Contrails - Research, comments and links

Contrails and aviation-cirrus 
Climate (06)

Posted on Tue, Jul. 23, 2002
Contrails: New studies warn of global effects


Special to the Mercury News

On clear winter days, the sky can resemble an enormous Etch A Sketch. White lines left in the wake of jets crisscross each other, often stretching from one horizon to the other. But as ephemeral as they may seem, these cloudy trails could be contributing to global warming by trapping heat, according to a growing body of scientific research. If counted all over the globe, contrails -- the vapor trails left by aircraft -- have been found to cover a small but noticeable portion of the Earth's surface.

From 35,000 feet below, contrails may look like wispy lines. But they can grow into clouds many miles wide and hundreds of feet tall, and they often contain pollutants left by the burning of thousands of gallons of jet fuel. The trails and the clouds they create help to trap heat, contributing to the greenhouse effect, several recent studies contend.

To be sure, power plants, heavy industry and automobiles are now believed to be the largest sources of greenhouse gases in the world. The airline industry says there is not enough evidence to single out contrails as a significant environmental factor. But as aviation grows, some scientists are concerned that contrails will become a significant part of the global warming problem. ``Although aviation does not make a large contribution to the warming problem today, over the course of the next 50 to 100 years, it could become as important a part of the problem as passenger cars, according to some estimates,'' said Michael Oppenheimer, professor of geosciences and international affairs at Princeton University.

Scientists estimate that a decade ago, air traffic was responsible for 3.5 percent of global warming. In 2000, there were 16.4 million flights, according to the International Air Transport Association. More than half of those were U.S. domestic flights. With airline traffic projected to grow more than six times the 1992 levels by 2050, experts from the United Nations to the National Aeronautics and Space Administration are now looking more carefully at aircraft emissions and their links to global warming.

Scrutinized before
The vapor trails have come under scrutiny before. In the late 1990s, conspiracy theories about contrails abounded. On talk radio and in Internet chat rooms, contrails or ``chemtrails,'' as they were sometimes called, were cited as part of a secret government plot to poison the population. Scientists found such theories to be unfounded.

Contrails arise from a mix of water, pollution particles and atmospheric conditions. They can form at any altitude if the right conditions are present. Their formation depends on the temperature and humidity of the surrounding air. When the air outside a jet is cold, the temperature forces the water vapor in jet exhaust to condense and form white plumes.

The soot in the airplane exhaust also plays a role in contrail formation. Sulfur and other particles found in jet exhaust act as seeds for the water vapor to land on, forming liquid droplets. The newly formed droplets freeze when mixed with cold air, forming ice particles that make up a contrail.

Contrails also can form as aerodynamic forces causing air pressure to drop over the wings, behind the wing tips and at the propeller tips. The vapor trails' life span depends on the surrounding level of humidity. They can be short-lived or linger for hours. If the air around the trail is wet, the ice crystals that make it up will grow, causing the trail to last longer.

Contrails have been around since aircraft were able to reach altitudes where the air was sufficiently cold, even before World War II. But scientists are beginning to better understand how they function, like cloud cover, to trap heat.

A cloud traps heat depending on where it is in the atmosphere. Low clouds, like fog, trap less because their temperature is almost equivalent to the temperature on the ground. High clouds trap more heat because they're colder, radiating less heat out to space and more back to Earth.

An insulating blanket
Airplane contrails perform the same function as these high clouds. Like a blanket, they insulate, keeping heat underneath them.
They can also spread. In one study, done on a clear day in the spring of 1996, a racetrack-sized oval appeared a few miles off the coast of Mendocino. Using a DC-8 airliner, scientists had drawn the oval using the vapor plumes extending from the back of the jet.

Tracking the cloudy oval from space for several hours, the researchers hoped to learn more about how jet trails affect the atmosphere. The oval floated as far as the Sierra near Lake Tahoe, growing and spreading to dozens of miles across. The DC-8's wispy trail, they found, had essentially grown into its own cloud system. By analyzing what parts of the globe encourage contrail formation and correlating that with air traffic patterns, scientists have been able to put a number on contrail presence in the atmosphere.

According to a 1999 report by scientists working with the United Nations' Inter-Governmental Panel on Climate Change, contrails cover 0.1 percent of the Earth's surface, roughly the combined areas of California and South Carolina. In more densely traveled areas, such as over North America and Central Europe, that percentage climbs into the low single digits.

Questions remain
The airline industry says the public should keep in mind the unanswered questions.
``The key with contrails is to recognize the tremendous scientific uncertainty associated with their climate effects,'' said Heather Miller, a spokeswoman for the Air Transport Association, the trade organization for commercial airlines based in Washington, D.C.

A recent European study estimates that the effect of linear contrails on global warming may be much less than previous studies showed. But critics of the study say it was skewed by the fact that humidity at flying altitudes, a key factor in contrail formation, has been relatively low in recent years. In 2001, it reached a 31-year low. Other aspects of the effects of jet emissions on climate change are already well-understood. Modern jet airplanes burn tremendous amounts of fuel. Fully loaded with 416 passengers, one 747 consumes 2,862 gallons of fuel per hour.

