Contrails - Research, comments and links

Contrails and Aviation-cirrus
Airtravel (01)

The Atmospheric Effects of Aviation Project
For the past 10 years NASA has held a conference on The Atmospheric Effects
of Aviation Project (AEAP). Several hundred researchers from around the
world attend annually. In 1997 Researchers from NASA Langley Research Center
in Hampton, Virginia, presented evidence that contrails are contributing to
global warming and causing local effects over areas with heavy air traffic.
This was reported by Jim Scanlon a journalist in attendance at the
conference. He also reports that Fred Singer held a session where he
presented a session that argued that the steady increase in air traffic for
the last 20 years was responsible for the nighttime warming detected over
North America.

- The Atmospheric Effects of Aviation Project (AEAP) consists of two major
efforts to assess the effect of aircraft on the atmosphere. The Atmospheric
Effects of Stratospheric Aircraft (AESA), sponsored by the High-Speed
Research (HSR) Program at Langley Research Center, is a study of the
potential effects of the operation of a projected future fleet of high speed
civil transport aircraft (HSCTs). The Subsonic Assessment program (SASS) is
a study of the effects of the present subsonic aircraft fleet and of
projected future subsonic fleets, and is sponsored by the Advanced Subsonics
Technology Program (AST) at Langley Research Center. Objective Develop
scientific basis for assessment of atmospheric impact of subsonic and
supersonic aviation, particularly commercial aircraft cruise emissions.

Possible Ozone Destruction Connection
A low ozone event occurred over The Northern Hemisphere in the days
following the busiest air traffic days in The United States in late November
and early December 1999. Traditionally the days leading up to and after
Thanksgiving  are the busiest of the year. Could there be a connection
between the air traffic and the low ozone event?

Ozone layer over Europe dwindling -European Space Agency 2 December 1999
PARIS - The ozone layer over Britain, Belgium, the Netherlands and
Scandinavia has dwindled to worrying levels nearly as low as those found in
the Antarctic, the European Space Agency (ESA) said Thursday.
Measurements taken in the Netherlands showed local ozone levels were some
two-thirds below the norm for this time of year, confirming the abnormally
low levels detected over northwest Europe this week, ESA said in a
The ozone layer, high up in the atmosphere, shields Earth from much of the
sun's harmful ultraviolet rays. A gradual thinning due to emissions of
damaging man-made chemicals has increased the occurrence of skin cancer and
other illnesses related to over-exposure to ultraviolet rays, scientists
say. "

Aviation Impact: Results and Open Questions

Ulrich Schumann
DLR Oberpfaffenhofen, Institut f�r Physik der Atmosph�re

Based on existing studies (EU 1998; IPCC, 1999; various national studies, 1990-2000), the aviation impact may be characterised as follows:

  • Regional increases in nitrogen oxides (NOx), aerosols (small particles), and cloudiness (contrails and cirrus) due to aviation measured.
  • Increases of carbon dioxide (CO2) and of ozone concentration (O3) computed.
  • No increase in ultraviolet radiation (UV-B).
  • Reduction of methane (CH4) computed.
  • Global aviation emits 2 % of all anthropogenic CO2 (10 to 13% of all CO2 from traffic), 1% of all NOx-emissions (7% from traffic).
  • Input into free troposphere and lower stratosphere has larger climate impact than the same emissions near the Earth's surface.
  • Global aviation causes about 4 % of the anthropogenic radiative forcing (greenhouse effect) today.
  • The radiative forcing by aviation today is 2 to 4 times larger than due to CO2-emissions in the past until today by aviation alone.
  • Growth rates of 3 to 5%/year imply a doubling within 24 to 14 years.
  • Scenarios imply traffic growth by factors 4 to 10 from 1992 to 2050.

Recent assessments, EU (1998: Atm. Env. 32, 2327-2422) and IPCC (1999), identified open questions that require urgent attention. Common to both reports are the following points:

  • The mechanism for transport of gases and particles in the UT and LS.
  • The influence of contrails and aerosols on cirrus clouds and chemical processes.
  • The response of the climate system from a non-uniform regional forcing and stratospheric perturbation.

The IPCC report also states:

  • The role of NOx in changing the ozone and methane concentration as a question that requires urgent attention.

The European assessment states:

  • The precise composition of the aircraft exhaust with regards to particle formation.

Example results will be shown from the many studies performed within the projects DFG, Pollutants in the Atmosphere, SULFUR 1-7; AERONOX, AEROCONTRAIL, AEROCHEM 1 and 2, POLINAT 1 and 2, EULINOX, MOZAIC; INCA, PartEMIS, CRYOPLANE. The results have been produced in cooperation with many scientists outside and inside DLR (within DLR-IPA, OP: R. Busen, M. Dameris, K. Gierens, V. Grewe, J. Hendricks, H. H�ller, H. Huntrieser, B. K�rcher, I. K�hler, H. Mannstein, R. Meyer, A. Minikin, A. Petzold, M. Ponater, R. Sausen, H. Schlager, F. Schr�der, P. Wendling, and others). Moreover, some ideas about how to address the open questions will be indicated.

