Operation Blue Skies: How Lower Flights Could Halve Aviation’s Warming

Good news on global warming is thin on the ground. But there may be some up in the air. Operation Blue Skies, announced on August 18th by Britain’s air-traffic-control service, NATS, and partners studying the climate effects of aircraft contrails, will test whether small flight-path tweaks can avoid the most warming contrails and halve aviation’s climate impact.

If the experiment works, the same techniques applied worldwide might cut the climate effects of air travel by as much as half.

Contrails look cooling — but warm on balance

On a sunny day the wispy cirrus clouds into which some contrails evolve can look like a delicate parasol that might cool the planet. And some contrails do cool things a bit beneath them. But all contrails — and the cirrus they become — also warm by trapping upwelling infrared radiation that would otherwise escape into space. On average that warming handsomely outweighs the cooling provided by some. In climate terms, contrails are a liability.

How flying a little lower could avoid persistent contrails

Happily, much of the harm appears avoidable with small changes to flight paths. Persistent contrails — the class with the largest effects — appear only in what are known as “ice-supersaturated regions” (ISSRs). Often planes can avoid such regions just by flying a little bit lower.

The purpose of Operation Blue Skies is to find out how well the presence of ISSRs can be predicted, how effective small shifts in altitude can be in avoiding persistent contrails, and how easily and safely the changes in flight path needed to bring them about can be integrated into routines for airlines and air-traffic control.

A cheap fix — but not one for a carbon market

The cut of up to half in the climate effects of air travel would come at a very small cost. The number of flights at lowered altitude might be small — as little as 5% of the total. Because altitude buys efficiency, those flights would consume more jet fuel, but less than 2% more.

Burning even that little extra fuel would be anathema to airlines. So some suggest the costs might be minimised, and airlines incentivised, by setting up a market which somehow links contrail avoidance to carbon prices. The European Union, which has had some success and plenty of experience with carbon markets, recently took a small step towards something along these lines. It is also the sort of idea to which economists — and The Economist — tend to be partial. But it would be a mistake nonetheless.

Why contrails do not suit market pricing

Markets for pollutants have worked best when both the aim and the remedy are fairly simple — most famously, reducing sulphur-dioxide emissions from power stations in America. Contrails, by contrast, are complicated.

Though there is no doubt that their net effect is warming, saying how much warming a particular contrail might produce is often difficult and sometimes next to impossible. The difficulties are made yet worse when the contrail being assessed is a purely notional one — as would be the case for the contrails avoided by changing flight paths. Valuing counterfactuals is always hard.

What air-traffic control should do instead

If contrail avoidance turns out to be as useful as it looks, it would be much simpler to have air-traffic controllers assign routes that limit persistent contrails when it is possible and safe to do so. If some airlines choose to avoid the affected routes, others will surely be happy to take their place.

And if such a scheme were also to show the benefits of innovations in air-traffic control — quite a few have been suggested — the industry would end up better off, as well as less polluting. ■

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