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The natural cycle of water in the atmosphere is complex, involving a suite of closely coupled physical processes. This is particularly true in the troposphere, where there is continual cycling between water vapour, clouds, precipitation, and ground water. Water vapour and clouds have large radiative effects on climate and directly influence tropospheric chemistry. The stratosphere is much drier than the troposphere. Nevertheless, water vapour is important in determining radiative balance and chemical composition, most dramatically in polar ozone loss through the formation of polar stratospheric clouds.
Emissions of water vapour by the global aircraft fleet into the troposphere are small compared with fluxes within the natural hydrological cycle; however, the effects of contrails and enhanced cirrus formation must be considered. Water vapour resides in the troposphere for about 9 days. In the stratosphere, the time scale for removal of any aircraft water emissions is longer (months to years) than in the troposphere, and there is a greater chance for aircraft emissions to increase the ambient concentration. Any such increase could have two effects: a direct radiative effect with a consequent influence on climate, and a chemical perturbation of stratospheric ozone both directly and through the potentially increased occurrence of polar stratospheric clouds at high latitudes.
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The behaviour of CO2 within the atmosphere is simple and well understood. There are no important formation or destruction processes that take place in the atmosphere itself. Atmospheric sources and sinks occur principally at the Earth’s surface and involve exchanges with the biosphere and the oceans. The effect of CO2 on climate change is direct and depends simply on its atmospheric concentration. CO2 molecules absorb outgoing infrared radiation emitted by the Earth’s surface and lower atmosphere. The observed 25–30% increase in atmospheric CO2 concentrations over the past 200 years has caused a warming of the troposphere and a cooling of the stratosphere.
There has been much discussion about how stabilization of CO2 concentrations might be achieved in the future. One of the most important factors is the accumulated emission between now and the time at which stabilization is reached.
The amount of CO2 formed from the combustion of aircraft fuel is determined by the total amount of carbon in the fuel because CO2 is an unavoidable end product of the combustion process (as is water). The subsequent transport and processing of this CO2 in the atmosphere follows the same pathways as those of other CO2 molecules emitted into the atmosphere from whatever source. Thus, CO2 emitted from aircraft becomes well mixed and indistinguishable from CO2 from other fossil fuel sources, and the effects on climate are the same. The rate of growth in aviation CO2 emission is faster than the underlying global rate of economic growth, so aviation’s contribution, along with those of other forms of transportation, to total emissions resulting from human activities is likely to grow in coming years.
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Nitrogen oxides (NO and NO2 are jointly referred to as NOx) are present throughout the atmosphere. They are very influential in the chemistry of the troposphere and the stratosphere, and they are important in ozone production and destruction processes. There are a number of sources (oxidation of N2O, lightning, fossil fuel combustion) whose contribution to NOx concentrations in the upper troposphere are not well quantified.
In all regions, the chemistry of the atmosphere is complex; aircraft NOx emissions are best viewed as perturbing a web of chemical reactions with a resultant impact on ozone concentrations that differs with location, season, and so forth. In the upper troposphere and lower stratosphere, aircraft NOx emissions tend to cause increased ozone amounts, so increased ozone and its greenhouse effects are the main issues for NOx emissions from subsonic aircraft. The pathways of other atmospheric constituents are also affected. Principal among these effects for NOx emissions is the reduction in the atmospheric lifetime and concentration of methane, another greenhouse gas. On the other hand, NOx emissions at the higher altitudes (18 km or above) of supersonic aircraft tend to deplete ozone.
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Although this covers a wide range of substances contained within aircraft exhaust emissions, the compounds of concern include sulphate aerosols and soot. These particles are heavily involved in the formation of contrails and cirrus clouds.
Sulphate aerosols play a critically important part in the stratosphere where they determine the NOx budget, and changes in sulphate levels would therefore have an effect on ozone levels.
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In 1992, aircraft line-shaped contrails were estimated to cover about 0.1% of the Earth’s surface on an annually averaged basis with larger regional values. Contrails tend to warm the Earth’s surface, similar to thin high clouds. The contrail cover is projected to grow to 0.5% by 2050 at a rate which is faster than the rate of growth in aviation fuel consumption.
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 vapour 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.
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Extensive cirrus clouds have been observed to develop after the formation of persistent contrails. Increases in cirrus cloud cover (beyond those identified as line-shaped contrails) are found to be positively correlated with aircraft emissions in a limited number of studies. About 30% of the Earth is covered with cirrus cloud. On average an increase in cirrus cloud cover tends to warm the surface of the Earth. An estimate for aircraft-induced cirrus cover for the late 1990s ranges from 0 to 0.2% of the surface of the Earth.
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