Emissions explained
February 23, 2012
Beginning this year, the European Union implemented a law forcing airlines to participate in a carbon-trading scheme. Representatives from countries opposed to the EU policy are concluding their two-day meeting on Wednesday to consider how to proceed.
Accounting for about 3 percent of human-caused global warming, airline emissions go through various chemical processes to produce the greenhouse gases that contribute to climate change.
Carbon dioxide from planes
Jet fuel is made up of about 14 percent hydrogen and 86 percent carbon, which binds with oxygen in the air after being burned.
For every kilogram of jet fuel burned, 3.15 kilograms (about 7 pounds) of carbon dioxide (CO2) are produced, explained Ulrich Schumann, director of the Institute of Atmospheric Physics in Germany.
"Because atmospheric CO2 is so long-lived, it spreads evenly above the earth," Schumann told DW.
The approximately 2.2 percent of human-caused carbon dioxide from air traffic is relatively small compared to 14 percent caused by automobile traffic, and sea and train traffic, which account for 3.8 percent of greenhouse gas emissions.
Burning fossil fuels to generate electricity is the largest source of man-made carbon dioxide emissions.
Besides producing carbon dioxide, burning jet fuel also generates nitric oxide and water vapor, which contribute to global warming.
With steam, it's harder to estimate air traffic's exact contribution to climate change. This depends to some extent on the altitude of the flight.
Seeding clouds
Burning one kilogram of jet fuel produces 1.23 kilos of water vapor. This combines with colder air in the surroundings, condensing into droplets, producing condensation trails, or contrails in the sky.
What happens next depends largely on whether the plane is in the lower troposphere or the upper stratosphere.
"Weather comes into play in the evenly mixed troposphere, but in the stable layers of the stratosphere, not much mixing occurs," Schumann explained.
In the extreme cold of the upper atmosphere, the droplets immediately turn into ice crystals. But in the turbulent troposphere, where much weather forms, the saturation of moisture in the air determines how long the contrails last.
In damp air, ice particles collect more moisture, which can build into cirrus clouds. "The ice particles act almost as a seed for moist air, which can build into thick clouds," Schumann said.
"In that sense, air traffic can actually make the Earth cloudier," Schumann added. This happens in 10 to 20 percent of all flights.
And the phenomenon of the clouds formed by contrails plays a role, as yet unclear, in the warming and cooling of the climate.
On the one hand, more clouds mean more shade. "And shade is cooler," Schumann said. But on the other hand, ice particles in these clouds absorb more infrared radiation in the long run, some of which gets reflected downward, making the Earth warmer.
Which of these effects gets the upper hand continues to be the crunch question of contrail research. Until now, research seems to indicate that "the warming effect prevails," Schumann said.
The soot question
Burning jet fuel also produces soot, or tiny bits of carbon grit. As it condenses, steam from the engine turbines sticks to these particles, which are five to 100 nanometers in diameter.
But even if water doesn't condense on the soot particles, they stay in the atmosphere for many days. Researchers think that they can also act to seed clouds, for example in conjunction with dust from deserts or drops of acid in a complicated web of ice production.
To resolve this question, researchers at the atmospheric institute are also examining the effects of ash clouds that spread through the atmosphere after forest fires.
To what extent soot, dust and ash particles act to build clouds remains unclear.
"It's not so easy, there's no answer on this as yet," Schumann added.
Good ozone, bad ozone
Airplane engines get pretty hot - up to 2,000 degrees Celsius. At this temperature, oxygen and nitrogen particles join in the air to form nitric oxide (NO) and nitrogen dioxide (NO2).
In the upper levels of the atmosphere, nitric oxide can lead to ozone reduction, while at lower altitudes, it can produce more ozone. "It's exactly the opposite effect," Schumann said.
It's known that ozone is being depleted at higher altitudes, where supersonic aircraft fly, while most air traffic - which flies at lower altitudes - is leading to excess ozone there.
And tropospheric ozone acts as a greenhouse gas - like carbon dioxide, absorbing infrared energy and keeping the Earth warm.
Although strides have been made on unravelling the mysteries of how air traffic affects climate, Schumann and other researchers still have their work cut out for them.
Author: Fabian Schmidt / sad
Editor: Cyrus Farivar