Contrails - Research, comments and links

Contrails and Aviation-cirrus

Aviation-emissions (02)

The Coastal Post - May, 1997

Jet Planes Pose Serious Environmental Threat; 
Can Anything Be Done About Contrails?

By Jim Scanlon

It is a criminal offense to mark or mar public or private property with chalk or paint or other material. This is understandable: what is hard to understand is the toleration afforded jet aircraft which leave long white streaks from one horizon to the other, some lasting for hours before turning slowly into thin gray clouds. This is not just a question of aesthetics, but of ever-increasing environmental degradation.

Just look up at any time, any clear day in winter or spring or fall and you will see a long white vapor trail following a barely visible jet, no doubt taking people like you and me someplace far away. Sometimes these lines braid and curl and wave slowly about before quickly disappearing. Sometimes they last longer, minutes, hours, maybe longer.

Sometimes no one on earth can see persistent contrails because lower level, natural clouds get in the way. But an endless queue of aircraft produce them in their wakes, seen or unseen, night and day, coming and going from airport to airport across the planet.

Whether or not one sees vapor trails, jet motors expel invisible oxides of nitrogen, carbon dioxide, carbon monoxide, soot and unburned hydrocarbon fuel besides the water which often condenses into the white lines we observe.

Simply burning fuel in the combustion chamber of a jet motor contributes toward "global warming" equally, in the same way as, say, power plants and buses, trucks, cars, etc. except that there are important differences-where the combustion products go, how long they stay and what their after effects are. A crude analogy might be that you can tolerate lots of dirt on your skin and some in your ears and mouth, but only a little in your eyes. It's not just how much, but where.

While taxiing on a runway, jet engines contribute significantly to urban smog just as other internal combustion engines---that is, unburnt fuel reacts with sunlight to produce ground level ozone, which is highly irritating to living things. Soot, in and of itself, is a irritant when breathed, but it also attracts acids which make it worse. This would continue during the initial stages of take off and steeply climbing to cruising altitude when the engines are going at full thrust. These effects are well known and usually accounted for in considering the environmental effects of civil aviation.

Once at cruising altitude in the upper troposphere (and often in the stratosphere) where the temperature is very low, very dry and relatively much cleaner than lower, closer to earth, the waste gases have different, sometimes very complicated effects---which are not accounted for. The earth really isn't a sphere, it really isn't round and neither is the boundary between the highly changeable troposphere and the stable stratosphere. One bobs and ripples and intermingles on top the other somewhat like a layer of oil on water. The stratosphere is warmer than the upper troposphere. There is a structure---a changing structure, but still a structure.

Warming a normally much colder part of the atmosphere breaks down this structure. Water is a very potent "greenhouse gas"---much more effective than carbon dioxide and other pollutants---and it is being introduced in massive, ever increasing amounts just under, and just over, the defining boundary in the structure. When the jet aircraft fly in the stratosphere the chemical reactions of exhaust gases reduce natural ozone, cooling the warmer air above the boundary, and allowing energetic ultra-violet radiation to penetrate lower where it warms the air and may even reach the surface of the earth. Flying just below the boundary exhaust gases produce ozone and other heat-trapping gases which warm cold air, further distorting the boundary. With large numbers of aircraft flying fixed routes, the effects become more pronounced.

On April 13th, 14th and 15th the sky over Marin was clear and criss-crossed with particularly long lasting clouds from contrails. Thin straight lines that gradually spread out forming a hazy layer. Towards evening they were particularly visible, with three, thick blood-red lines forming over Mount Tam as the sun set. (I will have a small selection of these photos on display at Smiley's Schooner Saloon in Bolinas during the month of May). How much do these clouds contribute towards changing the local climate, the regional climate and perhaps global climate? It's hard to tell. The weather is very complicated and confusing although occasionally exceptional weather conditions produce conditions which allow for unmistakable signs of pollution from aircraft. A stagnant air mass which moved slowly across the Atlantic over Southern Europe a few years ago is a good example.

