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were subsequently discussed by Schumann and Wendling (1990); but the efforts
made towards the IPCC report brought this issue to the fore.
Contrails form on particles that are emitted directly from the engine, such as
soot, and sulphate particles that form in the plume and perhaps in the engine itself.
The number density of sulphate particles in the plume is approximately one to two
orders of magnitude greater than that of soot particles (Anderson et al, 1998;
Schröder et al, 1998).
The formation of contrails arises from the increase in relative humidity that
occurs during the mixing of the warm and moist exhaust gases from the aircraft
engines with the colder and less humid ambient air. A contrail forms when satura-
tion with respect to liquid water is reached or surpassed in the plume. For contrails
to be persistent, the air mass through which the aircraft flies needs to be super-
saturated with respect to ice. Thus, the critical factors in persistent contrail forma-
tion are water vapour and particles from the engine exhaust, and particular environ-
mental conditions of temperature and humidity. In effect, since the engine exhaust
conditions are not so variable at cruise, it is the environmental parameters that dic-
tate whether a persistent contrail will form or not. Excellent reviews of contrail for-
mation and their dependencies have been provided by Schumann (1996), Kärcher
(1999) and IPCC (1999, Chapter 3).
The review and synthesis efforts associated with the IPCC (1999) report showed
that the radiative effects of persistent contrails and the enhancement of cirrus cloud-
iness were potentially large (see Figure 5.3). Sausen et al (1998) calculated the poten-
tial global coverage of persistent contrails for 1991/1992 and Gierens et al (1999)
for 2050. These coverages were utilized by Minnis et al (1999) to calculate radiative
forcing using a simple radiative transfer model for these two timelines. The globally
averaged radiative forcing from line-shaped contrails was shown to be ~0.02Wm -2 in
1992 and ~0.1Wm -2 in 2050 for the central Fa1 scenario (Minnis et al, 1999).
The other issue associated with contrails is the enhancement of cirrus clouds.
Cirrus clouds can have a powerful effect on surface temperatures (Liou, 1986;
Lohmann and Roeckner, 1995) and are a major source of uncertainty in global
climate-modelling studies. The above-mentioned estimation of radiative forcing is
for line-shaped persistent contrails only. As can be easily observed, persistent con-
trails may spread by diffusion and wind-shear to give a cirrus-like cloud coverage
that cannot be ultimately recognized as having originated from contrails. Moreover,
contrails and cirrus clouds (in common with all clouds) must nucleate on particles,
typically submicron in size, and aircraft introduce such particles from soot and sul-
phate particles in the plume. It is possible that these condensation nuclei can trigger
cirrus cloud formation long after the aircraft has passed, perhaps when temperature
and ice supersaturation conditions are more favourable for cirrus formation.
Boucher (1999) showed a correlation between increases of air traffic in the North
Atlantic Flight Corridor (NAFC) and increases in cirrus cloud coverage. A possible
relationship between the two was postulated, but it was acknowledged that other
causes could be contributory in a multicomponent way. Extending analyses of possi-
ble cirrus cloud increases, IPCC (1999, Chapter 3) concluded that 'a possible relation-
ship between air traffic and cirrus formation' may exist. However, the uncertainties
were so large in the IPCC's analysis that a best estimate of radiative forcing could
not be given, only an uncertainty estimate that ranged from 0 (no effect) through to
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