Impact of contrails on climate

At present, there is no agreed figure for the amount of climate altering effect that is produced by aircraft at high altitude, including cirrus cloud formed by contrails,  in addition to the effect of CO2 they emit. The UK government uses a multiplier of 1.9 for this extra impact by planes. An article in New Scientist in August 2011 presented evidence from a German scientist that contrail cirrus ended up covering 0.6% of Earth’s surface – an area 9 times as great as that covered by line contrails as they emerge from the planes. The  Germany researcher then used this figure to produce a more accurate estimate of the total energy trapped by contrails. Her calculations suggest a global figure of 31 mW/m2 – higher than that attributable to aviation CO2.



Sky above Chiswick, west London, on 8th September 2012 evening

Contrails warm the world more than aviation emissions

12 August 2011 (New Scientist)

by Michael Marshall

The innocuous white trails that criss-cross the sky may not be as harmless as they look. In fact, they might have contributed to more global warming so far than all aircraft greenhouse gas emissions put together.

High-altitude clouds like cirrus warm the planet by trapping heat. Contrail “cirrus” does the same thing, but the question is: how much? We know that contrails trap some extra energy in the atmosphere: their radiative forcing trapped 10 milliwatts per square metre (mW/m2) in 2005, according to an estimate by the Intergovernmental Panel on Climate Change. That compares with 28 mW/m2 trapped by all of the CO2 released by aircraft engines since the start of aviation.

However, the IPCC estimate only took into account relatively fresh, visible vapour trails that exist for just a few hours. Afterwards they spread out and become indistinguishable from normal cirrus. In this form they may trap energy in the atmosphere for many more hours.

“Only a small part of the problem has been studied,” says Ulrike Burkhardt of the Institute for Atmospheric Physics in Oberpfaffenhofen, Germany. With her colleague Bernd Kärcher, she set out to discover how much heat contrail cirrus traps.

Using satellite observations of spreading contrails as a guide, Burkhardt built a model that simulated how they form, spread out and dissipate. Then she embedded it in a global climate model and watched what happened. She found that contrail cirrus ended up covering 0.6 per cent of Earth’s surface – an area nine times as great as that covered by line contrails.

Burkhardt then used this figure to produce a more accurate estimate of the total energy trapped by contrails. Her calculations suggest a global figure of 31 mW/m2 – higher than that attributable to aviation CO2.

As the first to build contrail cirrus into a climate model, Burkhardt’s study is “an important leap forward”, says Olivier Boucher of the Met Office Hadley Centre in Exeter, UK.

There is a catch, though. “[The measurement] says nothing about what will happen tomorrow,” says David Lee of Manchester Metropolitan University in the UK. While a contrail lasts a day, the CO2 released from a plane lingers in the atmosphere for hundreds of years. For example, while contrails – and their warming potential – disappeared from European skies last April when anIcelandic volcanic ash cloud grounded flights, atmospheric CO2 continued to warm the world.

The flip side is that cutting contrails would make an immediate difference to atmospheric warming, whereas emissions cuts take years to have an effect, says Robert Noland of Rutgers University in New Brunswick, New Jersey.

Planes could avoid creating contrails by flying at lower altitudes, he says, or steering clear of water-rich patches of air – although such measures could prove counterproductive if they make flying more inefficient and lead to a greater increase in CO2 levels.

Journal reference: Nature Climate Change, DOI: 10.1038/nclimate1068


It's the way they spread out (Image: <a href="" target="ns">Mario Aurich/</a>)

It’s the way they spread out (Image: Mario Aurich/


Below is an extract from the Committee on Climate Change Report – April 2012

Page 33

Contrails and induced cirrus.

Contrails are lines of cloud caused by aircraft flying through supersaturated air. Induced cirrus cloud comes from spreading contrails, or from aircraft aerosol emissions. At the level aircraft fly, both have a significant warming effect but this only lasts up to a few hours. The actual extent of aircraft induced cirrus cloud and the magnitude of the current warming associated with the increased cloud is very uncertain. The best estimate is that it is comparable to the CO2 impact of aviation, but it could be a small fraction of it or up to three times as large. There is scope to reduce these along with CO2 emissions through flight path optimisation (e.g. routing of aircraft to avoid or make use of certain weather systems) although in some cases there may be a trade-off between CO2 and cloud reduction.
Page 34

Applying a multiplier to aviation CO2 emissions has been suggested in order to reflect total
aviation effects on the climate. However this would not provide a direct incentive to reduce
the non-CO2 effects, and may instead lead to CO2 measures being taken which actually
increase NOx or contrails and induced cirrus.

Page 35

Therefore the appropriate approach is to develop scientific understanding, and to ensure that options to markedly reduce contrails and induced cirrus are developed over the next decade.

Depending on progress made, there may be a need for a future adjustment to carbon
budgets. For example, if there were still to be significant NOx emissions or production of
contrails and induced cirrus in several decades, this would be a cause for concern, and could require that carbon budgets are tightened in order that the climate objective is achieved.

This is something that we will monitor in the context of our annual reports to Parliament and advice on setting carbon budgets.


CCC report, slides etc are at

Report “Scope of Carbon Budgets – Statutory advice on inclusion of international aviation and shipping” is at





2011 Guidelines to Defra / DECC’s GHG
Conversion Factors for Company Reporting:
Methodology Paper for Emission Factors

Page 59 of 102


Radiative Forcing


The emission factors provided in the 2011 GHG Conversion Factors Annex 6 and Annex 7 refer to aviation’s direct CO2, CH4 and N2O emissions only. There is currently uncertainty over the other non-CO2 climate change effects of aviation (including water vapour, contrails, NOX etc) which have been indicatively been accounted for by applying a
multiplier in some cases.


Currently there is no suitable climate metric to express the relationship between emissions and climate warming effects from aviation but this is an active area of research. Nonetheless, it is clear that aviation imposes other effects on the climate which are greater than that implied from simply considering its CO2 emissions alone.


The application of a multiplier‘ to take account of non-CO2 effects is a possible way of illustratively taking account of the full climate impact of aviation. A multiplier is not a straight forward instrument. In particular it implies that other emissions and effects are directly linked to production of CO2, which is not the case. Nor does it reflect accurately the different
relative contribution of emissions to climate change over time, or reflect the potential trade-offs between the warming and cooling effects of different emissions.

On the other hand, consideration of the non-CO2 climate change effects of aviation can be important in some cases, and there is currently no better way of taking these effects into account. A multiplier of 1.9 is recommended as a central estimate, based on the best available scientific evidence, as summarised in Table 50 below56. If used, this
factor would be applied to the emissions factors set out here.