You don’t have to live near an airport to be familiar with condensation trails, or “contrails”, slicing through the skies behind high-flying aircraft. They form due to aircraft exhausts and generally disappear fairly quickly.
However, under favourable conditions at the aircraft’s cruising altitude – too dry for clouds to form naturally yet cold enough for them to linger – these contrails may develop into more persistent contrail cirrus clouds or even widespread “cirrus fields” that are virtually indistinguishable from naturally occurring clouds.
In a study published in the journal Nature Communications, colleagues and I have now found planes passing through these clouds actually make them “brighter” and cause them to reflect more sunlight back into space. It won’t make up for all the CO₂ emissions, but this discovery assists the general cooling effect of contrails and will need to be factored into assessments of the impact of aviation on the climate.
Atmospheric scientists monitor cirrus clouds and contrails with the help of satellites. Feeding their observations into computer models show that the global climatic effect of contrails in a cloudless sky is rather small – though they certainly make a big difference locally. But what happens when aircraft fly through clouds that are already present in the atmosphere?
We already know what happens when ships move underneath fields of low-level clouds: water vapour condenses around the aerosol particles emitted from the funnels. This causes an increase in the number of droplets in the clouds, while those droplets are also smaller than those under undisturbed conditions. This is called the first indirect aerosol effect or Twomey effect. Clouds with denser, smaller droplets have more surface area for sunlight to bounce off, leading to an increase in brightness along ship tracks that is visible in aerial and space-borne imagery.
Cirrus clouds, in contrast to low-level water clouds, consist of ice crystals. But aircraft passing though them can be expected to have an effect all the same. Most likely, the soot particles emitted by the aircraft engines act as nuclei around which clouds grow. This leads to a larger number of ice crystals that, in turn, provide more surface area for incoming sunlight to hit.
When investigating the effect of contrails, previous studies looked at line-shaped features in satellite images of clouds and matched them up with known flight tracks of aircraft. The problem is these observations are purely two-dimensional and lack any possibility to look deeper into the clouds.
We instead looked for spots where flights from Hawaii to Seattle, San Francisco or Los Angeles would cross the ground track of a satellite that carries a special highly sensitive lidar (or “laser-radar”) instrument. Here we were most likely to see contrails.
Lidar allows us to easily identify cirrus clouds and to retrieve a profile of their optical properties, even from space. The extra vertical information means we are no longer restricted to a simple top-down view of these clouds, but can instead find the base and top of the cloud layer as well as its “optical thickness” – that is, how much light it scatters. The added data on flight tracks then tells us if an aircraft passed through a cirrus cloud before or after it was observed by the satellite.
We found a significant increase in the optical thickness of the clouds close to an aircraft’s flight track compared to those further away. In other words, clouds inside flight corridors were more reflective or “brighter”. This means incoming sunlight is more likely to be reflected back into space if it is met by cirrus clouds that have been penetrated by high-flying aircraft.
Learning more about the effect of contrails that are embedded in existing cirrus clouds improves our understanding of the impact of aviation on the climate – which may prove vitally important as the size of the aviation sector continues to grow.