Environmental effects of aviation

Between 1940 and 2018, aviation CO₂ emissions grew from 0.7% to 2.65% of all CO₂ emissions.^[1]

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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.^[2] Jet airliners contribute to climate change by emitting carbon dioxide (CO₂), 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 CO₂ alone, excluding induced cirrus cloud with a very low level of scientific understanding. In 2018, global commercial operations generated 2.4% of all CO₂ emissions.

Jet airliners have become 70% more fuel efficient between 1967 and 2007, and CO₂ emissions per Revenue Ton-kilometer (RTK) in 2018 were 47% of those in 1990. In 2018, CO₂ emissions averaged 88 grams of CO₂ 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 optimised to lower non-CO₂ effects on climate from NO
_x, particulates or contrails. Aviation biofuel, emissions trading and carbon offsetting, part of the ICAO's CORSIA, can lower CO₂ 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[]

Radiative forcings from aviation emissions estimated in 2020^[1]

Airplanes emit gases (carbon dioxide, water vapour, nitrogen oxides or carbon monoxide − bonding with oxygen to become CO₂ upon release) and atmospheric particulates (incompletely burned hydrocarbons, sulfur oxides, black carbon), interacting among themselves and with the atmosphere.^[3] While the main greenhouse gas emission from powered aircraft is CO₂, jet airliners contribute to climate change in four ways as they fly in the tropopause:^[4]

Carbon dioxide (CO₂)

CO₂ emissions are the most significant and best understood contribution to climate change.^[5] The effects of CO₂ 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.^[6]

Nitrogen oxides (NO
_x, nitric oxide and nitrogen dioxide)

In the tropopause, emissions of NO
_x favor ozone (O
₃) formation in the upper troposphere. At altitudes from 8 to 13 km (26,000 to 43,000 ft), NO
_x emissions result in greater concentrations of O
₃ than surface NO
_x emissions and these in turn have a greater global warming effect. The effect of O
₃ surface concentrations are regional and local, but it becomes well mixed globally at mid and upper tropospheric levels.^[7] NO
_x emissions also reduce ambient levels of methane, another greenhouse gas, resulting in a climate cooling effect, though not offsetting the O
₃ forming effect. Aircraft sulfur and water emissions in the stratosphere tend to deplete O
₃, partially offsetting the NO
_x-induced O
₃ increases, although these effects have not been quantified.^[8] Light aircraft and small commuter aircraft fly lower in the troposphere, not in the tropopause.

Contrails and Cirrus clouds

Contrails and cirrus clouds

Fuel burning produces water vapour, 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 CO₂ emissions.^[9] 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.^[10] Their global warming contribution is uncertain and estimating aviation's overall contribution often excludes cirrus cloud enhancement.^[5]

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.^[11] Contrails and cirrus clouds evolving from particles may have a greater radiative forcing effect than CO₂ emissions.^[12] 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.^[13][14][15]

In 1999, the IPCC estimated aviation's radiative forcing in 1992 to be 2.7 (2 to 4) times that of CO₂ alone − excluding the potential effect of cirrus cloud enhancement.^[4] This was updated for 2000, with aviation's radiative forcing estimated at 47.8 mW/m², 1.9 times the effect of CO₂ emissions alone, 25.3 mW/m².^[5]

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 CO₂ emissions alone, excluding induced cirrus cloud, with a very low level of scientific understanding.^[16] 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).^[17]

Uncertainties remain on the NOx–O3–CH4 interactions, aviation-produced contrails formation, the effects of soot aerosols on cirrus clouds and measuring non-CO2 radiative forcing.^[3]

In 2018, CO₂ represented 34.3 mW/m² of aviation's effective radiative forcing (ERF, on the surface), with a high confidence level (± 6 mW/m²), NO_x 17.5 mW/m² with a low confidence level (± 14) and contrail cirrus 57.4 mW/m², also with a low confidence level (± 40).^[1] All factors combined represented 43.5 mW/m² (1.27 that of CO₂ alone) excluding contrail cirrus and 101 mW/m² (±45) including them, 3.5% of the anthropogenic ERF of 2290 mW/m² (± 1100).^[1]

Volume[]

By 2018, airline traffic reached 4.3 billion passengers with 37.8 million departures, an average of 114 passengers per flight and 8.26 trillion RPKs, an average journey of 1,920 km (1,040 nmi), according to ICAO.^[18] 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.^[19] 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.^[20] At 6% per year, current flight demand increases outweigh the airline industry's fuel efficiency improvements of 1% per year.^{[21][dubious – discuss]} Between 1960 and 2018, RPKs increased from 109 to 8,269 billion.^[1]

