Aviation and the Environment: Aviation's Effects on the Global Atmosphere
Are Potentially Significant and Expected to Grow (Letter Report,
02/18/2000, GAO/RCED-00-57).

Aviation emissions are a potentially significant and growing percentage
of greenhouse gases and other emissions that are thought to contribute
to global warming. Aircraft emissions are potentially significant for
several reasons. First, jet aircraft are the main source of human
emissions deposited directly into the upper atmosphere, where they may
have a greater warming effect than if they were released at the earth's
surface. Second, carbon dioxide--the primary aircraft emission--is
relatively well understood and is the main focus of international
concern. For example, it survives in the atmosphere for nearly 100 years
and contributes to global warming, according to the Intergovernmental
Panel on Climate Change. The carbon dioxide emissions from worldwide
aviation roughly equal those of some industrialized countries. Third,
carbon dioxide emissions, combined with other gases and particles
emitted by jet aircraft, could have two to four times as great an effect
on the atmosphere as carbon dioxide alone. Fourth, the Intergovernmental
Panel recently concluded that the rise in aviation emissions due to
growing demand for air travel would not be fully offset by reductions in
emissions achieved solely through technological improvements. Experts
GAO interviewed, as well as the report of the Intergovernmental Panel,
have cited several options for better understanding and mitigating the
impact of aviation as the industry grows. These options include (1)
continuing research to improve the scientific understanding of
aviation's effects on the global atmosphere as a basis for guiding the
development of aircraft and engine technology to reduce them, (2)
promoting more efficient air traffic operations through the introduction
of new technologies and procedures, and (3) expanding the use of
regulatory and economic measures to encourage reductions in emissions.

--------------------------- Indexing Terms -----------------------------

 REPORTNUM:  RCED-00-57
     TITLE:  Aviation and the Environment: Aviation's Effects on the
	     Global Atmosphere Are Potentially Significant and
	     Expected to Grow
      DATE:  02/18/2000
   SUBJECT:  Aviation
	     Aircraft
	     Environmental policies
	     Hazardous substances
	     Air pollution control
	     Pollution monitoring
	     Atmospheric research
	     Climate statistics
	     Thermal pollution
	     Aviation fuels
IDENTIFIER:  Kyoto Protocol
	     NASA Ultra-Efficient Engine Technology Program
	     NASA Energy Efficient Engine Program
	     NASA Experimental Clean Combustor Program
	     FAA Air Traffic Control System
	     Montreal Protocol on Substances That Deplete the Ozone
	     Layer

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GAO/RCED-00-57

Report to the Honorable James L. Oberstar, Ranking Democratic Member,
Committee on Transportation and Infrastructure, House of Representatives

February 2000 AVIATION AND THE ENVIRONMENT

Aviation's Effects on the Global Atmosphere Are Potentially Significant and
Expected to Grow

GAO/ RCED- 00- 57

Letter 3 Appendixes Appendix I: Objectives, Scope, and Methodology 28

Appendix II: Level of Scientific Understanding of Aviation's Impact on the
Global Atmosphere 30

Appendix III: Comments From the Environmental Protection Agency 33

Appendix IV: Comments From the National Aeronautics and Space Administration
35

Appendix V: Comments From the Air Transport Association of America, Inc. 37

Figures Figure 1: Layers of the Atmosphere 11 Figure 2: Greenhouse Gas
Emissions From U. S. Aviation

and Other U. S. Sources 15 Figure 3: Total Carbon Emissions From Global
Aviation

and Selected Industrialized Countries 16 Figure 4: Radiative Forcing From
Aircraft in 1992 30 Figure 5: Radiative Forcing From Aircraft in 2050 31

Abbreviations

EPA Environmental Protection Agency FAA Federal Aviation Administration GAO
General Accounting Office ICAO International Civil Aviation Organization
IPCC Intergovernmental Panel on Climate Change NASA National Aeronautics and
Space Administration NOx A combination of nitric oxide and nitrogen dioxide

Resources, Community, and Economic Development Division

Lett er

B- 283769 February 18, 2000 The Honorable James L. Oberstar Ranking
Democratic Member Committee on Transportation

and Infrastructure House of Representatives

Dear Mr. Oberstar: Concerns about global warming are focusing increasingly
on the contribution of human activities, including aviation. Jet aircraft
are among many sources of greenhouse gases-gases that can trap heat,
potentially increasing the temperature of the earth's surface and leading to
changes in climate. 1,2 According to a recent report by the National
Research Council, the average global temperature at the earth's surface has
risen 0. 7 to 1. 4 degrees Fahrenheit over the last century. 3 Many experts
agree that, in total, greenhouse gases are warming the earth and that this
warming could have harmful effects on the environment and human health. 4
For example, some scientists are concerned that with global warming,
glaciers and ice sheets could melt, leading to a rise in sea levels and
subsequent coastal flooding. In addition, they expressed concern that the
incidence of malaria and other tropical infectious diseases could increase
in moderate climates.

1 Our report focuses on the effects of subsonic jet aircraft engine
emissions on the upper atmosphere- above 3,000 feet-and does not address the
effects of these emissions on local air quality. A recent international
review of aviation and the global atmosphere had a similar focus.

2 We focused on commercial civilian subsonic jet aircraft because they
account for the majority of aviation emissions; military and general
aviation aircraft account for the remainder. In the commercial fleet, fewer
than 15 civilian supersonic jets currently operate worldwide. Throughout
this report, we use the terms “aviation” and “jet
aircraft” interchangeably.

3 Reconciling Observations of Global Temperature Change, Panel on
Reconciling Temperature Observations, National Research Council (Jan. 2000).
4 Greenhouse gases emitted by jet aircraft include carbon dioxide and water
vapor. Other emissions from aircraft-nitric oxide and nitrogen dioxide (NOx)
and soot and sulfate particles-are not greenhouse gases, but are able to
produce gases (i. e., ozone) or other agents (i. e., clouds) that do act as
greenhouse gases. In combination, all of these aviation emissions are
believed to have a net warming effect on the earth's surface.

While aviation is believed to contribute less to global warming than some
other human activities, it is one of the fastest- growing sectors of the
world economy, with global air passenger travel projected by some experts to
grow 5 percent annually from 1990 through 2015. Hence, the impact of
aircraft emissions on the earth's atmosphere and climate is a concern for
transportation planners and policymakers.

Anticipating rapid growth in global air travel and potentially increasing
environmental effects from aircraft emissions, you asked us to provide
information on (1) what is currently known about aviation's contribution to
global warming and how aviation emissions, both domestic and global, compare
with emissions from other sources and (2) what options are available for
reducing aviation emissions. This report is based on our review of current
research and interviews with experts in the aviation, scientific, and
environmental communities on issues related to aviation and global warming.
In particular, we relied on a recent report on aviation and the earth's
atmosphere issued by the Intergovernmental Panel on Climate Change under the
auspices of the United Nations. Although we did not independently evaluate
the research in this report, it was reviewed by over 150 experts worldwide
and is generally considered the most comprehensive and up- to- date
information on the subject. (See app. I for more detailed information on our
scope and methodology.)

