Environmental Issues in 21st Century Aviation: Key Challenges

Introduction and Background

Environmental awareness regarding 21st-century aviation has been growing among the public and politicians since the 1960s. It became extensively documented that human activities were having damaging and large-scale effects on the environment (Suzanne & Fallacaro, 2011). Engineering and scientific research has also played a growing role in both protecting and understanding the environment. Research has established the significance of the environment to human health and well-being, along with the economic, social, and aesthetic harm that can result from poor environmental practices in aviation. Research has suggested that there are many issues, and that there are various methods to curb harmful practices without incurring unnecessarily high costs. For instance, scientific and engineering research has provided cost-effective ways to decrease water and air pollution in the United States; has established the significance of areas such as wetlands, which were once considered of little value to human societies; and has helped to preserve natural ecosystems and the species that inhabit them.

Over the years, aviation has faced numerous environmental issues. In 1983, for example, the International Civil Aviation Organization (ICAO) founded the Committee on Aviation Environmental Protection (CAEP) to evaluate aviation-associated noise and emissions issues. CAEP established three environmental objectives: reduce or limit the number of people impacted by aircraft noise; reduce or limit the impact of aviation emissions on local air quality (LAQ); and reduce or limit the influence of aviation greenhouse gas (GHG) emissions on the global climate.

The crucial aim for the industry will need to be sustainable development, where the environment is not sacrificed for growth and future generations will be able to benefit from air travel. However, the aviation industry has already begun to confront this difficult task, and continued creative effort is required to ensure the industry makes the most of its "environmental capacity."

Like any other form of public mass transport that depends on finite planetary resources, aviation cannot — in its present form — be considered sustainable in the very long term. Because of the finite nature of the resources upon which aviation depends, it is more accurate in the medium term to consider how best to improve the sustainability of air transport rather than to claim that it has attained sustainable development.

Aviation is widely regarded as a critical part of the national economy, facilitating the movement of goods and people around the world and enabling economic growth. In the last 35 years, there has been approximately a six-fold increase in the mobility provided by the United States air transportation system, accompanied by a 70% improvement in aircraft fuel efficiency and a 97% reduction in the number of individuals impacted by aircraft noise.

In spite of this progress, and despite aviation's comparatively small environmental impact in the United States, there is a compelling need to address the environmental aspects of air transportation. Because of strong growth in demand, emissions of some pollutants from aviation are rising against a background of declining emissions from many other sources. Furthermore, progress on noise reduction has slowed. Millions of people are adversely affected by these side effects of aviation. These factors, combined with the rising value placed on environmental quality, are creating constraints on the nation's mobility, economic vitality, and security. In many places, plans for airport expansion have been delayed or canceled due to concerns over local air quality, water quality, and community noise.

Even military readiness is being challenged by limitations on certain operations. These effects are expected to grow as the economy and demand for air transportation continue to develop (Iani & Wickens, 2007). If this concern is not addressed soon, environmental impacts may well become the fundamental constraint on air transportation growth in the 21st century.

Research shows that in the next 15 years, air travel is expected to grow substantially (Iani & Wickens, 2007). As a result, expansion and airport development projects will likely become increasingly important. A potential challenge to the success of these projects is community concern about airport environmental impacts. Airport operations involve an array of activities that affect the environment, including:

the operation of aircraft; the operation of airport and passenger vehicles and airport ground service equipment (GSE); cleaning and maintenance of aircraft, motor vehicles, and GSE; anti-icing and deicing of aircraft and airfields; fuel storage and fueling of aircraft and vehicles; airport facility maintenance and operations; and construction.

The environmental impacts of these activities could intensify if an airport is undergoing expansion. In many cases, before a local or state agency will permit an airport to proceed with a development project, the airport authority must commit to implementing positive environmental mitigation projects. Community concern about environmental impacts has caused projects to be cancelled or delayed (Iani & Wickens, 2007). All airports, regardless of location or size, are regulated to some degree under state, local, tribal, or federal environmental requirements.

Overview of Airport Environmental Issues

Many of the environmental regulatory requirements applicable to noise, water, and air quality have been in effect for years — airport managers are accustomed to their compliance responsibilities. However, the anticipated growth in air travel has intensified the significance and complexity of some environmental regulatory matters (Pezzullo, 2005). Several new requirements are expected to result in potentially significant changes to airport operations. The most notable issues include ongoing community concern about noise, changes to Environmental Protection Agency (EPA) procedures applicable to aircraft and airfield deicing operations (Maughan & Gillingwater, 2001), and revisions to EPA regulations applicable to oil spill prevention preparedness.