In 1992, aircraft emissions represented 2% of all man-made CO₂ emissions, having accumulated a little more than 1% of the total man-made CO₂ increase over 50 years.^[8] By 2015, aviation accounted for 2.5% of global CO₂ emissions.^[22] In 2018, global commercial operations emitted 918 million tonnes (Mt) of CO₂, 2.4% of all CO₂ emissions: 747 Mt for passenger transport and 171 Mt for freight operations.^[23] Between 1960 and 2018, CO₂ emissions increased 6.8 times from 152 to 1,034 million tonnes per year.^[1] Emissions from flights rose by 32% between 2013 and 2018.^[21]

Between 1990 and 2006, greenhouse gas emissions from aviation increased by 87% in the European Union.^[24] In 2010, about 60% of aviation emissions came from international flights, which are outside the emission reduction targets of the Kyoto Protocol.^[25] 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.^[26]

In 1992, aircraft radiative forcing was estimated by the IPCC at 3.5% of the total man-made radiative forcing.^[27]

Per passenger[]

Between 1950 and 2018, efficiency per passenger grew from 0.4 to 8.2 RPK per kg of CO₂.^[1]

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.^[28] Jet airliners have become 70% more fuel efficient between 1967 and 2007.^[28] Jetliner fuel efficiency improves continuously, 40% of the improvement come from engines and 30% from airframes.^[29] 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.^[30]

The average fuel burn of new aircraft fell 45% from 1968 to 2014, a compounded annual reduction of 1.3% with variable reduction rate.^[31] By 2018, CO₂ emissions per Revenue Ton-kilometer (RTK) were more than halved compared to 1990, at 47%.^[32] The aviation energy intensity went from 21.2 to 12.3 MJ/RTK between 2000 and 2019, a 42% reduction.^[33]

In 2018, CO₂ emissions totalled 747 million tonnes for passenger transport, for 8.5 trillion revenue passenger kilometres (RPK), giving an average of 88 gram CO₂ per RPK.^[23] The UK’s Department for BEIS calculate a long-haul flight release 102g of CO₂ per passenger kilometre, and 254g of CO₂ equivalent, including non-CO2 greenhouse gas emissions, water vapour etc; for a domestic flight in Britain.^[21]

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 CO₂ emissions in half by 2050 relative to 2005.^[33]

Evolution[]

By 2020, global international aviation emissions were around 70% higher than in 2005 and they could grow by over further 300% by 2050 in the absence of additional measures.^[34] The ICAO aims to lower carbon emissions through more fuel-efficient aircraft; sustainable aviation fuel; Improved air traffic management; and CORSIA

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.^[35] 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 CO₂ emissions budget would be entirely consumed by aviation emissions to keep the climate change temperature increase below 2 °C by mid-century.^[36] Given that growth projections indicate that aviation will generate 15% of global CO₂ 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.^[37]

In 2013, the National Center for Atmospheric Science at the University of Reading forecast 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.^[38]

Aviation CO₂ emissions grow despite efficiency innovations to aircraft, powerplants and flight operations.^[39][40] Air travel continue to grow.^[41][42]

In 2015, the Center for Biological Diversity estimated that aircraft could generate 43 Gt 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 3 Gt 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.^[43]

The International Energy Agency projects aviation share of global CO₂ emissions may grow from 2.5% in 2019 to 3.5% by 2030.^[44]

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.^[34]

By 2050, aviation's negative effects on climate could be decreased by a 2% increase in fuel efficiency and a decrease in NO_x emissions, due to advanced aircraft technologies, operational procedures and renewable alternative fuels decreasing radiative forcing due to sulfate aerosol and black carbon.^[3]

Noise[]

Noise map of Berlin Tegel Airport

Air traffic causes aircraft noise, which disrupts sleep, adversely affects children's school performance and could increase cardiovascular risk for airport neighbours.^[45] Sleep disruption can be reduced by banning or restricting flying at night, but disturbance progressively decreases and legislation differs across countries.^[45]

The ICAO Chapter 14 noise standard applies for aeroplanes submitted for certification after 31 December 2017, and after 31 December 2020 for aircraft below 55 t (121,000 lb), 7 EPNdB (cumulative) quieter than Chapter4.^[46] The FAA Stage 5 noise standards are equivalent.^[47] Higher bypass ratio engines produce less noise. The PW1000G is presented as 75% quieter than previous engines.^[48] Serrated edges or 'chevrons' on the back of the nacelle reduce noise.^[49]

A Continuous Descent Approach (CDA) is quieter as less noise is produced while the engines are near idle power.^[50] CDA can reduce noise on the ground by ~1-5 dB per flight.^[51]

Water pollution[]

