The environmental impact of aviation occurs because aircraft engines emit heat, noise, particulates, and gases which contribute to climate change and global dimming. Airplanes emit particles and gases such as carbon dioxide (CO2), water vapor, hydrocarbons, carbon monoxide, nitrogen oxides, sulfur oxides, lead, and black carbon which interact among themselves and with the atmosphere.
Despite emission reductions from automobiles and more fuel-efficient and less polluting turbofan and turboprop engines, the rapid growth of air travel in the past years contributes to an increase in total pollution attributable to aviation. From 1992 to 2005, passenger kilometers increased 5.2% per year. And in the European Union, greenhouse gas emissions from aviation increased by 87% between 1990 and 2006.
Comprehensive research shows that despite anticipated efficiency innovations to airframes, engines, aerodynamics and flight operations, there is no end in sight, even many decades out, to rapid growth in CO2 emissions from air travel and air freight, due to projected continual growth in air travel. This is because international aviation emissions have escaped international regulation up to the ICAO triennial conference in October 2016 agreed on the CORSIA offset scheme, and because of the lack of taxes on aviation fuel worldwide, lower fares become more frequent than otherwise, which gives a competitive advantage over other transportation modes. Unless market constraints are put in place, this growth in aviation’s emissions will result in the sector’s emissions amounting to all or nearly all of the annual global CO2 emissions budget by mid-century, if climate change is to be held to a temperature increase of 2 °C or less.
There is an ongoing debate about possible taxation of air travel and the inclusion of aviation in an emissions trading scheme, with a view to ensuring that the total external costs of aviation are taken into account.
Aircraft noise is seen by advocacy groups as being very hard to get attention and action on. The fundamental issues are increased traffic at larger airports and airport expansion at smaller and regional airports. Aviation authorities and airlines have developed Continuous Descent Approach procedures to reduce noise footprint. Current applicable noise standards effective since 2014 are FAA Stage 4 and (equivalent) EASA Chapter 4. Aircraft with lower standards are restricted to a time window or, on many airports, banned completely. Stage 5 will become effective between 2017-2020. Quantification and comparison of noise effects per seat-distance takes takes into account that noise from cruise levels usually does not reach the earth surface (as opposed to surface-transportation) but is concentrated on and in proximity of airports.
Airports can generate significant water pollution due to their extensive use and handling of jet fuel, lubricants and other chemicals. Airports install spill control structures and related equipment (e.g., vacuum trucks, portable berms, absorbents) to prevent chemical spills, and mitigate the impacts of spills that do occur.
In cold climates, the use of deicing fluids can also cause water pollution, as most of the fluids applied to aircraft subsequently fall to the ground and can be carried via stormwater runoff to nearby streams, rivers or coastal waters.:101 Airlines use deicing fluids based on ethylene glycol or propylene glycol as the active ingredient.:4
Ethylene glycol and propylene glycol are known to exert high levels of biochemical oxygen demand (BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen needed by aquatic organisms for survival. Large quantities of dissolved oxygen (DO) in the water column are consumed when microbial populations decompose propylene glycol.:2–23
Sufficient dissolved oxygen levels in surface waters are critical for the survival of fish, macroinvertebrates, and other aquatic organisms. If oxygen concentrations drop below a minimum level, organisms emigrate, if able and possible, to areas with higher oxygen levels or eventually die. This effect can drastically reduce the amount of usable aquatic habitat. Reductions in DO levels can reduce or eliminate bottom feeder populations, create conditions that favor a change in a community’s species profile, or alter critical food-web interactions.:2–30
Ultrafine particles (UFPs) are emitted by aircraft engines during near-surface level operations including taxi, takeoff, climb, descent, and landing, as well as idling at gates and on taxiways. Other sources of UFPs include ground support equipment operating around the terminal areas. In 2014, an air quality study found the area impacted by ultrafine particles from the takeoffs and landings downwind of Los Angeles International Airport to be of much greater magnitude than previously thought. Typical UFP emissions during takeoff are on the order of 1015-1017 particles emitted per kilogram of fuel burned. Non-volatile soot particle emissions are 1014-1016 particles per kilogram fuel on a number basis and 0.1-1 gram per kilogram fuel on a mass basis, depending on the engine and fuel characteristics.
