Contrails: a Threat to the Environment

Martina Roters
1 picture
6 minutes

It is something all children are familiar with: the white streaks stretched beautifully across the sky like finely distributed feathers. Researchers have now ascertained that the so-called contrails contribute to global warming. WingMag runs the rule over the facts:

Water vapour = greenhouse gas no. 1?

Yes! But only when it comes to the natural greenhouse effect. This effect has been enabling human life to thrive for millions of years and also remained in equilibrium for just as long. Water vapour accounts for 2/3 of this effect and only 1/3 is caused by CO2. Water vapour cannot accumulate in the atmosphere, as the latter absorbs only a certain amount of water vapour.

It is a wholly different matter when it comes to carbon dioxide (CO2) as a greenhouse gas, as it is able to accumulate. For this reason, CO2 is so instrumental in terms of the man-made greenhouse effect: the atmosphere’s CO2 content has increased by 36 per cent since industrialisation. This provides ammunition for climate campaigners, who can argue that this increase is indeed anthropogenic, that is: man-made, based on multiple factors: the difference in the concentration increase between the northern and southern hemisphere (90 per cent / 10 per cent) and the congruous rise in fossil fuel emissions, which differ isotopically from atmospheric CO2.

However, water vapour from contrails is now coming under ever-increasing scrutiny. As a (semi) layperson, you do have to wonder: just what can be so dangerous about water vapour?

Humans consist of about 80 per cent water – water is deemed the source of life. Many people dream of hydrogen cars, emitting from their exhaust pipes only water vapour not hazardous to the climate. Unfortunately, it is slightly more complicated than this. Let us see how we can describe this in a way everyone can understand:

What are contrails in the first place?

In the process, all of these phenomena relating to contrails can be explained quite naturally:

How contrails arise

By all accounts, engine waste gases meet relatively cold and moist air.  Approximately one litre of water is emitted for each litre of kerosene that is burned. These water droplets freeze to become tiny ice crystals that grow to become a visible size. Soot particles are particularly suited to be crystallisation germs.

This requires a short time lag (thus explaining the distance to the engines), and this takes place mostly at altitudes above 8,000 metres where the air is appropriately cold (therefore, not seen behind all aircraft in the sky). Contrails may also arise entirely without waste gases, e.g. amid high humidity in the centre of the wake vortices on the wing tips.

A dark grey contrail in the sky of course does not indicate that the aircraft has a problem (or is in the course of spraying nefarious chemicals). In fact, the grey colour results from the rather rare situation that there is another layer of cloud above casting a shadow onto the contrail.

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© Pixabay Myriams-Fotos 1699959

How do contrails pose a threat to the climate?

The danger has a name: cirrus homogenitus (man-made cirrus cloud) and homomutatus, a term for contrails that has achieved currency among international cloud experts since 2017.

If the contrails were to dissolve after a few minutes, a (natural) order would be retained. In this case, contrails of merely 300 to 500 metres would arise. Humidity decides whether such streaks are longer than this. However, the atmosphere is frequently over saturated particularly at high altitude. This means that wide cirrus clouds arise from thin contrails, with these clouds able to last for many hours and even days. The shape of the clouds depends on the wind speed and direction.

We need to bear in mind that air traffic increased by a factor of one hundred between 1950 and 2000. It should not therefore be a surprise, according to a study by the German Aerospace Center (DLR), that cirrus clouds resulting from contrails sometimes cover up to 10 per cent of the sky over central Europe. A study by the University of Leeds shows that in areas with high air traffic, 80 to 100 per cent of the vapour in the sky is artificially created. This layer of cloud has the effect of cooling during the day and warming during the night, as the infrared radiation is intercepted by the layers of water vapour. Unfortunately, this leads to a net contribution to global warming.

9/11, Eyjafjallajökull and Corona

There were three moments in recent history on which aviation ground to a halt for several days, meaning that no contrails were produced in the sky.

Once the Icelandic volcano Eyjafjallajökull erupted, numerous eyewitnesses reported enthusiastically that they had never seen such a blue sky before.

During the flight ban following the terrorist attacks on 11 September, researchers from the University of Wisconsin seized the opportunity and were indeed able to prove significant changes to the day/night temperature ranges owing to the lapse in cirrus clouds.

Finally, the Corona crisis with its significantly reduced air traffic makes the sky appear in a deeper shade of blue.

Searching for solutions

Hydrogen propulsion would be a suicide mission for aviation, as it bolsters the artificial cloud effect even more. There are also numerous further interactions that have not all been researched yet. In this way, the proportion of nitrogen oxide (NOx) in the contrails also has a – negative – effect on the ozone content.

The German researchers Bock and Burkhardt published the hypothesis in 2019 that it may be possible to reduce the emissions of soot particles by 50 per cent. This would reduce the formation of cirrus clouds due to a lack of crystallisation germs.

However this would reduce the radiative forcing (a measure of the difference of insolation (sunlight) absorbed by the Earth and energy radiated back to space) by the year 2050 by only approximately 15 per cent in the face of the expected increase in air traffic. Even a 90 per cent reduction in soot particles would not be enough to return to the level of 2006.

The coup: 1.7 % of flights could be re-directed with a 59 % improvement to the climate!

A group of researchers in London has published an ambitious study on the effects of contrails above Japanese airspace. At the end of their model test, they concluded that 2.2 per cent of all flights cause 80 per cent of homomutatus hazardous to the climate. This took into account meteorological conditions, contrail composition, albedo, natural cirrus cloud properties, day/night cycles and many more. This is the basis for their closing thesis that with a very selective intervention into air traffic, a maximum positive effect can be generated for the climate: by re-routing 1.7 per cent of the flights while retaining the flight routes. The aircraft only need to fly approx. 600 m higher (in winter) or 600 m lower (in summer).

Even if only the night flights were re-routed, this would lead to a 21 per cent improvement to the climate. The researchers believe, incidentally, that their approach could also be applied to other flight zones of medium width.

Let us cross our fingers that this study is heeded and the flights are re-routed!

contrails - aircraft - climate protection
© Pixabay Myriams-Fotos 2302083

Cover picture © Pixabay Alsen 670308

by Martina Roters

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