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LONGER flights burning MORE fuel can CUT planes' climate impact

Boffins show how airlines can choose between CO2 emissions or heat capture

The smart choice: opportunity from uncertainty

British scientists have suggested that the world's airlines could cut their climate impact – by flying further and therefore burning more fuel.

The seemingly-paradoxical claim is based on a calculation of the climate impact of contrails on the world's climate. The researchers suggest that rather than simply calculating the cheapest route based on distance and fuel, airlines could change routes to avoid the weather conditions that lead to increased contrails.

It's an important trade-off for airlines, since the more they can demonstrate mitigation of their climate impact, the less money they'll be asked to outlay on offsets.

Contrails – short for condensation trails – form in cold, moist air, and are caused either by the condensation of water vapour in the engine exhaust, or by the turbulence at wingtips causing water vapour in the air to condense.

As they spread out to wispy clouds, contrails have a measurable radiative forcing impact on the climate. Although they reflect some of the sun's energy outwards, they trap more infrared energy that would otherwise be emitted by Earth, resulting in a small net increase in warming.

The question for scientists is how to balance contrail-driven warming with that caused by burning dinosaurs to keep the plane in the air, and whether the two can be traded off for a net benefit.

The University of Reading researchers, writing in Environmental Research Letters, believe they've come up with a framework that lets this trade-off be calculated so that aircraft can fly around the “ice-supersaturated regions” (ISSRs) where contrails form, without increasing their flight path so much that the extra fuel burn outweighs the reduced contrail formation.

The scientists, led by Reading's Dr Emma Irvine, also believe the route-around approach could be more efficient than trying to avoid contrails formation by changing altitude (which takes extra fuel two ways: on the climb, and by cruising at a sub-optimal altitude).

As explained by Eco-Business: “With a small aircraft that is predicted to form a contrail 20 miles long, an alternative route would have a smaller climate impact if it adds less than 200 miles to its journey. For larger aircraft, the alternative route could still be preferable, but only if it added less than 60 miles to the journey.”

The complexity of the trade-off, Dr Irvine explained, this is a non-trivial calculation: while contrails can last for hours, CO2 lasts decades.

And, of course, building variations to routes into flight plans is non-trivial from other viewpoints: it would need weather forecasting accurate enough to predict contrail-likely conditions, and enough flexibility in air traffic control to deal with changes to flight plans.

There is, at least, this: if aircraft were to generate fewer contrails, a tiny bit of steam would be taken out of “chemtrail” conspiracies. That can't be a bad thing. ®

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