By the year 2050, increased flights by jet airplanes will impact global climate through the greater number of contrails they will produce, according to a study completed in 1999 and published in a journal named Geophysical Research Letters.
A research team of American and German scientists, headed by Patrick Minnis of the NASA Langley Research Center in Hampton, Virginia, reports evidence that contrails cause a warming of the Earth's atmosphere. Currently their impact is small as compared to other greenhouse effects. They predict, however, that it may grow by a factor of six over the next 50 years. The researchers emphasize that these are conservative estimates, which take into account only the thicker contrails that can be readily observed. Thinner contrails and contrails that have developed into natural-looking cirrus clouds also affect climate, but their impact cannot yet be predicted. Other factors that would play a role include natural cloud cover, overlapping of contrails, and size of the ice particles that form in them. They call for further research into the full extent of current contrail coverage and the specific effect of contrails in forcing climate change.
Several scientific studies have suggested that aviation may contribute to detrimental chemical changes in the atmosphere (particularly ozone content), as well as possible climate modification. The most widely accepted assessments are those conducted by United Nations (U.N.) scientific organizations. Ozone trends are monitored by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO).
Air traffic and, therefore, contrails, are not evenly distributed around the globe. They are concentrated over parts of the United States and Europe, where local warming reaches up to 0.7 watts per square meter, or 35 times the global average. The ghostly white trails following airplanes and rockets through the sky, called contrails, are probably adding to global warming, according to scientists at NASA’s Langley Research Center, Hampton, Va. The contrails often turn into cirrus clouds, a thin, wispy type of cloud made of ice crystals. The most common form of high-level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet (6,000 meters), cirrus clouds are composed of ice crystals that originate from the freezing of super cooled water droplets. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation. While some clouds tend to help cool the globe and negate the affects of global warming, thin cirrus clouds are heat trappers, holding in more heat than they reflect back into space.
Contrails are human-induced clouds that only form at very high altitudes (usually above 8 km) where the air is extremely cold (less than -40°C). If the air is very dry, they do not form behind the plane. If the air is somewhat moist, a contrail will form immediately behind the aircraft and make a bright white line that lasts for a short while. Persistent contrails form immediately behind the airplane in very moist air. Persistent contrails can exist long after the airplane that made them has left the area. They can last for a few minutes or longer than a day. However, because they form at high altitudes where the winds are usually very strong, they will move away from the area where they were born. Persistent contrails are those most likely to affect climate.
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 vapor, 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 reactions also the concentration of methane (CH4), another greenhouse gas, decreases. These changes can have effects on climate: Ozone, CO2, and water vapor are greenhouse gases and their increase has a warming effect. Methane is also a greenhouse gas and its decrease has a cooling effect. Aerosols (sulfate particles, soot) could have a cooling effect. Contrails formed due to the emission of particles and water vapor can increase the cloud cover in the upper troposphere. This may result in a cooling or heating depending on the size and optical depth of the ice crystals of which the contrails consist. Presently it is believed that contrails lead to a net warming effect. There may be changes in (non-contrail) upper level clouds: Most contrails decay after minutes to hours, but some continue to exist and are then not distinguishable from natural cirrus clouds. ....