Some groups and individuals erroneously believe that the long-lasting condensation trails, or contrails, left behind aircraft are evidence of a secret large-scale spraying programme. They call these imagined features ‘chemtrails’. Adherents of this conspiracy theory sometimes attribute this alleged spraying to the government and to industry.

The authors of a recent survey, including Carnegie’s Ken Caldeira, conducted a survey of the world’s leading atmospheric scientists, who categorically rejected the existence of a secret spraying programme. The team’s findings, published by Environmental Research Letters, are based on a survey of two groups of experts: atmospheric chemists who specialise in condensation trails and geochemists working on atmospheric deposition of dust and pollution.

In the study, 77 scientists – experts in atmospheric chemistry and in the deposition of air pollutants – were shown some of this evidence and asked whether a chemical spraying programme was the simplest explanation for it or whether there were others that were more straightforward. With the photographs, all of the experts chose science over conspiracy theory. For instance, in a photograph of a contrail broken by a gap, chemtrails supporters say the gap is proof the spraying was turned on and off. The scientists, however, said there were reasonable scientific explanations for the gap, the most commonly cited being that the aircraft had passed through an area of drier air, which would make that part of the contrail dissipate more quickly.

“Despite the persistence of erroneous theories about atmospheric chemical spraying programmes, until now there were no peer-reviewed academic studies showing that what some people think are ‘chemtrails’ are just ordinary contrails, which are becoming more abundant as air travel expands. Also, it is possible that climate change is causing contrails to persist for longer periods than they used to.” Caldeira said.

More on ‘chemtrails’ at

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