“We find that for smaller aircraft, a diversion which is up to 10 times longer than the contrail avoided might reduce the climate impact,” Emma Irvine of the University of Reading, UK, told environmentalresearchweb. “For larger aircraft, which emit more carbon dioxide per kilometre flown, the diversion can be up to three times longer.”

To come up with these values, Irvine and colleagues considered the climate savings made by flying planes around a patch of ice-supersaturated air, without changing their altitude. On a journey from London to New York avoiding an air patch with a radius of 2° would add 22.5 km, or 0.4%, to the length of the flight. (Other studies have considered flying at a height lower than is optimum for fuel efficiency.)

How much a contrail affects climate depends on its width and length, which scales with the distance the plane flies through ice-supersaturated air. This can vary from 10 to 1000 km but MOZAIC (Measurements of Ozone and Water Vapour by Airbus In-Service Aircraft) have shown that the most frequent distance a plane spends in such a zone is 150 km. Contrail width, on the other hand, can be up to 10 km, depending on lifetime and the vertical wind shear. The trails can persist for as long as 24 hours but around five hours is more typical – this is the value the team used for their calculations.

The amount of warming a contrail causes also depends on the time of day, the amount of natural cloud cover, its thickness and the surface beneath.

“Comparing the relative climate impacts of carbon dioxide and contrails is not trivial,” said Irvine. “One complicating factor is their vastly differing lifetimes – contrails may last for several hours, carbon dioxide for decades. There are many potential methods for estimating the impact of emissions today on future climate, and these depend, for example, on how far into the future you look.”

To assess climate impact, the team used both absolute global warming potential and absolute global temperature potential, calculated over 20, 50 or 100 years. Analysis involving absolute global warming potential gave values of the threshold extra-travel distance at which it’s no longer worth rerouting the plane three to ten times higher than using the absolute global temperature potential did, because this metric gives more weight to short-term forcings such as contrails. What’s more, the uncertainty in the calculations affected the threshold distance by a factor of 20.

“There is much discussion on reducing the climate impact of aviation,” said Irvine. “Many of the suggested approaches concentrate on modification of the aircraft or its engines, to improve their fuel efficiency. Because of the long lead-times in replacing the global aircraft fleet, it can take many years for such advances to actually be put into practice.” So Irvine and colleagues have been investigating whether there are ways to reduce aviation’s climate impact through more day-to-day decisions.

There’s some way to go before avoiding contrails becomes viable as a climate strategy. “As well as the need to reduce the scientific uncertainties in estimating the climate impact of contrails, policymakers would need to decide what they believe is the most important climate effect they want to avoid (e.g. long-term or short-term climate change), air traffic control agencies would need to consider whether such flight-by-flight re-routing is feasible and safe, and weather forecast agencies would need to establish whether they were able to forecast reliably when and where contrails are likely to form,” said Irvine.

The team hopes the study will help interpret the results from complex re-routing calculations. “We are involved in an EC-funded collaborative project which has been assessing the feasibility of climate-optimal routing for trans-Atlantic air traffic,” said Irvine. “Here, climate optimal means that the routing calculations take into account many other aviation climate impacts, not just carbon dioxide and contrails. The calculations also incorporate air traffic restrictions, such as minimum separation requirements between aircraft.”

Irvine and colleagues, who reported their work in Environmental Research Letters (ERL), are also trying to understand how climate change will affect aviation, including its impact on contrail formation and whether changes in the jet stream will alter aircraft routes.

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