Some 167,000 piston engine aircraft—about three-quarters of private planes in the United States—release lead (Pb) into the air due to leaded aviation fuel. From 1970 to 2007, general aviation aircraft emitted about 34,000 tons of lead into the atmosphere according to the Environmental Protection Agency. Lead is recognized as a serious environmental threat by the Federal Aviation Administration if inhaled or ingested leading to adverse effects on the nervous system, red blood cells and cardiovascular and immune systems with infants and young children especially sensitive to even low levels of lead, which may contribute to behavioral and learning problems, lower IQ and autism.
Flying 12 kilometres (39,000 ft) high, passengers and crews of jet airliners are exposed to at least 10 times the cosmic ray dose that people at sea level receive. Every few years, a geomagnetic storm permits a solar particle event to penetrate down to jetliner altitudes. Aircraft flying polar routes near the geomagnetic poles are at particular risk.
Land use for infrastructure
Airport buildings, taxiways and runways take possession of a part of our ecosystem. Most of aircraft movement however is positioned in air at altitude and so away from direct interaction with sensitive nature or human detection. This opposed to roads, railways and canals being very significant in use of area and dividing of ecological structures while required for surface transportation for as many miles as the distance traveled.
Like all human activities involving combustion, most forms of aviation release carbon dioxide (CO2) and other greenhouse gases into the Earth’s atmosphere, contributing to the acceleration of global warming and (in the case of CO2) ocean acidification. These concerns are highlighted by the present volume of commercial aviation and its rate of growth. Globally, about 8.3 million people fly daily (3 billion occupied seats per year), twice the total in 1999. U.S. airlines alone burned about 16.2 billion gallons of fuel during the twelve months between October 2013 and September 2014.
In addition to the CO2 released by most aircraft in flight through the burning of fuels such as Jet-A (turbine aircraft) or Avgas (piston aircraft), the aviation industry also contributes greenhouse gas emissions from ground airport vehicles and those used by passengers and staff to access airports, as well as through emissions generated by the production of energy used in airport buildings, the manufacture of aircraft and the construction of airport infrastructure.
While the principal greenhouse gas emission from powered aircraft in flight is CO2, other emissions may include nitric oxide and nitrogen dioxide (together termed oxides of nitrogen or NOx), water vapour and particulates (soot and sulfate particles), sulfur oxides, carbon monoxide (which bonds with oxygen to become CO2 immediately upon release), incompletely burned hydrocarbons, tetraethyllead (piston aircraft only), and radicals such as hydroxyl, depending on the type of aircraft in use. Emissions weighting factor (EWFs) i.e., the factor by which aviation CO2 emissions should be multiplied to get the CO2-equivalent emissions for annual fleet average conditions is in the range 1.3–2.9.
Mechanisms and cumulative effects of aviation on climate
In 1999 the contribution of civil aircraft-in-flight to global CO2 emissions was estimated to be around 2%. However, in the case of high-altitude airliners which frequently fly near or in the stratosphere, non-CO2 altitude-sensitive effects may increase the total impact on anthropogenic (human-made) climate change significantly. A 2007 report from Environmental Change Institute / Oxford University posits a range closer to 4 percent cumulative effect. Subsonic aircraft-in-flight contribute to climate change in four ways:
Carbon dioxide (CO2)
CO2 emissions from aircraft-in-flight are the most significant and best understood element of aviation’s total contribution to climate change. The level and effects of CO2 emissions are currently believed to be broadly the same regardless of altitude (i.e. they have the same atmospheric effects as ground-based emissions). In 1992, emissions of CO2 from aircraft were estimated at around 2% of all such anthropogenic emissions, and that year the atmospheric concentration of CO2 attributable to aviation was around 1% of the total anthropogenic increase since the industrial revolution, having accumulated primarily over just the last 50 years.
Oxides of nitrogen (NOx)
At the high altitudes flown by large jet airliners around the tropopause, emissions of NOx are particularly effective in forming ozone (O3) in the upper troposphere. High altitude (8–13 km) NOx emissions result in greater concentrations of O3 than surface NOx emissions, and these in turn have a greater global warming effect. The effect of O3 concentrations are regional and local (as opposed to CO2 emissions, which are global).
