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Are contrails dangerous ?

The innocuous white vapour trails that criss-cross the sky may not be as harmless as they look. In fact, they might have contributed to more global warming so far than all aircraft greenhouse gas emissions put together.

High-altitude clouds like cirrus warm the planet by trapping heat. Contrail “cirrus” does the same thing, but the question is: how much? We know that contrails trap some extra energy in the atmosphere: their radiative forcing trapped 10 milliwatts per square metre (mW/m2) in 2005, according to an estimate by the Intergovernmental Panel on Climate Change. That compares with 28 mW/m2 trapped by all of the CO2 released by aircraft engines since the start of aviation.

However, the IPCC estimate only took into account relatively fresh, visible vapour trails that exist for just a few hours. Afterwards they spread out and become indistinguishable from normal cirrus. In this form they may trap energy in the atmosphere for many more hours.

“Only a small part of the problem has been studied,” says Ulrike Burkhardt of the Institute for Atmospheric Physics in Oberpfaffenhofen, Germany. With her colleague Bernd Kärcher, she set out to discover how much heat contrail cirrus traps.

Using satellite observations of spreading contrails as a guide, Burkhardt built a model that simulated how they form, spread out and dissipate. Then she embedded it in a global climate model and watched what happened. She found that contrail cirrus ended up covering 0.6 per cent of Earth’s surface – an area nine times as great as that covered by line contrails.

Burkhardt then used this figure to produce a more accurate estimate of the total energy trapped by contrails. Her calculations suggest a global figure of 31 mW/m2 – higher than that attributable to aviation CO2.

As the first to build contrail cirrus into a climate model, Burkhardt’s study is “an important leap forward”, says Olivier Boucher of the Met Office Hadley Centre in Exeter, UK.

There is a catch, though. “[The measurement] says nothing about what will happen tomorrow,” says David Lee of Manchester Metropolitan University in the UK. While a contrail lasts a day, the CO2 released from a plane lingers in the atmosphere for hundreds of years. For example, while contrails – and their warming potential – disappeared from European skies last April when an Icelandic volcanic ash cloud grounded flights, atmospheric CO2 continued to warm the world.

The flip side is that cutting contrails would make an immediate difference to atmospheric warming, whereas emissions cuts take years to have an effect, says Robert Noland of Rutgers University in New Brunswick, New Jersey.

Planes could avoid creating contrails by flying at lower altitudes, he says, or steering clear of water-rich patches of air – although such measures could prove counterproductive if they make flying more inefficient and lead to a greater increase in CO2 levels.

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