Results in Brief Aviation emissions comprise a potentially significant and
growing percentage of human- generated greenhouse gases and other emissions
that

are thought to contribute to global warming. The Intergovernmental Panel on
Climate Change recently estimated that global aircraft emissions accounted
for approximately 3.5 percent of the warming generated by human activities.
Of the various emissions generated by aviation, scientists know a great deal
about carbon dioxide, which is the primary aircraft emission, but less about
the other emissions. As a result, the scientific community has identified
areas that need further study to enable them to more precisely estimate
aviation's effects on the global atmosphere. As for the contributions of U.
S. aviation relative to other U. S. industrial sources, data from the
Environmental Protection Agency show that in 1997, aviation accounted for
about 3 percent of U. S. greenhouse gas emissions. This compares with 23
percent for other transportation sources and 41 percent for other industrial
sources. Global aviation emissions of carbon dioxide (measured in million
metric tons of carbon) are a small percentage of carbon emissions worldwide;
however, they are roughly equivalent to the carbon emissions of certain
industrialized countries. According to data

from a 1999 report of the Intergovernmental Panel on Climate Change, global
aviation contributed about 145 million metric tons of carbon in 1996, or
about 2.4 percent of all human- generated carbon emissions- an amount
roughly equivalent to the total carbon emissions of Canada.

Aircraft emissions are potentially significant for several reasons: Jet
aircraft are the primary source of human emissions deposited

directly into the upper atmosphere. The Intergovernmental Panel on Climate
Change and experts noted that some of these emissions have a greater warming
effect than they would have if they were released in equal amounts at the
surface- by, for example, automobiles. Carbon dioxide is relatively well
understood and is the main focus of

international concern. According to the Intergovernmental Panel on Climate
Change, it survives in the atmosphere for about 100 years and contributes to
warming the earth. Moreover, as noted, global aviation's carbon dioxide
emissions (measured in million metric tons of carbon) are roughly equivalent
to the carbon emissions of certain industrialized countries. Carbon dioxide
emissions combined with other gases and particles

emitted by jet aircraft-including water vapor, nitrogen oxide and nitrogen
dioxide (collectively termed NOx), and soot and sulfate- could have two to
four times as great an effect on the atmosphere as carbon dioxide alone.
According to the Intergovernmental Panel on Climate Change the atmospheric
effects of these combined emissions will require further scientific study.
The Intergovernmental Panel on Climate Change recently concluded

that the increase in aviation emissions attributable to a growing demand for
air travel would not be fully offset by reductions in emissions achieved
through technological improvements alone.

Experts in the aviation, scientific, and environmental communities agree
that the aviation industry will continue to grow globally and contribute
increasingly to human- generated emissions. The experts differ, however, in
the rates of growth they project and the effects they anticipate.

Recognizing aviation's potentially significant impact on the global
atmosphere, experts we interviewed and the report of the Intergovernmental
Panel on Climate Change identified a range of options to better understand
and mitigate aviation's impact as the industry grows. These options include
(1) continuing research to improve the scientific understanding of
aviation's effects on the global atmosphere as a basis for

guiding the development of aircraft and engine technology to reduce these
effects, (2) promoting more efficient air traffic operations through the
introduction of new technologies and procedures, and (3) expanding the use
of regulatory and economic measures to encourage reductions in emissions.
Governments are pursuing these options, although they have not agreed on
specific regulatory and economic measures.

Background Human activities, primarily those related to producing and using
energy, are increasing concentrations of the heat- trapping greenhouse gases
that

many experts believe are warming the planet. According to the
Intergovernmental Panel on Climate Change (IPCC), 5 when greenhouse gases
are added to the atmosphere, they increase the effectiveness of the earth's
atmospheric blanket, warming the earth's surface and potentially leading to
changes in climate. Greenhouse gases are produced through both natural
activities (e. g., decaying organic matter) and human activities (e. g.,
manufacturing and transportation). Greenhouse gas levels in the atmosphere
are the net result of processes that generate greenhouse gases (sources) and
processes that destroy or remove them (sinks). The ability to accurately
quantify the current impact of human activity on the global climate is
limited by a lack of understanding about how much the climate would vary
without these activities. Also uncertain, because of incomplete scientific
understanding, is how the atmosphere and climate system will react to human-
induced changes in greenhouse gas concentrations over the long term.
According to the IPCC, to date, the balance of the evidence suggests that
there is a discernable human influence on the global climate.

5 The IPCC was established by the United Nations Environment Programme and
the World Meteorological Organization to assess information on the science,
impacts, economics, and options for mitigating and adapting to climate
change.

Global aviation is the first industrial subsector whose potential impact on
the global atmosphere has undergone an international assessment by the IPCC.
According to the IPCC, this assessment was important because the aviation
industry has grown rapidly and become an integral and vital part of modern
society. The Air Transport Action Group reported that the aviation industry
contributed $1,140 billion in annual gross output to the global economy in
1994 6 -a contribution that is expected to increase to $1,800 billion by
2010. Increased environmental impact can be expected to accompany this
increased economic activity. To its credit, the aviation industry is working
with the U. S. government, the International Civil Aviation Organization,
and other international organizations to assess and manage the effects of
aviation on the environment.

International concerns about the contribution of human activities, including
aviation, to global climate change have led to several efforts to reduce
their impact. For example, in 1992, 155 nations, including the United
States, signed the United Nation's Framework Convention on Climate Change- a
convention designed to stabilize concentrations of greenhouse gases in the
atmosphere at a level that would prevent human activities from interfering
dangerously with the climate system. By 1995, parties to the convention,
including the United States, realized that progress toward this goal was not
sufficient. In December 1997, the parties reconvened in Kyoto, Japan, to
establish binding measures to reduce greenhouse gas emissions. Under the
resulting Kyoto Protocol, 38 developed nations (the United States, France,
Japan, and others) pledged to reduce their emissions of carbon dioxide and
other greenhouse gases from 2008 through 2012. The protocol directed the
parties to work through the International Civil Aviation Organization (ICAO)
to reduce or limit emissions from aviation.

6 The Air Transport Action Group is an independent coalition of
organizations with a worldwide membership that includes airlines, airports,
pilots, air traffic controllers, and manufacturers, among others. Air
Transport Action Group, The Economic Benefits of Air Transport: 1994 Data(
Mar. 1997).

If the U. S. Senate ratifies the protocol, the United States could be
required to significantly reduce its greenhouse gas emissions. Specifically,
the United States agreed in the protocol to reduce its annual emissions,
during the 5- year period from 2008 through 2012, to a level 7 percent below
the 1990 emissions level. To achieve this new level, the United States may
have to cut projected emissions levels by 31 percent by 2010 (the midpoint
of the 5- year period)- a reduction equivalent to about 548 million metric
tons of carbon. 7 In addition, the United States, along with other parties
to the protocol, is already required under the Framework Convention on
Climate Change to report periodically to the secretariat of the convention
on its greenhouse gas emissions, plans for developing programs to mitigate
climate change, and strategies for adapting to the impact of climate change.
The Kyoto Protocol would add the requirement that parties to the protocol
establish national systems for estimating greenhouse gas emissions using
methodologies adopted by the parties to the protocol and estimate and report
their emissions estimates annually. In addition, under the protocol,
emissions targets would be established and participants would be allowed to
use a market- based mechanism for trading emissions credits. 8

In an effort to understand the impact of aviation emissions on the global
atmosphere, ICAO and parties to the Montreal Protocol on Substances that
Deplete the Ozone Layer 9 requested that the IPCC assess the effects of
aircraft on climate and atmospheric ozone. A group of over 100 international
experts reviewed the most current research available and incorporated the
findings into a report entitled Aviation and the Global Atmosphere. 10 This
report then went through a rigorous peer review process that involved 150
international academic, technical, and scientific experts; nongovernmental
organizations; and industry before being published in 1999. Throughout this
process, the participants attempted to develop a consensus view on the
effects of aviation on the global atmosphere. ICAO will consider the results
of this report as it addresses its

7 Energy Information Administration, International Energy Outlook 1999( Mar.
1999). 8 Such trading allows parties to exchange emissions credits to meet
an environmental goal. Parties that exceed emissions requirements can thus
trade with others that are operating below emissions requirements to achieve
compliance at a lower overall cost.

