On clear days you can sometimes see them well: the white condensation trails of airplanes. When it’s cold and humid enough, this mixture of frozen water vapour and soot particles lingers for a long time and the stripes form visible clouds. What not everyone knows is that these man-made clouds have an influence on the weather, and ultimately on the climate.
Climate researchers have been trying for some time to find out how great this influence is. The corona crisis now offers an unexpected opportunity to compare theory with practice.
Stripes have a cooling and warming effect
Aircraft clouds affect the climate in two opposite ways. The clouds block solar radiation, like a mirror. So the stripes have a cooling effect. But the earth also gives off heat. And then those same clouds act as a blanket that retains heat.
“Ultimately, that warming effect wins,” says Piet Siebesma, a cloud expert at KNMI and Delft University of Technology. “The net effect is that aircraft stripes make the weather, and ultimately the climate, slightly warmer
During the lockdown in March, April and May, the number of flights worldwide dropped dramatically. According to some measurements, this resulted in up to 90 percent fewer aircraft stripes. If those stripes influence the weather, their disappearance should also have consequences. But how do you measure that?
That’s not so easy, explains Andrew Gettelman, climate researcher at Oxford University. After all, the weather is always different. Comparing 21 April 2020 with 21 April 2019 is of little use.
An international research team of which Gettelman is a member has an advanced computer model in which they can carry out various weather scenarios. “As if you have a laboratory with the earth with which you can do various experiments,” says Gettelman. “Our assumption is that the condensation trails have a measurable influence and we’ll see consequences when they’re gone.”
By comparing the data from their simulations with actual weather data during corona, the researchers hope to be able to determine the climate contribution of the condensation tracks more accurately. Gettelman expects to be able to observe slight cooling due to the decrease in aircraft stripes.
Effect barely measurable
However, both Siebesma and Gettelman stress that the influence of aircraft stripes on the climate is not so great. “It is hardly measurable. If there were only those aircraft stripes, we wouldn’t have to worry about climate change,” says Gettelman.
However, it is important that there is more clarity about that influence. After all, just like the CO2 emissions of airplanes, the traces of condensation also contribute to the climate problem.
Some studies even conclude that stripes contribute as much to global warming as the total contribution of aviation. This contribution is currently estimated at around two percent, but may therefore be twice as large if the condensation trails are included.
Sector falls outside climate agreements
In 2015, the world made agreements to reduce CO2 emissions in order to keep global warming well below two degrees. Aviation falls outside those agreements. Because the sector expects to grow in the coming decades, the share of emissions will increase considerably. Partly for this reason, there is more and more discussion whether aircraft can remain outside the climate agreements.
“We have to map the impact of aviation and pay for it. And in doing so, we have to take the impact of aircraft stripes into account,” says Gettelman. His Dutch colleague Siebesma agrees. “Although it should be noted that the effect of the stripes would disappear immediately if we stopped flying, while the emission of CO2 is of course permanent”
The first results of the international study on the effects of fewer aircraft lines on the climate are expected before the end of the year.