Aviation and the environment

Like other emissions resulting from fossil fuel combustion, aircraft engines produce gases, noise, and particulates, raising environmental concerns over their global effects and their effects on local air quality.

Jet airliners contribute to climate change by emitting carbon dioxide (), the best understood greenhouse gas, and, with less scientific understanding, nitrogen oxides, contrails and particulates.
Their radiative forcing is estimated at 1.3–1.4 that of alone, excluding induced cirrus cloud with a very low level of scientific understanding.
In 2018, global commercial operations generated 2.4% of all emissions.

Jet airliners have become 70% more fuel efficient between 1967 and 2007, and emissions per revenue ton-kilometer (RTK) in 2018 were 47% of those in 1990. In 2018, emissions averaged 88 grams of per revenue passenger per km.

While the aviation industry is more fuel efficient, overall emissions have risen as the volume of air travel has increased. By 2020, aviation emissions were 70% higher than in 2005 and they could grow by 300% by 2050.

Aircraft noise pollution disrupts sleep, children's education and could increase cardiovascular risk.

Airports can generate water pollution due to their extensive handling of jet fuel and deicing chemicals if not contained, contaminating nearby water bodies.

Aviation activities emit ozone and ultrafine particles, both of which are health hazards. Piston engines used in general aviation burn Avgas, releasing toxic lead.

Aviation's environmental footprint can be reduced by better fuel economy in aircraft or Air Traffic Control and flight routes can be optimized to lower non- effects on climate from , particulates or contrails.
Aviation biofuel, emissions trading and carbon offsetting, part of the ICAO's CORSIA, can lower emissions. Aviation usage can be lowered by short-haul flight bans, train connections, personal choices and aviation taxation and subsidies. Fuel-powered aircraft may be replaced by hybrid electric aircraft and electric aircraft or by hydrogen-powered aircraft.

Climate change

Factors

Airplanes emit gases (carbon dioxide, water vapor, nitrogen oxides or carbon monoxide − bonding with oxygen to become upon release) and atmospheric particulates (incompletely burned hydrocarbons, sulfur oxides, black carbon), interacting among themselves and with the atmosphere.
While the main greenhouse gas emission from powered aircraft is , jet airliners contribute to climate change in four ways as they fly in the tropopause:

; Carbon dioxide ()
: emissions are the most significant and best understood contribution to climate change. The effects of emissions are similar regardless of altitude. Airport ground vehicles, those used by passengers and staff to access airports, emissions generated by airport construction and aircraft manufacturing also contribute to the greenhouse gas emissions from the aviation industry.

; Nitrogen oxides (, nitric oxide and nitrogen dioxide)
: In the tropopause, emissions of favor ozone () formation in the upper troposphere. At altitudes from , emissions result in greater concentrations of than surface emissions and these in turn have a greater global warming effect. The effect of surface concentrations are regional and local, but it becomes well mixed globally at mid and upper tropospheric levels. emissions also reduce ambient levels of methane, another greenhouse gas, resulting in a climate cooling effect, though not offsetting the forming effect. Aircraft sulfur and water emissions in the stratosphere tend to deplete , partially offsetting the -induced increases, although these effects have not been quantified. Light aircraft and small commuter aircraft fly lower in the troposphere, not in the tropopause.

; Contrails and cirrus clouds
: Fuel burning produces water vapor, which condenses at high altitude, under cold and humid conditions, into visible line clouds: condensation trails (contrails). They are thought to have a global warming effect, though less significant than emissions. Contrails are uncommon from lower-altitude aircraft. Cirrus clouds can develop after the formation of persistent contrails and can have an additional global warming effect. Their global warming contribution is uncertain and estimating aviation's overall contribution often excludes cirrus cloud enhancement.

; Particulates
: Compared with other emissions, sulfate and soot particles have a smaller direct effect: sulfate particles have a cooling effect and reflect radiation, while soot has a warming effect and absorbs heat, while the clouds' properties and formation are influenced by particles. Contrails and cirrus clouds evolving from particles may have a greater radiative forcing effect than emissions. As soot particles are large enough to serve as condensation nuclei, they are thought to cause the most contrail formation. Soot production may be decreased by reducing the aromatic compound of jet fuel.