With regard to water quality compliance matters, the management of anti-icing and deicing chemicals has posed the greatest challenge to many airport operators. The deicing and anti-icing of aircraft and airfield surfaces is required by the Federal Aviation Administration (FAA) to ensure passenger safety. However, when performed without discharge controls in place, airport deicing procedures can result in environmental impacts (Suzanne & Fallacaro, 2011). Discharges from deicing operations have the potential to cause algae blooms, fish kills, and contamination of surface and groundwaters. In addition to potential water-life and human health impacts from the toxicity of anti-icing and deicing chemicals, the biodegradation of ethylene glycol or propylene glycol in surface waters can significantly impair water quality, including causing important reductions in dissolved oxygen levels (Woodcock & Roberts, 2007).

Studies have likewise revealed toxicological effects of deicer solutions that cannot be attributed to either ethylene glycol or propylene glycol (Westermark, 2001). This has led to concern that these effects are attributable to unknown, proprietary additives, the environmental impact and fate of which are not yet understood (Suzanne & Fallacaro, 2011). Typically, airlines are responsible for aircraft deicing and anti-icing operations, while airports are accountable for the deicing and anti-icing of airstrip surfaces. The airfield is ultimately responsible for managing the resulting wastewater, and this responsibility is typically absorbed into the airport's stormwater authority (Wayson & Iovinelli, 2009).

As noted above, significant differences exist among airport NPDES permits. A local permitting authority can impose specific requirements — for example, restrictions on where deicing operations may take place, an obligation to use deicing collection units to recover deicing fluid before it enters the stormwater system, or provisions requiring monitoring equipment to ensure permit compliance. Other permits may allow the airport to discharge deicing fluids directly into a neighboring water body (Woodcock & Roberts, 2007).

According to the EPA, the disparity in airport permitting requirements led the agency to consider applying national standards in the form of effluent limitation guidelines (ELGs) for airport deicing and anti-icing operations (Westermark, 2001). Effluent limitation guidelines are national standards for regulating wastewater discharges to surface waters. They are technology-based and industry-specific. ELGs applicable to airport deicing would be intended to deliver consistent guidance for NPDES permit writers across the country, thereby establishing a baseline standard for all airports (Wayson & Iovinelli, 2009). In 2004, the EPA began developing effluent limitation guidelines for airport deicing operations.

Initial estimates from the EPA suggest that treatment technology and pollution-prevention practices could decrease deicing discharges from the current level of 25 million gallons per year to 6 million gallons per year (Suzanne & Fallacaro, 2011). Numerous airports already have strict permit requirements stipulating the management of deicing chemicals, while others have few controls. Those with few controls may be required to make capital investments to comply with new permitting requirements.

Because airports must store fuel on-site to refuel aircraft and ground equipment, most airports are required to develop a Spill Prevention, Control, and Countermeasure (SPCC) plan (Wayson & Iovinelli, 2009). These requirements are designed to ensure that facilities storing oil have planned for and taken measures to prevent environmental harm from oil spills. An SPCC plan must include operating procedures intended to prevent oil spills (e.g., procedures to inspect tanks and associated piping for leaks) and control measures designed to prevent a spill from reaching navigable waters (e.g., construction of a dike, containment curb, or pit around a tank or tank farm).

One of the primary control measures required under SPCC regulations is the use of a secondary containment system for oil storage containers. Such a system must be large enough to temporarily hold the entire contents of the largest oil tank at the storage location in the event of a breach. For instance, if a tank farm contained four 14,000-gallon tanks, two 7,000-gallon tanks, and ten mobile refueling trucks with 500-gallon tanks, the tank farm would be required to have secondary containment sufficient to hold the contents of the largest single tank — 16,000 gallons (Wayson & Iovinelli, 2009).

When the EPA proposed new SPCC requirements in 2002, airport operators and the EPA disagreed about the secondary containment requirements applicable to mobile airport refueling trucks. In particular, airport operators argued that it was impractical to require mobile refuelers to provide secondary containment equal to the tank size because, during refueling operations, the trucks would need to move to many parts of the airfield that could not be retrofitted with secondary containment systems (Westermark, 2001).

