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If aircraft flew at lower altitudes they would lose the contrails that have such a big impact on global climate

AIRLINES could boost their emissions of the greenhouse gas carbon dioxide and still halve their impact on global warming. That's the paradoxical conclusion of a new study into the effects of commercial aviation on the environment.

The CO[sub2] emitted from their engines is not the only way aircraft affect climate. They also do so through their contrails, the long trails of water vapour and ice that form in an aircraft's wake and which can persist for several hours. Contrails trap heat in the atmosphere by reflecting infrared radiation emitted from the Earth's surface.

In 1999 the Intergovernmental Panel on Climate Change (IPCC) calculated that contrails from the world fleet of 12,000 civil airliners contribute as much to globalwarming as the CO[sub2] their engines pour out as they burn jet fuel. But with global air traffic growing by around 3.5 per cent per year, and many of those extra flights being long-haul, high-altitude contrail-forming journeys, by 2050 contrails will be having a great deal more of an impact on global warming than the CO[sub2] emissions from aircraft engines.

Contrails could be eliminated if aircraft reduced their altitude from about 33,000 feet to between 24,000 feet and 31,000 feet, depending on the weather. But this would come at a price: lower altitude means denser air and higher air resistance, so planes have to burn more fuel. And this means more CO[sub2] emissions, which would apparently negate any benefits from eliminating contrails.

But according to researchers at Imperial College, London, the idea may work after all. “It seems counterintuitive,” admits Robert Noland, one of the authors of the study. But Noland and his colleagues have calculated that if planes flew low enough they leave no contrails behind, their fuel consumption would increase by only 4 per cent, boosting CO[sub2] emissions by the same amount. The team based their calculations on a simulation of a year's worth of traffic over the busiest part of Europe, taking into account the need for different aircraft to fly at different altitudes to avoid collisions.

Contrails form when warm, moist exhaust gases from an aircraft's engines mix with the surrounding air. If the air is cold enough, the vapour reaches saturation point and condenses to form water droplets that rapidly freeze, leaving a visible trail.

Even though contrails may only be formed for part of a plane's journey, and start out very narrow, they can quickly spread out and are thought eventually to form high-altitude cirrus clouds. “They can spread out several kilometres if the conditions are right and can be tens of kilometres long in extreme cases,” says Toumi.

At present, contrails cover 0.1 per cent of the Earth's surface area. By 2050 this is predicted to rise to 0.5 per cent. The effects of aircraft CO[sub2] emissions are cumulative: once it has been emitted, the greenhouse gas lingers in the atmosphere. The impact of contrails, by contrast, only lasts for as long as the trails themselves. Despite this, contrails contribute as much to global warming as all the CO[sub2] ever produced by the aviation industry since heavier-than-air powered flight began nearly a century ago.

But this proposed trade-off between cumulative CO[sub2] emissions and short-lived contrails should be approached with caution, says Ben Matthews, a climate researcher at the Catholic University of Leuven, in Belgium. “Such a policy might reduce the warming in the short term in regions where most planes are flying, but still increase the warming in the longer term in southern developing countries.”

And the Imperial team admits that reducing altitudes may not be a cure-all, since its burden on air traffic controllers has yet to be fully assessed.

Eliminating contrails offsets extra CO[sub2] from engines

At 33,000 ft, planes produce linear ice clouds called contrails. They can persist for hours, growing to form extended cirrus cloud cover

Between 24,000 and 31,000 ft, contrails do not form. The denser air menas planes burn 4% more fuel and produce 4% more CO[sub2], but the immediate net effect is to reduce global warming

CONTRAILS REFLECT LARGE AMOUNT OF HEAT RADIATED BY THE EARTH, HEATING THE ATMOSPHERE AND ADDING TO GLOBAL WARMING

PHOTO (BLACK & WHITE): Trails of ice and water trap the Earth's heat

PHOTO (BLACK & WHITE): CONTRAILS REFLECT SMALL AMOUNT OF SOLAR ENERGY

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By Duncan Graham-Rowe

Source: New Scientist, 10/19/2002, Vol. 176, p6, 2p
Item: 500941489