It has taken a half century for our government and the tobacco industry to officially come to the realization that cigarette smoking is unhealthy, addictive and has caused millions of early deaths. Who wants to face the unpleasant possibility that a trillion dollar industry which provides millions of high paying, glamorous jobs and the magic carpets which carry us across continents in a few drowsy hours, might be highly dangerous to life on earth. Who wants to face it ?

The Federal Aviation Administration recently announced that domestic air passenger traffic will rise from 546.2 million in 1996 to 827.1 in by 2008. Aircraft are expected to get bigger. The average seat capacity of planes flying Pacific Ocean routes will rise from 326 to 366 and might rise to 736. The market for "super jumbos" is estimated to be 1,400 planes in 20 years. Civil air traffic is expected to increase 200 percent in 20 years. The effects of this traffic on the structure of the atmosphere are not being considered and it does not appear likely that they will.

It seems that this technology, as with others, has a life of its own, and is unreformable and irreversible. We are entering a new world.


There are three gases emitted by aircraft which contribute to global warming: H2O, CO2 and NOx

The most obvious is the water vapour (created by burning jet fuels -see below) which forms condensation trails -clouds of frozen ice crystals. Since the air in the upper troposphere (the level at which most commerical planes fly) is naturally very dry, water vapour emitted by aircraft can make a big difference. Sometimes the contrails cover the whole sky as shown by this satellite photo taken over Germany, and the average coverage in this region is about 6%. Have you ever wondered, why the sky is so much clearer in remoter locations?

Although these contrails reflect a little sunlight away from earth, they reflect back to earth much more invisible infra-red (heat) radiation which would otherwise escape to space -and therefore they have an overall warming effect. This is hard to measure accurately, because the contrails eventually spread out and become indistinguishable from natural cirrus clouds.

Not all of the water vapour forms contrails, but water is itself a "greenhouse gas" which also traps this outgoing infra-red radiation. Each water molecule traps much more heat and also survives much longer at this height than it would do at sea-level.

Jet-fuel - kerosene - is a mixture of substances produced by distilling crude oil, which can be represented by C13H28 (this is assumed in the "how much" calculations" <mapcalc.html>). The chemical equation for burning it is as follows:
2C13H28 + 40O2 =>26CO2 + 28H2O

So you can see, that for every 14 water molecules produced, the aircraft must also emit 13 of CO2. This is also a greenhouse gas and will stay in the atmosphere warming the earth for an average of 100 years, some of it for 1000s of years. There's no way that you can get the energy from such fossil fuel without producing that much CO2. It's not a by-product that can be "scrubbed" from the exhaust.

A Boeing-747 can burn over 200 tons of fuel in one flight. If you have already clicked your travel route on the world map <mapcalc.html>, the "how much is this" section <mapcalc.html> will calculate the volume of CO2 that will be emitted per passenger. The "global context" section <mapcalc.html> compares this amount to the sustainable average level of emissions per person per year.
If you understand German, you can find a similar calculation on the web page Klimabelastung durch Flugverkehr <>
(For more about greenhouse warming click here <climint.html>)

A simple "rule of thumb" to remember, is that a plane uses about as much fuel, and therefore produces about as much CO2, as would every passenger driving one car the same distance. So next time you're thinking of flying thousands of miles, think how much petrol you would need to put in a car to go so far. Since take-off uses a disproportionate amount of fuel, short-haul flights emit a bit more per passenger-kilometer, and long-haul flights a bit less.
Note: the car comparison is for typical european cars, not for the less efficient american gas-guzzlers. Trains produce, on the other hand, about 1/3rd as much CO2 per passenger-kilometer (for more details on this see the "ecobalance of the Climate Train" <../climatetrain/ecobal.html> and the bar-chart below), and could potentially be run from renewable sources of electricity.

So if they use so much fuel, why are flights so cheap now? One reason is that not a penny of tax is paid on aircraft fuel <cheap.html>.

But fuel use is not the end of the story. Aircraft also emit NOx gases, a product of combustion of atmospheric nitrogen and oxygen in the very hot jet engines. NOx is not in itself a greenhouse gas but it catalyses the production of ozone (O3), which is a powerful greenhouse gas. Nearer the ground, ozone also leads to the formation of photochemical smog which is harmful to human health.

