Reading University research indicates the extent of non-CO2 aviation emissions on climate

Some research from the University of Reading, published in Environmental Research Letters, indicates just how much of the impact of aircraft is not only from the CO2 they emit, but also from the water vapour they emit. This will form contrails, in some weather conditions. These contrails can then expand and create a layer of high cloud, which has significant climate effects as it traps heat below it. The exact extent of the climate impact of the non-CO2 emissions from planes at high altitude is not established. It is likely to have around double the climate impact of the CO2.  The research implies that it may be better for some planes to fly longer distances, burning more fuel and emitting yet more CO2, in order to avoid areas where contrails will form the most, and be the most long lasting. Contrails form where the air is very cold and moist, which is often in the ascending air around high-pressure systems. On average, 7% of the total distance flown by aircraft is in such areas. However, it is hard to compare the climate impacts of contrails and short term warming, and CO2 because the former can last just hours while the latter is irreversible and will last decades.



The research is at

A simple framework for assessing the trade-off between the climate impact of aviation carbon dioxide emissions and contrails for a single flight

E A Irvine1, B J Hoskins1,2 and K P Shine1

1 Department of Meteorology, University of Reading, Reading, UK
2 Grantham Institute for Climate Change, Imperial College London, London, UK


Persistent contrails are an important climate impact of aviation which could potentially be reduced by re-routing aircraft to avoid contrailing; however this generally increases both the flight length and its corresponding CO emissions. Here, we provide a simple framework to assess the trade-off between the climate impact of CO emissions and contrails for a single flight, in terms of the absolute global warming potential and absolute global temperature potential metrics for time horizons of 20, 50 and 100 years. We use the framework to illustrate the maximum extra distance (with no altitude changes) that can be added to a flight and still reduce its overall climate impact. Small aircraft can fly up to four times further to avoid contrailing than large aircraft. The results have a strong dependence on the applied metric and time horizon. Applying a conservative estimate of the uncertainty in the contrail radiative forcing and climate efficacy leads to a factor of 20 difference in the maximum extra distance that could be flown to avoid a contrail. The impact of re-routing on other climatically-important aviation emissions could also be considered in this framework.



Persistent contrails are a climate impact of aviation whose radiative forcing may be comparable with that from aviation carbon dioxide (CO) emissions (Burkhardt and Kärcher 2011). There are few viable technological options for reducing contrail formation (Haglind 2008, Gierens et al 2008), meaning that the easiest way of mitigating this climate impact is to avoid routing aircraft through regions where contrails can form. As the ice-supersaturated regions (ISSRs) where contrails form frequently occur in relatively shallow layers (Rädel and Shine 2007), much of the previous work in this area has concentrated on avoiding contrail formation by altitude changes (Williamset al 2002, Fichter et al 2005, Mannstein et al 2005, Rädel and Shine 2008, Schumann et al 2011, Deuber et al 2013). Reducing the cruise altitude of the entire global fleet of aircraft by 6 000 ft can substantially reduce contrail formation (Fichter et al 2005); however this requires aircraft to fly at a sub-optimal altitude, leading to an increase in fuel burn and CO emissions. Assessing the viability of such a strategy requires calculating the trade-off between CO emissions and contrails. Zou et al (2013) use a monetization approach, which involves making value judgements on the relative ‘cost’ of each climate impact. Deuber et al (2013) use climate metrics which are based on the response of the atmosphere to the relative forcings, providing a framework which is useful in a policy context. For an individual flight, however, a framework is required which can be adapted to take into account the characteristics of the aircraft and the prevailing weather conditions since the altitude at which contrails are formed is highly dependent on the weather pattern (Irvine et al 2012).

Moreover, less attention has been paid to re-routing aircraft without altitude changes; such a strategy might be preferable where the increase in flight distance is small, since it allows an aircraft to remain at the altitude where it is most fuel-efficient. As motivation for this approach we provide an idealised example. Figure 1 shows a circular ISSR of radius 2 degrees, located along the great circle route between two airports. As shown on figure 1 the shortest alternative route avoiding the ISSR (in zero wind conditions) is to fly great circle routes from LON-A, and A-NY. This increases the flight distance by 22.5 km, 0.4% of the original route. We note also that since the increase in flight distance is dependent on how wide the ISSR is in the direction perpendicular to the original flight, it is independent of the contrail length. Together this implies that if regions in which contrails may be formed can be predicted, and routes recalculated to avoid them, then the added flight distance and therefore the CO penalty may be small.



We have developed a simple framework to enable the trade-off between contrail and CO climate impacts to be estimated for a single flight. The framework currently considers re-routing without altitude changes, which has the advantage of allowing the aircraft to fly at its most fuel-efficient altitude. The trade-off calculation depends on aircraft parameters such as fuel flow rate which are known a priori, and meteorological parameters such as the contrail size, lifetime and radiative forcing, which would be required to be known a priori were such a strategy to be implemented operationally.