The Clean Water Act (CWA) prohibits any "point source" — a discrete conveyance such as a drainage ditch, pipe, or outfall — from discharging pollutants into the waters of the United States (Suzanne & Fallacaro, 2011). The primary mechanism for controlling pollutant releases is the National Pollutant Discharge Elimination System (NPDES) permit program, which is administered in most cases by individual states (Peter, 2012). The NPDES permit program regulates discharges of both stormwater and wastewater. Because of the nature of their outdoor operations, and because airports fall under one of the industrial categories covered by the NPDES stormwater permitting program (under the Standard Industrial Classification code "Transportation by Air"), all airports are required to obtain a stormwater permit (Suzanne & Fallacaro, 2011).

Airports that discharge other wastewater — for example, from equipment maintenance and cleaning operations — require an additional NPDES wastewater permit. Stormwater discharges frequently pose the greatest challenge to airport managers because airports may spread across a wide surface area with most operations exposed to the elements. For instance, Dallas Fort Worth International Airport encompasses 20,000 acres and has 72 stormwater outfalls (Woodcock & Roberts, 2007). Monitoring or controlling every outfall is extremely difficult. The primary technique for managing stormwater discharges is the application of best management practices (BMPs) that prevent or reduce the release of pollutants into a water body (e.g., construction of a stormwater retention pond to prevent direct drainage into receiving waters). BMPs appropriate for one airport are not necessarily appropriate for another. Factors that may affect permit requirements include local climate, the size or type of neighboring water bodies, the water quality of those water bodies, and airport size (Suzanne & Fallacaro, 2011).

Aviation is considered a major source of local air pollution, and its emissions also lead to important public health impacts. Jet emissions have been associated with throat, lung, larynx, nasal, and brain cancer, lymphoma, leukemia, asthma, and various birth defects. Research shows that benzpyrene — a highly carcinogenic byproduct of jet fuel combustion that attaches to airborne dust — can cause tumors and cancer in humans through skin and lung absorption; it is released into the atmosphere across the United States at levels that routinely exceed safety limits (Lund & Shine, 2012). Jet emissions can affect an area up to 25 miles centered on an airport. People, animals, children, and plants are exposed to toxic substances whenever jet emissions are present within 12 miles of a runway's end. A typical commercial airport releases hundreds of tons of harmful chemicals and regulated pollutants into the atmosphere on a daily basis. These substances drift over densely populated areas and settle onto water bodies and crops.

Aircraft and airports cause several types of air pollution at various altitudes and at significant distances from their source. Of primary concern to those living and working many miles from a local airport or under aircraft flight tracks are toxic and hazardous air emissions. Aircraft release these toxic mixtures in large quantities; such emissions are typically dispersed over an area approximately 12 miles long and 12 miles wide on take-off.

The area heavily polluted by a light-to-medium-traffic, two-runway airport extends approximately 12 miles around the airfield and 20 or more miles downwind. A single-runway airport with light to medium traffic pollutes an area roughly 8 miles around the airfield and up to 30 miles downwind. Chicago's O'Hare Airport, for example, has approximately eight runways, yet a detailed area-wide environmental study has never been conducted. Newer aircraft, though their emissions are comparatively less visible, may be at least as problematic as older aircraft in terms of pollution because of their production of smaller particulate matter, different combustion processes, and different fuel compositions.

As a result, the number of people exposed to aviation pollutants in an airport's surrounding area can be enormous. In Chicago, a physician who teaches clinical medicine at a university school of public health estimated that as many as 7 million people's health could be affected by O'Hare Airport alone. There are four major airports in the Chicago metropolitan area, and similar conditions exist in other communities nationwide.

Health Effects and Local Air Quality

The following table summarizes the health effects of key aviation-related pollutants:

Ozone: Lung function impairment, effects on exercise performance, increased airway responsiveness, increased susceptibility to respiratory infection.
Carbon Monoxide: Cardiovascular effects, especially in persons with heart conditions.
Nitrogen Oxides: Lung irritation and lower resistance to respiratory infections.
Particulate Matter: Eye and respiratory tract irritation, headaches, dizziness, visual disorders, and memory impairment.
Volatile Organic Compounds: Contribute to smog formation and respiratory distress.

The environmental effects of the same pollutants include crop damage to trees, bushes, grass, and flowers (ozone); acid rain, visibility degradation, and particle formation contributing to ozone depletion (nitrogen oxides and particulate matter); and structural effects on buildings (Pezzullo, 2005).

Research shows that the chemicals used to de-ice aircraft — propylene glycol and ethylene glycol — are toxic substances in sufficient quantities. Ethylene glycol causes central nervous system depression as well as kidney and liver damage; propylene glycol, when used at airports alongside anti-corrosion substances, is equally toxic (Lee, 2011). The lethal dose in adults is approximately 1.4 ml/kg. No long-term studies on their effects on human populations have been completed. However, each winter large numbers of fish and wildlife are poisoned by aircraft de-icing chemicals. Additional pollutants, including oils and other toxic materials, are washed off aircraft during deicing events. Airports such as O'Hare currently have no method for recycling de-icing fluids, which are instead discharged into the surrounding environment.

