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1. Introduction
This report assesses the effects of aircraft on climate and atmospheric
ozone and is the first IPCC report for a specific industrial subsector. It was
prepared by IPCC in collaboration with the Scientific Assessment Panel to the
Montreal Protocol on Substances that Deplete the Ozone Layer, in response to
a request by the International Civil Aviation Organization (ICAO)
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because of the potential impact of aviation emissions. These are the predominant
anthropogenic emissions deposited directly into the upper troposphere and lower
stratosphere.
Aviation has experienced rapid expansion as the world economy has grown. Passenger
traffic (expressed as revenue passenger-kilometers 2
) has grown since 1960 at nearly 9% per year, 2.4 times the average Gross Domestic
Product (GDP) growth rate. Freight traffic, approximately 80% of which is carried
by passenger airplanes, has also grown over the same time period. The rate of
growth of passenger traffic has slowed to about 5% in 1997 as the industry is
maturing. Total aviation emissions have increased, because increased demand
for air transport has outpaced the reductions in specific emissions
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from the continuing improvements in technology and operational procedures. Passenger
traffic, assuming unconstrained demand, is projected to grow at rates in excess
of GDP for the period assessed in this report.
The effects of current aviation and of a range of unconstrained growth projections
for aviation (which include passenger, freight, and military) are examined in
this report, including the possible effects of a fleet of second generation,
commercial supersonic aircraft. The report also describes current aircraft technology,
operating procedures, and options for mitigating aviation's future impact on
the global atmosphere. The report does not consider the local environmental
effects of aircraft engine emissions or any of the indirect environmental effects
of aviation operations such as energy usage by ground transportation at airports.
2. How Do Aircraft Affect Climate and Ozone?
Aircraft emit gases and particles directly into the upper troposphere and
lower stratosphere where they have an impact on atmospheric composition. These
gases and particles alter the concentration of atmospheric greenhouse gases,
including carbon dioxide CO2), ozone (O3), and methane (CH4); trigger formation
of condensation trails (contrails); and may increase cirrus cloudiness-all of
which contribute to climate change (see Box 1).
The principal emissions of aircraft include the greenhouse gases carbon dioxide
and water vapor (H2O). Other major emissions are nitric oxide (NO) and nitrogen
dioxide (NO2) (which together are termed NOx), sulfur oxides (SOxO), and soot.
The total amount of aviation fuel burned, as well as the total emissions of
carbon dioxide, NOx, and water vapor by aircraft, are well known relative to
other parameters important to this assessment. The climate impacts of the gases
and particles emitted and formed as a result of aviation are more difficult
to quantify than the emissions; however, they can be compared to each other
and to climate effects from other sectors by using the concept of radiative
forcing.
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Because carbon dioxide has a long atmospheric residence time (�100 years) and
so becomes well mixed throughout the atmosphere, the effects of its emissions
from aircraft are indistinguishable from the same quantity of carbon dioxide
emitted by any other source. The other gases (e.g., NOx, SOxO, water vapor) and
particles have shorter atmospheric residence times and remain concentrated near
flight routes, mainly in the northern mid-latitudes. These emissions can lead
to radiative forcing that is regionally located near the flight routes for some
components (e.g., ozone and contrails) in contrast to emissions that are globally
mixed (e.g., carbon dioxide and methane).
The global mean climate change is reasonably well represented by the global
average radiative forcing, for example, when evaluating the contributions of
aviation to the rise in globally averaged temperature or sea level. However,
because some of aviation's key contributions to radiative forcing are located
mainly in the northern mid-latitudes, the regional climate response may differ
from that derived from a global mean radiative forcing. The impact of aircraft
on regional climate could be important, but has not been assessed in this report.
Ozone is a greenhouse gas. It also shields the surface of the earth from harmful
ultraviolet (UV) radiation, and is a common air pollutant. Aircraft-emitted
NOx participates in ozone chemistry. Subsonic aircraft fly in the upper troposphere
and lower stratosphere (at altitudes of about 9 to 13 km), whereas supersonic
aircraft cruise several kilometers higher (at about 17 to 20 km) in the stratosphere.