The framework calculates the maximum extra distance that can be added to a flight, before the additional CO emissions outweigh the benefit of not contrailing. As the quantity of CO emissions depends on aircraft type, any decision to avoid making the contrail would be highly dependent on aircraft type. For example, using the AGWP metric with 100 year time horizon, the extra distance that a small jet can fly is more than ten times the avoided contrail length, whilst for a very large jet this reduces to three times. As discussed by Deuber et al (2013), it is important to choose a suitable metric, depending on the required outcome. Here, we find a factor of 3–10 difference between the AGTP and AGWP results, depending on the time horizon used.

This framework is useful to show where the major uncertainties are. Joos et al (2013) find that calculations of the atmospheric COresponse agree within 15%, thus the climate impact of the flightʼs CO emissions can be calculated with a relatively small uncertainty, given knowledge of aircraft fuel burn. The calculation of the climate impact of the contrail has a much larger uncertainty. The uncertainty chiefly arises from two sources: an inability to estimate, a priori, the eventual size and therefore climate impact of the contrail that would be formed, and second the radiative forcing (which has a potential dependence on the time of day, not taken into account here) and climate efficacy of that forcing. Even if the radiative forcing were calculated operationally within a forecast model, there would still be an uncertainty in the size of the calculated radiative forcing due to the radiative forcing codes (Myhre et al 2009), and also due to uncertainty in the contrail characteristics. Taking into account the uncertainty in the eventual climate impact of a contrail of 100 km length, the estimate of the maximum diversion distance varies by a factor of 20.

The application of such a strategy in the real world would require highly accurate forecasts of ISSRs where potential contrails form, and the ability to know a priori the climate impact of a potential contrail, as well as being highly dependent on air traffic control and other operational and economic considerations. In addition, the overall climate impact of the flight should take into account the chemical forcings from aircraft  emissions; detailed calculations of such ‘climate optimal’ routings are currently being performed by the REACT4C project. We note here that for small horizontal diversions it is possible that the chemical forcings between the two routes would be comparable; however since the impact depends on where the  emissions are advected, small diversions could potentially result in large differences in impact (Grewe et al 2014). The impact of black carbon and other aerosol emissions may also be important and could be incorporated in more detailed estimates (Jacobson et al 2012).

Nevertheless, despite the uncertainties, the calculations presented here indicate that once a metric (and time horizon) choice has been made, guidance can be given as to whether it is beneficial to divert to avoid contrails. So for example, adding 100 km distance to a flight to avoid making a contrail would seem beneficial for many of the cases presented here, and other parameter choices, such as the extreme high values in Haywood et al (2009), could allow significantly longer diversions.

Full research paper at



Longer flights could cut global warming caused by contrails

JUNE 23, 2014


Passengers may be forced to spend longer in the air because their aeroplane’s flight path could be altered at short notice to reduce the formation of condensation trails, or contrails.
Contrails disperse into wispy clouds which trap heat in the atmosphere, a study showed. These clouds, which can be 1600km long, could contribute more to global warming than the carbon dioxide in the fuel burnt by the aircraft which formed them.

Dr Irvine said it was difficult to compare the climate impacts of contrails and CO2 because the former can last hours while the latter can last decades.

She said that governments needed to consider the impacts of aviation when setting green targets because a measure designed to reduce fuel use could be counterproductive for some flights.
“Current mitigation targets do not yet address the non-CO2 climate impacts of aviation, such as contrails, which may cause an impact as large, or larger, than aviation CO2 emissions.”

Full article at




Re-routing flights could reduce climate impact, research suggests


By Pete Castle (EurekAlert)    44-011-837-87391
Institute of Physics

Aircraft can become more environmentally friendly by choosing flight paths that reduce the formation of their distinctive condensation trails, new research suggests.

In a study published today, 19 June 2014, in IOP Publishing’s journal Environmental Research Letters, researchers from the University of Reading have shown that aircraft contribute less to global warming by avoiding the places where the thinly shaped clouds, called contrails, are produced – even if that means flying further and emitting more carbon dioxide.

Contrails only form in regions of the sky where the air is very cold and moist, which is often in the ascending air around high pressure systems. They can sometimes stay in the air for many hours, eventually spreading out to resemble natural, wispy clouds.

The findings suggest that policymakers need to consider more than carbon emissions in discussions about how to make aviation less environmentally damaging. Recent research has shown that the amount of global warming caused by contrails could be as large, or even larger, that the contribution from aviation CO2 emissions.

The work was carried out by Dr Emma Irvine, Professor Keith Shine, and Professor Sir Brian Hoskins, at the Department of Meteorology at the University of Reading.

Dr Irvine said: “If we can predict the regions where contrails will form, it may be possible to mitigate their effect by routing aircraft to avoid them.

“Our work shows that for a rounded assessment of the environmental impact of aviation, more needs to be considered than just the carbon emissions of aircraft.”

Just like natural clouds, contrails reflect some of the Sun’s incoming energy, resulting in a cooling effect, but also trap some of the infrared energy that radiates from Earth into space, therefore having a warming effect. Detailed calculations indicate that generally the warming effect wins over the cooling effect.