Research shows there has been a 95% reduction in the number of individuals affected by aircraft noise in the United States over the past 38 years. However, this dramatic reduction has been measured in terms of the number of people living in areas above 65 dB Day-Night Noise Level (DNL), where more than 15% of the population may be highly annoyed, and in terms of the number of people in areas above 58 dB DNL, where more than 5% may be highly annoyed (Lund & Shine, 2012).

The FAA identifies 65 dB DNL as the threshold for federal support of noise mitigation. Whereas existing FAA policy acknowledges that impacts below 65 dB DNL may be assessed, federal funds for mitigation cannot be applied to these lower-level impacts (Westermark, 2001).

The reductions in the number of people exposed to aircraft noise were achieved during a period of six-fold growth in traffic, largely through key technological advances such as the introduction of high-bypass-ratio engines, which delivered both fuel savings and noise reductions. Most of these improvements were driven by new noise standards and the mandatory phase-out of 60% of the older, noisier fleet resulting from the Airport Noise and Capacity Act of 1990 (ANCA). The phase-out is estimated to have cost the industry roughly $6 billion (Maughan & Gillingwater, 2001).

Aircraft noise worldwide remains a significant issue and is expected to grow over time. In 2000, approximately 0.5 million people in the United States were living in areas where noise levels exceeded 65 dB DNL (Iani & Wickens, 2007), while around 6 million lived in areas with noise levels above 55 dB DNL. A further 20% reduction in the number of people impacted has been noted since 2000 due to the earlier-than-expected retirement of certain aircraft following the post-9/11 traffic decline and the ongoing reduced traffic levels in the U.S. system relative to 2000 (Maughan & Gillingwater, 2001).

Such dramatic improvements are not expected to continue. The environmental impact of aircraft noise is expected to remain roughly stable in the United States for the next several years and then rise as air travel growth outpaces likely operational and technological improvements (Maughan & Gillingwater, 2001). Ongoing increases in noise impact are expected for Asia and Europe. Additionally, new concerns are emerging, such as the loudness of aircraft noise in certain national parks, low-frequency noise impacts near some airfields, and efforts to develop supersonic commercial jets with sonic boom signatures that might be acceptable for flight over populated areas.

Research indicates that while industry and federal investment can reduce aircraft noise, it is local authorities that actually control land-use decisions near airports. There are numerous examples where federal land-use guidance intended to mitigate impacts has not been followed by local authorities, worsening the problem (Peter, 2012). Even when airports are situated in once-sparsely populated areas (such as Naval Air Landing Field Fentress, Dallas/Fort Worth International Airport, and Denver International Airport), difficulties eventually arise as local decisions lead to increased land-use near the airports.

By 2020, the European Union aimed to reduce the perceived noise from new large jet aircraft to one-half of average 2002 levels (Iani & Wickens, 2007). NASA has planned to develop technology allowing a 50% reduction in operational perceived noise level for a new aircraft relative to the 1997 state-of-the-art. Research within the FAA currently focuses on the development of better tools and metrics to measure aviation noise impacts and on the development and application of operational procedures to mitigate aviation noise (Maughan & Gillingwater, 2001). It is widely understood that a balanced approach is necessary, with the greatest near-term opportunities lying in operational procedures, and reductions in source noise (engines and airframes) being required in the long term for further reductions (Iani & Wickens, 2007). Ongoing policy efforts to encourage appropriate land use will be required throughout.

Although noise is the primary environmental constraint on airport operations and development, many airports are placing local air quality concerns on equal footing with noise, or anticipate doing so in the near future (Lund & Shine, 2012). Emissions of carbon monoxide (CO), unburned hydrocarbons (UHC), nitrogen oxides (NOx), and particulate matter (PM) from a variety of airport sources contribute to local air quality degradation, resulting in human health and welfare impacts.