In 1999, the IPCC estimated aviation's radiative forcing in 1992 to be 2.7 (2 to 4) times that of alone − excluding the potential effect of cirrus cloud enhancement.
This was updated for 2000, with aviation's radiative forcing estimated at 47.8 mW/m², times the effect of emissions alone, 25.3 mW/m².

In 2005, research by David S. Lee, et al, published in the scientific journal ''Atmospheric Environment'' estimated the cumulative radiative forcing effect of aviation at 55 mW/m², which is twice the 28 mW/m² radiative forcing effect of its emissions alone, excluding induced cirrus cloud, with a very low level of scientific understanding.
In 2012, research from Chalmers university estimated this weighting factor at 1.3–1.4 if aviation induced cirrus is not included, 1.7–1.8 if they are included (within a range of 1.3–2.9).

Uncertainties remain on the NO_x–O₃–CH₄ interactions, aviation-produced contrails formation, the effects of soot aerosols on cirrus clouds and measuring non-CO₂ radiative forcing.

In 2018, represented 34.3 mW/m² of aviation's effective radiative forcing (ERF, on the surface), with a high confidence level (± 6 mW/m²), 17.5 mW/m² with a low confidence level (± 14) and contrail cirrus 57.4 mW/m², also with a low confidence level (± 40).
All factors combined represented 43.5 mW/m² ( that of alone) excluding contrail cirrus and 101 mW/m² (±45) including them, 3.5% of the anthropogenic ERF of 2290 mW/m² (± 1100).

Volume

By 2018, airline traffic reached 4.3 billion passengers with 37.8 million departures, an average of passengers per flight and 8.26 trillion RPKs, an average journey of , according to ICAO.
The traffic was experiencing continuous growth, doubling every 15 years, despite external shocks − a 4.3% average yearly growth and Airbus forecasts expect the growth to continue.
While the aviation industry is more fuel efficient, halving the amount of fuel burned per flight compared to 1990 through technological advancement and operations improvements, overall emissions have risen as the volume of air travel has increased.
Between 1960 and 2018, RPKs increased from 109 to 8,269 billion.

In 1992, aircraft emissions represented 2% of all man-made emissions, having accumulated a little more than 1% of the total man-made increase over 50 years.
By 2015, aviation accounted for 2.5% of global emissions.
In 2018, global commercial operations emitted 918 million tonnes (Mt) of , 2.4% of all emissions: 747 Mt for passenger transport and 171 Mt for freight operations.
Between 1960 and 2018, emissions increased 6.8 times from to 1,034 million tonnes per year.
Emissions from flights rose by 32% between 2013 and 2018.

Between 1990 and 2006, greenhouse gas emissions from aviation increased by 87% in the European Union.

In 2010, about 60% of aviation emissions came from international flights, which are outside the emission reduction targets of the Kyoto Protocol. International flights are not covered by the Paris Agreement, either, to avoid a patchwork of individual country regulations. That agreement was adopted by the International Civil Aviation Organization, however, capping airlines carbon emissions to the year 2020 level, while allowing airlines to buy carbon credits from other industries and projects.

In 1992, aircraft radiative forcing was estimated by the IPCC at 3.5% of the total man-made radiative forcing.

Per passenger

As it accounts for a large share of their costs, 28% by 2007, airlines have a strong incentive to lower their fuel consumption, reducing their environmental footprint.
Jet airliners have become 70% more fuel efficient between 1967 and 2007.
Jetliner fuel efficiency improves continuously, 40% of the improvement come from engines and 30% from airframes.
Efficiency gains were larger early in the jet age than later, with a 55–67% gain from 1960 to 1980 and a 20–26% gain from 1980 to 2000.

The average fuel burn of new aircraft fell 45% from 1968 to 2014, a compounded annual reduction of 1.3% with variable reduction rate.
By 2018, emissions per revenue ton-kilometer (RTK) were more than halved compared to 1990, at 47%.
The aviation energy intensity went from 21.2 to 12.3 MJ/RTK between 2000 and 2019, a % reduction.

