Despite conspiracy theories about so-called chemtrails, there's no evidence that the white plumes seen trailing from high-flying airplanes are part of a secret government program to spray toxic chemicals into the atmosphere for mass sterilization or mind control.
But contrails do pose a threat. Scientists say they contribute to climate change by trapping heat that radiates upward from Earth's surface. A new study published in the journal Atmospheric Chemistry and Physics suggests that the global warming effect will triple by 2050 as air travel grows in popularity and new technology enables planes to reach the higher cruising altitudes where contrails tend to form.
"Given the forecast for the increase in air traffic, which is very large, this contrail effect will increase even more than the carbon dioxide impact," says study co-author Ulrike Burkhardt, an atmospheric physicist at the German Aerospace Center's (DLR) Institute of Atmospheric Physics. "And so it will remain the largest aviation impact on the climate," outpacing the contribution to warming from the carbon dioxide and other greenhouse gases in airplane exhaust.
The warming effect from contrails already represented the largest contributor to aviation's climate impact back in 2005, when aviation accounted for 5 percent of the human impact on climate. That's less than the overall contribution from automobiles and other ground vehicles, but aviation's impact will likely increase given the growing air traffic.
To make their projections about the rising threat posed by contrails, Burkhardt and his colleagues ran several simulations to study the impact of increased air traffic, overall climate change and how greater aircraft engine efficiency combined with alternative fuels could curb the formation of contrails.
Critics of the research caution that the simulations could overstate the threat, though they agree that policymakers should take steps to address the climate impact of contrails regardless of the unknowns — steps that could include changing jet fuel blends or rerouting aircraft flight paths.
Cloudy with a chance of warming
Scientists have been trying to understand contrails since World War II, when massive air raids over Europe filled the skies with the now-familiar white lines. But there's still considerable uncertainty about them.
One issue is that just 10 percent of flights create contrails. The icy plumes form when water vapor in the air gathers and condenses on soot particles in aircraft engine exhaust. But they tend to form only at high altitudes — five miles or higher above the planet's surface, where temperatures of minus 40 degrees Fahrenheit are common.
Airplane traffic, of course, plays a key role in the impact of contrails. North America and Europe have the most commercial flights, so contrails there are common. Flights close to polar regions tend to produce only thin, hazy contrails because the dry air there lacks sufficient moisture to form significant ice crystals; flights over tropical regions tend to produce contrails only rarely because of the warmer atmospheric temperatures.
"Some days there aren't contrails forming, some days they're short-lived, and other days they're long-lasting and soon covering the whole sky," Burkhardt says.
Many contrails last just a few minutes before the frozen water turns back into vapor and the plumes dissipate. Others persist in the atmosphere for more than 10 minutes, and some spread out to become cirrus clouds.
These persistent contrails are believed to have a more significant effect on global temperatures.
Another complication is that contrails can have both the heat-trapping warming effect and a separate cooling effect by reflecting light from the sun back into space. For example, contrails at night tend to have a warming effect, because they mainly trap heat coming from the Earth without reflecting any incoming sunlight.
The new research may overstate the climate-warming effect of contrails by treating them all like heat-trapping cirrus clouds, says Judith Rosenow, an aeronautical meteorologist at the Dresden University of Technology in Germany, who was not involved in the study.
"The impact of contrails on global warming is often reduced to a warming effect, derived from global estimations, where contrails are considered as homogeneous, infinitely extended cirrus cloud layer," Rosenow says. "This assumption doesn't do justice to the true character of contrails."
Making future flight plans
Satellite images have proven to be a useful tool for studying contrail patterns from commercial flights. But the images often lack the detail researchers need to distinguish contrails from clouds or, in crowded skies, tell where one airplane's contrails end and another's begins.
Another way scientists study contrails is by having chase planes fly behind a test aircraft and measure the amount of soot being produced by its engines. Using these flights, researchers have been testing whether contrails can be reduced by switching jets to biofuels made from plants, fat, oil and grease rather than the standard kerosene-based jet fuel that has long been the mainstay of commercial aviation.
In an ongoing series of flight tests, NASA researchers working with Canadian and German teams found that biofuel blends can reduce soot by up to 70 percent and help minimize formation of contrails.
"We're interested in reducing greenhouse gas emissions, but a nice secondary benefit is the reduction in soot particles," Rich Moore, a physical scientist at the NASA Langley Research Center in Virginia, said.
During their recent study, Burkhardt and her colleague Lisa Bock at the German Aerospace Center ran a simulation that found biofuel blends for jet fuel could reduce the estimated warming effect of contrails by 14 percent through 2050. But that alone would not be enough to offset the climate impact.
Another idea involves rerouting flights so that they avoid "ice supersaturated" regions where contrails are most likely to form — a challenge when such regions frequently change in size, shape and location. The tradeoff is that taking extra time to fly around those regions could also burn more jet fuel and raise aircraft CO2 emissions, which Rosenow says would be counterproductive to the goal of reducing aviation's climate impact.
Current international efforts to reduce the climate impact of flying have focused on greenhouse gas emissions while ignoring contrails. But individual nations could take steps to tighten jet fuel standards.
Regulators might consider adding extra fees that discourage airlines from planning flight paths with a higher chance of creating contrails that have a warming effect, such as night flights over the ocean, Rosenow says. France already plans to curb aviation's climate impact by applying an "ecotax" on almost all air travel starting in 2020. Similar efforts could make the biggest difference in North American and European skies where most commercial flights take place.
"Let's do something for those regions where the major part of air traffic is anyway," Burkhardt says. "That would be good enough for a start."
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