Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Concern that chlorofluorocarbons might destroy ozone was first raised in the 1970s. Following the general realization that these human-generated chemicals
posed a real threat to the ozone layer, the Vienna Convention for the Protection of the Ozone Layer was adopted in 1985. Shortly afterwards, the Antarctic
ozone hole was discovered, leading to renewed pressure to control ozone-depleting substances. In 1987, the Montreal Protocol on Substances that Deplete
the Ozone Layer was agreed; it has since been ratified by more than 160 countries. Initially, the Montreal Protocol imposed clear limits on the future
production of chlorofluorocarbons and halons only; as scientific evidence about ozone depletion has mounted, however, the Protocol has been modified to
include other chemicals.
As a direct result of these controls, there have been marked reductions in emissions of these substances into the atmosphere; assuming full compliance with
the Protocol, these reductions will result in reduced atmospheric amounts of chlorine and bromine. However, the magnitude of the ozone loss at any given
time depends on a number of factors. Although the amount of chlorine and bromine clearly is important, other influences include the temperature of the
ozone layer, the atmosphere's chemical composition, and long-term changes in atmospheric circulation related to climate change. How all of these factors
evolve over coming decades will determine future ozone amounts as chlorine and bromine are reduced.
Source: WMO, 1999.
For both of these issues, the effects from aviation are part of a larger picture. Human-generated emissions at the Earth's surface can be carried aloft and affect the
global atmosphere. The unique property of aircraft is that they fly several kilometers above the Earth's surface. The effects of most aircraft emissions depend strongly
on the flight altitude and whether aircraft fly in the troposphere or stratosphere. The effects on the atmosphere can be markedly different from the effects of the same
emissions at ground level.
A number of aircraft emissions can affect climate. Carbon dioxide CO2 ) and water (H2O) do so directly; other effects (e.g., production of ozone in the troposphere,
alteration of methane lifetime, formation of contrails and modified cirrus cloudiness) are indirect. The emissions that can affect stratospheric ozone (i.e., nitrogen
oxides, particulates, and water vapor) do so indirectly by modifying the chemical balance in the stratosphere. There has been sustained long-term growth in civil air
transportation. For example, over the past 10 years, passenger traffic on scheduled airlines has increased by 60%. Over the next 10 to 15 years, demand for air travel
is expected to grow by about 5% per year (Airbus, 1997; Boeing, 1997; Brasseur et al., 1998), though there are likely to be regional variations in demand. In contrast,
no such increase in the numbers of military aircraft is anticipated; they are expected to remain static or even decrease. As a consequence, fuel use and emissions
produced by future military activities are expected to be a decreasing part of the total from aviation (see Chapter 9).
Aviation fuel currently corresponds to 2-3% of the total fossil fuels used worldwide. Of this total, the majority (> 80%) is used by civil aviation. By comparison, the whole
transportation sector currently accounts for 20-25% of all fossil fuel consumption. Thus, the aviation sector consumes 13% of the fossil fuel used in transportation; it is
the second biggest sector after road transportation, which consumes 80% (IPCC, 1996b).
Given the continued growth of aviation, a number of questions have been raised regarding the future effects of aviation emissions on the global environment. For
example, if supersonic aircraft (which fly primarily in the stratosphere) were to be introduced in significant numbers, what special effects might there be, and what tradeoffs
might be possible?
Answering such questions involves consideration of a number of complex issues and
assumptions about the future growth, technology trends, and operational practices of the aircraft
industry. In the past, for example, fuel efficiency has improved dramatically over time, so total
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