F R 6 - 9 8
Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 6/98
NEW STUDY: AIR TRANSPORT POLLUTANTS HAVE IMPACT ON CLIMATE
by Christopher HessA six-year research program under the lead of the German Aerospace Center (DLR) concerning the climatic effects of pollutants from air traffic was recently completed. The result is not a surprise: International air traffic has a small but measurable impact on the climate of the Earth. In the past, aviation was often accused of being the biggest climate killer _ often without any scientific backing.
With the completion of the research program, now, for the first time, there is a fundamental knowledge about the real effects of air traffic available. The research program included 25 different single projects and was funded by the German research ministry with DM23 million.
Jet airliner burn kerosene with air. The exhaust products are emitted into the atmosphere. The emissions include carbon dioxide (CO2), water vapour, nitrogen oxides (NOX), sulphuric particles and soot. The majority of international air traffic is typically flying at altitudes between nine and twelve kilometres. This is the area of the tropopause which is the boundary layer between the troposphere and the stratosphere. Because of the low temperatures and the low background concentration of nitrogen oxides, emissions at these altitudes have larger climatic impact than the same amount of emissions on the ground.
When the research activities started in the beginning of the nineties, the aviation emitted nitrogen oxides were suspected of destroying the stratospheric ozone layer. Stratospheric ozone is making up the ultraviolet protection shield of the Earth. The troposphereÕs ozone layer is assumed to act as a greenhouse gas.
Since there was not much knowledge about the NOX concentration at the tropopause, several measuring campaigns were conducted. High altitude test chambers were used and DLR test aircraft flew directly in the exhaust plume of airliners over the North Atlantic flight corridor. Furthermore, the scientist developed climate models to compute the impact of the emissions.
One result of the campaigns was that there is presently no measurable variation in the atmosphereÕs ozone content which can be related to aviation. However, the test flights did reveal a noticeable regional increase in exhaust products coming from aviation. The DLR then computed the effects of these emissions with the help of the climate and atmosphere models.
Due to different chemical processes of the air in the various atmospheric altitudes, the emissions have a different effect on the climate depending if they are generated in the troposphere or in the stratosphere.
Based on the model computations, NOX emissions in the typical flight altitudes of today's air traffic (right at the tropopause) are (surprisingly)increasing the ozone concentration. Since ozone, like carbon dioxide, absorbs infrared radiation, the increase in ozone concentration is increasing the greenhouse effect.
The DLR researchers from the Institute of physics of the atmosphere in Oberpfaffenhofen, Germany, under the lead of Professor Ulrich Schumann, are expecting that globally, the air traffic related increase in ozone has increased the radiative force by 0,01 to 0,04 W/m2 (Watts per square meter). As a comparison: It is assumed that because of the increase in all human generated greenhouse gases from 1850 to 1990, the radiation has increased by 2,5 W/m2.
Further calculations show that air traffic might have caused a global warming of 0,02 degrees Kelvin with increasing tendency.
The emission of nitrogen oxides in the stratosphere has a different effect. In this area of the atmosphere it destroys ozone. While todayÕs NOX emission of the subsonic traffic at the tropopause level does not effect the stratosphere (because of the short lifetime of the NOX molecules) it may look different in 20 years. NASA is forecasting that by 2015, a fleet of 500 supersonic airliners will be cruising at 2.4 Mach at an altitude of 20 km, which is the heart of the stratosphere. If this happens, DLR forecasts that these aircraftÕs emission will destroy up to three to four percent of the stratospheric ozone every year.
While the ozone changes are not visible from the ground air traffic generated contrails are. Following the processing of satellite data, the scientists assume that contrails over central Europe are covering approximately 0,5 percent of the ground in a yearly average (at noon). Again based on the climate models, the DLR researchers compute an increase in radiation of 0,02 W/m2 related to this additional cloud coverage. While a 0,5 percent additional coverage reportedly is responsible for an increase in temperature of 0,05 degrees Kelvin, an additional five percent cloud coverage due to contrails will be responsible for an overproportional regional increase in temperature of one degree.
Following the first results of the research campaign, the soot and sulphur content of the jet fuel affects the contrail generation by air traffic also. Since the exact study of these processes were not a topic of the research program, the DLR final report recommends that further investigation and research should be done to find out more about the effect of contrails and soot.
With 130 to 180 million tons of jet fuel per year, aviation burns approximately five to six percent of the global petrol products. Based on the higher number, this results in an emission of 567 million tons of carbon dioxides per year, which would be appropriate to 2,2 percent of the human generated CO2 emissions. Of two emitted CO2 molecules, one may live as long as 100 years. Due to this, it is not relevant for the greenhouse effect if these carbon dioxides are emitted on the ground or in the air.
Based on above aviation related CO2 emissions, the climate model computes an increase in radiation by 0,02 W/m2. According to the DLR figures, the aviation related increase in CO2 emissions from 1800 until today has caused a temperature increase of 0,005 degrees Celsius.
At the bottom line, the report states that aviation has globally led to an increase in radiation of 0,1 W/m2 until 1995. This number means that air traffic has contributed to the global greenhouse effect by four percent.
However, Professor Schumann is pointing out that the confidence level of the published numbers is still very uncertain and should be accounted for as "the best of today's possible estimate". Many of the data are computed with today's available climate models.
The climate scientists see the necessity for further investigation and research involving the other means of transportation and other emission sources. The measuring flights in the North Atlantic corridor already revealed a large background concentration of emissions coming from ground sources on the North American continent.
The industry itself has already achieved a major progress in engine technology over the past years, developing new combustion chamber concepts with considerably less nitrogen oxide emissions. Following successful testing in the laboratories these technologies are still due to be implemented in safe and operational aircraft engines.
The researchers go even farther, saying that, based on the recent research results, the engine manufacturers are now faced with the challenge of developing engines which in 50 years will have less impact on the climate, still satisfying the higher need of transportation at that point in time.
From page 118 of FLUG REVUE 6/98
Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 6/98
Copyright 1998 by Motor-Presse Stuttgart. All rights reserved.
Last updated May 7, 1998
FLUG REVUE, Ubierstr. 83, 53173 Bonn, Germany