9 The Montreal Protocol on Substances that Deplete the Ozone Layer, as
amended, is an international agreement established in 1987 to phase out the
use of chlorofluorocarbons and other ozone- depleting substances.

10 Aviation and the Global Atmosphere, IPCC (1999).

responsibilities under the Kyoto Protocol to reduce greenhouse gas emissions
from international aviation.

Aviation's Effects on The IPCC estimated aviation's impact on global warming
on the basis of

the Global Atmosphere emissions that aviation generates. However, scientific
understanding of the

impact of aviation emissions on global warming varies. For example, the Are
Potentially

influence of carbon dioxide is well understood, while the effects of other
Significant

aircraft emissions, such as nitric oxide and nitrogen dioxide (commonly
referred to as NOx), are less certain. In the United States, aviation's
contribution to increases in human- generated greenhouse gases is about 3
percent and can be compared with the contributions of other U. S. industrial
sources. Higher greenhouse gas levels are, in turn, thought to contribute to
global warming. Global aviation's carbon dioxide emissions (measured in
million metric tons of carbon) are a small percentage of carbon emissions
worldwide; however, they are roughly comparable to the contributions of
certain industrialized countries. Some experts believe that aviation's
emissions are potentially significant, in part because some aircraft
emissions deposited directly into the upper atmosphere are thought to have a
greater warming effect than the same volume of emissions generated at ground
level. In addition, the scientific, aviation, and environmental communities
agree that the global aviation industry will continue to grow well into the
next century. However, they disagree on the pace of growth relative to that
of the world economy as a whole and, thus, on whether global aviation will
contribute an increasing proportion of human- generated climate effects.

The IPCC Estimated Global To estimate global aviation's impact on global
warming, and, hence, on

Aviation's Contribution to climate change relative to other human sources,
the IPCC used the concept

Global Warming of radiative forcing- a measure of the importance of a
potential climate

change mechanism in affecting the heat balance of the earth's atmospheric
system. 11 The 1999 IPCC report estimated that global aviation contributed
3.5 percent of human- generated radiative forcing (warming) in 1992. 12 The
IPCC further estimated that by 2050, aviation's contribution could increase
to 5 percent of the total human- generated radiative forcing. According to
the IPCC, when measured in watts per square meter, the increase in warming
attributable to aviation alone is projected to be over 260 percent between
1992 and 2050. 13 In terms of climate change, the IPCC estimated an increase
in the earth's temperature of approximately 1.6 degrees Fahrenheit by 2050,
of which about 0.09 degrees would be attributable to aviation. 14, 15
However, it should be noted that the earth's average temperature has risen
only about 7 degrees Fahrenheit since the last ice age. According to
officials from the National Aeronautics and Space Administration (NASA), the
projection that aviation's contribution would increase to only 5 percent by
2050 assumes a robust contribution from technology that would increase fuel
efficiency and reduce NOx emissions.

11 Radiative forcing, in watts per square meter, measures the impact on
climate (warming or cooling) of changes in greenhouse gases, aerosols, and
clouds. 12 In 1992, the IPCC developed several scenarios that included the
contributions of greenhouse gases from all sectors. These scenarios were
inherently uncertain because they incorporated assumptions- to project
warming trends worldwide-about future economic and population growth,
technological changes, and land use. The authors of the 1999 IPCC report on
aviation started with these projections and built a new scenario
specifically for aviation growth based on key assumptions about economic and
population growth contained in the 1992 scenario. Experts believe the 1992
scenario is still applicable. Throughout the report, the IPCC uses a mid-
range estimate to illustrate the possible effects of aircraft emissions on
the atmosphere.

13 According to the IPCC, aviation contributed about 0. 05 watts per square
meter in 1992, and its contribution is estimated to increase to
approximately 0. 19 watts per square meter by 2050- an increase of 263
percent.

14 According to an IPCC expert, the authors of the report were able to
estimate the temperature change associated with aviation's future
contribution because they used the same variables over the same time period,
allowing for a more direct calculation. By contrast, calculating the
temperature change for the past would have been much more difficult because
of the complexity of making the calculation, the large number of variables
involved, and the resulting lack of confidence in the outcome.

15 The IPCC used the scenarios discussed in footnote 12 to estimate these
temperature changes.

They noted, however, that achieving these reductions would require
significant improvements in technology that are not currently funded.

Scientific Understanding of The IPCC experts agree on the types of emissions
from jet aircraft that may

Aviation Emissions Varies contribute to a warming of the earth's surface but
know more about the

impact of carbon dioxide than of the other emissions. Jet aircraft deposit
most of their emissions at cruise altitudes, primarily in the troposphere-
altering concentrations of greenhouse gases directly by emitting carbon
dioxide and indirectly by emitting NOx. In addition, emissions of water
vapor and soot and sulfate particles have both direct and indirect effects.
See figure 1 for an overview of the layers of the atmosphere, including the
altitudes flown by jet aircraft.

Figure 1: Layers of the Atmosphere

UV/ Visible Sunlight

MESOSPHERE

infrared radiation

STRATOSPHERE

0ï¿½F ~ 50 km

(30 miles) TROPOSPHERE

~ 8 km Ozone

Mt. Everest (5 miles) Layer

~ 9 km -80ï¿½F (5.5 miles)

infrared radiation

~9 - 12 km (5.5 - 7.5 miles)

60ï¿½F

Source: The National Oceanic and Atmospheric Administration's Aeronomy
Laboratory.

According to the IPCC report, the atmospheric effects of carbon dioxide
emissions from jet aircraft are relatively well understood. Carbon dioxide
emissions remain in the atmosphere for about 100 years and contribute to a
warming of the earth's surface. Scientists estimate that as a result of
human activity, the level of carbon dioxide in the atmosphere has risen by
almost 30 percent since pre- industrial times (e. g., about 250 years ago).
In addition, carbon dioxide becomes well mixed throughout the atmosphere,
meaning that the total impact of carbon dioxide on the climate will be the
same irrespective of the point of origination. (See app. II for the levels
of scientific understanding of aviation's impact on the global atmosphere.)

The impact of other aviation emissions on the global atmosphere is less
certain. Such emissions include the gases NOx, water vapor, and, to a lesser
extent, soot and sulfate particles (aerosols) that result, in part, from the
incomplete combustion of jet fuel. While the IPCC states that scientists
cannot calculate the precise impact of these gases and particles, it
maintains that in certain layers of, or locations in the atmosphere, the
gases and particles in combination have a warming effect. Moreover,
according to the IPCC, the atmospheric effects of these aircraft- related
gases and particles in combination with carbon dioxide could be two to four
times greater than the atmospheric effects of carbon dioxide alone.

Current scientific models predict that NOx, in combination with water vapor
and sulfate, depletes ozone- a greenhouse gas-in the mid- and upper
stratosphere. In contrast, NOx increases ozone in the troposphere and lower
stratosphere, warming the earth's surface by trapping radiation as it is
being reflected back toward space. These increases in ozone are the primary
effects of NOx emissions. However, increases in NOx emissions also reduce
methane- another greenhouse gas. 16

In the troposphere, precipitation quickly removes emissions of water vapor-
a greenhouse gas. However, in the lower stratosphere, water vapor emissions
can build up and lead to higher concentrations that, in turn, are predicted
to warm the earth's surface. Water vapor can enhance the formation of
contrails-the thin white- line clouds often seen behind jet

16 Methane is a greenhouse gas with many natural and human sources.
According to the IPCC, reducing its concentration in the atmosphere leads to
cooling. However, the warming effect resulting from increases in ozone in
the troposphere offsets the cooling effect of reductions in methane
attributable to aviation- resulting in a net warming.

aircraft-that are also expected to warm the earth's surface. In addition,
extensive cirrus clouds have been observed to develop after the formation of
persistent contrails. These increases in cirrus cloud cover have been
positively correlated with aircraft emissions in a limited number of
studies. On average, an increase in cirrus cloud cover tends to warm the
earth's surface. Finally, increases in particles emitted by aircraft have
mixed effects: Soot tends to warm the earth's surface, while sulfate
particles tend to cool it. While the direct effects of these particles are
believed to be small, increases in their emissions by jet aircraft may
potentially influence the formation of clouds in the upper atmosphere. This,
in turn, may contribute to future climate change. Given the limited
scientific understanding of the impact of many aviation emissions-in
particular, their influence on cirrus cloud formation-experts noted that
further research would help to clarify and more accurately quantify
aviation's impact on the global atmosphere and contribution to climate
change.