NOx emissions also reduce ambient levels of methane, another greenhouse gas, resulting in a climate cooling effect. But this effect does not offset the O3 forming effect of NOx emissions. It is now believed that aircraft sulfur and water emissions in the stratosphere tend to deplete O3, partially offsetting the NOx-induced O3 increases. These effects have not been quantified. This problem does not apply to aircraft that fly lower in the troposphere, such as light aircraft or many commuter aircraft.
Water vapor (H2O), and contrails
One of the products of burning hydrocarbons in oxygen is water vapour, a greenhouse gas. Water vapour produced by aircraft engines at high altitude, under certain atmospheric conditions, condenses into droplets to form Condensation trails, or contrails. Contrails are visible line clouds that form in cold, humid atmospheres and are thought to have a global warming effect (though one less significant than either CO2 emissions or NOx induced effects). Contrails are uncommon (though by no means rare) from lower-altitude aircraft, or from propeller-driven aircraft or rotorcraft.
Cirrus clouds have been observed to develop after the persistent formation of contrails and have been found to have a global warming effect over-and-above that of contrail formation alone. There is a degree of scientific uncertainty about the contribution of contrail and cirrus cloud formation to global warming and attempts to estimate aviation’s overall climate change contribution do not tend to include its effects on cirrus cloud enhancement. However, a 2015 study found that artificial cloudiness caused by contrail “outbreaks” reduce the difference between daytime and nighttime temperatures. The former are decreased and the latter are increased, in comparison to temperatures the day before and the day after such outbreaks. On days with outbreaks the day/night temperature difference was diminished by about 6F° in the U.S. South and 5F° in the Midwest.
Least significant on a mass basis is the release of soot and sulfate particles. Soot absorbs heat and has a warming effect; sulfate particles reflect radiation and have a small cooling effect. In addition, particles can influence the formation and properties of clouds, including both line-shaped contrails and naturally-occurring cirrus clouds. The impact of “spreading contrails and cirrus clouds that evolve from them — collectively known as contrail cirrus — have a greater radiative forcing (RF) today than all aviation CO2 emissions since the first powered airplane flight”. Of the particles emitted by aircraft engines, the soot particles are thought to be most important for contrail formation since they are large enough to serve as condensation nuclei for water vapor. All aircraft powered by combustion will release some amount of soot; although, recent studies suggest that reducing the aromatic content of jet fuel decreases the amount of soot produced.
Greenhouse gas emissions per passenger kilometre
Emissions of passenger aircraft per passenger kilometre vary extensively because of differing factors such as the size and type aircraft, the altitude and the percentage of passenger or freight capacity of a particular flight, and the distance of the journey and number of stops en route. Also, the effect of a given amount of emissions on climate (radiative forcing) is greater at higher altitudes: see below. Some representative figures for CO2 emissions are provided by LIPASTO’s survey of average direct emissions (not accounting for high-altitude radiative effects) of airliners expressed as CO2 and CO2 equivalent per passenger kilometre:
Domestic, short distance, less than 463 km (288 mi): 257 g/km CO2 or 259 g/km (14.7 oz/mile) CO2e
Domestic, long distance, greater than 463 km (288 mi): 177 g/km CO2 or 178 g/km (10.1 oz/mile) CO2e
Long distance flights: 113 g/km CO2 or 114 g/km (6.5 oz/mile) CO2e
These emissions are similar to a four-seat car with one person on board; however, flying trips often cover longer distances than would be undertaken by car, so the total emissions are much higher. For perspective, per passenger a typical economy-class New York to Los Angeles round trip produces about 715 kg (1574 lb) of CO2 (but is equivalent to 1,917 kg (4,230 lb) of CO2 when the high altitude “climatic forcing” effect is taken into account). Within the categories of flights above, emissions from scheduled jet flights are substantially higher than turboprop or chartered jet flights. About 60% of aviation emissions arise from international flights, and these flights are not covered by the Kyoto Protocol and its emissions reduction targets. However, in a more recent development:
The United Nations’ aviation arm overwhelmingly ratified an agreement Thursday (06.Oct.2016) to control global warming emissions from international airline flights, the first climate-change pact to set worldwide limits on a single industry. The agreement, adopted overwhelmingly by the 191-nation International Civil Aviation Organization at a meeting in Montreal, sets airlines’ carbon emissions in the year 2020 as the upper limit of what carriers are allowed to discharge.