What is left in the jet's wake is a mixture of water, carbon dioxide, other chemicals and soot. The carbon dioxide and nitrogen oxides, a key ingredient in smog, are known greenhouse gases. Because of their chemical properties, greenhouse gases hold heat. They allow radiation from the sun to pass through, but they don't let that radiation back out. A simple analogy would be a car left in the sun that gets very hot inside with the windows closed. Greenhouse gases are like the car windows -- they let heat in and trap it.

Relative to all other human sources, current scenarios put the overall climate change caused by aviation at 3.5 percent, according to the 1999 IPCC study. Including projected improvements in jet engine efficiency and cleaner emissions, that number is expected to climb to 5 percent by 2050, according to the IPCC's report. The document also noted that the impact could be as much as four times greater.

Reducing contrails
There are a number of ways to reduce contrail formation, but the simplest, scientists say, would be to avoid the conditions that give rise to them, including changing plane routes. But the aviation industry says that would lead to other environmental problems.
``It's not enough to simply change air traffic patterns to avoid contrails,'' said Miller, of the industry trade group. ``That has other consequences like unnecessary fuel burn.''

The composition of jet engine fuel is one of the biggest factors in contrail formation. One of the large byproducts of burning fossil fuel is water. ``In the long run, we'll have to produce propulsion that doesn't create water vapor,'' said Patrick Minnis, a senior research scientist at the NASA Langley Research Center in Hampton, Va.

Water vapor is the main ingredient in contrails. But to remove it from jet emissions would be difficult or impossible, according to engineers. ``As you make engines more efficient, you form more contrails,'' said Steve Baughcum, an engineer at Boeing. That's because cooler exhaust creates conditions that increase the likelihood of contrail formation. ``Ironically, exhaust is cooler in more efficient engines,'' he said. And cooler conditions mean more contrails. Baughcum said he doesn't see engine design solving the contrail problem.

Other strategies for reducing aviation's impact on climate include more stringent airline emissions laws or even environmental taxes or charges for emissions.``If aviation doesn't do its 5 percent, some other sector will need to,'' Oppenheimer said. ``It would be wise to consider how to reduce this impending contribution to warming as the industry designs its next-generation aircraft.''

Contact Dan McKinney at
New evidence for man-made global warming

For years the debate about climate change has had a contentious sticking point � satellite measurements of temperatures in the troposphere, the layer of atmosphere where most weather occurs, were inconsistent with fast-warming surface temperatures.

But a team led by a University of Washington atmospheric scientist has used satellite data in a new and more accurate way to show that, for more than two decades, the troposphere has actually been warming faster than the surface. The new approach relies on information that better separates readings of the troposphere from those of another atmospheric layer above, which have disguised the true troposphere temperature trend.

"This tells us very clearly what the lower atmosphere temperature trend is, and the trend is very similar to what is happening at the surface," said Qiang Fu, a UW associate professor of atmospheric sciences.

He is lead author of a paper documenting the work published in the May 6 edition of the journal Nature. Co-authors are Celeste Johanson, a UW research assistant and graduate student in atmospheric sciences; Stephen Warren, a UW professor of atmospheric sciences and Earth and space sciences; and Dian Seidel, a research meteorologist with the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Md.

The team examined measurements from devices called microwave-sounding units on NOAA satellites from January 1979 through December 2001. The satellites all used similar equipment and techniques to measure microwave radiation emitted by oxygen in the atmosphere and determine its temperature.

Different channels of the microwave-sounding units measured radiation emitted at different frequencies, thus providing data for different layers of the atmosphere. In the case of the troposphere � which extends from the surface to an altitude of about 7.5 miles � it was believed there was less warming than what had been recorded at the surface.

The troposphere temperature was measured by channel 2 on the microwave sounding units, but those readings were imprecise because about one-fifth of the signal actually came from a higher atmospheric layer called the stratosphere.

"Because of ozone depletion and the increase of greenhouse gases, the stratosphere is cooling about five times faster than the troposphere is warming, so the channel 2 measurement by itself provided us with little information on the temperature trend in the lower atmosphere," Fu said.

Stratosphere temperatures are measured by channel 4 on the microwave units. Fu's team used data from weather balloons at various altitudes to develop a method in which the two satellite channels could be employed to deduce the average temperature in the troposphere. The scientists correlated the troposphere temperature data from balloons with the simulated radiation in the two satellite channels to determine which part of the channel 2 measurement had come from the cooling stratosphere and should be removed.

What remained indicated that the troposphere has been warming at about two-tenths of a degree Celsius per decade, or nearly one-third of a degree Fahrenheit per decade. That closely resembles measurements of warming at the surface, something climate models have suggested would result if the warmer surface temperatures are the result of greenhouse gases. The previous lack of demonstrable warming in the troposphere has prompted some to argue that climate models are missing unrecognized but important physical processes, or even that human-caused climate change is not happening.

One reason previous data have not shown enough warming in the troposphere, Fu said, is because the stratosphere influence on the channel 2 temperature trend has never been properly quantified, even though there have been attempts to account for its influence. Those attempts had large uncertainties, so many researchers had simply used the unadjusted channel 2 temperature trends to represent the temperature trends in the middle of the troposphere.

Fu's work is supported by the U.S. Department of Energy, the National Science Foundation and the National Aeronautics and Space Administration.

The findings, he said, could offer a new context for climate change discussion.

"I think everyone can understand our approach," he said. "I think this could convince not just scientists but the public as well."

Source: University of Washington


Contrails are Bad News  (Startpage)