The researchers estimate that smaller aircraft can fly much further to avoid forming contrails than larger aircraft. For example, for a small aircraft that is predicted to form a contrail 20 miles long, if an alternative route adds less than 200 miles onto the route (i.e. 10 times the length of contrail that would have been produced) then the alternative route would have a smaller climate impact.

For larger aircraft, which emit more CO2 than smaller aircraft for each mile flown, the alternative route could still be preferable, but only if it added less than 60 miles (i.e. 3 times the contrail length) onto the route.

Dr Irvine added: “Comparing the relative climate impacts of CO2 and contrails is not trivial. One complicating factor is their vastly differing lifetimes. Contrails may last for several hours, whilst CO2 can last for decades. In terms of mitigating these impacts, air traffic control agencies would need to consider whether such flight-by-flight re-routing is feasible and safe, and weather forecasters would need to establish if they can reliably predict when and where contrails are likely to form.

“The mitigation targets currently adopted by governments all around the world do not yet address the important non-CO2 climate impacts of aviation, such as contrails, which may cause a climate impact as large, or even larger, than the climate impact of aviation CO2 emissions.

“We believe it is important for scientists to assess the overall impact of aviation and the robustness of any proposed mitigation measures in order to inform policy decisions. Our work is one step along this road.”


Fast Facts

  • Aviation CO2 emissions accounted for 6% of UK total greenhouse gas emissions in 2011.
  • Global CO2 emissions from aviation were estimated at 630 million tonnes of CO2 for 2005. This is 2.1% of the global emissions of CO2 in that year.
  • Previous research by scientists at the University of Reading has shown that, on average, 7% of the total distance flown by aircraft is in cold, moist air where long-lasting contrails can form (2.4 billion km out of a global total of 33 billion km flown in 2005).
  • Aircraft engines emit a number of other gases and particles that can alter climate (such as oxides of nitrogen and sulphur gases) and their effects might also depend on the route taken.





Aviation now contributes 4.9% of climate change worldwide

Work by the IPCC now estimates that aviation accounted for 4.9% of man-made climate impacts in 2005. This contrasts with the 2% figure that is constantly quoted by aviation lobbyists, and 3% which the same authors quoted two years ago. They have now revised their estimates with 2 important changes: including for the first time estimates of cirrus cloud formation and allowing for aviation growth between 2000 and 2005. The effect of these is to increase aviation’s impacts to 3.5% without cirrus and 4.9% including cirrus. 23.5.2009

More  …


Work by the IPCC (International Panel on Climate Change) has been updated by
the same authors. They estimate that aviation accounted for 4.9% of man-made climate
impacts in 2005.   This contrasts with the 2% figure that is constantly quoted
by aviation lobbyists.

Just two years ago the authors came up with a figure of 3% for aviation’s worldwide
contribution to climate change.   They have now revised their estimate for 2005
(David Lee et al ‘Aviation and global change in the 21st century’). There are
two important changes:

* Including for the first time estimates of cirrus cloud formation

* Allowing for aviation growth between 2000 and 2005

The effect of these is to increase aviation’s impacts to:

3.5% without cirrus

4.9% including cirrus

In quite a long and complex paper, the authors estimate the radiative forcing or RF due to aviation emissions and express these as a % of worldwide RF from
all sectors.   Several gases have climate impacts (some cause cooling rather than
warming) and there are considerable uncertainties about the exact impacts and
thus wide error limits.   The range of uncertainly around the 3.5% figure (excluding
cirrus) is given as 1.2% to 10%.

The uncertainties about cirrus formation are particularly great, which is why
scientists have previously been reluctant to quote figures.   The range of uncertainty
around the 4.9% (including cirrus) is 2% to 14%.

The figure of 3.5% (excluding cirrus), includes CO2, O3, CH4, NOx, H2O vapour,
contrails, SO4 and soot.   The total impact of these is 1.96 times greater than
CO2 alone.   This illustrates how important it is it to assess the full RF and
not just the effect of CO2.

The figure of 4.9% includes cirrus as well as all these other substances.  
The total impact is then 3.06% greater than CO2.   This illustrates even more
the importance of looking at all aviation’s emissions

All the figures quoted are for 2005.   Because of the high rate of aviation growth,
the %s would be higher if re-calculated for 2009.

The relative impact of aviation in the UK is much higher.   The government (Department
for Transport) estimates that CO2 accounts for 6.3% of total UK emissions and
9.8% of all greenhouse gases, but excluding cirrus.   These figures are not on
the quite the same basis as the RFs of Lee et al, but they illustrate that aviation
is a specially important issue for the UK.

Note – Radiative forcing (RF)

There is no one measure or ‘metric’ that expresses climate or global warming
impacts. Different metrics have different roles and different pros and cons. Radiative
forcing (RF) is a measure of the amount of atmospheric warming in a period, eg
a year, caused by historical emissions up to that year.   Thus the RF due to aviation
in 2009 is a function of emissions from aircraft up to 2009. The relationship
between emissions and RF is complex because different substances last a different
amount of time in the atmosphere.   For example, CO2 can last a hundred years or
more whereas H2O may only last a matter of days.