Why do aircraft leave contrails in the sky?

However, the present understanding is that “on average over all countries all over the world, in over a year, contrails do warm”, says Schumann. The latest major paper on the climate impacts of contrails, published in 2020, estimated that non-CO2 impacts of aviation – dominated by contrails – lead to a tripling of the “radiative forcing” of CO2 emissions alone.

In some ways it’s a tricky comparison, because the warming effects of contrails and CO2 occur on very different timelines. “Contrails have a very strong forcing effect, far stronger than the CO2. But contrails are short lived, they are gone after an hour or so. Whereas the CO2 lives very long, it can live for 100 years,” says Schumann.

Still, scientists have warned that the heat-trapping effect of contrail clouds could triple by 2050 if no action is taken.

The good news is that this could actually be a fairly simple thing to tackle. Researchers have shown that just 2.2% of flights contribute 80% of this forcing, and that relatively small adjustments to the altitudes of these flights – at a small fuel cost – could hugely reduce contrails’ warming effect. Contrail formation can also be reduced by reducing the amount of soot particles emitted by flights, research has indicated, as these provide the nuclei for ice crystals to form.

Avoiding flying through very humid air, where persistent contrails can form, is the most important measure here, says Schumann, by flying above, below or around these regions. Better weather predictions are needed to facilitate this, however, he says. “The weather predictions which we have today are not accurate enough for that purpose.”

In one letter to the journal Nature last year, two researchers argued that tweaking of flight altitudes to minimise contrail formation could be one of the most cost effective climate measures around. They calculated that preventing the most damaging contrails would cost $1bn (£850,000) per year, with a benefit worth over 1,000 times this amount. “We know of no comparable climate investment with a similarly high likelihood of success,” they wrote.

However, while governments recognise the climate challenges posed by contrails, there has been little policy action to date, says Johnson. “The climate targets set for the sector by industry, countries, and the UN, all relate to CO2 emissions only. The debate around scientific uncertainty and appropriate metrics is often cited as a reason for focusing on more research rather than action.”

In the UK, the Government’s climate change advisers have said non-CO2 effects like those from contrails need to be brought into sharper focus. Considering these effects now is essential, especially in considering how future technologies such as hydrogen can reduce the sector’s emissions, says Johnson.