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Contrails and Aviation-cirrus
Climate (02)

Environment News:
Aircraft Contrails May Fuel Global Warming


LILLE, France, January 11, 1999 (ENS) - Contrails may be increasing the Earth's cloud cover, which could, in turn, increase global warming. A new ten-year study by French climate scientist Olivier Boucher of the Laboratoire d'Optique Atmosphrique, Universit de Lille-I found a greater number of high altitude, long, fleecy cirrus clouds formed where air traffic is heaviest.

Global warming may be increased by a thicker blanket of high altitude clouds. Cirrus clouds, which form above 16,000 feet, may help to warm the planet by trapping the heat radiating from the Earth's surface.

Boucher, who completed his investigation in June, 1998, concluded that an increased occurrence of cirrus clouds follows the world's air traffic corridors.

Long, thin condensation trails, contrails, are formed when the warm, humid exhaust gases from jet planes mix with the cold, dry air of high altitudes. The particles in the exhaust plumes allow the formation of clouds by acting as the nuclei around which ice crystals can form.

Boucher has found that in the decade between 1982 and 1991, a time when air traffic had an annual average growth rate of 3.2 percent, there was a tendency towards a larger increase in cirrus occurrence over regions of higher air traffic with the exception of the North Pacific Ocean.

Over the Earth as a whole, a increase in cirrus occurrence of 1.7 percent over land and 6.2 percent over ocean was found. "Over North America, the trend is much larger at 5.6 percent in annual average, and up to 9.9 percent in March-April-May when the persistent contrail frequency is largest," Boucher wrote.

Over three regions of the Northern Hemisphere - Eurasia, North America, and the North Atlantic Ocean - the trend is always larger over the grid-boxes with most air traffic than over the region as a whole, whatever season is considered, Boucher found. An exception is Eurasia in September-October-November.

Over the North Atlantic Ocean, "the tendency of larger increase in cirrus occurrence over regions of higher air traffic is particularly evident," Boucher wrote. The trend in cirrus occurrence is 8.4 in December-January-February and 8.7 percent in June-July-August for the portion of the flight corridor with most air traffic.

Over North America, Boucher found, the trend in cirrus occurrence is between 2.4 percent and 9.9 percent depending on the season. In December-January-February and March-April-May the persistent contrail frequency is greatest over North America. Close to the Great Lakes, the trend in the annual-mean cirrus occurrence is 13.3 percent, more than twice the value over North America as a whole.

Over Europe and Asia taken as a single continent, the trend in cirrus occurrence is 0.6 percent in annual average, compared to 1.6 percent for the 20 grid-boxes with most air traffic.

Bouchard examined and discarded other reasons for the observed changes in cirrus occurrence such as the effect of volcanic eruptions like El Chich�n and Pinatubo, changes in the upper-troposphere, natural ocean-atmosphere variability, or a temporary response to some global or regional climate occurrences.

Cautiously, Boucher left the door open for other explanations of the correlations between contrails and the formation of cirrus clouds. "We cannot rule out that the observed change in cirrus amount is not due solely to aviation, or inversely that other climate processes have masked an even larger aviation impact on cirrus cloudiness. This study does not demonstrate undoubtedly that aviation is the cause for the observed changes in cirrus cloudiness, but taken with other studies, the results presented here are consistent with the hypothesis that cirrus cloudiness is affected by aviation," he wrote.

Boucher's study is reported in the current issue of the journal "Nature."

Aircraft contrails could be contributing to global warming by creating unnatural clouds in the upper atmosphere, German and American scientists said in 1997. "Aircraft are contributing to climate change, but we really don't know yet how much," said Howard Wesoky of the U.S. National Aeronautics and Space Administration (NASA). Ulrich Schumann of the German Aerospace Research Establishment said up to one-tenth of all cirrus clouds are produced by jets.

NASA scientist Jim Spinhirne confirms that the condensation trails created by planes passing through water-bearing supercool air causes clouds to form that have the same altitude, temperature and acidity as cirrus clouds. His 1997 experiment sent an infrared spectral imaging radiometer into space aboard the space shuttle Discovery to document cloud profiles. The data are now being analyzed.

1998. All Rights Reserved.
Environmental Press Releases  

Present commercial aircraft fly at altitudes of 8-13 km. The emissions from such air traffic can change the atmospheric composition: Directly: by emitting carbon dioxide (CO2), nitrogen oxides (NOx = NO + NO2), water vapor,  hydrocarbons, soot, and sulfate particles. Indirectly: by a chemical reaction chain similar to smog-formation the greenhouse gas ozone (O3) can be formed. In this reaction chain nitrogen oxides act as a catalyst under the influence of sunlight. As a result of these chemical reactions also the concentration of methane (CH4), another greenhouse gas, decreases. These changes can have effects on climate: Ozone, CO2, and water vapor are greenhouse gases and their increase has a warming effect. Methane is also a greenhouse gas and its decrease has a cooling effect. Aerosols (sulfate particles, soot) could have a cooling effect. Contrails formed due to the emission of particles and water vapor can increase the cloud cover in the upper troposphere. This may result in a cooling or heating depending on the size and optical depth of the ice crystals of which the contrails consist. Presently it is believed that contrails lead to a net warming effect. There may be changes in (non-contrail) upper level clouds: Most contrails decay after minutes to hours, but some continue to exist and are then not distinguishable from natural cirrus clouds . 


