Photo: Clive Simpson
Contrails - a cirrus-like cloud - are a relatively new type of high-level cloud that appear in the wake of high-altitude aircraft as water vapour from engine exhausts coalesces.
They can be up to 150km in length and last up to 24 hours, forming when planes fly through very cold and moist air causing engine exhausts condense into a visible vapour.
Founder of The Cloud Appreciation Society Gavin Pretor-Pinney writes in The Cloudspotter’s Guide that we may be surprised to learn that contrails are even classified as clouds.
But clouds they are - and the only difference between them and what nature produces is that contrails are definitely man-made.
Contrails reflect sunlight back into space and so have a cooling effect - but they also trap infrared energy in the atmosphere, which adds to global warming.
Scientists at the University of Reading in the UK have been looking at how the warming impact - which is more significant overall than the cooling - could be reduced by altering the flight paths of long and short haul aircraft.
Previous work has suggested that planes could fly at lower altitudes to limit the trails but this would burn significantly more fuel and adding to CO2 emissions overall.
The Reading study attempted to see if the benefits of curbing contrails would outweigh the extra fuel burned if flights were re-routed whilst at the optimum flying altitude.
They suggest that avoiding the creation of a major contrail on a flight between New York and London would only add 22km to the journey but could curb the flight’s warming impact.
"You think that you have to do some really huge distance to avoid these contrails," said Dr Emma Irvine, the study’s lead author.
"But because of the way the Earth curves you can actually have quite small extra distances added onto the flight to avoid some really large contrails."
The researchers found that short haul aircraft are more fuel efficient and can add up to 10 times the length of the contrail to their journeys and still reduce overall warming potential.
So, if a flight from the UK to Spain is predicted to create a 20km long contrail, as long as the plane flew less than 200km extra to avoid it, the overall warming impact would be reduced.
But for large planes on longer routes over oceans and unpopulated areas - which offer more flexibility to minimally alter flight paths - this reduces to three times the contrail length.
"The key things you need to know are the temperature of the air and how moist it is, these are things we forecast at the moment, so the information is already in there," said Dr Irvine.
On average, 7% of the total distance flown by aircraft is in the type of air where long lasting contrails form but none of the calculations on the impact of aviation on global warming currently include them.
The carbon restrictions being introduced from 2017 for long haul flights originating or arriving in the EU will not include this significant source of warming from aviation.
"The mitigation targets currently adopted by governments all around the world do not yet address the important non-CO2 climate impacts of aviation," said Dr Irvine.
"Contrails 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."
Four years ago when flights across Europe were grounded - due to the dust cloud created during the eruption of an Icelandic volcano - our skies and sunsets could be viewed without visible signs of human intrusion.
Contrails and their delicate linear patterns often add a seemingly innocent beauty to the skies above but Pretor-Pinney is not so sympathetic and describes them as "the bastard sons" of the cloud family.
"They may cut dashing figures across the rosaceous autumn evening but these icy swathes of progress must be the writing on the sky for cloudspotters and everyone else besides," he says.
The Lighthouse Keeper is written by Clive Simpson - for more information, commission
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