The environmental impact of aviation occurs because aircraft engines emit heat, noise, particulates and gases which contribute to climate change and global dimming. Among others 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 recent 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.
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 Edit
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) Edit
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.
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.
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 extremely 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 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, they can influence the formation and properties of clouds. All aircraft powered by combustion will release some amount of soot.
Greenhouse gas emissions per passenger kilometre Edit
Averaged emissions Edit
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.
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 Edit
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.
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).
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 Edit
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 Edit
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.
In a 2008 presentation and paper  Professor Kevin Anderson of the Tyndall Centre for Climate Change Research showed how continued aviation growth in the UK threatens the ability of that nation to meet CO2 emission reduction goals necessary to contain the century-end temperature increase to even 4 or 6C°. (See also: the 4 Degrees and Beyond International Climate Conference (2009) and its proceedings.) His charts show the projected domestic aviation carbon emission increase for the UK as growing from 11 MT in 2006 to 17 MT in 2012, at the UK’s historic annual emission growth rate of 7%. Beyond 2012 if the growth rate were reduced to 3% yearly, carbon emissions in 2030 would be 28 MT, which is 70% of the UK’s entire carbon emissions budget that year for all sectors of society. This work also suggests the foreseeable future which confronts many other nations that have high dependency on aviation. “Hypermobile Travelers”, an academic study by Stefan Gössling et al. (2009) in the book “Climate Change and Aviation”, also points to the dilemma caused by the increasing hypermobility of air travelers both in particular nations and globally.
Scope for improvement Edit
Aircraft efficiency Edit
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 first decade of the 21st Century 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.
A 2014 life-cycle assessment of the cradle-to-grave reduction in CO2 by a carbon-fiber-reinforced polymer (CFRP) airliner such as a Boeing 787 – including its manufacture, operations and eventual disposal – has shown that by 2050 such aircraft could reduce the airline industry’s CO2 emissions by 14-15%, compared use of conventional airliners. The benefit of CFRP technology is not higher than that amount of reduction, despite the lighter weight and substantially lower fuel consumption of such aircraft, “because of the limited fleet penetration by 2050 and the increased demand for air travel due to lower operating costs.”
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 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 spend during take off, and in comparison, less during flight – 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 optimisation of airline timetables, route networks and flight frequencies to increase load factors (minimise the number of empty seats flown), together with the optimisation 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.
In December 2008, an Air New Zealand jet completed the world’s first commercial aviation test flight partially using jatropha-based fuel. Jatropha, used for biodiesel, can thrive on marginal agricultural land where many trees and crops won’t grow, or would produce only slow growth yields. Air New Zealand set several general sustainability criteria for its Jatropha, saying that such biofuels must not compete with food resources, that they must be as good as traditional jet fuels, and that they should be cost competitive with existing fuels.
In January 2009, Continental Airlines used a sustainable biofuel to power a commercial aircraft for the first time in North America. This marks the first sustainable biofuel demonstration flight by a commercial carrier using a twin-engined aircraft, a Boeing 737-800, powered by CFM International CFM56-7B engines. The biofuel blend included components derived from algae and jatropha plants.
One fuel biofuel alternative to avgas that is under development is Swift Fuel. Swift fuel was approved as a test fuel by ASTM International in December 2009, allowing the company to continue their research and to pursue certification testing. Mary Rusek, president and co-owner of Swift Enterprises predicted at that time that “100SF will be comparably priced, environmentally friendlier and more fuel-efficient than other general aviation fuels on the market”.
As of June 2011, revised international aviation fuel standards officially allow commercial airlines to blend conventional jet fuel with up to 50 percent biofuels. The renewable fuels “can be blended with conventional commercial and military jet fuel through requirements in the newly issued edition of ASTM D7566, Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons”.
In December 2011, the FAA announced it is awarding $7.7 million to eight companies to advance the development of drop-in commercial aviation biofuels, with a special focus on ATJ (alcohol to jet) fuel. As part of its CAAFI (Commercial Aviation Alternative Fuel Initiative) and CLEEN (Continuous Lower Emissions, Energy and Noise) programs, the FAA plans to assist in the development of a sustainable fuel (from alcohols, sugars, biomass, and organic matter such as pyrolysis oils) that can be “dropped in” to aircraft without changing current infrastructure. The grant will also be used to research how the fuels affect engine durability and quality control standards.
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.
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 Edit
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. (Reay 2004)
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. (Le Quéré et al. 2015).
Ending incentives to fly—frequent flyer programs Edit
Over 130 airlines have “frequent flyer programs” based at least in part on miles, kilometers, points or segments for flights taken. Globally, such programs included about 163 million people as reported in 2006. These programs benefit airlines by habituating people to air travel and, through the mechanics of partnerships with credit card companies and other businesses, in which high profit margin revenue streams can amount to selling free seats for a high price.