Contrails and aviation-cirrus
Posted on Tue, Jul. 23, 2002
Contrails: New studies warn of global effects
EMISSIONS OF VAPOR, GASES MAY CONTRIBUTE TO WARMING
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
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.
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.
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
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.
The airline industry says the public should keep in mind the unanswered
``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.
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
Contact Dan McKinney at firstname.lastname@example.org.
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
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
"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
"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