Contrails And Climate Change

Five years ago I wrote an article in this magazine saying that science was starting to suggest that contrails, which we’re all used to seeing in clear skies anywhere in the world, are a significant player in climate change.

- Dr Guy Gratton

In those five years, the science has moved on a lot, but has definitely not proved me wrong.

So, first of all, let’s remind ourselves what contrails are, and why they matter. A contrail is a product of two things. The first is air which is supersaturated with respect to ice – a term you won’t find in most pilot textbooks yet. This is partly the same as any other kind of supersaturated air – as in it is over 100% relative humidity: holding more water vapour than it technically should. The extra factor is that the air is also below the freezing point of water. So, if anything happens to cause that excess water to be precipitated out, it’ll come out not as water, but as ice.

Now if you fly an aeroplane through that supersaturated air, pretty much any aeroplane so long as it has fuel-burning engines, those engines will emit various combustion products, including sooty particles. They’re exactly what they sound like – tiny bits of solid carbon compound, leftover from the process of burning fuel. When the air is disturbed behind the aeroplane, and those sooty particles mix into the disturbed air, they provide nuclei around which ice can deposit – and it does. Very small ice crystals, but in enormous numbers, and those create a contrail, that we can often see from the ground. Physically a contrail is not much different to cirrus cloud and, in fact if it persists in the sky for more than a few minutes, it does become what meteorologists now call contrail cirrus. In effect, artificial cirrus clouds.