Contributions of U. S. and The relative impact of aviation emissions on the
global atmosphere can be

Global Aviation Emissions assessed by comparing (1) greenhouse gas emissions
from U. S. aviation

Can Be Compared With and other U. S. industrial sources and (2) carbon
emissions (measured in

Contributions of Emissions million metric tons of carbon) from global
aviation and industrialized

countries. From Other Sources

The relative contributions to greenhouse gases from U. S. aviation and other
U. S. industrial sources can be estimated by comparing the amounts of
emissions generated by each source. According to the most recent data
available from the Environmental Protection Agency (EPA), in 1997,
greenhouse gas emissions from U. S. aviation accounted for about 3 percent
of the total U. S. greenhouse gas emissions from human sources, while the
remainder of the transportation sector accounted for approximately 23
percent. 17 Passenger cars and light duty trucks were responsible for well
over half of these emissions. In comparison, other industrial sectors
accounted for approximately 41 percent of the total U. S. greenhouse gases
from human sources, and other miscellaneous sources accounted for the
remaining 33 percent. 18 (See fig. 2)

17 EPA measured the greenhouse gases carbon dioxide, methane, nitrous oxide,
and hydroflourocarbons for transportation- related emissions. Carbon dioxide
emissions from international passenger and cargo aviation are reported
separately as bunker fuels and are not included in the U. S. totals. See
Inventory of U. S. Greenhouse Gas Emissions and Sinks; 1990- 1997, EPA (Apr.
1999).

18 For the industrial sector, EPA measured the four gases emitted by the
transportation sector plus two other gases, PFCs and SF6, which are not
emitted by the transportation sector. Sources in the “other”
category include such things as landfills and wastewater treatment
facilities. See EPA's Apr. 1999 report.

Figure 2: Greenhouse Gas Emissions From U. S. Aviation and Other U. S.
Sources

3%

Aviation

Nonaviation transportation

23%

Industry

41%

Other

33% Source: GAO's analysis of data from EPA.

Carbon dioxide emissions result from the burning of fossil fuels and are the
primary component of aviation emissions. 19 Globally, aviation- related
carbon dioxide emissions (measured in million metric tons of carbon) can be
compared with the carbon emissions of some industrialized countries (also
measured in million metric tons of carbon). For example, according to data
from the 1999 IPCC report, global aviation produced about 145 million metric
tons of carbon, or about 2.4 percent of all human- generated carbon
emissions in 1996. In the same year, Canada's total carbon emissions were
140 million metric tons, or also about 2. 4 percent of the world's total
carbon emissions, and the United Kingdom's were 153 million metric tons, or
about 2.6 percent of the world's total. (See fig. 3.) While these individual
percentages are small, collectively they add to the world's total carbon
emissions, which are known to have a warming effect on the earth.

19 Fossil fuels include coal, oil, and natural gas that are formed in the
earth from plant or animal remains.

Figure 3: Total Carbon Emissions From Global Aviation and Selected
Industrialized Countries

350 Metric tons of carbon in millions 300

291 250

238 200

153 145 150 140

116 101 100

50 0

1996 data Japan

Germany Kingdom

Aviation Canada

Italy France

United Global Sources of carbon emissions

Notes: In 1996, the United States produced 1,463 million metric tons of
carbon, or about 24 percent of the world's total.

As mentioned earlier, the focus of our report is civil global aviation,
which accounted for about 145 million metric tons of carbon in 1996.
However, if total global aviation (including military operations) is
included, the figure rises to about 170 million metric tons of carbon.

Sources: International Energy Outlook, Energy Information Administration
(1999) and Aviation and the Global Atmosphere, IPCC (1999).

Some Experts Believe That According to some experts, aviation's contribution
to human- generated

Aviation's Effects on the emissions and its effects on the global atmosphere
are potentially

Global Atmosphere Are significant for several reasons. First, jet aircraft
are the major source of

Potentially Significant human emissions deposited directly into the upper
atmosphere. The IPCC

and experts noted that when these emissions are released into the upper

atmosphere, some of them have a greater warming effect than when they are
released in the same amounts at the surface, by sources such as automobiles.
Next, carbon dioxide-aviation's primary emission-is the central focus of
international attention because, as the IPCC noted, it has a long
atmospheric life span (about 100 years) and has a warming effect on the
earth's surface. Furthermore, the carbon dioxide emissions (measured in
million metric tons of carbon) of global aviation are roughly equivalent to
the carbon emissions of selected industrialized countries.

In addition, according to the IPCC, jet aircraft flying at cruise altitudes
also emit other atmospheric gases (water vapor and NOx) and particles (soot
and sulfate) whose effects are not as well understood as carbon dioxide's.
However, the combined effects of carbon dioxide and these gases on the
atmosphere could be two to four times greater than the effects of carbon
dioxide alone. Furthermore, the IPCC recently concluded that the increases
expected in aviation emissions from a growing demand for air travel would
not be fully offset by technological improvements alone.

An EPA official noted that aviation's contribution to U. S. emissions of
carbon dioxide is small compared with the contributions of electric
utilities and automobiles, which are substantially larger. In addition,
officials from both EPA and the Federal Aviation Administration (FAA) told
us that aviation contributes as much or more than some industrial
subsectors- including the chemical, iron and steel, and cement-
manufacturing subsectors.

Aviation's Effect on The scientific, aviation, and environmental communities
agree that the

Greenhouse Gases Is global aviation industry will continue to grow well into
the next century

Expected to Grow, but and its operations will result in increased emissions
and environmental

Experts Differ on the effects. However, they disagree on the pace at which
aviation will grow

relative to that of the world economy, especially when projected 50 years
Magnitude of Growth

into the future. They also disagree on whether global aviation will
contribute an increasing proportion of human- generated climate effects. The
IPCC report used a range of scenarios to estimate future air passenger
demand. These included a high- range scenario that projected annual growth
of 4.7 percent between 1990 and 2050, a mid- range scenario that projected
annual growth of 3. 1 percent, and a low- range scenario that projected
annual growth of 2. 2 percent. 20, 21

While some environmental groups from the United States and Europe have
expressed concern that the mid- range estimate to 2050 was understated, some
in the aviation industry found the estimate overstated. 22 Specifically,
some of the aviation industry representatives we interviewed noted that the
growth of global air travel depends on a wide range of factors that are very
difficult to project with much precision 50 years into the future. The IPCC
report acknowledged the uncertainty involved in projecting conditions this
far into the future.

In addition, the IPCC assessment assumed that future aviation growth would
not be constrained by aviation or airport infrastructure. 23 According to
some industry and government representatives, this assumption was
unrealistic and contributed to an overstatement of aviation's future impact
on the global atmosphere. Another industry representative stressed that the
demand for air travel has historically paralleled economic growth as
measured by the Gross Domestic Product, not by population- the indicator
used by a leading environmental group in its projection.

