Contrail avoidance not likely to damage climate

A new study allays fears that rerouting flights to avoid forming climate-warming contrails could result in inadvertently making climate warming worse.

Researchers from Sorbonne Universite and the 色狗导航 found that for most flights that form contrails in the North Atlantic, the climate benefit of avoiding the contrail outweighs the extra carbon dioxide emitted from flying a different route. 

Contrail avoidance requires comparing the climate impacts of carbon dioxide and contrails, called CO2 equivalence. Different methods have been proposed, and the choice of which has been largely political. Scientists feared that some choices could be misleading, making avoidance seem beneficial for climate when it is in fact damaging.

The study, published today (Sunday, 15 September) in Atmospheric Chemistry and Physics, finds that for a large majority of North Atlantic flights, contrail avoidance would benefit climate regardless of the choice of CO2 equivalence. 

Contrails explained

Contrails - the white lines left behind planes in the sky - can trap heat in the atmosphere and contribute to global warming.

The new study builds on previous research that suggested planes could be rerouted to avoid contrail formation, potentially reducing climate impact. However, the benefits of avoiding contrails against the drawbacks of extra CO2 emissions were unclear.

Prof Nicolas Bellouin, co-author at the 色狗导航, said: “Rerouting flights to avoid contrails could in theory reduce the climate impact of aviation and make air travel more sustainable. Our findings lift a major obstacle against implementing contrail avoidance, but we now need better forecasting and real-world trials to make this work in practice."

The new findings show that regardless of how the trade-off between contrail avoidance and increased CO2 emissions is measured, rerouting rarely worsens climate effects unintentionally. The study looked at nearly half a million flights over the North Atlantic in 2019 to estimate how much warming was caused by the carbon dioxide emissions from these flights and any contrails they formed. 

The researchers first examined how current flight routes would warm the world over time. They estimate that the CO2 emissions and contrails from these flights will have warmed the climate by about 17 microKelvins (μK) in 2039, 20 years later, and 14 μK in 2119, 100 years later. A microKelvin is a very tiny unit of temperature change. 

Then the researchers imagined a situation where planes could avoid all contrails by using just 1% more fuel. In this case, the total warming would decrease significantly. By 2039, warming would be reduced by about 5 μK, which is 29% less than without rerouting. By 2119, it would be about 2 μK (14%) less.

The researchers used nine different ways to measure climate impact. In most cases, all these methods agreed that rerouting flights would be good for the climate, as long as the planes successfully avoid contrails as predicted. 

The researchers emphasise that there is still much uncertainty in predicting exactly where contrails will form and how much warming they cause. They suggest focusing initial rerouting efforts on flights that form the most warming contrails, where the climate benefit is clearest.

by Jan van der Wolf.