Long, white vapor trails blending into cirrus clouds and cooling over southeastern England during World War II have led researchers to believe that contrails could influence climate.

A new study based on historical data examines Allied air raids, including one on May 11, 1944, in which 1,980 aircraft took flight into Germany. Behind the planes, witnesses described the sky turning white, and temperature records show that the air turned slightly, but significantly, cooler.

The study, published in the International Journal of Climatology, highlights these observations and other historical data, weather reports and flight logs of Allied air campaigns between 1943 and 1945.

Professor Rob MacKenzie at the University of Birmingham in England, who helped lead the study, said that evaluating the climate effects of contrails is challenging.

"As soon as the contrails stop being these pencil-thin lines in the sky, it becomes difficult to differentiate natural cirrus from aviation-induced clouds," said MacKenzie.

To study the role of artificial clouds on climate, the team needed a period of time when civil aviation was scarce in order to compare the atmosphere with and without contrails. Unadulterated skies suddenly filling with hundreds of bombers and fighters made an ideal assay.

The inverse scenario was observed after the Sept. 11, 2001, terrorist attacks, when busy skies over the United States were suddenly empty after all flights were grounded. The widest changes in temperature were observed in areas with typically heavy air traffic, according to the Aviation Climate Change Research Initiative (ACCRI), a project by the Federal Aviation Administration.

"It's not an easy study to do," said MacKenzie. "It required a lot of mining of data."

Too early for policy measures

MacKenzie said the idea for the study was suggested by co-author Roger Timmis of the U.K. Environment Agency, who heard a radio program in which survivors of World War II discussed their experiences. Timmis heard one woman recall that as a girl, she saw the sky turn white in the wake of hundreds of aircraft over England. That led him to suspect that contrails could play a role in climate and suggest to MacKenzie that they look at aviation records from the time.

Other studies have resorted to modeling to understand the effects of contrails, which have shown that they can have an appreciable impact on global climate, despite their transient nature. A study published in Nature Climate Change in March demonstrated that contrails have a net warming effect and can also affect natural cloud patterns.

ACCRI also commissioned a study that suggests that the Earth's climate is affected by contrails and aviation-induced cirrus clouds.

"Cirrus clouds are known to warm up the atmosphere by attracting infrared radiation," said Guy Brasseur, director of the Climate Service Center in Germany and a coordinator for ACCRI. "The issue appears to be small on the global scale. However, planes are not flying all over the place. They are flying in corridors." Certain routes, like those over the North Atlantic between Europe and the United States, are heavily blanketed with contrails, leading to an increased local impact.

"It's not a major effect compared to other sources of greenhouse warming, but when it contributes up to 10 percent [of aircraft-produced warming emissions], you have to take it into account," said Brasseur.

However, MacKenzie said he is wary of deriving laws to regulate contrails from aircraft the way carbon emissions are regulated and taxed. "I don't think there is enough data ... that we can develop policy from," he said.

MacKenzie acknowledged that the World War II study was limited in scope and was largely a proof of concept. "The next step would be to do it again and again," he said. "We'd like to do the study and do more and more."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500