Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Persistent contrails often develop into more extensive contrail cirrus in ice-supersaturated air masses. Ice particles in such persistent contrails grow by uptake of water
vapor from the surrounding air. The area of the Earth covered by persistent contrails is controlled by the global extent of ice-supersaturated air masses and the number
of aircraft flights in those air masses. Present contrail cover will increase further as air traffic increases. The properties of persistent contrails depend on the aerosol
formed in exhaust plumes. Regions of ice-supersaturation vary with time and location and are estimated to cover an average of 10 to 20% of the Earth's surface at midlatitudes.
Ice-supersaturation in these regions is often too small to allow cirrus to form naturally, so aircraft act as a trigger to form cirrus clouds.
The mean coverage of line-shaped contrails is currently greatest over the United States of America, Europe, and the North Atlantic; it amounts to 0.5% on average
over central Europe during the daytime. The mean global linear contrail coverage represents the minimum change in cirrus cloud coverage from air traffic; its present
value is estimated to be 0.1% (possibly 0.02 to 0.2%).
Aviation-induced aerosol present in exhaust plumes and accumulated in the background atmosphere may indirectly affect cirrus cloud cover or other cloud properties
throughout the atmosphere. Observations and models are not yet sufficient to quantify the aerosol impact on cirrus cloud properties.
Satellite and surface-based observations of seasonal and decadal changes in cirrus cover and frequency in main air traffic regions suggest a possible relationship
between air traffic and cirrus formation. Cirrus changes in main air traffic regions suggest global cirrus cover increases of up to 0.2% of the Earth's surface since the
beginning of jet aviation, in addition to the 0.1% cover by line-shaped contrails. Observed cirrus cover changes have not been conclusively attributed to aircraft
emissions or to other causes.
Contrails cause a positive mean radiative forcing at the top of the atmosphere. They reduce both the solar radiation reaching the surface and the amount of longwave
radiation leaving the Earth to space. Contrails reduce the daily temperature range at the surface and cause a heating of the troposphere, especially over warm and
bright surfaces. The radiative effects of contrails depend mainly on their coverage and optical depth.
For an estimated mean global linear-contrail cover of 0.1% of the Earth's surface and contrail optical depth of 0.3, radiative forcing is computed to be 0.02 W m -2 , with
a maximum value of 0.7 W m -2 over parts of Europe and the United States of America. Radiative forcing by linear contrails is uncertain by a factor of about 3 to 4
(range from 0.005 to 0.06 W m -2 ), reflecting uncertainties in contrail cover (x 2) and contrail mean optical depth (x 3).
In the current atmosphere, the direct radiative forcing of accumulated aircraft-induced aerosol is smaller than that of contrails. The optical depth of aircraft-induced
aerosol is less than 0.0004 in the zonal mean and is much smaller than that of stratospheric aerosol from large volcanic eruptions or mean tropospheric aerosol
abundances.
Indirect radiative forcing is caused by aviation-induced cirrus that is produced in addition to line-shaped contrail cirrus. This forcing is likely positive and may be larger
than that from line-shaped contrails. Radiative forcing from additional cirrus may be as large as 0.04 W m -2 in 1992. Indirect forcing from other cloud effects has not yet
been determined and may be either positive or negative.
In the future, contrail cloudiness and radiative forcing are expected to increase more strongly than global aviation fuel consumption because air traffic is expected to
increase mainly in the upper troposphere, where contrails form preferentially, and because aircraft will be equipped with more fuel-efficient engines. More efficient
engines will cause contrails to occur more frequently and over a larger altitude range for the same amount of air traffic. For the threefold increase in fuel consumption
calculated for a 2050 scenario (Fa1), a fivefold increase in contrail cover and a sixfold increase in radiative forcing are expected. The contrail cover would increase
even more strongly if the number of cruising aircraft increases more than their fuel consumption. For other 2050 scenarios (Fc1 and Fe1), the expected cirrus cover
increases by factors of 3 and 9, respectively, compared to 1992. Higher cruise altitudes will increase contrail cover in the subtropics; lower cruise altitudes will increase
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