20 The high- range scenario was developed by the Environmental Defense Fund-
a nonprofit environmental advocacy group that has conducted research on the
impact of aviation emissions on the atmosphere. The other scenarios were
developed by ICAO.

21 The IPCC measured this growth in revenue passenger- kilometers flown. 22
The IPCC report used the mid- range scenario throughout to illustrate the
possible effects of aircraft on the global atmosphere. 23 The IPCC report
also assumed that by 2050, improvements in fuel efficiency would be realized
and optimal air traffic management would be achieved. The report further
noted that if these improvements do not materialize, then fuel use and
emissions will be higher.

According to this representative, the group's use of population as an
indicator overestimated future demand.

A Range of Options Are Given that aviation currently contributes a
potentially significant and

Available to Reduce growing proportion of the human additions to greenhouse
gases believed to

contribute to global warming, some experts we interviewed and the IPCC
Aviation's Effects on

report identified a range of options to help limit the effects of aviation
the Global Atmosphere

emissions as the industry grows. These options would (1) continue research
to improve the scientific understanding of aviation's effects on the global
atmosphere as a basis for guiding the development of aircraft and engine
technology to reduce these effects, (2) promote more efficient air traffic
operations through the introduction of new technologies and procedures, and
(3) expand the use of regulatory and economic measures to encourage
reductions in aviation emissions.

Further Research Would Experts have stated that to more fully understand
aviation's impact on the

Help Improve Scientific global atmosphere and to guide future improvements
in engine and aircraft

Understanding and Guide technology, additional research would be beneficial.
For example,

Technological Development according to the IPCC, further work is required to
reduce scientific and

other uncertainties to better understand the options for reducing emissions
and better inform decisionmakers.

Despite steady improvements in characterizing the potential effects of human
activities on the atmosphere- including those of aviation- significant
scientific uncertainties have been identified. According to experts and the
IPCC report, further study is needed to answer the following questions:

How do the effects of aircraft flying in the lower stratosphere compare with
those of aircraft flying in the troposphere? Under what conditions do
contrails and particles emitted from aircraft

lead to the formation of cirrus clouds? To what extent do aerosols from
aviation compare with aerosols from

other sources, increase cloud formation, and change the degree to which
clouds warm the earth? To what extent do the warming and cooling effects of
methane and

ozone offset each other? Many experts believe that the scientific
uncertainties associated with aviation emissions other than carbon dioxide
make it difficult for

decisionmakers to identify, rank, and then target the most critical needs
first. For example, one expert noted that a sound scientific footing would
enable the aviation and scientific communities to address global warming in
the most efficient and cost- effective manner. Furthermore, experts we
interviewed cited the importance of both technological and scientific
research to reduce the impact of aviation on the global atmosphere because
there is currently no economically feasible alternative to the kerosene-
based jet fuel used by aircraft.

NASA expects to continue its research to improve aircraft and engine
technology, although its opportunities may be limited because of reductions
in current and projected funding. For example, the funding for NASA's
Atmospheric Effects of Aviation Project- the only U. S. government program
that assesses the potential effects of aircraft emissions at cruise
altitudes on climate change- is scheduled to be terminated after fiscal year
2000. In addition, funding for the Advanced Subsonic Technology and
HighSpeed Research programs has already come to an end; however, NASA has
incorporated some elements from these programs into its new UltraEfficient
Engine Technology program. This new program will fund engine technology
research and development to reduce aircraft emissions. NASA believes that
this 6- year program (running from fiscal year 2000 through fiscal year
2005) will continue efforts to reduce NOx emissions to 70 percent below
ICAO's 1996 standard. Through this program, NASA will develop technology to
further reduce carbon dioxide emissions by 15 percent below today's
emissions. 24 However, an aviation industry official noted that current
aircraft engine technologies generally require a trade- off between NOx and
carbon dioxide emissions- when engines are designed to minimize NOx
emissions, they generally emit more carbon dioxide, and vice versa.

Aerodynamic improvements are expected to reduce the amount of fuel burned by
aircraft. These improvements include manufacturing smoother fuselage and
wing surfaces to reduce drag and using lighter and stronger materials, such
as aluminum alloys, titanium components, and composite materials.
Improvements in aircraft engines- anticipated through reductions in weight
and applications of new engine technologies-are also expected to
significantly improve fuel efficiency. However, an aviation

24 According to FAA officials, there is no assurance that industry will
complete the development of any technology developed in NASA programs. Such
development, they said, will occur in response to the marketplace or
regulatory standards.

industry official pointed out that there are trade- offs between applying
these new engine technologies and ensuring safety and performance.

In Europe as well as the United States, efforts are being funded to develop
aircraft that will produce fewer emissions per passenger carried and,
therefore, have less of an impact on the environment. The IPCC estimated
that these types of improvements in aircraft and engine technology could
increase fuel efficiency 20 percent between 1997 and 2015. 25 The IPCC also
stated that fuel efficiency could increase 40 to 50 percent over current
levels by 2050 if further technological improvements are realized. However,
the IPCC report concluded that technological improvements alone would not
fully offset the expected environmental impact of increases in aviation
emissions attributable to a growing demand for air travel. As noted in the
IPCC report, the amount of fuel burned by global aviation in 1990 will
increase 2.7 times by 2050 as a result of increased demand for air travel,
thereby increasing the aviation's industry's overall emissions.

Experts noted that in the past, NASA has made significant contributions
toward reducing aircraft emissions through the development of new
technologies. These improvements-in combination with those of the aviation
industry-have helped aircraft to burn fuel more efficiently and, hence,
reduce emissions on a per- passenger- seat basis by 70 percent over the past
40 years. In the late 1970s and early 1980s, two NASA programs, the Energy
Efficient Engine Program and the Experimental Clean Combustor Program,
developed engine technologies that reduce aviation emissions. According to
NASA, the combustor technologies resulting from these programs reduced NOx
levels by 40 to 50 percent compared with the original first- generation
annular combustor technologies.

25 The IPCC used forecasts developed by a group of aerospace industry
experts to project future improvements in fuel efficiency.

Improved Air Traffic Operational improvements in such areas as
communications, navigation,

Operations Could Reduce surveillance, and air traffic management could also
lead to reductions in

Aviation Emissions aircraft emissions. For example, in the United States, a
range of new

technologies and procedures are expected to give pilots and air traffic
controllers more precise information about the location of aircraft and
allow them to exchange information more efficiently. With better and more
efficient communication, pilots will have more flexibility to change their
routes, speed, and altitude (under certain conditions) with fewer
restrictions, thus saving time and money. Better communication will also
allow the FAA to improve the air traffic control system's safety and use
airspace and airport resources more efficiently. According to the IPCC
report, improvements in air traffic management worldwide could reduce the
annual consumption of aircraft fuel by 6 to 12 percent over the next 20
years. The scenarios developed and used in the IPCC report assume that
substantial enhancements to air traffic management systems will be
implemented in a timely manner to substantially reduce emissions. However,
our past work has noted the magnitude of effort required to implement these
types of improvements globally. For example, we reported that coordinating
air traffic management improvements worldwide would require the development
of compatible operational concepts, technologies, and systems architectures.
26, 27 While efforts are under way in some parts of the world, much work
remains to achieve worldwide coordination.

The IPCC report also observed that the environmental benefits of fuel
efficiencies expected from improved air traffic operations could be offset
by increases in the demand for air travel. An industry official noted that
inefficiencies in the present air traffic control system currently have led
to higher levels of aircraft emissions than are necessary and that
improvements in communication, navigation, surveillance, and air traffic
management would help reduce aircraft emissions. While improvements in air
traffic management would be beneficial, our past work has identified a lack
of airport surface capacity as potentially limiting the benefits of such
improvements. For example, new air traffic control technologies and
procedures may allow aircraft to arrive at their destinations sooner, but

26 A systems architecture is a blueprint/ framework used to guide and
constrain the development and evolution of a collection of related systems,
such as the nation's air traffic control system.

