Aircraft vapour trails beef up – and brighten – clouds

That aircraft zooming overhead may be helping cool the planet by making clouds bright, a satellite study shows.

Researchers from Sweden and the UK, led by Stockholm University’s Matthias Tesche, found the condensation trails left in an aeroplane’s wake thicken cirrus clouds below, which reflect more of the sun’s radiation into space.

The work was published in Nature Communications.

In a cloudless sky, contrails – white streaks across the blue formed when water vapour condenses and freezes around exhaust particles – have little effect on climate, purely because they’re so thin. Any radiation from the sun barrels straight through.

But in a cloudy sky?

“Though contrails and their effect on climate have been studied for quite a while now, we have not yet investigated what happens when they form in already existing clouds,” Tesche says. 

This was, partly, because imaging sensors mounted on satellites couldn’t see what was happening inside clouds, he adds.

This changed with the launch of the cloud-aerosol lidar and infrared pathfinder satellite observations (CALIPSO) satellite in 2006. Finally, atmospheric scientists could measure the thickness of clouds from above – and how much radiation they let through – using a laser instrument on the satellite.

So Tesche and colleagues combined CALIPSO data with aircraft track data between the US west coast and Hawaii in 2010 and 2011.

They mainly looked at cirrus clouds – wispy clouds that usually form above 5,500 metres, and lower than an aircraft’s cruising altitude of around 11,000 metres. Cirrus clouds tend to be optically thin – that is, they let much more light through than, say, a big fluffy cumulus cloud.

And they found cirrus clouds directly below a contrail optically thickened by, on average, 22%. But cirrus cloud directly either side of the flight track remained thin.

The reason behind the thickening is unknown yet, but the researchers speculate that specks of soot falling from a contrail and dropping directly onto cirrus clouds below may “seed” ice crystals that beef up the clouds.

The effect, they write, occurs only in certain bands of latitude, so flights near the equator or towards the North Pole are unlikely to have the same effect. But in the southern and northern mid-latitudes, it appears to be a common effect.

Of course, the effect is nowhere near enough to reverse the rise in global temperatures due to carbon emissions. But how much has aircraft-generated cirrus cloud cover affected our climate? The researchers don’t know, but intend to calculate it and find out.