Nationally, local air quality has been steadily improving as a result of the Clean Air Act, which has driven reductions in pollution from most sources (Lee, 2011). However, many of the technologies employed for land-based sources are not suitable for aircraft because of the more stringent volume, weight, and safety constraints. Therefore, even though aviation is a relatively small overall contributor to local air quality impacts, some aircraft emissions are rising against a background of generally declining emissions from other sources. The most problematic pollutant to control for aviation has been NOx. Air travel operations below 4,000 feet contribute approximately 0.4% to the total national NOx inventory. Forty-one of the 80 major airports are in ozone maintenance or non-attainment areas. In serious and severe non-attainment areas, airport contributions to the area NOx inventory range from 0.8% to 7.1%, with an average below 4% (Pezzullo, 2005). The contribution of aviation to NOx emissions around airports is anticipated to grow.

There are chemical and physical phenomena that make it more complex to reduce NOx emissions from aircraft engines that use high pressures and temperatures to improve fuel efficiency. However, there are alternatives for reducing NOx that do not require trade-offs with fuel efficiency; improvements in combustor technology and airframe aerodynamics and weight have led to reductions in NOx emissions without adverse effects on fuel productivity. Over the past 40 years, fuel burn per passenger-mile has been reduced by 70%, with roughly half of this reduction due to advances in engine technology and the remainder owing to improvements in weight, aerodynamics, and procedures (Iani & Wickens, 2007).

Continued technology research is expected to reduce fuel consumption at a slower rate — approximately 4% per year over the next 25 to 40 years — with more opportunities for improvement in airframes than in engines (Lee, 2011). On the other hand, demand for air transportation is projected to increase by 4% to 6% per year (Lund & Shine, 2012). Low-emissions technology and operations must therefore make up the difference to avoid increased pollutant emissions from aircraft.

Two areas of increasing importance and high uncertainty related to local air quality have been emerging for aviation over the last decade. The first is fine particulate matter (PM). The morbidity and mortality costs of PM are several hundred times greater than those resulting from NOx emissions (Maughan & Gillingwater, 2001). Despite the EPA's progressively more stringent national ambient air quality standards for particulate matter, there are currently no universally accepted approaches for quantifying PM and PM precursors from aviation. The aviation community is therefore required first to characterize and measure the pollutants, then to measure their impact, and ultimately to adopt approaches to reduce them if necessary. NASA, FAA, EPA, and academic and industry institutions have joined together to develop a National Roadmap for Aviation Particulate Matter Research (Peter, 2012) to plan the efforts needed in this area.

The topic of greatest contention and uncertainty is the climate change impact of aviation. In Europe, this is considered the single most important environmental impact of air travel (Lund & Shine, 2012), while in the United States it is still regarded by many as less pressing than community noise and local air quality. It is established that aircraft emit chemical species and produce physical effects — such as condensation trails, or contrails — that most scientists believe affect the climate. Scientific analyses suggest that the resulting chemical and physical effects of aviation are such that air travel may have a disproportionate effect on climate per unit of fuel burned, compared to terrestrial sources.

In 1999, a special study was performed by the Intergovernmental Panel on Climate Change (IPCC), which estimated that aviation accounted for approximately 3.6% of anthropogenic radiative forcing of the climate in 1996. Per unit of fuel burned, radiative forcing from aircraft is estimated to be roughly twice that of land-based combustion of hydrocarbon fuels (Iani & Wickens, 2007). Since the IPCC study, scientific understanding of some of the physical and chemical effects — chiefly contrails and the cirrus clouds they may encourage — has progressed. A report by the UK Royal Commission on Environmental Protection (RCEP) indicated that the net result of contrail and aviation-induced cirrus is anticipated to be three to four times the radiative forcing due to CO2 alone, though further revisions to these estimates are likely (Pezzullo, 2005).

If these estimates are accurate and aviation growth forecasts are realized, aviation may be responsible for between 5% and 20% of anthropogenic climate forcing by 2070 (Maughan & Gillingwater, 2001). Given the uncertainty in understanding aviation's impacts on climate, appropriate technological, policy, and operational mitigation options are also uncertain. Most mitigation options are currently focused on reducing fuel burn. However, it is likely that this is not the most effective strategy for reducing aviation's contribution to climate change. Furthermore, although fuel use per passenger-mile has been reduced by 70% over the last 35 years, most projections suggest a slower rate of improvement over the next 20 to 25 years — approximately 2% per year (Pezzullo, 2005) — falling short of anticipated growth in demand. NASA has established a five-year goal to advance technologies to reduce CO2 emissions of new aircraft by 30%.

Despite the fact that the United States has increased investment to reduce uncertainty in climate change impacts generally, there are currently no major U.S. research programs specifically evaluating the climate impacts of aviation (Lund & Shine, 2012). This could put the United States at a disadvantage in assessing technology and policy options, and in negotiating appropriate standards and regulations with other nations. It could also lead to dependence on data produced by others who may favor restricting aviation activity to mitigate environmental impacts, despite aviation's important contribution to the economy.