European Workshop on Aviation, Aerosols, Contrails and Cirrus Clouds

Seeheim (Germany), 10-12 July 2000
  Last update: 27.03.2000 |

This focussed Workshop will address persisting uncertainties on the role of aerosols and particles from aviation and other sources to contrails and cloud formation with impact on stratospheric ozone and climate. The recent European and international assessments (WMO, IPCC, ICAO) stress the urgent need to clarify these uncertainties, which limit our ability to project the atmospheric impacts of aviation. The workshop will provide a forum for presentation of the results of current and past EC and national research activities in this field (including THESEO). The outcome of the Workshop will contribute to the regulatory process within ICAO and the Montreal Protocol. For aviation industry is also of great importance to know which type of aircraft emissions has the largest impact on contrails and cloud formation.
Further information: or

Air travel releases 600 million tons of carbon dioxide in atmosphere

- 1 round trip from NY to LA or Trans Atlantic round trip = 2,000 pounds of
CO2. In a year air travel releases 600 million tons of carbon dioxide into the
(A visitor wrote: "Lest there be any misunderstanding by non-technical members 
of the public, you might want to make it quite clear that this figure is 
per passenger and not per plane. Thanks for an impressive collection of photographs.")

- By the year 2050, increased flights by jet airplanes will impact global
climate through the greater number of contrails they will produce, according
to a study completed in 1999 and published in The Journal of Geophysical
Research Letters

- A research team of American and German scientists, headed by Patrick
Minnis of the NASA Langley Research Center in Hampton, Virginia, reports
evidence that contrails cause a warming of the Earth's atmosphere. Currently
their impact is currently small as compared to other greenhouse effects.
They predict, however, that it may grow by a factor of six over the next 50
years. The researchers emphasize that these are conservative estimates,
which take into account only the thicker contrails that can be readily

- Thinner contrails and contrails that have developed into natural-looking
cirrus clouds also affect climate, but their impact cannot yet be predicted.
Other factors that would play a role include natural cloud cover,
overlapping of contrails, and size of the ice particles that form in them.
They call for further research into the full extent of current contrail
coverage and the specific effect of contrails in forcing climate change.

- Several scientific studies have suggested that aviation may contribute to
detrimental chemical changes in the atmosphere (particularly ozone content),
as well as possible climate modification. The most widely accepted
assessments are those conducted by United Nations (U.N.) scientific
organizations. Ozone trends are monitored by the United Nations Environment
Programme (UNEP) and the World Meteorological Organization (WMO).

- Contrails are human-induced clouds that only form at very high altitudes
(usually above 8 km) where the air is extremely cold (less than -40�C). If
the air is very dry, they do not form behind the plane. If the air is
somewhat moist, a contrail will form immediately behind the aircraft and
make a bright white line that lasts for a short while. Persistent contrails
form immediately behind the airplane in very moist air.  Persistent
contrails can exist long after the airplane that made them has left the
area. They can last for a few minutes or longer than a day. However, because
they form at high altitudes where the winds are usually very strong, they
will move away from the area where they were born.  Persistent contrails are
those most likely to affect climate

- 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

- Schematic of aerosol and contrail formation processes in an aircraft plume
and wake as a function of plume age and temperature. Reactive sulfur gases,
water vapor, chemi-ions, soot aerosols, and metal particles are emitted from
the nozzle exit planes at high temperatures. H2SO4 increases as a result of
gas-phase oxidation processes. Soot particles become chemically activated by
adsorption and binary heterogeneous nucleation of SO3 and H2SO4 in the
presence of H2O, leading to the formation of a partial liquid H2SO4/H2O
coating. Upon further cooling, volatile liquid H2SO4/H2O droplets are formed
by binary homogeneous nucleation, whereby the chemi-ions act as preferred
nucleation centers. These aerosols grow in size by condensation and
coagulation processes. Coagulation between volatile particles and soot
enhances the coating and forms a mixed H2SO4/H2O-soot aerosol, which is
eventually scavenged by background aerosol particles at longer times. If
liquid H2O saturation is reached in the plume, a contrail forms. Ice
particles are created in the contrail mainly by freezing of exhaust
aerosols. Scavenging of exhaust particles and further deposition of H2O
leads to an increase of the ice mass. The contrail persists in
ice-supersaturated air and may develop into a cirrus cloud. Short-lived and
persistent contrails return residual particles into the atmosphere upon
evaporation. The scavenging timescales are highly variable and depend on the
exhaust and background aerosol size distributions and abundances, as well as
on wake mixing rates

- Clouds play a complex role in the Earth's radiation budget. Low Clouds
reflect much of the sunlight that falls on them, but have little Effect on
the emitted energy. Thus, low clouds act to cool the Current climate. High
clouds reflect less energy, but trap more of The energy emitted by the


The Atmospheric Effects of Aviation Project

September 25th: Mr D. Stellingwerf, member of the Dutch Parliament (RPF) has  shown foresight in various matters concerning transport. When asked by reporter Elaine De Boer what he thought of the future of the Dutch Amsterdam Airport Schiphol, Mr. Stellingwerf replied that within a few years flying for fun should be made unbelievably expensive. 'People should get used to the fact that not everything is possible at all times', he says. He does not think that this would be unfair for families who want to fly to the sun. 'I don't think we're discussing human rights here', he replies. (Source: De Volkskrant, Sept. 25th, 1999)

GreenSkies is a worldwide information network of environmental organisations concerned with aviation's environmental affects. The network started as a European initiative (see below), and its members are predominantly based in Europe. However, organisations from all over the world are increasingly joining.

