Myth-busting: Chemtrails

Despite official reassurances, many people believe that the thin white lines left behind high-flying aircraft are chemicals that are being deliberately sprayed into the atmosphere for some sinister purpose.

Believers think that toxic chemicals in the trails settle to the ground causing effects ranging from immune dysfunction to memory loss. Some – including Ian Wishart of New Zealand’s Investigate magazine – even claim to have suffered serious health effects after gazing at chemtrails. It’s a theory that has concerned many: if you type 'chemtrails' into Google, more than seven million web pages come up.

So are they anything to worry about?

Not poisonous

Normally called ‘contrails’, or ‘vapour trails’, chemtrails are in reality made of little more than water vapour and are about as sinister as the puff of cloud your breath makes on a chilly morning.

Contrails form behind aircraft flying at high altitude when the air is colder than –40°C. One of the main products of combustion in an engine is water, which comes out of the engine hot and then quickly condenses into mist-like water vapour or tiny ice particles in the surrounding frigid air.

Humidity and air turbulence determine how long the contrail persists. If the air around the contrail is dry, the contrail will disappear quickly, with the ice turning into invisible water vapour.

However, if the air is calm and more humid the trail may hang in the sky for half a day. It’s these long-lived trails that conspiracy theorists regard as menacing chemtrails, failing to appreciate that they are merely markers of humid, still air.

Contribution to global warming

However, even non-conspiracy theorists could be forgiven for taking some interest in persistent contrails.

Where numerous, contrails can spread and merge to form cirrus-like clouds. In areas with lots of jet traffic, like Europe and North America, they can increase cloudiness by up to 20 per cent.

So is this a good or bad thing? Clouds reflect back incoming sunlight (which reduces heat), but they also trap existing heat radiating from the earth below – and studies suggests this heat trapping effect outweighs the cooling.

It’s well known that CO₂ released by jet engines contributes to global warming, yet few people are aware that – according to recent research published by the journal Nature Climate Change – contrails actually cause more warming than the CO₂ produced by planes!

So while the conspiracy theory about an evil plan to poison the world with chemtrails is definitely false, it is true that the convenience of inexpensive high-speed travel comes at an increasing cost to the world at large in the form of global warming.

Handy links

Fact sheet from the United States Environmental Protection Agency
Article in Nature journal
Technical study in Nature Climate Change
New Zealand conspiracy site