The graph referenced in the original report illustrates the effect of flying altitude on climate impact from various aircraft emissions (NOx, CO2, and water vapor, not accounting for contrails or cirrus cloud changes). The value of reference (100) corresponds to CO2 radiative forcing at an altitude of 12 km (Iani & Wickens, 2007). Even if the significance of CO2 climate impacts tends to decrease with increasing altitude, the total climate effect of the measured emissions grows rapidly because of the amplified effects of NOx and H2O. As a result, the total climate impact of an aircraft at 12 km is approximately twice as large as the effect at 8 km.

The major global assessment of total climate impact from the aviation sector was made by the IPCC special report "Aviation and the Global Atmosphere" (1999). The principal "greenhouse gas" pollutant released from aviation is CO2 (carbon dioxide). Overall CO2 emissions from aviation represent approximately 2.0–2.5% of total annual CO2 emissions (Lee et al., 2009a). Other emissions from aviation that affect the radiative balance include nitrogen oxides (NOx), soot particles, sulfate, and water vapor, each of which leads to a variety of effects.

More recent outcomes from European projects show that the estimated contribution to radiative forcing due to NOx (through ozone and methane) and due to linear contrails has been revised downward since the IPCC 1999 report. However, these projects provide a first estimate for the radiative forcing from increasing cirrus clouds that is comparable in magnitude to the sum of all other gases, including CO2. The uncertainty range for cirrus remains extremely large, while that for other gases has been reduced (Maughan & Gillingwater, 2001).

The geographical coverage of persistent contrails is expected to expand significantly between 1992 and 2050, in line with projected air traffic growth. The regional impact is apparent, since most contrails form along main air corridors and the intensity of coverage increases with traffic growth on key routes. Belgium, for instance, sits in the center of major European air routes and is expected to experience one of the most significant contrail coverages — largely due to overflights of Belgian territory.

Climate Change and Greenhouse Gas Emissions

CO2: Increases CO2 concentration in the atmosphere, leading to global warming.

Water Vapor (H2O): Increases water vapor concentration in the atmosphere; contributes to condensation trail and cloud formation; effects are primarily local and regional.

NOx: Increases ozone concentration in the atmosphere; destroys methane; produces both local/global warming and global cooling effects.

Particulate Matter (soot): Influences cloud formation; local and regional effects.

Aerosols: Influence cloud formation; local and regional effects.

Most research on interdependencies demonstrates that noise, climate effects, and local air quality from aviation result from an interdependent set of operations and technologies, so that action to address impacts in one area can produce undesirable consequences in another. For instance, both technological and operational measures to reduce noise can result in higher fuel burn, thereby increasing aviation's impact on local air quality and climate change (Lund & Shine, 2012). Emissions interrelationships make it difficult to optimize engine design as a mitigation approach, since they force trade-offs among individual pollutants as well as between noise and emissions (Suzanne & Fallacaro, 2011). As yet, interdependencies among various policy, operational, and technological choices — and their full economic implications — have not been adequately assessed.

The NRC has recommended that government and industry invest in comprehensive interdisciplinary studies that assess the marginal costs of environmental protection strategies (Suzanne & Fallacaro, 2011). NASA and the FAA plan to contribute $10 million per year over six years to develop a complete framework of aviation environmental analytical methodologies and tools to assess interdependencies among noise, emissions, and economic performance (Maughan & Gillingwater, 2001). These tools will be essential for informing decisions on new emissions and noise standards, potential fleet phase-outs, and possible cruise emissions criteria. They will also guide appropriate research and development investment in technological and operational options for reducing emissions and noise. The development of such tools represents a key step forward for the nation.

Environmental issues in 21st-century aviation also concern the ozone layer. Certain chemicals released from airports are affecting the stratospheric ozone layer. In the early 1970s, researchers began to study the atmospheric fate of chlorofluorocarbons (CFCs), which had been widely used as refrigerants and in industrial processes. These scientists demonstrated that, when CFCs rise into the stratosphere, they react with sunlight to produce chemical radicals that destroy ozone molecules. The stratospheric ozone layer protects the earth's surface from harmful ultraviolet radiation. In 1984, researchers measured severe decreases in stratospheric ozone over Antarctica in early spring, directly attributable to the catalytic action of CFCs.

