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Airplane Contrails Boost Global Warming, Study Suggests

Brian Handwerk
for National Geographic News
June 14, 2006
 
Moving flight times from night to day could reduce air travel's
contributions to global warming, a new study suggests.

Scheduling more daytime flights may lessen the impact of contrails—the visible streaks of condensation that many planes leave in their wake.

The role of contrails in climate change is still under study, but some scientists believe that they contribute to the greenhouse effect by trapping heat in Earth's atmosphere.

Nicola Stuber, first author of the study, to be published in tomorrow's edition of the journal Nature, suggests that contrails' overall impact on climate change is similar in scope to that of aircrafts' carbon dioxide (CO2) emissions over a hundred-year period.

Aircraft are believed to be responsible for 2 to 3 percent of human CO2 emissions. Like other high, thin clouds, contrails reflect sunlight back into space and cool the planet.

However, they also trap energy in Earth's atmosphere and boost the warming effect, the study says.

(See National Geographic magazine's "Global Warning: Signs From Earth.")

Stuber and other scientists believe that the effect of the contrails is significant.

"On average the greenhouse warming effect dominates [the effects of contrails]," said Stuber, a meteorologist at England's University of Reading.

Global Warming and Contrails

This warming effect is far greater for contrails left by night flights, Stuber added.

"The solar cooling effect [wherein contrails reflect the sun's rays back into space] only happens during the day, when the sun is up," she explained.

"During the night the greenhouse warming is no longer balanced, and that is why the contribution of nighttime flights is so large."

Most commercial airline traffic occurs during daylight hours.

For example, only one in four United Kingdom flights is a night flight, but those flights create some 60 percent of the warming attributed to contrails, the study reports.

Contrails are artificial clouds that form around the tiny aerosol particles in airplane exhaust.

They appear only in moist, very cold (less than 40ºF/4ºC) air—usually at altitudes of 5 miles (8 kilometers) or higher.

Some contrails can last for a day or longer, though they gradually disperse and begin to resemble natural clouds.

Contrails Mystery
Scientists disagree about the extent of contrails' climate impact.

"The jury is out on the impact of contrails," said Patrick Minnis, an atmospheric scientist at NASA's Langley Research Center in Langley, Virginia.

David Travis, a climatologist at the University of Wisconsin-Whitewater, notes that some recent studies suggest that contrails have little impact on global climate change but have a greater regional warming impact.

"I prefer to think of contrails as a regional-scale climate problem, as they are most common in certain regions of the world, such as western Europe, eastern and central U.S., and parts of eastern Asia," he said.

"This is due to a combination of dense air traffic in these areas and favorable atmospheric conditions to support contrail persistence once they form."

Because of their locations and short life spans, contrails are a difficult study subject.

"The greatest impediment to understanding the contrail impacts on weather and climate is the poor state of knowledge of humidity in the upper troposphere [3.8 to 9.3 miles/6 to 15 kilometers in altitude]," NASA's Minnis said.

"Until we can measure it properly and extensively, and model it and its interaction with cirrus clouds and contrails, we will continue to have large uncertainties about the effect of contrails."

Winter is Contrail Season

At the high altitudes favored by commercial airlines, the air is much more humid in winter, so contrails are twice as likely in that season, study co-author Stuber said.

"We also found that flights between December and February contribute half of the annual mean climate warming, even though they account for less than a quarter of annual air traffic," she said of her U.K.-based research.

Study leader Piers Forster, of England's University of Leeds, suggests that contrails' current impact on the atmosphere is likely to increase as air traffic grows.

"Aircraft currently only have a small effect on climate," he said.

"However, the fact that the volume of air traffic is set to rapidly grow in coming years makes it important to investigate the effects of contrails on our climate."

Shifting airline schedules will surely prove far easier in theory than in reality.

"The problem is that this is not something that can be done easily," said the University of Wisconsin's Travis.

"With fuel prices on the rise and airlines going bankrupt, it is a difficult time to try and convince airlines to consider issues such as this one."

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