27 National Airspace System: FAA Has Implemented Some Free Flight
Initiatives, but Challenges Remain( GAO/ RCED- 98- 246, Sept. 28, 1998).

limits on airport surface capacity, such as too few runways and, gates may
then delay them. Limits on surface capacity can also lead to increased
aircraft emissions when they cause aircraft to idle at gates and/ or on
taxiways or circle in the air while waiting to land.

Regulatory and Economic The IPCC report and others have reviewed regulatory
and economic

Measures Could Also options to reduce aircraft emissions and their effects.
These options

Reduce Aviation Emissions include mandated policies, emissions trading,
international charges, and

voluntary agreements. ICAO's Committee on Aviation Environmental Protection
is currently evaluating many of these options. 28 However, because of a lack
of agreement among countries and the options' potential economic impact on
the industry, these types of measures will require further study and debate
before being implemented.

The Kyoto Protocol directs the parties to work through ICAO to address
aviation's contribution to greenhouse gas emissions. Currently, the
Committee on Aviation Environmental Protection, through its working groups,
is studying various measures for reducing emissions from aviation. To reduce
emissions, one working group is looking at technical issues, another is
focusing on operational issues, and a third is looking at marketbased
options. These working groups are expected to report at the next regular
session of the Committee on Aviation Environmental Protection in January
2001.

Emissions trading could give airlines the flexibility to reduce their own
emissions or to purchase equivalent reductions from others if doing so would
be less expensive. Emissions trading creates an economic incentive to employ
innovative technologies and reduce emissions below the level any specific
technological standard might require. 29 For example, each airline could be
given an emissions budget for its fleet of aircraft. If one airline exceeds
its budget by choosing to fly aircraft that produce higher levels of carbon
dioxide or NOx, it would be required to purchase

28 An Air Transport Association official noted that the U. S. aviation
industry has been working within this committee to assess and manage the
effects of aviation on the environment.

29 As we have reported in the past, because of possible environmental and
economic benefits, emissions trading could be part of a regulatory approach
to curb carbon dioxide emissions. See Air Pollution: Allowance Trading
Offers an Opportunity to Reduce Emissions at Less Cost( GAO/ RCED- 95- 30,
Dec. 16, 1994).

emissions credits from another airline or other regulated source that is
operating below its allowable emissions budget. According to FAA, while the
United States favors the use of emissions trading and voluntary measures,
Europe prefers that emissions trading be one part of an entire package of
options that would include charges and voluntary measures for reducing
emissions. Discussions between the United States and Europe on these matters
are ongoing within the Committee on Aviation Environmental Protection.

International charges on air service providers, such as landing fees based
on the amounts of emissions produced and other factors, could also serve as
incentives to improve operational efficiency and operate newer aircraft with
lower emissions. 30 The practical implication of instituting such a charge
is that the polluter would pay more to operate higher- emitting aircraft
than to operate newer aircraft with lower emissions. According to one
aviation industry expert, because the U. S. fleet tends to be older and may
in some cases produce more pollution than the fleets owned by European
airlines, U. S. airlines could face higher costs if charges were assessed on
the basis of emissions.

Voluntary agreements within the aviation industry to meet environmental
targets, such as reductions in greenhouse gases from aviation, could be used
to achieve lower emissions. According to an EPA official, the aviation
industry in Europe recently put forward such a proposal to reduce carbon
dioxide emissions from aviation.

Agency Comments We provided the Department of Transportation, FAA, EPA,
NASA, the Air Transport Association of America, Inc., and the Environmental
Defense

Fund with a copy of our draft report for review and comment. The Department
of Transportation and the Environmental Defense Fund did not provide
comments. FAA, EPA, and NASA generally agreed with the facts presented and
provided us with technical and clarifying comments

30 According to the IPCC report, Zurich Airport has added an emissions
surcharge to its landing fee based on engine certification information. This
charge is intended to provide an incentive to operators to fly their lowest-
emitting aircraft into Zurich and accelerate the use of the best available
technology. The IPCC report also noted that a similar emissions- related
charge was applied at 10 Swedish airports in 1998. In addition, the Air
Transport Association noted that the Zurich airport charge was based on the
cost of taking certain measures to reduce emissions attributable to aviation
operations, such as improving taxiways to decrease taxi time.

and information, which we included in the report as appropriate. EPA's and
NASA's written comments appear in appendixes III and IV.

According to the Air Transport Association, the report did not establish a
basis for concluding that aviation's emissions of greenhouse gases are
“potentially significant.” In addition, the Association said
that a comparison of aviation emissions with other industrial sources should
await equally thorough analyses of other industrial sectors before a
comparison of the effects of aviation emissions to other sources is made.
The Association also commented that our draft report did not adequately
acknowledge (1) the benefits of aviation's role in the transportation
system- both domestically and internationally- and (2) the progress and
participation of the industry in meeting established environmental goals and
advancing the scientific understanding of aviation's proper contribution to
environmental challenges, such as greenhouse gas emissions. The Association
provided technical and clarifying comments, which we have incorporated as
appropriate.

Our conclusion that aviation's effect on the global atmosphere is
potentially significant is based on our assessment of the 1999 IPCC report
and our consultations with knowledgeable agency officials and other experts.
We carefully considered where jet aircraft deposit the bulk of their
emissions, what types of emissions they produce, and how these emissions
affect the atmosphere, both by themselves and in combination with each
other. We also took into account the IPCC's finding that the aviation
emissions attributable to a growing demand for air travel will not be fully
offset by technological improvements alone.

As for our comparisons of the amount of emissions from aviation and other
sources, FAA and EPA, as well as leading experts involved in the IPCC
report, concurred with our use of comparisons to establish a context for
assessing aviation's relative contribution to potential changes in the
global atmosphere. Furthermore, we were specifically asked to compare
aviation's contribution with contributions from other sources of emissions.
While we agree that data are not available to compare the relative effects
of emissions from aviation and other industrial sources on the global
atmosphere, data are available to compare the relative amounts of emissions
from aviation and other sources- both industries and nations.

We agree that aviation provides significant benefits to the transportation
system and the world economy. We added information in the background section
of our report to provide this context. Our report also notes that

international efforts are under way to address the effects of aviation on
the global atmosphere, primarily through ICAO's Committee on Aviation
Environmental Protection, as specified by the Kyoto Protocol.

The complete text of the Air Transport Association's comments and our
response are included as appendix V.

As arranged with your office, unless you publicly announce it contents
earlier, we plan no further distribution of this report until 14 days after
the date of this letter. At that time, we will provide copies of this report
to interested Members of Congress; the Honorable Rodney Slater, Secretary of
Transportation; the Honorable Jane Garvey, Administrator, Federal Aviation
Administration; the Honorable Carol M. Browner, Administrator, Environmental
Protection Agency; and the Honorable Daniel Goldin, Administrator, National
Aeronautics and Space Administration.

Should you or your staff need further information, please contact me or
Belva M. Martin at (202) 512- 2834. Key contributors to this assignment were
Sandra Cantler and Beverly Dulaney.

Sincerely yours, Gerald L. Dillingham Associate Director, Transportation
Issues

Appendi Appendi xes xI

Objectives, Scope, and Methodology In light of the growing demand for global
air travel and the potentially increasing effects of aircraft emissions, the
Ranking Democratic Member of the House Committee on Transportation and
Infrastructure asked us to provide information on (1) what is currently
known about aviation's contribution to global warming and how aviation
emissions, both domestic and global, compare with emissions from other
sources and (2) what options are available for reducing aviation's
emissions.

