropolitan University, said:

“This is an excellent piece of research that builds on a solid development of model development and evaluation. However, when interpreting this in terms of policy and regulation, the devil is in the detail. Comparing the present-day warming from contrail cirrus to that from historical emissions of the sector’s CO2 is not necessarily the most useful thing to do when considering the future. The ‘radiative forcing’ (RF) of contrail cirrus is indeed several times larger than that of CO2 from aviation but comparing the RF of a short-lived climate effect to that of a long-lived greenhouse gas is fraught with difficulties as the effect of CO2 last many thousands of years because of its long lifetime; the warming effect of contrail-cirrus would be a few decades at most. Moreover, the uncertainties on contrail cirrus forcing remain much larger than that of CO2, particularly the ‘effective radiative forcing’ which accounts better for the effect on surface temperature – which the paper acknowledges. Early estimates suggest that the effective radiative forcing of contrails could be around 1/2 the radiative forcing. Reducing the soot emissions through changes to the aircraft engine may help decrease the contrail-cirrus signal but this also has large uncertainties and can actually results in small CO2 increases because of engine technology tradeoffs, although reducing the aromatic content of the fuel could achieve soot reductions with no combustion technology changes. The authors’ caution on ‘tactical avoidance’ of contrail formation is well placed as the atmospheric tradeoff between increased fuel and CO2 emissions from such measures to reduce contrail formation is not well quantified or understood, and will vary over time. CORSIA is a first attempt to offset CO2 emissions from international aviation but even this will not reduce emissions sufficiently for the Paris Agreement goals; the non-CO2 signal of contrail-cirrus may add to warming in a more uncertain way but CO2 should remain the number one policy priority because CO2 emissions are so long-lasting and impacts well-understood”


Prof William Collins, Professor of Meteorology, University of Reading, said:

“While the vapour trails (contrails) made by aircraft may look pretty in the sky they have a strong warming effect on climate that is often neglected. This research highlights the importance of contrails if air traffic were allowed to continue to increase unchecked for the next 30 years. These contrails have so far caused more warming than the CO2 emitted by the same aircraft. However contrails only last a short time in the sky whereas the CO2 persists for centuries, so if we are able to reduce air traffic, or reduce the contrails it produces, the climate benefit would appear quickly. Much research is underway to investigate the feasibility of diverting aircraft above or below regions forecast to be conducive to contrails.”

Prof Paul Williams, Professor of Atmospheric Science, University of Reading, said:

“Aviation makes a small but growing contribution to climate change. In return, we expect climate change to cause bumpier flights by increasing the amount of turbulence in the atmosphere. Air travel looks set to be a victim of global warming as well as a cause.”


Prof Keith Shine, Professor of Physical Meteorology, University of Reading, said:

“There is no surprise that contrails are causing a bigger warming than CO2. The IPCC Special Report on “Aviation and the Global Atmosphere” published in 1999 was already saying this, although (as is still true today) the uncertainties in the contrail forcing are larger than that for CO2. The work by Bock and Burkhardt is, in my view, the most advanced set of calculations on the contrail effect to date.

“One thing to note though, is the longevity of the emissions. If aviation was halted today, the contrails would disappear in a matter of hours. The CO2 emitted by aviation would persist for many decades and so is a longer-term commitment to the impact of aviation on the environment. Nevertheless, contrails currently make a very significant contribution to aviation climate effects.”

‘Contrail cirrus radiative forcing for future air traffic’ by Lisa Bock and Ulrike Burkhardt et al. was published in Atmospheric Chemistry and Physics at 1pm UK TIME on Thursday 27 June 2019.