Figures from British Airways suggest carbon dioxide emissions of 100g per passenger kilometre for large jet airliners (a figure which does not account for the production of other pollutants or condensation trails).
Emissions by passenger class, and effects of seating configuration
In 2013 the World Bank published a study of the effect on CO2 emissions of its staff’s travel in business class or first class, versus using economy class. Among the factors considered was that these premium classes displace proportionately more economy seats for the same total aircraft space capacity, and the associated differing load factors and weight factors. This was not accounted for in prior standard carbon accounting methods. The study concluded that when considering respective average load factors (percent of occupied seats) in each of the seating classes, the carbon footprints of business class and first class are three-times and nine-times higher than economy class. A related article by the International Council on Clean Transport notes further regarding the effect of seating configurations on carbon emissions that:
The A380 is marketed as a “green giant” and one of the most environmentally advanced aircraft out there. But that spin is based on a maximum-capacity aircraft configuration, or about 850 economy passengers. In reality, a typical A380 aircraft has 525 seats. Its fuel performance is comparable to that of a B747-400 ER and even about 15% worse than a B777-300ER on a passenger-mile basis (calculated using Piano-5 on a flight from AUH to LHR, assuming an 80% passenger load factor, and in-service fleet average seat counts).
Total climate effects
In attempting to aggregate and quantify the total climate impact of aircraft emissions the Intergovernmental Panel on Climate Change (IPCC) has estimated that aviation’s total climate impact is some 2–4 times that of its direct CO2 emissions alone (excluding the potential impact of cirrus cloud enhancement). This is measured as radiative forcing. While there is uncertainty about the exact level of impact of NOx and water vapour, governments have accepted the broad scientific view that they do have an effect. Globally in 2005, aviation contributed “possibly as much as 4.9% of radiative forcing.” UK government policy statements have stressed the need for aviation to address its total climate change impacts and not simply the impact of CO2.
The IPCC has estimated that aviation is responsible for around 3.5% of anthropogenic climate change, a figure which includes both CO2 and non-CO2 induced effects. The IPCC has produced scenarios estimating what this figure could be in 2050. The central case estimate is that aviation’s contribution could grow to 5% of the total contribution by 2050 if action is not taken to tackle these emissions, though the highest scenario is 15%. Moreover, if other industries achieve significant cuts in their own greenhouse gas emissions, aviation’s share as a proportion of the remaining emissions could also rise.
Future emission levels
Even though there have been significant improvements in fuel efficiency through aircraft technology and operational management as described here, these improvements are being continually eclipsed by the increase in air traffic volume.
A December 2015 report finds that aircraft could generate 43 Gt of carbon pollution through to 2050, consuming almost 5% of the remaining global climate budget. Without regulation, global aviation emissions may triple by mid-century and could emit more than 3 Gt of carbon annually under a high-growth, business-as-usual scenario. Efforts to bring aviation emissions under an effective global accord have so far largely failed, despite there being a number of technological and operational improvements on offer.
Continual increases in travel and freight
From 1992 to 2005, passenger kilometers increased 5.2% per year, even with the disruptions of 9/11 and two significant wars. Since the onset of the current recession:
During the first three quarters of 2010, air travel markets expanded at an annualized rate approaching 10%. This is similar to the rate seen in the rapid expansion prior to the recession. November’s results mean the annualized rate of growth so far in Q4 drops back to around 6%. But this is still in line with long run rates of traffic growth seen historically. The level of international air travel is now 4% above the pre-recession peak of early 2008 and the current expansion looks to have further to run.
Air freight reached a new high point in May (2010) but, following the end of inventory restocking activity, volumes have slipped back to settle at a similar level seen just before the onset of recession. Even so, that means an expansion of air freight during 2010 of 5–6% on an annualized basis – close to historical trend. With the stimulus of inventory restocking activity removed, further growth in air freight demand will be driven by end consumer demand for goods which utilize the air transport supply chain. … The end of the inventory cycle does not mean the end of volume expansion but markets are entering a slower growth phase.
