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Saturday, January 04, 2003

According to Dutch research on present commercial aircraft flying at altitudes of 8-13 km over the North Atlantic, emissions from such air traffic can change the atmospheric chemical composition directly (carbon dioxide (CO2), nitrogen oxides (NOx = NO + NO2), water vapour, unburnt hydrocarbons, soot, and sulfate particles) and indirectly (by a chemical reaction chain similar to smog-formation the greenhouse gas ozone (O3) can be formed while methane (CH4), another greenhouse gas, decreases).

These changes can have effects on climate:

Ozone, CO2 + water vapour 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 vapour 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 anymore from natural cirrus clouds (thin upper level clouds) for the human eye. The climate effect of changes in cirrus cloud cover due to aviation are not well known.

Interestingly, the grounding of flights for three days after the 11 September attacks gave scientists in the US a unique opportunity to see what the weather would be like without these vapour trails. Their findings, reported in the journal Nature, show that the gap between daytime and night time temperatures was more than one degree Celsius larger than normal when flights were at a standstill.