Since 1987, more than 200 countries have signed a series of international agreements beginning with the Montreal Protocol, calling for a phased reduction in the production and ultimate elimination of CFCs. Although stratospheric ozone depletion will persist for several decades, the ozone layer is expected to recover eventually, and the potential penalties will be far less severe than they would have been had the research not led to early recognition of the issue.

Aviation produces a number of pollutants that alter the chemical composition of the atmosphere, modifying its radiative balance and thus influencing the climate. The principal greenhouse gas pollutant released from aviation is CO2. Other emissions from aviation that affect the radiative balance include nitrogen oxides (NOx), soot particles, sulfate, and water vapor. Research shows that the only true "greenhouse gas" emissions from aviation are CO2 and water vapor; other emissions such as NOx and particles result in changes in radiative forcing but are not greenhouse gases in themselves. Emissions of water vapor from current subsonic aviation are minor and contribute insignificantly to warming. CO2 emissions are proportionally linked to fuel (kerosene) consumption by a factor of approximately 3.15 (Peter, 2012).

Several events have impacted the sector (disease, oil crises, conflicts) and produced a response in demand and emissions, demonstrating that the sector is unusually resilient and adaptable to various external pressures. The typical pattern is a weakening or slump in demand that recovers within 3 to 4 years, sometimes so strongly that growth returns to its prior trajectory.

In the immediate vicinity of airports, emissions of nitrogen oxides (NOx), volatile organic compounds (VOCs), particulate matter (PM), and carbon monoxide are recognized as contributors to local air quality concerns. All of these are believed to have potentially harmful health effects at defined ambient concentrations. The influence of other trace species — such as hydroxyl radicals, sulfur dioxide (SO2), and nitrous and nitric acids — still requires a better understanding but is currently believed to be minor (Maughan & Gillingwater, 2001).

Emissions of VOCs and NOx contribute to the formation of ground-level ozone through chemical reactions with oxygen in the presence of sunlight. Sufficient studies link NOx and ground-level ozone — the most important constituent of smog — to heightened respiratory difficulties in certain population segments. Although the aviation industry has achieved important reductions in CO and VOCs, "odor nuisance" is becoming an increasingly common complaint by local residents at some airports. At a local level, emissions of NOx, combined with SO2, may also contribute to eutrophication and acidification of ecosystems (Peter, 2012).

Climate change may be defined as the disturbance of the earth's energy equilibrium through natural or anthropogenic emissions of greenhouse gases, primarily consisting of water vapor, CO2, methane, nitrous oxide, halocarbons, and ozone (Lee, 2011). Consequences of climate change could include alterations in global average surface temperatures (commonly referred to as global warming) and local changes such as average rainfall patterns or the frequency and intensity of heat waves. Numerous scientists agree that human activity over the past 200 years has contributed to a measurable increase in greenhouse gas emissions, primarily in the form of CO2 (approximately 79%) (Pezzullo, 2005).

At present, CO2 emissions from air travel represent roughly 5% of total CO2 emissions from fossil fuel combustion and 15% of those from all transport sources. CO2 and water vapor are natural products of fossil fuel combustion and are expected to persist in the atmosphere for more than 100 years (Westermark, 2001). Aircraft water vapor emissions have a positive but insignificant direct effect on climate change, contributing little to naturally occurring water vapor concentrations in the troposphere.

One focus of aviation efficiency improvements is achieving reduced fuel consumption, which translates to reduced emissions and a decrease in greenhouse gases that directly affect climate change. General aviation, which includes business aviation, is a relatively small contributor to greenhouse gases, accounting for 0.8–0.9% of the air transportation total. However, aviation as a whole has made great strides in utilizing advanced technology to achieve efficiency gains and accompanying emissions reductions (Pezzullo, 2005). All cost-effective ways to advance air transport growth and environmental protection should be explored and implemented, including improvements in operating procedures, land use management, airport infrastructure, technology and equipment, ground systems, and air traffic management.

The most significant fuel savings are expected to come from improvements in air traffic communications and management, surveillance, and navigation (ATM/CNS) systems. The IPCC estimated that ATM/CNS enhancements worldwide could lead to fuel savings and CO2 emission reductions of between 6% and 12%. Other possible operational improvements could yield savings of 2–6%, resulting in a possible combined saving of 9–19%.

Economic incentives such as emission trading arrangements are intended to cap and reduce emissions to pre-determined targets. Although effective as a partial solution, such arrangements must operate in an open market and must also consider thresholds to differentiate between low- and high-utilization activities and the administrative burden they may impose on small operators.

