Contrails - Research, comments and links

Astronomy (01)

"There are several reasons for the growth in cloudiness,
   but aviation is one of the more notorious"

(Holger Pedersen (Danish astronomer)

"A blanket over the world" 

(Holger Pedersen in Berlingske Tidende)

"The frequency of days with a clear, blue sky has decreased significantly during the last decade. One important reason is due to the condensation trails left by jet engines. 
The globally growing aviation places a heavy blanket over the Earth, contributing to the greenhouse effect."
(Miriam Katz in The Danish national newspaper  Berlingske Tidende", 14-09-1999)

One upon a time, they gave a craving for exotic travels, the white lines behind lofty airliners. Today, they are so many, that they threaten to deprive humanity of a fundamental right: that of seeing a clear, blue sky. Astronomer Holger Pedersen, guest researcher from Copenhagen University observatory, warns against what he terms "celestial grafitti".

"On sunny days, you can clearly see the numerous white lines, which the airplanes pull behind them. But few people know that the lines give rise to cloudiness, which is a problem for the global climate, and for those of us, who have to endure a shrinking number of clear, cloudless days", he says.

The cloud cover over the northern hemisphere is growing rapidly. One important factor is the accelerating aviation. This is apparent from e.g. a French study published in the scientific magazine "Nature" earlier this year. The paper shows how the jet contrails cause increased cloudiness in and near the most trafficked flight corridors. The total amount of clouds over the Pacific, USA, and North Atlantic rose by almost three percent, during a period as short as ten years, from 1982 to 1991.

"Perhaps this appears little, but it is an alarming figure over so short an interval", says Holger Pedersen, who tells that other researchers found similar results. "There are several reasons for the growth in cloudiness, but aviation is one of the more notorious", he adds. The contrails give problems for the astronomers. They are having a harder time viewing the stars and the celestial events far out in space because the contrails interfere, often at unexpected moments of the night. "Then we have to turn the telescope in another direction. But it is not just astronomy, that has a problem. It is mankind as such", says Holger Pedersen.

The white lines arise because aviation fuel combines with oxygen from the atmosphere and forms water vapor and carbon dioxide. When the hot, invisible water vapor leaves the jet-engine, it encounters the cold, ambient air. Thereby it condenses and becomes visible, exactly as steam from a water kettle. Most obvious is it, if some smoke is present, on which the droplets can form. Often the freshly formed contrail persists, and grows ever stronger, at the expense of the atmosphere's own water vapor.

Turbulent winds slowly smear the contrail. What could have been a deep, blue summer-sky now becomes a greyish-blue pele-mele of stripes in different degrees of dissolution, explains Holger Pedersen. "The really clear days are fewer. And today, kids grow up thinking that is the way it is".

Even though the white contrails, which cut across the blue sky, slowly disappear from view, a trace is left in the atmosphere. The soot and the sulphur, which is contained in the contrails, stays around for several months. When the right conditions of pressure and temperature arises, the particles will again draw water, to form what looks like natural clouds. It is an almost impossible task to tell, what is man-made, and what is natural cloudiness. But the effect of the clouds is the same: The more clouds on the sky, the less can the Earth cool at night, and the more will it heat up. "The rise in temperature will lead to more evaporation, and thereby even more cloudiness. This is an evil circle" says Holger Pedersen.

The problem grows quickly due to the thousands of aircraft ploughing through airspace. ICAO, the United Nations organization for civilian aviation has found that during 1997 17.000 airliners were in commercial operation. They carried no less than 1.4 billion paying passengers. That corresponds to one quarter of the entire population of the Earth. To this adds the smaller civilian airplanes (< 9 tons), all military planes, rescue services, etc.

Aviation is growing by three to five percent per year, globally, and even more in the third world. Over USA, Europe, and Japan, the fuel consumption is as large as 10 gram per square meter per year.

Heaps of reports from scientific institutions all over the world confirm the warnings about the pollution caused by aviation. Most recently, the United Nations Intergovernmental Panel for Climate Change in Geneva has issued a (detailed) report on the damaging influence of aviation on climate. All the considered scenarios point to strong growth in aviation, with serious consequences for the environment.

The IPCC estimates that aviation is responsible for 2 percent of all carbon dioxide produced, but the experts add, that the influence on the environment probably is three times larger, due to positive feed-back of cloudiness.

"This is an accelerating situation, which we cannot gauge mathematically. If you wait taking action till 50 years from now, it may be too late" says Holger Pedersen, who tells that there are several ways to tackle the problem: "One way would be to fly at lesser altitude, and to increase the efficiency of the jet engines. Furthermore, taxes could be imposed on jet fuel, and quotas placed on civil aviation, at large. Today, neither economic arguments, nor international agreements work in the right direction.

If you want to know more: Holger Pedersern (holger@astro.ku.dk) has on his homepage https://www.astro.ku.dk/~holger/ collected a number of pictures of contrails, among which are some obtained by satellites launched by the National Oceanographic and Atmospheric Administration (USA).

https://www.astro.ku.dk/~holger/IDA/notes.html

A letter from the Danish astronomer Holger Pedersen:

Having read correspondance with Mr.VanWaning, may I add my few lines of comment: I agree with all parts that contrail formation is an unfortunate side-effect of aviation.

It is my view-point that no bad will (conspiration theories, etc) needs be invoked to explain the large numbers of contrails, which have been documented on the web.

Aviation is rapidly increasing, and this alone should lead to more contrails (all jet engines are prone to contrail formation, as they emit water vapor and soot). Aviation itself perturbes the 'system' by
depositing water vapor at cruise level, from where it escapes only slowly (time scale of months to years).  Hence water vapor concentrations must be building up, and this is likely to increase the life-times of contrails as they depend on RH of ambient air. The result is MORE contrails and
likely more LONG-LASTING contrails (exact measurements are very difficult). 

To this comes, that we may be exceeding a 'critical mass' level, where the public (and thereafter the politicians) start becoming concerned. Here, the web has a key role.

There is no need to introduce a new term (chem trails) to describe long-lasting contrails. Scientists usually say 'persistent contrails'. It is well known that lifetimes may be many hours, and much longer life-times (days) cannot be ruled out.

https://www.astro.ku.dk/~holger/IDA/S/page005.html

[...] ( a part about chemtrails)

It is a sad fact that no convention exists, limiting formation of contrails (or for that matter other effects of cruise level aviation). As an astronomer, I surely prefer clear, blue skies, and I know that other parts of society also suffer (solar collectors, nature photographers, nature lovers, tourism
propaganda). The concerns of many such groups have been amassed on https://www.astro.ku.dk/~holger/IDA/notes.html
(best used with a printed version in hand).

For the most detailed account on contrails, see the IPCC report "Aviation and the Global Atmosphere", Cambridge University Press, 1999.

Sincerely,
Holger Pedersen
cc R.C.VanWaning