Climate Change Out of the Blue

By Douglas Page

02:00 AM May. 10, 2004 PT

Those wispy streams of vapor that follow jetliners across the sky may not be as innocuous as they appear.

A new NASA study claims man-made cirrus clouds formed by commercial jet engine exhaust may be responsible for the increased surface temperatures detected in the United States between 1975 and 1994.

Climate data shows there has been a 1 percent per decade increase in cirrus cloud cover over the United States, which the NASA paper says is likely due to commercial air traffic.

Cirrus clouds, whether natural or artificial, play an important climatological role because they trap heat in the atmosphere by reflecting infrared radiation emitted from the Earth's surface.

The study, which appeared in the April 15 issue of the Journal of Climate, estimates that cirrus clouds from jet engine condensation trails, or contrails, increased the temperature of the lower atmosphere by anywhere from 0.36 to 0.54 degrees Fahrenheit per decade. These findings tend to agree with National Weather Service data that shows temperatures at the surface and lower atmosphere rising by almost 0.5 degrees per decade between 1975 and 1994.

Using 25 years of global surface observations of cirrus clouds, temperature and humidity from the National Centers for Environmental Prediction, the researchers confirmed the cirrus trends with 13 years of satellite data from NASA's International Satellite Cloud Climatology Project.

"Both air traffic and cirrus coverage increased during the period of warming, despite no changes in the NCEP humidity at jet-cruise altitudes over the United States," said Patrick Minnis, senior research scientist at NASA's Langley Research Center in Hampton, Virginia.

By contrast, humidity at flight altitudes decreased over other land areas, such as Asia, and was accompanied by less cirrus coverage, except over Western Europe, where air traffic is very heavy, Minnis said.

The trends in cirrus cover and warming over the United States were greatest during winter and spring, when contrails are most frequent. These results led to the conclusion that contrails caused the increase in cirrus clouds.

Exhaust from aircraft engines is hot and moist, the water vapor in them coming mostly from combustion of hydrogen in the aircraft's fuel. The exhaust takes a moment to cool and mix with the surrounding air, so there is normally a 50- to 100-meter gap behind an aircraft before the contrail appears.

Once formed, contrails are distorted and spread by upper winds. Curtains of ice crystals can sometimes be seen falling from them.

Humidity in the air determines how long contrails remain in the atmosphere. Persistent trails sometimes form large patches of fibrous clouds indistinguishable from natural cirrus, cirrocumulus or cirrostratus clouds, according to Malcolm Walker, of England's Royal Meteorological Society.

Contrails that persist for an extended period of time are most likely to affect the climate. Minnis has estimated that a contrail that begins as a thin gossamer line across the sky can spread to cover more than 20,000 square kilometers in just a few hours.

Not everyone was immediately convinced by Minnis' contrail conclusions.

"The idea that the Earth is warming and high cloudiness is increasing and therefore part of the warming is due to increasing high cloudiness is not logically valid, if one is considering observations only," said Andy Detwiler, a professor of atmospheric sciences at South Dakota School of Mines and Technology.

Correlation does not equate to causation, he said.

"So many processes affect the temperature of the Earth that contrails could easily be acting to cool the Earth, and yet the overall temperature trend could be increasing," Detwiler said.

Still, this is not the first study to connect contrails to the issue of global warming. In 1999, the Intergovernmental Panel on Climate Change estimated that contrails from the world fleet of 12,000 civilian jetliners contribute as much to global warming as the carbon dioxide their engines emit burning jet fuel.



Informant: NHNE


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