Modern jet aircraft are significantly more fuel-efficient — and therefore emit less CO2 per unit — than their counterparts from 30 years ago (Peter, 2012). Manufacturers have committed to achieving reductions in both NOx and CO2 emissions with each new generation of engine and aircraft design (Pezzullo, 2005). The more rapid introduction of modern aircraft therefore represents an opportunity to reduce emissions per passenger-kilometre. However, aircraft are major capital investments that remain in service for many decades, and renewal of the global fleet is consequently a long-term proposition that will significantly delay the realization of the climate benefits of new designs. Engines can be upgraded over time, but airframes may have very long service lives.

Another potential improvement is the electrification of the aircraft nose wheel, which would allow taxiing to be performed using an electric motor rather than the main engines (Costabel, 2011). Other opportunities arise from the optimization of airline schedules, route networks, and flight frequencies to increase load factors and minimize the number of empty seats flown. However, each of these represents a one-time improvement, and as these opportunities are successively captured, diminishing returns can be expected.

Another potential reduction in climate impact is the restriction of aircraft cruise altitude. This would lead to an important decrease in high-altitude contrails with a trade-off of increased flight time and an estimated 6% rise in CO2 emissions. Disadvantages include limited airspace capacity, particularly in North America and Europe, and increased fuel burn because jet aircraft are less efficient at lower cruise altitudes (Pezzullo, 2005).

Although not suitable for long-haul or transoceanic flights, turboprop aircraft used for passenger flights offer two important advantages: they typically burn significantly less fuel per passenger-mile, and they naturally fly at lower altitudes well within the troposphere, where there are no concerns about contrail formation or ozone production.

Some scientists and companies — including GE Aviation and Virgin Fuels — have been investigating biofuel technology for use in jet aircraft (Costabel, 2011). As part of one such test, Virgin Atlantic Airways flew a Boeing 747 from London Heathrow Airport to Amsterdam Schiphol Airport on 21 March 2009, with one engine burning a mixture of babassu oil and coconut oil. Greenpeace's chief scientist, Doug Parr, described the flight as "high-altitude greenwash" and noted that growing organic oils for biofuel could lead to deforestation and a significant increase in greenhouse gas emissions (Costabel, 2011). Also of concern is the large amount of land that would be required to provide the biomass feedstock needed to sustain civil and military aviation (Ramel, 2012).

In March 2009, an Air New Zealand jet completed the world's first commercial aviation test flight partly using jatropha-based fuel. Jatropha, which is sometimes substituted for biodiesel, can thrive on marginal agricultural land where many crops would grow poorly or not at all (Costabel, 2011). Air New Zealand established several sustainability standards for its jatropha supply, stipulating that such biofuels must not compete with food crops, must perform comparably to conventional jet fuels, and should be price-competitive with existing fuels (Peter, 2012).

In December 2012, Continental Airlines used a sustainable biofuel to power a commercial flight — the first such demonstration flight in North America by a commercial carrier using a twin-engine aircraft (a Boeing 737-800 powered by CFM International CFM56-7B engines). The biofuel blend involved components derived from jatropha plants and algae (Pezzullo, 2005).

One alternative biofuel under development is Swift Fuel, which was accepted as a test petroleum by ASTM International in December 2009. Swift Enterprises president Mary Rusek predicted that "100SF will be comparably priced, environmentally friendlier, and more fuel-efficient than other general aviation fuels on the market" (Keen & Strand, 2007).

As of July 2011, updated global aviation fuel standards formally allow commercial airlines to blend up to 50% biofuels with conventional jet fuel. In March 2012, the FAA announced $8.8 million in grants to eight companies to advance the development of drop-in commercial aviation biofuels, with a particular emphasis on alcohol-to-jet (ATJ) fuel. As part of its CAAFI (Commercial Aviation Alternative Fuels Initiative) and CLEEN (Continuous Lower Emissions, Energy, and Noise) programs, the FAA has planned to support the development of sustainable fuels — from alcohols, sugars, biomass, and organic matter such as pyrolysis oils — that can be "dropped in" to aircraft without altering existing infrastructure (Pezzullo, 2005).

Conclusion

It is clear that the environmental issues faced in the 21st century are many. The aviation sector's fuel efficiency improvements have slowed since the 1970s because of the slower pace of technological progress in aerodynamic designs and in engine and airframe materials. Even with all the technological advances of the 21st century, some environmental issues show no signs of improvement. Noise complaints from affected communities have continued to rise. Long lead times in product development and fleet turnover, combined with the high costs associated with radical technological advances, have also been key barriers to progress.