In the stratosphere, on the other hand, ozone is valuable as it blocks harmful ultra-violet radiation. Unfortunately NOx emissions from aircraft which venture into the stratosphere (such as concorde) catalyse the destruction of ozone there. Confused? -Yes, ozone chemistry is complicated, but in a nutshell, NOx emissions are bad news -producing ozone where we don't want it, and destroying it where we do..

To make matters even more complicated, NOx emissions also catalyse the destruction of methane (CH4), another greenhouse gas, although this cooling effect is much smaller than the ozone warming effect (see IPCC graphic below). Aircraft emissions of sulphate aerosols also have a slight cooling effect, but also contribute to acid rain.

It's not easy to quantify the exact greenhouse warming due to water vapour and NOx emissions from aircraft, but a scientific consensus is now emerging, that the total warming effect of all emissions (CO2, H2O and NOx) put together, is in the range 2-5 times greater than that of CO2 alone. This is confirmed by analysis in the "Special Report on Aviation" recently published by the "Intergovernmental Panel on Climate Change" (IPCC) <> in April 1999. The image below, taken from the summary of this report, shows the relative effect of all the different gases ("fair", "poor", etc. indicates the relative certainty of the estimates).


9/11 study: Air traffic affects climate

August 8, 2002 Posted: 1:29 PM EDT (1729 GMT)
By Richard Stenger CNN

(CNN) -- The thin wisps of condensation that trail jet airliners have a significant influence on the climate, according to scientists who studied U.S. skies during a rare interruption in national air traffic after the September 11 terrorist attacks.

Contrails, such as in this satellite image over California, have an impact on temperatures, scientists say. The trail of condensation forms in an aircraft's wake. Contrails, such as in this satellite image over California, have an impact on temperatures, scientists say. The trail of condensation forms in an aircraft's wake.

During the three-day commercial flight hiatus, when the artificial clouds known as contrails all but disappeared, the variations in high and low temperatures increased by 1.1 degrees Celsius (2 degrees Fahrenheit) each day, said meteorological researchers.

While the temperature range is significant, whether the jet clouds have a net effect on global warming remains unknown.

"I think what we've shown are that contrails are capable of affecting temperatures," said lead scientist David Travis of the University of Wisconsin, Whitewater. "Which direction, in terms of net heating or cooling, is still up in the air."

Contrails Explainer:
  • Long white wisps of artificial clouds high in the atmosphere, contrails are the condensation trails left behind by jet airplanes.
  • Similar to human exhalation making a fog in chilly weather, contrails form when warm humid engine exhaust meets extremely cool air in the atmosphere.
  • Air temperatures where contrails form are generally lower than minus 40 degrees Fahrenheit (minus 40 degrees Celsius).
  • Like natural cirrus clouds, contrails insulate the planet, blocking out incoming solar energy from above and keeping in heat down below.
  • Scientists estimate that contrails cover some 0.1 percent of the Earth's overall surface, with regional concentrations as high as 20 percent.

In many ways, contrails behave in the same manner as cirrus clouds, thin high-altitude floaters that block out solar energy from above and trap in heat below.

As a result, they help reduce the daily range in daytime highs and nighttime lows. Contrails, by providing additional insulation, further reduce the variability.

With air traffic growing and contrails becoming more prevalent, the natural variation will further decline and could disrupt regional ecosystems, some scientists speculate.

Certain trees, crops and insect species depend on specific daily temperature variations for their survival.

In some ways, contrails differ from their natural brethren. Cirrus clouds let less heat out than in overall, producing a net increase in the Earth's temperatures, according to climate scientists. With contrail clouds, they said they are not so sure.

"Contrails are denser and block sunlight much more than natural cirrus clouds," said Travis, who conducted the study with Andrew Carleton of Penn State University in University Park, Pennsylvania. They reported the findings this week in the journal Nature.

"And contrails are much more prevalent when the sun is out," he said. "When this is factored in, there is a possibility that they offset global warming, and this is what we are trying to determine now."

The researchers plan more studies to tackle that question, but they said they expect to rely on circumstantial evidence only.

"We can only hope that the September 11 tragedy never happens again," Travis said.

Contrails are Bad News