Excess aircraft deicing fluid may contaminate nearby water bodies

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.^[52]

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.^[53]: 101 Deicing fluids are based on ethylene glycol or propylene glycol.^[53]: 4 Airports use pavement deicers on paved surfaces including runways and taxiways, which may contain potassium acetate, glycol compounds, sodium acetate, urea or other chemicals.^[53]: 42

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.^{[54]: 2–23} 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.^{[54]: 2–30}

Air pollution[]

Aviation is the main human source of ozone, a respiratory health hazard, causing an estimated 6,800 premature deaths per year.^[55]

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,^[56] while significant differences are observed depending on the engine.^[57] Other estimates include 4 to 200 × 10¹⁵ particles for 0.1–0.7 gram,^[58] or 14 to 710 × 10¹⁵ particles,^[59] or 0.1-10 × 10¹⁵ black carbon particles for 0.046–0.941  g.^[60]

In the United States, 167,000 piston aircraft engines, representing three-quarters of private airplanes, burn Avgas, releasing lead into the air.^[61] The Environmental Protection Agency estimated this released 34,000 tons of lead into the atmosphere between 1970 and 2007.^[62] 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, lower IQ,^[63] and autism.^[64]

Mitigation[]

In February 2021, Europe's aviation sector unveiled its Destination 2050 sustainability initiative towards zero CO₂ 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.^[65] The initiative is led by ACI Europe, , A4E, CANSO and ERA.^[65]

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.

Improved Air Traffic Control would allow more direct routes

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%.^[28] 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.^[66] By 2007, 12 million tons of CO₂ emissions per year were caused by the lack of a Single European Sky.^[28] 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 CO₂ emissions.^[67]

CO₂ price in the European Union Emission Trading Scheme

Emissions trading

ICAO has endorsed emissions trading to reduce aviation CO₂ emission, guidelines were to be presented to the 2007 ICAO Assembly.^[68] 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.^[69][70] 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.^[71]

Short-haul flight ban

A short-haul flight ban is a prohibition imposed by governments on airlines to establish and maintain a flight connection over a certain distance, or by organisations or companies on their employees for business travel using existing flight connections over a certain distance, in order to mitigate the environmental impact of aviation. In the 21st century, several governments, organisations and companies have imposed restrictions and even prohibitions on short-haul flights, stimulating or pressuring travellers to opt for more environmentally friendly means of transportation, especially trains.^[72]

The Aéroport Charles de Gaulle 2 TGV station offers train connections

Train connections

Train connections reduce feeder flights.^[73] By March 2019, Lufthansa offered connections through Frankfurt with the Deutsche Bahn (AIRail Service) and Air France offered TGV connections through Paris.^[74] In October 2018, Austrian Airlines and the Austrian Federal Railways introduced train connections through Vienna Airport.^[75] In March 2019, the Dutch cabinet was working on an Amsterdam connection via NS International or Thalys.^[73] By July 2020, Lufthansa and Deutsche Bahn expanded their offer through Frankfurt Airport to 17 major cities.^[76]

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.^[77] By 2003, Access Grid technology had hosted several international conferences.^[77] The Tyndall Centre has reported means to change common institutional and professional practices.^[78][79]

Flight shame

In Sweden the concept of "flight shame" or "flygskam" has been cited as a cause of falling air travel.^[80] 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.^[81] Swedish airports operator Swedavia reported 4% fewer passengers across its 10 airports in 2019 compared to the previous year: a 9% drop for domestic passengers and 2% for international passengers.^[82] The ICCT estimates that 3% of the global population take regular flights.^[21]

ICAO regulation and CORSIA

In 2016, the International Civil Aviation Organization committed to improve aviation fuel efficiency by 2% per year and to stabilise carbon emissions from 2020.^[83] To achieve these goals, multiple measures have been planned: more fuel-efficient aircraft technology; development and deployment of sustainable aviation fuels; Improved air traffic management; market-based measures like emission trading, levies, and carbon offsetting,^[83] the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA).^[84]

Developed by the International Civil Aviation Organization (ICAO) and adopted in October 2016. Its goal is to have a carbon neutral growth from 2020. CORSIA uses Market-based environmental policy instruments to offset CO₂ emissions: aircraft operators have to purchase carbon credits from the carbon market. Starting in 2021, the scheme is voluntary for all countries until 2027.

Taxation and subsidies[]

Financial measures can discourage airline passengers and promote other transportation modes and motivates airlines to improve fuel efficiency. Aviation taxation include:

• Air passenger taxes, paid by passengers for environmental reasons, may be variable by distance and include domestic flights;
• Departure taxes, paid by passengers leaving the country, sometimes also applies outside aviation;
• Jet fuel taxes, paid by airlines for the consumed jet fuel, like the kerosene tax for the European Union or fuel taxes in the United States.