The key objectives of the GreenSkies Network are to;

a) reduce noise at airports, especially at nights; and
b) reduce the industry's growing contribution to global climate change.

To promote these objectives the GreenSkies Network works together to exchange information and raise awareness of the issues.

Air travel a threat to the future of the planet

- air travel is emerging as one of the greatest threats to the future of the planet.
- aircraft pump out carbon dioxide and other greenhouse gases high in the
atmosphere, where they do more damage.
- airports outdo almost all factories as sources of dangerous pollution -
while the noise from planes blights millions of lives.
- emissions from international flights are excluded from the treaties agreed
to combat global warming and ozone depletion
- airports in Britain are exempt from pollution control
- air travel receives billions of pounds every year in subsidies from the taxpayer
- the fastest growing form of transport worldwide is air travel.
- the Institute of Public Policy Research (IPPR) says, "flying by jet plane
is the least environmentally sustainable way to travel and transport goods"
- airports also emit massive air pollution from refuelling and storing fuel,
from maintenance and supply operations, and from the huge amount of road
traffic they generate.
- Frankfurt airport contributes nearly three-quarters of its city's
emissions of unburnt hydrocarbons and almost half its load of pollution by
carbon monoxide, sulphur dioxide and nitrogen dioxide.
- if Terminal 5 is built, Heathrow will emit Britain's second highest amount
of volatile organic compounds, many of which cause cancer, after a factory
on Teesside
- pollution from aviation has increased rates of cancer around
Chicago-Midway airport, about the same size as Gatwick
- Although individual planes are getting quieter, noise is also increasing
with the number of flights.
- the European Environment Agency last year concluded that 440,000 people
around Heathrow are plagued with noise above the standards set down by the
World Health Organisation. Excessive noise damages health, with effects
ranging from hearing damage to heart disease. And it blights children's
education. Studies around Munich airport and New York's La Guardia and JFK
conclude that it impairs their memory and reading ability.
- the economic benefits of increased air travel, long cited as the
justification for its expansion, are looking uncertain
- Fast rail services can replace some short-haul flights, but experience in
Germany and from the Channel Tunnel shows these just free up slots for more
long-haul ones.
- Subsidies must be ended, aircraft fuel taxed, and exemption from pollution
control ended. New charges can be levied on polluting aircraft, as has been
done in Switzerland, and on all flights, as in Denmark. But these will only
slow, not stop the explosive growth of air travel. Reducing its increasingly
devastating impact on the planet would require a reversal of one of the most
liberating trends of the last half century. And this bank holiday weekend
that's about as likely as pigs taking wing.

(bron: )

Measuring and modelling the effects of aviation on the atmosphere 

The use of aircraft in transporting people and goods is an important part of today's economy. But what is the environmental impact of all those airplanes, flying ceaselessly through the atmosphere? This contribution to the ‘Recent highlights’ aims to give a short overview of our present knowledge of the effects of aviation on the atmosphere and also of the work that has recently been done in this field in the Atmospheric Composition Division at KNMI.


The T-39, DC-8 and ER-2 flew a coordinated mission over the CART site, under patchy cirrus clouds. The T-39 created contrails (by flying circles) upwind of the CART site. At the same altitude, but downwind of the CART site, the DC-8 flew perpendicular to the wind-direction to sample the remnants of the T-39 contrails, which were not too persistent to be easily identified, and to profile the patchy cirrus cloud-field. The ER-2, while flying an wind pattern, observed the other aircraft from above them. After the T39 exhausted its fuel and returned to Salina, the DC-8 continued to fly under, and with the same pattern as the ER-2. At 2006 UTC measurements were coordinated with the NOAA-14 satellite overpass.
Link to
no longer valid

Those who are responsible for the growth of aviation should prove that this development is without any harm. At the moment the burden of proof rests upon those who are worried about the environmental and climatic consequences of too much flying.
(The actual airtraffic (real time!) over Los Angeles gives you a fair idea of what we are talking about: )

Too many people fly
too many times,
for too little money
and for too silly reasons
to too distant destinations

Flying from
Amsterdam to Geneve
is cheaper by plane than by bus.
That is: cheaper only in terms of money.
The costs to the world, our living environment,
are still unknown. 
What counts, and who accounts for it?

Vliegverkeer boven Los Angeles (real time!)


Contrails are Bad News