Scope for improvement
While it is true that late model jet aircraft are significantly more fuel efficient (and thus emit less CO2 in particular) than the earliest jet airliners, new airliner models in the 2000s were barely more efficient on a seat-mile basis than the latest piston-powered airliners of the late 1950s (e.g. Constellation L-1649-A and DC-7C). Claims for a high gain in efficiency for airliners over recent decades (while true in part) has been biased high in most studies, by using the early inefficient models of jet airliners as a baseline. Those aircraft were optimized for increased revenue, including increased speed and cruising altitude, and were quite fuel inefficient in comparison to their piston-powered forerunners.
Today, turboprop aircraft – probably in part because of their lower cruising speeds and altitudes (similar to the earlier piston-powered airliners) compared to jet airliners – play an obvious role in the overall fuel efficiency of major airlines that have regional carrier subsidiaries. For example, although Alaska Airlines scored at the top of a 2011–2012 fuel efficiency ranking, if its large regional carrier – turbo-prop equipped Horizon Air – were dropped from the lumped-in consideration, the airline’s ranking would be somewhat lower, as noted in the ranking study.
Aircraft manufacturers are striving for reductions in both CO2 and NOx emissions with each new generation of design of aircraft and engine. While the introduction of more modern aircraft represents an opportunity to reduce emissions per passenger kilometre flown, aircraft are major investments that endure for many decades, and replacement of the international fleet is therefore a long-term proposition which will greatly delay realizing the climate benefits of many kinds of improvements. Engines can be changed at some point, but nevertheless airframes have a long life. Moreover, rather than being linear from one year to the next the improvements to efficiency tend to diminish over time, as reflected in the histories of both piston and jet powered aircraft.
Research projects such as Boeing’s ecoDemonstrator program have sought to identify ways of improving the efficiency of commercial aircraft operations. The U.S. government has encouraged such research through grant programs, including the FAA’s Continuous Lower Energy, Emissions and Noise (CLEEN) program, and NASA’s Environmentally Responsible Aviation (ERA) Project.
Adding an electric drive to the airplane’s nose wheel may improve fuel efficiency during ground handling. This addition would allow taxiing without the use of the main engines.
Another proposed change is the integrating of an Electromagnetic Aircraft Launch System to the airstrips of airports. Some companies such as Airbus are currently researching this possibility. The adding of EMALS would allow the civilian aircraft to use considerably less fuel (as a lot of fuel is used during take off, in comparison to cruising, when calculated per km flown). The idea is to have the aircraft take off at regular aircraft speed, and only use the catapult for take-off, not for landing.
Other opportunities arise from the optimization of airline timetables, route networks and flight frequencies to increase load factors (minimize the number of empty seats flown), together with the optimization of airspace. However, these are each one-time gains, and as these opportunities are successively fulfilled, diminishing returns can be expected from the remaining opportunities.
Another possible reduction of the climate-change impact is the limitation of cruise altitude of aircraft. This would lead to a significant reduction in high-altitude contrails for a marginal trade-off of increased flight time and an estimated 4% increase in CO2 emissions. Drawbacks of this solution include very limited airspace capacity to do this, especially in Europe and North America and increased fuel burn because jet aircraft are less efficient at lower cruise altitudes.
While they are not suitable for long-haul or transoceanic flights, turboprop aircraft used for commuter flights bring two significant benefits: they often burn considerably less fuel per passenger mile, and they typically fly at lower altitudes, well inside the tropopause, where there are no concerns about ozone or contrail production.
Some scientists and companies such as GE Aviation and Virgin Fuels are researching biofuel technology for use in jet aircraft. Some aircraft engines, like the Wilksch WAM120 can (being a 2-stroke Diesel engine) run on straight vegetable oil. Also, a number of Lycoming engines run well on ethanol.
In addition, there are also several tests done combining regular petrofuels with a biofuel. For example, as part of this test Virgin Atlantic Airways flew a Boeing 747 from London Heathrow Airport to Amsterdam Schiphol Airport on 24 February 2008, with one engine burning a combination of coconut oil and babassu oil. Greenpeace’s chief scientist Doug Parr said that the flight was “high-altitude greenwash” and that producing organic oils to make biofuel could lead to deforestation and a large increase in greenhouse gas emissions. Also, the majority of the world’s aircraft are not large jetliners but smaller piston aircraft, and with major modifications many are capable of using ethanol as a fuel. Another consideration is the vast amount of land that would be necessary to provide the biomass feedstock needed to support the needs of aviation, both civil and military.