In 2018, emissions totalled 747 million tonnes for passenger transport, for 8.5 trillion revenue passenger kilometres (RPK), giving an average of 88 gram per RPK.
The UK's Department for calculate a long-haul flight release 102g of per passenger kilometre, and 254g of equivalent, including non-CO₂ greenhouse gas emissions, water vapor etc; for a domestic flight in Britain.

The ICAO targets a 2% efficiency improvement per year between 2013 and 2050, while the IATA targets 1.5% for 2009–2020 and to cut net emissions in half by 2050 relative to 2005.

Evolution

In 1999, the IPCC estimated aviation's radiative forcing may represent 190 mW/m² or 5% of the total man-made radiative forcing in 2050, with the uncertainty ranging from 100 to 500 mW/m². If other industries achieve significant reductions in greenhouse gas emissions over time, aviation's share, as a proportion of the remaining emissions, could rise.

Alice Bows-Larkin estimated that the annual global emissions budget would be entirely consumed by aviation emissions to keep the climate change temperature increase below 2 °C by mid-century. Given that growth projections indicate that aviation will generate 15% of global emissions, even with the most advanced technology forecast, she estimated that to hold the risks of dangerous climate change to under 50% by 2050 would exceed the entire carbon budget in conventional scenarios.

In 2013, the National Center for Atmospheric Science at the University of Reading forecast that increasing levels will result in a significant increase in in-flight turbulence experienced by transatlantic airline flights by the middle of the 21st century.

Aviation emissions grow despite efficiency innovations to aircraft, powerplants and flight operations.
Air travel continue to grow.

In 2015, the Center for Biological Diversity estimated that aircraft could generate of carbon dioxide emissions through 2050, consuming almost 5% of the remaining global carbon budget. Without regulation, global aviation emissions may triple by mid-century and could emit more than of carbon annually under a high-growth, business-as-usual scenario.
Many countries have pledged emissions reductions for the Paris Agreement, but the sum of these efforts and pledges remains insufficient and not addressing airplane pollution would be a failure despite technological and operational advancements.
*

The International Energy Agency projects aviation share of global emissions may grow from 2.5% in 2019 to 3.5% by 2030.

By 2020, global international aviation emissions were around 70% higher than in 2005 and the ICAO forecasts they could grow by over further 300% by 2050 in the absence of additional measures.

By 2050, aviation's negative effects on climate could be decreased by a 2% increase in fuel efficiency and a decrease in emissions, due to advanced aircraft technologies, operational procedures and renewable alternative fuels decreasing radiative forcing due to sulfate aerosol and black carbon.

Noise

Air traffic causes aircraft noise, which disrupts sleep, adversely affects children's school performance and could increase cardiovascular risk for airport neighbours. Sleep disruption can be reduced by banning or restricting flying at night, but disturbance progressively decreases and legislation differs across countries.

The ICAO Chapter 14 noise standard applies for aeroplanes submitted for certification after 31 December 2017, and after 31 December 2020 for aircraft below , 7 EPNdB (cumulative) quieter than Chapter4. The FAA Stage 5 noise standards are equivalent. Higher bypass ratio engines produce less noise. The PW1000G is presented as 75% quieter than previous engines. Serrated edges or 'chevrons' on the back of the nacelle reduce noise.

A Continuous Descent Approach (CDA) is quieter as less noise is produced while the engines are near idle power. CDA can reduce noise on the ground by ~1–5 dB per flight.

Water pollution

Airports can generate significant water pollution due to their extensive use and handling of jet fuel, lubricants and other chemicals. Chemical spills can be mitigated or prevented by spill containment structures and clean-up equipment such as vacuum trucks, portable berms and absorbents.

Deicing fluids used in cold weather can pollute water, as most of them fall to the ground and surface runoff can carry them to nearby streams, rivers or coastal waters. Deicing fluids are based on ethylene glycol or propylene glycol. Airports use pavement deicers on paved surfaces including runways and taxiways, which may contain potassium acetate, glycol compounds, sodium acetate, urea or other chemicals.

