Aviation and the Global Atmosphere

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1.1. Background

Aviation is an integral part of the infrastructure of today's society. It plays an important role in the global economy; it supports both commerce (through business travel and air freight) and private travel. Aviation also plays an important role in military activity. As such, aviation affects the lives of citizens in every country in the world, regardless of whether they fly. The activities of the civil air transport industry have long been circumscribed by matters of public interest in addition to economic factors. Of most importance historically are matters related to safety and environmental issues associated with local noise and air pollution. Two global environmental issues have emerged for which aviation may have potentially important consequences: Climate change, including changes to weather patterns (i.e., rainfall, temperature, etc.), and, for supersonic aircraft, stratospheric ozone depletion and the resultant increase in UV-B radiation at the Earth's surface. Boxes 1-1 and 1-2 contain general descriptions of the basic science and the political process related to these two issues (without addressing aviation in particular), respectively.

Box 1-1. Climate Change and the Framework Convention

Human activities release greenhouse gases into the atmosphere. The atmospheric concentrations of carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and tropospheric ozone have all increased over the past century. These rising levels of greenhouse gases are expected to cause climate change. By absorbing infrared radiation, these gases change the natural flow of energy through the climate system. The climate must somehow adjust to this "thickening blanket" of greenhouse gases to maintain the balance between energy arriving from the sun and energy escaping back into space. This relatively simple picture is complicated by increased amounts of sulfate aerosol from human activities that modulate incoming solar radiation and tend to cause a cooling effect on climate, at least on regional and hemispheric scales.

Global mean surface air temperatures have increased by 0.3-0.6C since the late 19th century, and recent years have been among the warmest on record. Any human-induced effect on climate, however, is superimposed on natural climate variability resulting from climate fluctuations (e.g., El Nio) and external causes such as solar variability and volcanic eruptions. More sophisticated approaches are now being applied to the detection and attribution of the causes of change in climate by looking, for example, for spatial patterns expected from climate-forcing change by greenhouse gases and aerosols. To date, the balance of the evidence suggests that there is a discernible human influence on the global climate.

Model projections of future climate, based on the present understanding of climate processes and using emission scenarios (IS92) based on a range of economic and technological assumptions, estimate a rise in global mean temperature of 1-3.5C (best estimate 2C) between 1990 and 2100. In all cases, the average rate of warming would probably be greater than any in the past 10,000 years, though 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. Regional temperature changes could differ substantially from the global mean value, and there are many uncertainties about the scale and impacts of climate change, particularly at the regional level. The mean sea level is expected to rise 15-95 cm (best estimate 50 cm) by 2100, with some flooding of low-lying areas. Forests, deserts, rangelands, and other unmanaged ecosystems would face new climatic stresses, partly as a result of changes in the hydrological cycle; many could decline or fragment, with some individual species of flora or fauna becoming extinct. Because of the delaying effect of the oceans, surface temperatures do not respond immediately to greenhouse gas emissions, so climate change would continue for many decades even if atmospheric concentrations were stabilized.

Achieving stabilized atmospheric concentrations of greenhouse gases would demand a major effort. For CO2 alone, freezing global emissions at their current rates would result in a doubling of its atmospheric concentrations from pre-industrial levels soon after 2100. Eventually, emissions would have to decrease well below current levels for concentrations to stabilize at doubled CO2 levels, and they would have to continue to fall thereafter to maintain a constant CO2 concentration. The radiative forcing of greenhouse gas levels (including methane, nitrous oxide, and others, but not aerosols) could equal that caused by a doubling of pre-industrial CO2 concentrations by 2030 and a trebling or more by 2100.

The international community is tackling this challenge through the United Nations Framework Convention on Climate Change (UNFCCC). Adopted in 1992, the Convention seeks to stabilize atmospheric concentrations of greenhouse gases at safe levels. More than 170 countries have become Parties to the Convention. Developed countries have agreed to take voluntary measures aimed at returning their emissions to 1990 levels by the year 2000, with further legally binding emissions cuts after the year 2000 proposed at Kyoto in late 1997. Developed countries have also agreed to promote financial and technological transfers to developing countries to help them address climate change.

Source: IPCC, 1996a.

Box 1-2. Stratospheric Ozone Depletion, UV-B Radiation, and the Montreal Protocol

Although ozone can be measured throughout much of the atmosphere, most of it is found in the stratosphere in a layer centered about 20 km above the Earth's surface. Stratospheric ozone is beneficial to life on Earth because it blocks much of the dangerous ultraviolet light (UV-B) radiated by the sun. If unnaturally high levels of UV-B radiation reach the Earth's surface, many forms of life can be harmed. For instance, UV-B can cause skin cancers in humans and may reduce crop yields.

Natural ozone amounts in the stratosphere result from a balance of production and loss processes involving chemistry, meteorology, and solar radiation. Since the 1960s, however, increases in atmospheric concentrations of human-generated chlorine- and bromine-containing compounds (principally chlorofluorocarbons and halons) have caused additional ozone loss. This trend has resulted in declines in stratospheric ozone amounts at middle and high latitudes in both hemispheres. The most dramatic manifestation is the Antarctic ozone hole, where more than half of the ozone is destroyed in a 6-week period each spring. In recent winters, similar features-but with half the ozone loss (20-30%)-have been observed over the Arctic, and at northern mid-latitudes a long-term decline of 5-10% has occurred over the past 20-30 years. Annual amounts of biologically active UV-B radiation have increased by about 10% over mid-latitudes since 1979. No significant loss of ozone or increase in UV-B radiation has been found in the tropics.

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 trade-offs might be possible?

Figure 1-1: Major links between chapters in this report

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 aviation fuel use did not increase as fast as passenger or freight traffic. Fuel use, CO2 emissions, and NOx emissions per passenger-kilometer have decreased, but the increase in total NOx emissions has been larger than the increase in total fuel use. If it is not technologically possible to reduce all aircraft emissions simultaneously, then some kind of trade-off may be needed. The relative environmental benefits of further reductions in all emissions need to be carefully considered; this analysis is one of the underlying aims of this report.

This report covers issues relating both to the atmosphere and to the aviation industry that address these questions. It is the first such assessment of a single industry and its global environmental impact. (Issues such as local air quality and ground transportation around airports are outside its scope, however, and are not discussed.)

The chapter structure and the major relationships between the chapters are depicted schematically in Figure 1-1. This chapter provides a background for the main part of the report and indicates where detailed information on each subject can be found. In Section 1.2, the present-day aviation industry is described. In Section 1.3, aircraft emissions that are most relevant for climate change and stratospheric ozone depletion are summarized. In Section 1.4, the use and development of future scenarios are discussed, and in Section 1.5, possible options for mitigating the atmospheric effects of aviation are outlined.

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