The condensation trails left in the sky by high-flying aircraft could be increasing the amount of cloud cover around the globe.

This is the conclusion of a French scientist who has compared the increase in air traffic with the apparent increase in the occurrence of cirrus clouds.

Planes produce a trail when their warm, humid exhaust gases mix with the cold, dry air of high altitudes. The particles in the exhaust plumes are thought to trigger the formation of clouds by acting as the nuclei around which ice crystals can form.

At altitudes above 6,000 metres (20,000 feet) this means cirrus-type formations which typically produce long, thin, wispy streaks.

An increase in global cloud cover has implications for climate change. The cirrus variety, in particular, may help to warm the planet by trapping in the heat trying to escape from the Earth's surface.

Air corridors

Olivier Boucher of the Laboratoire d'Optique Atmosph�rique, Universit� de Lille-I, Villeneuve d'Ascq, says cirrus occurrence follows air traffic corridors worldwide.

He used cloud reports from land and ship stations compiled between 1982 and 1991, a period when air traffic had an annual average growth rate of 3.2%. The occurrence of cirrus clouds increased markedly over the same period, and disproportionately so over busy flight corridors.

"It depends on the region, but for the North Atlantic Flight Corridor the increase is seven per cent which is a big increase," he told BBC News Online. Other possible causes - the eruptions of Mt Pinatubo or El Chich�n, or other long-term climatic variations - were insufficient on their own to explain the observed trend in cirrus frequency.

"We had great volcanic events in 1982 and 1991 but they don't seem to match the data because they cannot explain the trends we are observing," he said.

"If there are more cirrus clouds there could be a warming of the climate. This warming is expected to be small because the perturbation due to aviation is still small. But it is very possible that it will become much larger in the future because air traffic is expected to grow at a very high rate."

Cloud formation

Professor Jan-Peter Muller, who heads Cloudmap, an EC-funded project to improve the monitoring of cloud formation, says Boucher's study fits with previous research.

"We are talking at the moment about a very, very small percentage of cloud - of the order 0.2-0.8 % of the total amount of upper-level cirrus," he said.

"The warning is that all the future predictions of increases in subsonic and, even more importantly, supersonic transport certainly indicate that there would be a significant impact and that the impact would be an increase in global warming."

He said the warming effect would be greatest if the cirrus clouds triggered by aircraft were long-lasting.

"The dangerous ones are thin, so-called semi-transparent, cirrus, which will reflect only a small amount of radiation received from the Sun but trap a large amount of thermal infrared radiation emitted by the Earth and by clouds at lower cloud depths."

Olivier Boucher's study is published in the science journal Nature.