Finally, liquified natural gas is another fuel that is used in some airplanes. Besides the lower GHG emissions (depending from where the natural gas was obtained from), another major benefit to airplane operators is the price, which is far lower than the price for jet fuel.
Reducing air travel
Personal choices and social pressure
The German video short The Bill explores how travel and its impacts are commonly viewed in everyday developed-world life, and the social pressures that are at play. British writer George Marshall has investigated common rationalizations that act as barriers to making personal choices to travel less, or to justify recent trips. In an informal research project, “one you are welcome to join”, he says, he deliberately steered conversations with people who are attuned to climate change problems to questions about recent long-distance flights and why the travel was justified. Reflecting on actions contrary to their beliefs, he noted, “(i)ntriguing as their dissonance may be, what is especially revealing is that every one of these people has a career that is predicated on the assumption that information is sufficient to generate change – an assumption that a moment’s introspection would show them was deeply flawed.”
Business and professional choices
With most international conferences having hundreds if not thousands of participants, and the bulk of these usually traveling by plane, conference travel is an area where significant reductions in air-travel-related GHG emissions could be made….This does not mean non-attendance.
For example, by 2003 Access Grid technology has already been successfully used to host several international conferences, and technology has likely progressed substantially since then. The Tyndall Centre for Climate Change Research has been systematically studying means to change common institutional and professional practices that have led to large carbon footprints of travel by research scientists, and issued a report.
Potential for governmental constraints on demand
One means for reducing the environmental impact of aviation is to constrain demand for air travel, through increased fares in place of expanded airport capacity. Several studies have explored this:
The UK study Predict and Decide – Aviation, climate change and UK policy, notes that a 10% increase in fares generates a 5% to 15% reduction in demand, and recommends that the British government should manage demand rather than provide for it. This would be accomplished via a strategy that presumes “… against the expansion of UK airport capacity” and constrains demand by the use of economic instruments to price air travel less attractively.
A study published by the campaign group Aviation Environment Federation (AEF) concludes that by levying £9 billion of additional taxes, the annual rate of growth in demand in the UK for air travel would be reduced to 2%.
The ninth report of the House of Commons Environmental Audit Select Committee, published in July 2006, recommends that the British government rethink its airport expansion policy and considers ways, particularly via increased taxation, in which future demand can be managed in line with industry performance in achieving fuel efficiencies, so that emissions are not allowed to increase in absolute terms.
International regulation of air travel GHG emissions
Kyoto Protocol 2005
Greenhouse gas emissions from fuel consumption in international aviation, in contrast to those from domestic aviation and from energy use by airports, are excluded from the scope of the first period (2008–2012) of the Kyoto Protocol, as are the non-CO2 climate effects. Instead, governments agreed to work through the International Civil Aviation Organization (ICAO) to limit or reduce emissions and to find a solution to the allocation of emissions from international aviation in time for the second period of the Kyoto Protocol starting from 2009; however, the Copenhagen climate conference failed to reach an agreement.
Recent research points to this failure as a substantial obstacle to global policy including a CO2 emissions reduction pathway that would avoid dangerous climate change by keeping the increase in the average global temperature below a 2 °C rise.
Approaches toward emissions trading
As part of that process the ICAO has endorsed the adoption of an open emissions trading system to meet CO2 emissions reduction objectives. Guidelines for the adoption and implementation of a global scheme are currently being developed, and will be presented to the ICAO Assembly in 2007, although the prospects of a comprehensive inter-governmental agreement on the adoption of such a scheme are uncertain.
Effects of climate change on aviation
A report published in the science journal Nature Climate Change forecasts that increasing CO2 levels will result in a significant increase in in-flight turbulence experienced by transatlantic airline flights by the middle of the 21st century. The lead author of the study, Paul Williams, a researcher at the National Center for Atmospheric Science, at the University of Reading stated, “air turbulence does more than just interrupt the service of in-flight drinks. It injures hundreds of passengers and aircrew every year – sometimes fatally. It also causes delays and damage to planes.”
Source from Wikipedia