Contrail avoidance is less likely to damage c

A new study allays fears that diverting flights to avoid climate-warming contrails could inadvertently worsen global warming.

Researchers at Sorbonne University and Reading University have found that for most flights that create contrails over the North Atlantic, the climate benefit of avoiding these contrails outweighs the additional carbon dioxide emissions caused by flying a different route.

To avoid contrails, one must compare the climate impacts of carbon dioxide and contrails, which is called CO2 equivalence. Various methods have been proposed, the choice of which has been largely politically motivated. Scientists have worried that some methods could be misleading, giving the impression that avoidance is beneficial to the climate when in fact it is harmful.

The study published today (Sunday, September 15) Atmospheric chemistry and physics, concludes that for a large majority of flights over the North Atlantic, avoiding contrails would benefit the climate, regardless of the choice of CO2 equivalence.

Contrails explained

Contrails – the white lines left by airplanes in the sky – can trap heat in the atmosphere and contribute to global warming.

The new study builds on previous research that suggested aircraft could be rerouted to avoid contrails, potentially reducing climate impacts. However, the benefits of avoiding contrails versus the downside of additional CO2 emissions were unclear.

Prof Nicolas Bellouin, co-author at the University of Reading, said: “Rerouting flights to avoid contrails could theoretically reduce the climate impact of air travel and make air travel more sustainable. Our results remove a major obstacle to implementing contrail avoidance, but we now need better predictions and real-world trials to make this a reality in practice.”

The new findings show that no matter how you measure the trade-off between contrail avoidance and increased carbon emissions, rerouting rarely inadvertently worsens climate effects. The study examined nearly half a million flights over the North Atlantic in 2019 to estimate how much warming was caused by the carbon dioxide emissions of those flights and the contrails they created.

The researchers first examined how current flight paths would warm the Earth 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 by 14 µK in 2119, 100 years later. A microkelvin is a very small unit of temperature change.

The researchers then imagined a situation in which airplanes could avoid all contrails by burning just 1% more fuel. In this case, the overall warming would decrease significantly. By 2039, warming would be reduced by about 5 μK, which is 29% less than without diversion. By 2119, it would be about 2 μK (14%) less.

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

The researchers stress that there are still large uncertainties in predicting exactly where contrails form and how much warming they cause. They suggest focusing diversion efforts initially on flights that form the warmest contrails, as that is where the climate benefit is most evident.

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of press releases submitted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert! system.