To address the first objective, we conducted a literature search on aviation
and global warming and consulted with scientific and aviation experts in the
United States and Europe through briefings and interviews. Our search
identified approximately 50 reports and articles published in the past 5
years. We reviewed those that were repeatedly recommended by the experts we
interviewed. Through the interviews, we confirmed the adequacy of the 5-
year period selected for our literature review and identified other key
experts to contact. Many of the experts we interviewed recommended the 1999
report, Aviation and the Global Atmosphere, prepared by the
Intergovernmental Panel on Climate Change (IPCC), as the most comprehensive
and up- to- date source of information on the subject. Although we did not
independently assess the validity of the reported research, the report was
written by over 100 experts and was peer reviewed by another 150 worldwide
experts on this subject and represents an attempt to reach a consensus
opinion. In addition, we analyzed statistics from the Environmental
Protection Agency (EPA) and Energy Information Administration to determine
the quantity of emissions from various human sources, including aviation.
Although we did not independently verify the accuracy and reliability of
these statistics, EPA's statistics represent the official U. S. submission
to the United Nations Framework Convention on Climate Change to comply with
existing commitments. The Energy Information Administration is a statistical
agency of the Department of Energy-mandated by the Congress to develop
information independently of the Department's policy objectives-to provide
data forecasts and analyses on energy and its interaction with the economy
and the environment.

We also interviewed officials from the Federal Aviation Administration, EPA,
the National Aeronautics and Space Administration, and the National Oceanic
and Atmospheric Administration; the National Center for Atmospheric
Research; airframe and engine manufacturers; nongovernmental organizations,
including the Center for Clean Air Policy and the Environmental Defense
Fund; aviation industry associations, including the Air Transport
Association of America, Inc., the Aerospace

Industries Association, the Airports Council International- North America,
and the International Air Transport Association; representatives of the
European Union/ European Commission; and professors from the University of
California, Irvine, the University of Michigan, and the Massachusetts
Institute of Technology.

For the second objective, we relied on the IPCC report, interviews with the
previously cited officials, and our past work to identify the range of
options for reducing aviation's impact on global warming in the future.

Throughout the review, the following expert reviewers helped us identify the
most recent literature and reviewed our report for accuracy and balance: Dr.
Daniel Albritton, Director, Aeronomy Laboratory, National Oceanic and
Atmospheric Administration; Dr. Joyce Penner, Professor, Atmospheric,
Oceanic and Space Sciences, University of Michigan; and Dr. Michael Prather,
Professor of Earth System Science, University of California, Irvine.

We conducted our work from July 1999 through February 2000 in accordance
with generally accepted government auditing standards.

Level of Scientific Understanding of Aviation's

Appendi xII

Impact on the Global Atmosphere Figures 4 and 5 represent the IPCC's
estimates of the globally and annually averaged radiative forcing from
subsonic aircraft emissions in 1992 and 2050.

Figure 4: Radiative Forcing From Aircraft in 1992

0.10 Radiative forcing (Wm -2 )

0.08 0.06 0.04 0.02

Direct CH 4

sulfate 0.00

CO 2 O 3

H 2 O Contrails Cirrus

Direct Total

clouds soot (without 0.02

cirrus clouds)

-0.04 from NO x -0.06

good poor poor poor fair very fair fair poor

Level of scientific understanding

Note: For radiative forcing, a positive value indicates warming, while a
negative value represents cooling.

Source: IPCC, Aviation and the Global Atmosphere( 1999).

Figure 5: Radiative Forcing From Aircraft in 2050

0.5 Radiative forcing (Wm -2 )

0.4 0.3 0.2 0.1

Direct CH 4

sulfate 0.0

CO 2 O 3

H 2 O Contrails Cirrus

Direct Total

clouds soot

(without cirrus -0.1

clouds) -0.2

from NO x good poor poor poor fair very

fair fair poor

Level of scientific understanding

Notes: For radiative forcing, a positive value indicates warming, while a
negative value represents cooling. The 2050 figure, from the 1999 IPCC
report Aviation and the Global Atmosphere, was based on a 1992 scenario that
was modified to reflect aviation's impact on the global atmosphere.

The scale used by the IPCC in the figure for 2050 is greater than the scale
used in the figure for 1992 by about a factor of four.

Source: IPCC, Aviation and the Global Atmosphere( 1999).

A rating- ranging from very poor to good- is assigned (below the graph) to
each bar to indicate the level of scientific understanding for each
component, including carbon dioxide (CO 2 ), ozone (O 3 ), methane (CH 4 )
water vapor (H 2 O), contrails, cirrus clouds, and sulfate and soot
particles. Each bar represents the “best guess” or estimate of
warming, while the line associated with each bar represents the range of
uncertainty for each component at the 67- percent confidence level.
Specifically, this means the

IPCC is 67 percent confident that the true value falls within the range
indicated by each line. Conversely, there is a 33- percent chance that the
true value falls outside of these ranges.

Comments From the Environmental

Appendi xI II

Protection Agency See comment 1.

The following are GAO's responses to the Environmental Protection Agency's
letter dated January 27, 2000.

GAO Comments 1. We added the tables summarizing the consensus estimates, for
1992 and 2050, of the impact of subsonic aircraft emissions on global
warming.

These tables come from the 1999 IPCC report Aviation and the Global
Atmosphere.

Comments From the National Aeronautics

Appendi xI V

and Space Administration Now on p. 10 See comment 1. Now on p. 20 See
comment 2.

Now on p. 20. See comment 1.

The following are GAO's responses to the National Aeronautics and Space
Administration's (NASA) letter dated January 20, 2000.

GAO Comments 1. We added information to the report in response to NASA's
comment. 2. We made the corrections to the report in response to NASA's
comments.

Comments From the Air Transport

Appendi xV Association of America, Inc.

See comment 2. See comment 1.

See comment 3. See comment 1.

See comment 4. See comment 5.

See comment 6.

See comments 6 and 7.

See comment 8. See comment 9. See comment 10.

Now on p. 5. See comment 11.

Now on p. 5. See comment 12.

Now on p. 6. See comment 13. Now on p. 6. See comment 14. Now on p. 7. See
comment 15.

Now on p. 8. See comment 16.

Now on p. 9. See comment 1.

Now on p. 10. See comment 17.

Now on p. 10. See comment 18. Now on p. 18. See comment 19.

Now on pp. 16- 17. See comment 20.

See comment 10. Now on p. 11. See comment 21.

Now on p. 13. See comment 22.

Now on pp. 13- 14. See comment 23.

Now on pp. 18- 19, 22- 23. See comment 24.

See comment 25. See comments 1, 9, &26.

Now on p. 23. See comment 15.

Now on p. 24. See comment 27.

Now on p. 24. See comment 7.

The following are GAO's responses to the Air Transport Association of
America, Inc. 's (ATA) letter of January 21, 2000.

GAO Comments 1. Our draft report acknowledged that technological
improvements by NASA and the aviation industry have helped aircraft burn
fuel more

efficiently and have thus reduced emissions on a per- passenger- seat basis
by 70 percent over the past 40 years. We acknowledge that energy consumption
per passenger- mile (energy intensity) improved 70 percent between 1980 and
1996- particularly through changes in seating capacity that increase the
number of passengers or modifications to aircraft that increase the amount
of cargo carried. Improved energy intensity does not necessarily enhance
fuel efficiency and thereby lead to fewer emissions. As a result, we did not
add the information on energy intensity because our report focuses on the
effects of aviation emissions and on ways to reduce these effects.