During degradation in surface waters, ethylene and propylene glycol exert high levels of biochemical oxygen demand, consuming oxygen needed by aquatic life. Microbial populations decomposing propylene glycol consume large quantities of dissolved oxygen (DO) in the water column.
Fish, macroinvertebrates and other aquatic organisms need sufficient dissolved oxygen levels in surface waters. Low oxygen concentrations reduce usable aquatic habitat because organisms die if they cannot move to areas with sufficient oxygen levels. Bottom feeder populations can be reduced or eliminated by low DO levels, changing a community's species profile or altering critical food-web interactions.

Air pollution

Aviation is the main human source of ozone, a respiratory health hazard, causing an estimated 6,800 premature deaths per year.

Aircraft engines emit ultrafine particles (UFPs) in and near airports, as does ground support equipment. During takeoff, 3 to 50 × 10¹⁵ particles were measured per kg of fuel burned, while significant differences are observed depending on the engine. Other estimates include 4 to 200 × 10¹⁵ particles for 0.1–0.7 gram, or 14 to 710 × 10¹⁵ particles, or 0.1–10 × 10¹⁵ black carbon particles for 0.046–0.941 g.

In the United States, 167,000 piston aircraft engines, representing three-quarters of private airplanes, burn Avgas, releasing lead into the air. The Environmental Protection Agency estimated this released 34,000 tons of lead into the atmosphere between 1970 and 2007. The Federal Aviation Administration recognizes inhaled or ingested lead leads to adverse effects on the nervous system, red blood cells, and cardiovascular and immune systems. Lead exposure in infants and young children may contribute to behavioral and learning problems and lower IQ.

Mitigation

In February 2021, Europe's aviation sector unveiled its ''Destination 2050'' sustainability initiative towards zero emissions by 2050:
* aircraft technology improvements for 37% emission reductions;
* sustainable aviation fuels (SAFs) for 34%;
* economic measures for 8%;
* air traffic management (ATM) and operations improvements for 6%;
while air traffic should grow by 1.4% per year between 2018 and 2050.
The initiative is led by ACI Europe, ASD Europe, A4E, CANSO and ERA.

Reducing air travel

Aviation's environmental footprint would be mitigated by reducing air travel, route optimization, emission caps, short-distance restrictions, increased taxation, and decreased subsidies.

;Route optimization
An improved Air Traffic Management system, with more direct routes than suboptimal air corridors and optimized cruising altitudes, would allow airlines to reduce their emissions by up to 18%. In the European Union, a Single European Sky has been proposed since 1999 to avoid overlapping airspace restrictions between EU countries and to reduce emissions. By 2007, 12 million tons of emissions per year were caused by the lack of a Single European Sky. As of September 2020, the Single European Sky has still not been completely achieved, costing 6 billion euros in delays and causing 11.6 million tonnes of excess emissions.

;Emissions trading
ICAO has endorsed emissions trading to reduce aviation emission, guidelines were to be presented to the 2007 ICAO Assembly. Within the European Union, the European Commission has included aviation in the European Union Emissions Trading Scheme operated since 2012, capping airline emissions, providing incentives to lower emissions through more efficient technology or to buy carbon credits from other companies. The Centre for Aviation, Transport and Environment at Manchester Metropolitan University estimates the only way to lower emissions is to put a price on carbon and to use market-based measures like the EU ETS.

;Short-haul flight ban

; Train connections
Train connections reduce feeder flights. By March 2019, Lufthansa offered connections through Frankfurt with the Deutsche Bahn (AIRail Service) and Air France offered TGV connections through Paris. In October 2018, Austrian Airlines and the Austrian Federal Railways introduced train connections through Vienna Airport. In March 2019, the Dutch cabinet was working on an Amsterdam connection via NS International or Thalys. By July 2020, Lufthansa and Deutsche Bahn expanded their offer through Frankfurt Airport to 17 major cities.

; International conferences
Most international professional or academic conference attendants travel by plane, conference travel is often regarded as an employee benefit as costs are supported by employers. By 2003, Access Grid technology had hosted several international conferences. The Tyndall Centre has reported means to change common institutional and professional practices.

; Flight shame
In Sweden the concept of " flight shame" or "flygskam" has been cited as a cause of falling air travel. Swedish rail company SJ AB reports that twice as many Swedish people chose to travel by train instead of by air in summer 2019 compared with the previous year. Swedish airports operator Swedavia

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