Aviation and the Global Atmosphere Aviation and the Global Atmosphere Get Javascript Other reports in this collection 4.4. Most subsonic aircraft water vapor emissions are released in the troposphere where they are rapidly removed by precipitation within 1 to 2 weeks. A smaller fraction of water vapor emissions is released in the lower stratosphere where it can build up to larger concentrations. Because water vapor is a greenhouse gas, these increases tend to warm the Earth s surface, though for subsonic aircraft this effect is smaller than those of other aircraft emissions such as carbon dioxide and NO x . 4.5.
Contrails tend to warm the Earth s surface, similar to thin high clouds.
This faster growth in contrail cover is expected because air traffic will increase mainly in the upper troposphere where contrails form preferentially, and may also occur as a result of improvements in aircraft fuel efficiency. Contrails are triggered from the water vapor emitted by aircraft and their optical properties depend on the particles emitted or formed in the aircraft plume and on the ambient atmospheric conditions. The radiative effect of contrails depends on their optical properties and global cover, both of which are uncertain. Cirrus Clouds Extensive cirrus clouds have been observed to develop after the formation of persistent contrails. On average an increase in cirrus cloud cover tends to warm the surface of the Earth.