We also added language to the report acknowledging the efforts of the
aviation industry to work effectively with the U. S. government, the
International Civil Aviation Organization, and other international
organizations to assess and manage the effects of aviation on the
environment.

2. We agree that aviation provides significant benefits to the
transportation system and the world economy. For example, our draft report
stated that global aviation contributed $1,140 billion in 1994 to the world
economy and this contribution was expected to increase to $1, 800 billion by
2010. We did not revise this statement in our report.

3. Our draft report stated that the industry was involved in the preparation
of the IPCC report.

4. Our draft report stated that we were focusing on jet aircraft emissions
in the upper atmosphere and did not address the effects of aircraft
emissions on local air quality. The figures cited by ATA from our 1992
report Air Pollution: Global Pollution From Jet Aircraft Could Increase in
the Future( GAO/ RCED- 92- 72) referred to reductions in aircraft emissions
below 3, 000 feet- ground- level emissions. The effects of these emissions
were outside the scope of our review.

5. While we recognize the efforts of the aviation industry to reduce
aircraft noise, this report focused exclusively on aviation's effects on the
global atmosphere, not on noise reduction.

6. We added language to the report noting the importance of recognizing the
trade- offs between applying new engine technologies and considering safety
and performance.

7. We deleted references to older aircraft as more polluting except in our
discussion of regulatory and economic options to reduce aircraft emissions.
Specifically, an aviation industry representative noted that the U. S. fleet
tends to be older and in some cases more polluting than younger fleets owned
by European airlines; therefore, U. S. airlines could face higher costs if
charges were assessed on the basis of emissions.

8. We agree. Our draft report stated that except for aviation emissions of
carbon dioxide whose atmospheric effects are well understood, the scientific
understanding of the effects of the other aviation emissions is less
certain. In addition, we included two charts from the 1999 IPCC report
Aviation and the Global Atmosphere. These charts provide an overview of the
various levels of scientific understanding of aviation's impact on the
global atmosphere.

9. We deleted the reference to the dual annular combustor and, instead,
referred to combustor technologies in general.

10. As stated in our draft report, we concluded that aviation's effect on
the global atmosphere is potentially significant on the basis of our
assessment of the IPCC report Aviation and the Global Atmosphere, as well as
our consultations with knowledgeable agency officials and other experts. In
reaching this conclusion, we carefully considered where jet aircraft deposit
the bulk of their emissions, what types of emissions they produce, and how
these emissions affect the atmosphere, both by themselves and in combination
with each other. We also took into account the IPCC's finding that the
aviation emissions attributable to a growing demand for air travel will not
be fully offset by technological improvements alone.

As for our comparisons of the amounts of emissions from aviation and other
sources, the Federal Aviation Administration and EPA, as well as leading
experts that were involved in the IPCC report, concurred with our use of
these comparisons to establish a context for assessing aviation's relative
contribution to potential changes in the global atmosphere.

Furthermore, we were specifically asked to compare aviation's contribution
with contributions from other sources of emissions. While we agree that data
are not available to compare the relative effects of emissions from aviation
and other industrial sources on the global atmosphere, data are available to
compare the relative amounts of emissions from aviation and other sources-
both industries and nations.

11. We disagree. The IPCC report specifically states that “aircraft
emissions of NOx are more effective at producing ozone in the upper
troposphere than an equivalent amount of emissions at the surface. Also,
increases in ozone in the upper troposphere are more effective at increasing
radiative forcing [warming] than increases at lower altitudes.”
Similarly, the IPCC report states that water vapor emissions released in the
troposphere are rapidly removed by precipitation, whereas in the lower
stratosphere, water vapor emissions can build up to larger concentrations
that tend to warm the earth's surface. While we agree that the IPCC report
found that some of these emissions tend to warm the earth and others tend to
cool it, the report concluded that the net effect of these emissions is a
warming of the earth's surface. See appendix II for the IPCC's overview of
this net warming.

12. We clarified this point. 13. We disagree and believe that it would be
misleading to imply that other compounds “counter” this warming.
We made no change.

14. We agree, and our draft report stated that “The ability to
accurately quantify the impact of human activity on the global climate is
currently limited by a lack of understanding about how much the climate
would vary without these activities.”

15. We clarified the report to indicate that the Kyoto Protocol directs
parties to pursue limitation or reduction of emissions. In addition, we
agree that the discussion under Article 2.2 is not limited to international
aviation, but also addresses maritime emissions. However, maritime emissions
are not the subject of this report.

16. We recognize that the IPCC report was prepared in collaboration with a
Montreal Protocol Panel. However, the statement in our draft report referred
to the requesters of the special report on aviation and the global
atmosphere, not to those who prepared it. As stated in the Forward of the
IPCC report, “This Special Report was prepared following a request
from

ICAO and the Parties to the Montreal Protocol on Substances that Deplete the
Ozone Layer.”

17. We added language to the footnote acknowledging that the IPCC scenarios
are inherently uncertain.

18. We added a footnote indicating that different scenarios were used to
make these estimates.

19. We modified this language to make clear that (1) the industry was
concerned about the imprecision involved in estimating 50 years into the
future and (2) the IPCC report agreed, as noted in our draft report, that
such estimates are imprecise.

20. Much of the information used was taken from the IPCC report, and this
attribution has been added. In addition, many of the experts we interviewed
were directly involved in developing the IPCC report. Furthermore, the three
expert reviewers of our draft report were key participants in the
preparation of the IPCC report and are widely considered leading
international experts.

21. We made this change in our final report. 22. Reductions in UV- B
radiation are a result of NOx- induced increases in ozone in the
stratosphere. However, NOx also increases ozone in the troposphere, and such
increases tend to warm the surface of the earth. Therefore, the UV- B
benefits are somewhat offset. In addition, these effects are regional,
rather than global. Given the trade- offs and regional effects, we did not
make the suggested change.

23. We disagree. Our characterizations of clouds and particles closely
parallel those of the IPCC report. For example, our draft report said
“contrails ... may warm the earth's surface.” The IPCC report
stated that contrails “tend to warm the earth's surface.” We
changed “may” to “expected to” to match the strength
of our statement with that of the IPCC. In addition, our draft report said
“. . . cirrus cloud cover tends to warm the surface of the
earth.” Regarding cirrus clouds, the IPCC states that “On
average, an increase in cirrus cloud cover tends to warm the surface of the
earth.” Our draft report stated that “. . . increases in soot
tend to warm, while increases in sulfate tend to cool.” This is the
same language used in the IPCC report.

24. Our report presents the views of various interested or affected groups
without endorsing one particular position. Our draft report cited
information from our past work on future air traffic management issues and
potential constraints on airports' surface capacity to provide the reader
with additional context.

25. Our intent was not to suggest that the IPCC's efforts were unreliable or
not objective. However, because the reference to an environmental group's
characterization of the IPCC as an industry- led effort could give this
inference, we deleted this statement.

26. We deleted the footnote in the final report. 27. We revised this
footnote in the final report.

(348196) Lett er

GAO United States General Accounting Office

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Contents

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Page 3 GAO/ RCED- 00- 57 Aviation's Effects on the Global Atmosphere United
States General Accounting Office

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Appendix I

Appendix I Objectives, Scope, and Methodology

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Appendix II

Appendix II Level of Scientific Understanding of Aviation's Impact on the
Global Atmosphere

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Appendix II Level of Scientific Understanding of Aviation's Impact on the
Global Atmosphere

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Appendix III

Appendix III Comments From the Environmental Protection Agency

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Appendix IV

Appendix IV Comments From the National Aeronautics and Space Administration

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Appendix V

Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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Appendix V Comments From the Air Transport Association of America, Inc.

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