Do contrails affect climate?

What happens to the long tracks of exhaust left behind by commercial jets? NASA scientists are discovering that some of those tracks, known as contrails, linger for hours and actually become clouds. NBC’s Robert Hager reports on the phenomenon and what it may mean for our global climate.

Study focuses on contrails and climate, ENN Daily News -- June 3, 1997

With more than 62 million commercial and military flights weaving trails of jet exhaust across the skies above the United States each year, there's an emerging question on the radar screens of some climatologists.

And the short answer, according to Steven Ackerman, a professor of atmospheric science, is that jet exhaust plumes -- commonly referred to as contrails -- can indeed influence regional climate.

Addressing scientists at a meeting of the American Geophysical Union, Ackerman detailed the results of a year-long study of contrails and their fate in the upper atmosphere.

"Contrails are of concern in climate studies because increased jet traffic may result in an increase in cloud cover," said Ackerman. "It's been estimated that in certain heavy air-traffic corridors, cloud cover has increased by as much as 20 percent."

Contrails, in essence, are a type of cloud, formed by two parcels of air -- warm, wet air from the jet exhaust and the frigid air of the upper atmosphere. The process, said Ackerman, is analogous to seeing one's breath on a cold day.

There are two broad areas of concern when it comes to contrails, he said: One, are they changing the chemistry of the upper atmosphere? And, two, are they responsible for increased regional cloud cover, a phenomenon that could disrupt the radiation balance, and thus the climate, of a region?

Ackerman and his colleagues, supported by the National Science Foundation and NASA, have been focusing on the latter question for the past year. The Wisconsin scientists are exploring the geometry and fate of contrails in a set of field experiments employing four different aircraft. One lays down a contrail, the second tailgates and samples the contrail at close range, the third observes from an intermediate distance and the fourth high-flying aircraft looks down on the contrail much like a satellite would.

Depending on the conditions in the upper atmosphere, Ackerman said, some contrails may be short lived while others can last for many hours, spread out and cover large portions of the sky. Under some circumstances, contrails don't form at all.

And, he said, contrails appear to be different than other types of ice-crystal-based clouds, such as thunderstorms, mountain wave clouds and cirrus clouds. The field experiments indicate that contrails process infrared radiation differently. Those differences, Ackerman suspects, arise from the varying cloud particle sizes and shapes in contrails.

Clouds of all kinds, including contrails, affect weather and climate because they can both reflect the sun's radiation back into space, and trap the infrared radiation emitted by the surface of the Earth.

"If contrails spread out, then they can modify the energy balance of a region," Ackerman said. "But how it affects the energy balance is not just a function of how much cloud is up there, but what kind of cloud is up there."

All clouds are composed of water, and at high altitudes the water that makes up a cloud exists in the form of ice crystals. A key issue of contrails, which are formed in the cold upper atmosphere, said Ackerman, is the size of the ice crystals that make up the contrail.

"What we don't know about contrails is, are they big particles or small particles? The smaller the ice crystals, the more efficient the reflector," he explained. "They may not only be modifying energy balance, but how energy is distributed."

Another unanswered question, according to Ackerman, is whether contrails feed other types of cloud formation processes. They might, for example, contribute to the formation of cirrus clouds, thin high-altitude clouds that are only now being implicated as an important factor in long-term regional climates.

But are contrails, a relatively small percentage of the clouds in a given area, a significant enough phenomenon to affect climate?

"If we are going to change the climate of a region," said Ackerman, "we're going to do it indirectly," by setting a series of events into motion. "We've got planes in the upper atmosphere and we're changing things. But how big is that effect? They're small clouds to begin with, but they grow and they can stay around for a long time."

For more information, contact Steven A. Ackerman, University of Wisconsin, (608)263-3647, email:, or visit the University of Wisconsin web site.

Copyright 1997, Environmental News Network, All Rights Reserved
ENN Daily News -- June 3, 1997

Large-Eddy-Simulation of Contrails (Andreas Chlond, Max Planck Institute)

Engine water vapor emissions by aircraft are expected to be of significant importance because they may lead to an increase in high altitude ice clouds (Gra�l, 1990; Liou et al., 1990). Ice clouds enhance reflection of sunlight as well as thermal emission. The net effect is uncertain because the competition of solar albedo and IR greenhouse effects involving ice clouds is dependent on such factors as cloud position, cloud thickness and microphysical properties. Contrails are expected to enhance the greenhouse effect, owing to their small thickness.

The objectives of our study have been firstly to investigate the secondary flow structure within clouds to understand the processes that cause contrails to spread out to form cirrus clouds. Secondly, to examine the respective roles of external conditions, such as temperature, humidity, static stability and baroclinicity of the ambient atmosphere in the evolution of contrails and to determine the importance of physical processes such as radiation on the development of contrails. 

Contrails are Bad News  (Startpage)