Longer flights 'could curb impact of vapour trails'
Large condensation trails in the sky caused by aircraft could be eliminated by re-routing flight paths, say scientists.
Researchers are concerned about the climate change potential of these wispy, man-made clouds.
But a new study suggests that making changes to existing flight routes could curb their warming impact.
Avoiding a major contrail on a flight to New York from London would only add 22km to the journey, experts say.
Contrails are formed when planes fly through very cold, moist air and the exhausts from their engines condense into a visible vapour.
These can be very large in size: they can be up to 150km in length and can last up to 24 hours.
Scientists have been arguing about the climate impact of contrails for many years, as the clouds that they form impact both cooling and warming.
Contrails reflect sunlight back into space and cool the Earth but they also trap infrared energy in the atmosphere, adding to warming. Researchers believe that the warming effect is more significant than the cooling.
Now scientists at the University of Reading have tried to work out how this impact 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 means burning significantly more fuel and adding to CO2 emissions.
The Reading study attempted to see if the benefits of curbing contrails would outweigh the extra fuel burned if flights were re-routed at the optimum flying altitude.
"You think that you have to do some really huge distance to avoid these contrails," lead author Dr Emma Irvine told BBC News.
"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.
For large planes on longer routes, this reduces to three times the contrail length.
But longer routes over oceans and unpopulated areas, offer more flexibility to minimally alter flight paths.
The researchers found that large contrails could be avoided on flights between London and New York by adding just an extra 22km to the route.
"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.
"Whether the forecasts are accurate enough to do this is another question."
On average, 7% of the total distance flown by aircraft is in the type of air where long lasting contrails form. But at present, calculations on the impact of aviation on global warming don't include them.
The European Union has attempted to include flights in its emissions trading scheme with limited success.
Long haul flights originating or arriving in the EU will be subject to carbon restrictions from 2017. But the Reading team say that these efforts will still miss out on a 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, such as contrails, which may cause a climate impact as large, or even larger, than the climate impact of aviation CO2 emissions," said Dr Irvine.
"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."
The research has been published in the journal Environmental Research Letters.
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