Ozone in the upper troposphere and lower stratosphere is expected to increase
in response to NOx increases and methane is expected to decrease. At higher
altitudes, increases in NOx lead to decreases in the stratospheric ozone layer.
Ozone precursor (NOx) residence times in these regions increase with altitude,
and hence perturbations to ozone by aircraft depend on the altitude of NOx injection
and vary from regional in scale in the troposphere to global in scale in the
stratosphere.
Water vapor, SOxO (which forms sulfate particles), and soot 5
play both direct and indirect roles in climate change and ozone chemistry.
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Box 1. The Science of Climate Change
Some of the main conclusions of the Summary for Policymakers of Working
Group I of the IPCC Second Assessment Report, published in 1995, which
concerns the effects of all anthropogenic emissions on climate change,
follow:
- Increases in greenhouse gas concentrations since pre-industrial times
(i.e., since about 1750) have led to a positive radiative forcing of
climate, tending to warm the surface of the Earth and produce other
changes of climate.
- The atmospheric concentrations of the greenhouse gases carbon dioxide,
methane, and nitrous oxide (N2O), among others, have grown significantly:
by about 30, 145, and 15%, respectively (values for 1992). These trends
can be attributed largely to human activities, mostly fossil fuel use,
land-use change, and agriculture.
- Many greenhouse gases remain in the atmosphere for a long time (for
carbon dioxide and nitrous oxide, many decades to centuries). As a result
of this, if carbon dioxide emissions were maintained at near current
(1994) levels, they would lead to a nearly constant rate of increase
in atmospheric concentrations for at least two centuries, reaching about
500 ppmv (approximately twice the pre-industrial concentration of 280
ppmv) by the end of the 21st century.
- Tropospheric aerosols resulting from combustion of fossil fuels,
biomass burning, and other sources have led to a negative radiative
forcing, which, while focused in particular regions and subcontinental
areas, can have continental to hemispheric effects on climate patterns.
In contrast to the long-lived greenhouse gases, anthropogenic aerosols
are very short-lived in the atmosphere; hence, their radiative forcing
adjusts rapidly to increases or decreases in emissions.
- Our ability from the observed climate record to quantify the human
influence on global climate is currently limited because the expected
signal is still emerging from the noise of natural variability, and
because there are uncertainties in key factors. These include the magnitude
and patterns of long-term natural variability and the time-evolving
pattern of forcing by, and response to, changes in concentrations of
greenhouse gases and aerosols, and land-surface changes. Nevertheless,
the balance of evidence suggests that there is a discernible human influence
on global climate.
- The IPCC has developed a range of scenarios, IS92a-f, for future
greenhouse gas and aerosol precursor emissions based on assumptions
concerning population and economic growth, land use, technological changes,
energy availability, and fuel mix during the period 1990 to 2100. Through
understanding of the global carbon cycle and of atmospheric chemistry,
these emissions can be used to project atmospheric concentrations of
greenhouse gases and aerosols and the perturbation of natural radiative
forcing. Climate models can then be used to develop projections of future
climate.
- Estimates of the rise in global average surface air temperature by
2100 relative to 1990 for the IS92 scenarios range from 1 to 3.5�C.
In all cases the average rate of warming would probably be greater than
any seen in the last 10,000 years. Regional temperature changes could
differ substantially from the global mean and the actual annual to decadal
changes would include considerable natural variability. A general warming
is expected to lead to an increase in the occurrence of extremely hot
days and a decrease in the occurrence of extremely cold days.
- Average sea level is expected to rise as a result of thermal expansion
of the oceans and melting of glaciers and ice-sheets. Estimates of the
sea level rise by 2100 relative to 1990 for the IS92 scenarios range
from 15 to 95 cm.
- Warmer temperatures will lead to a more vigorous hydrological cycle;
this translates into prospects for more severe droughts and/or floods
in some places and less severe droughts and/or floods in other places.
Several models indicate an increase in precipitation intensity, suggesting
a possibility for more extreme rainfall events.
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