Jet contrails may add to warming

HAMPTON, Va. — Next time you’re on a plane, think of this: The plane’s exhaust might be adding just enough moisture to the atmosphere to create a cloud and keep it floating.

That cloud could stretch 1,000 miles long and 37 miles wide, depending on the weather and your flight distance.

Add that cloud to all the other clouds produced by airplane exhaust, and it creates a blanket effect — trapping heat that’s radiating from the Earth. The end result: warmer temperatures on the surface.

In other words, your flight might be contributing to the greenhouse effect.

A recently published study by researchers at NASA Langley in Hampton, Va., concludes that clouds from airplane exhaust, or contrails, contributed to a .5-degree per decade warming trend in the United States between 1975 and 1994.

That might seem like a small amount, but those half-degrees add up over time. Environmentalists fear that global warming will bring about radical changes to weather patterns and agricultural production. Some worry that the polar icecaps could melt and cause the sea level to rise. Scientists vary on their level of alarm over global warming, but most agree that it’s worth a serious look.

Greenhouse gasses like carbon dioxide are still the main focus of global warning research. But scientists had suspected that contrails might contribute to the greenhouse effect as well. The NASA study is the first time someone used real weather observations to document temperature change relating to contrails, said Patrick Minnis, a senior research scientist at Langley.

Minnis studied contrails and the feathery white cirrus clouds that develop from contrails. He also studied surface temperatures across the globe to see what, if any, effect the clouds were having down below.

To understand his conclusions, you need to know a few facts about contrails, cirrus clouds and global radiation.

Not every flight creates a contrail. The conditions have to be just right, with temperatures of minus 40 degrees or colder at typical flying altitudes of 25,000 to 43,000 feet.

Clear skies cannot hold enough moisture to support a cloud. If a plane comes along, its exhaust can add just enough moisture to the air to condense and freeze, forming a sustainable cloud.

“You’re basically turning ‘on’ a cloud that wouldn’t form on its own,” Minnis said.

The moisture levels over the United States did not increase during the study period. But the amount of cirrus clouds increased at a rate of about 1 percent per decade.

“Cirrus clouds can have a net warming or net cooling effect on the Earth, depending on how thick they are,” Minnis said. “Cirrus clouds from contrails tend to be thin, and the effect of thin clouds tends to be warming.”