Contrails and Aviation-cirrus
Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate
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A leading North West academic will tell a House of Lords inquiry into climate
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The suspension of flights after the Sept. 11 attacks has provided scientists with a rare chance to improve their analysis of how aviation affects climate.
When the air above 25,000 feet is humid, the skies are laced with drifting streaks called contrails, created as ice crystals form on the exhaust plumes of hundreds of jets. The wind and gravity disperse the narrow contrails, which form wispy cirrus clouds.
This veil-like type of cloud allows sunlight in to warm the earth and traps some of the rising heat before it can radiate into space. This effect is thought to add slightly to a warming trend that scientists say is caused by heat-trapping gases released when fossil fuels are burned. The ideal way to measure such an effect would be to look at the same piece of sky when air traffic is heavy and when it is absent, said Dr. Patrick Minnis, a senior research scientist at the National Aeronautics and Space Administration's Langley Research Center in Hampton, Va.
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 vapour, unburnt 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 reations also the concentration of methane (CH4), another greenhouse gas, decreases.
These changes can have effects on climate
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For Immediate Release:
NEW ANALYSIS SHOWS AIR TRAFFIC INFLUENCE ON CLIMATE, CONFOUNDING IPCC GLOBAL WARMING ESTIMATES;
Regional Warming Likely Produced by Ice Particles in Upper Troposphere
FAIRFAX, VA, JUNE 26, 1997---Global temperature data gathered by satellites over the past 18 years--the most reliable data available--have consistently shown a slight downward trend, contrary to climate model forecasts. Analyzing satellite data compiled by scientists John Christy of the University of Alabama and Roy Spencer of the NASA Marshall Space Flight Center, however, atmospheric physicist S. Fred Singer has discovered an unusual and previously unexplained regional warming trend over northern mid-latitudes (which includes Europe and the United States), where commercial airline traffic is at its maximum. In a paper just submitted for publication, Dr. Singer demonstrates that this warming has been increasing in line with the growth of air traffic--a correlation that is particularly striking over the last decade.
Unrelated to carbon dioxide emissions or any large-scale "urban heat island" effect, the mechanism, as Dr. Singer explains it, is this: burning jet fuel releases not only pollutants, such as nitrogen oxides and sulfur dioxide, but also large quantities of water vapor, approximately 1.2 pounds for every pound of fuel burned. With airliners routinely flying at altitudes above 30,000 feet, this water vapor condenses into ice particles (contrails) that fade into thin cirrus clouds. These cirrus clouds have radiative properties capable of producing a measurable warming at the Earth's surface.
In a research paper published in Meteorology & Atmospheric Physics (Vol. 38, pp. 228-239, 1988), Singer had already calculated that these thin, virtually invisible clouds could produce a surface warming; direct measurements of infra-red (heat) emissions from cirrus particles appear to support this view. Singer speculates that the same physical mechanism could also explain decreases in diurnal temperature range (the difference between high and low temperatures over a 24-hour period) that have been reported over northern mid-latitudes by Thomas Karl and colleagues at the NOAA Climate Data Center in Asheville, North Carolina.
"The UN Intergovernmental Panel on Climate Change (IPCC) never calculated the climate impact of commercial airline traffic, even though air traffic has been increasing at the rate of 5 percent per year," said Singer. "If it is confirmed that air traffic produces this regional climate affect, then IPCC predictions of future warming must be reduced substantially."
Dr. Singer, who earned his Ph.D. in physics at Princeton University, presented a preliminary version of his research paper at the Annual Meeting of the American Geophysical Union in December 1996, and later before a group of scientific specialists at the NASA Conference on the Atmospheric Effects of Aviation, March 1997. For a printed copy of this press release, with graphs from the research paper, please fax your request to The Science & Environmental Policy Project at (703) 352-7535.
Condensation trails add to Earth's insulation blanket
Global warming increasingly fueled by jet contrails
06/24/99: The condensation trails, or contrails, left by jet airplanes already
cover more than 5 percent of the sky over some heavily traveled parts of the
eastern United States. It's long been known that they are useful indicators of
weather to come. Now, it seems, they may affect the
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Contrails may enhance the greenhouse effect, but microphysical and optical properties as well as the size and global distribution of contrails are not adequately known for conclusive model calculations.
In this study a groundbased scanning lidar system is used to investigate the vortex regime (10-100 sec), the dispersion regime (100 - 1000 sec), and the diffusion regime of persistent contrails with respect to geometric and optical parameters.
Lidar measurements show that the geometric growth of contrails in a sufficiently humid environment is not only determined by turbulence but also by the structure of the local windfield. Horizontal growth can be dominated by windshear spreading a contrail to a band of several km width. The depolarization of the laser light backscattered from contrails changes from about 10 % in the early vortex regime to about 50 % in the dispersion/diffusion regime. This change has to be interpreted with a particle format n mechanism typical for contrails. The lidar also provides the optical depth at 532 nm in contrail cross sections. By calibrating CCD-camera images with this information the optical depth of extended areas of contrails can be determined. This techique will be used to improve existing algorithms which investigate contrails in NOAA AVHRR satellite images with respect to the detection limit and optical depth.