Consumer behaviour can be influenced by cutting subsidies for unsustainable aviation and subsidising the development of sustainable alternatives. By September–October 2019, a carbon tax on flights would be supported by 72% of the EU citizens, in a poll conducted for the European Investment Bank.^[85]

Aviation taxation could reflect all its external costs and could be included in an emissions trading scheme.^[86] International aviation emissions escaped international regulation until the ICAO triennial conference in 2016 agreed on the CORSIA offset scheme.^[87] Due to low or nonexistent taxes on aviation fuel, air travel has a competitive advantage over other transportation modes.^[88][89]

Alternative fuels[]

Refueling an Airbus A320 with biofuel

An aviation biofuel or bio-jet-fuel^[90] or bio-aviation fuel (BAF)^[91] is a biofuel used to power aircraft and is said to be a sustainable aviation fuel (SAF). The International Air Transport Association (IATA) considers it to be one of the key elements to reduce the carbon footprint within the environmental impact of aviation.^[92] Aviation biofuel could help decarbonize medium- and long-haul air travel generating most emissions, and could extend the life of older aircraft types by lowering their carbon footprint.

Biofuels are biomass-derived fuels, from plants or waste; depending on which type of biomass is used, they could lower CO₂ emissions by 20–98% compared to conventional jet fuel.^[93] The first test flight using blended biofuel was in 2008, and in 2011 blended fuels with 50% biofuels were allowed in commercial flights. In 2019, the IATA was aiming for a 2% penetration by 2025.

Aviation biofuel can be produced from plant sources like Jatropha, algae, tallows, waste oils, palm oil, Babassu and Camelina (bio-SPK); from solid biomass using pyrolysis processed with a Fischer–Tropsch process (FT-SPK); with an alcohol-to-jet (ATJ) process from waste fermentation; or from synthetic biology through a solar reactor. Small piston engines can be modified to burn ethanol.

Sustainable biofuels do not compete with food crops, prime agricultural land, natural forest or fresh water. They are an alternative to electrofuels.^[94] Sustainable aviation fuel is certified as being sustainable by a third-party organisation.

Hydrogen and e-fuel[]

In 2020, Airbus unveiled liquid-hydrogen-powered aircraft concepts as zero-emissions airliners, poised for 2035.^[95] Aviation, like industrial processes that cannot be electrified, should use primarily Hydrogen-based fuel.^[96]

The Potsdam Institute for Climate Impact Research reported a €800–1,200 mitigation cost per ton of CO₂ for hydrogen-based e-fuels.^[97] Those could be reduced to €20–270 per ton of CO₂ in 2050, but maybe not early enough to replace fossil fuels.^[97] Climate policies could bear the risk of e-fuel uncertain availability, and Hydrogen and e-fuels may be prioritised when direct electrification is inaccessible.^[97]

Non-CO₂ emissions[]

Economic cost and climate influence relation for transatlantic traffic

Besides carbon dioxide, aviation produces nitrogen oxides (NO
_x), particulates, unburned hydrocarbons (UHC) and contrails. Flight routes can be optimised: modelling CO₂, H
₂O and NO
_x effects of transatlantic flights in winter shows westbound flights climate forcing can be lowered by up to 60% and ~25% for jet stream-following eastbound flights, costing 10–15% more due to longer distances and lower altitudes consuming more fuel, but 0.5% costs increase can reduce climate forcing by up to 25%.^[98] A 2000 feet (~600 m) lower cruise altitude than the optimal altitude has a 21% lower radiative forcing, while a 2000 feet higher cruise altitude 9% higher radiative forcing.^[99]

Nitrogen oxides (NO
_x)

As designers work to reduce NO
_x emissions from jet engines, they fell by over 40% between 1997 and 2003.^[49] Cruising at a 2,000 ft (610 m) lower altitude could reduce NO
_x-caused radiative forcing from 5 mW/m² to ~3 mW/m².^[100]

Particulates

Modern engines are designed so that no smoke is produced at any point in the flight while particulates and smoke were a problem with early jet engines at high power settings.^[49]

Unburned hydrocarbons (UHC)

Produced by incomplete combustion, more unburned hydrocarbons are produced with low compressor pressures and/or relatively low combustor temperatures, they have been eliminated in modern jet engines through improved design and technology, like particulates.^[49]

Contrails

Contrail formation would be reduced by lowering the cruise altitude with slightly increased flight times, but this would be limited by airspace capacity, especially in Europe and North America, and increased fuel burn due to lower efficiency at lower altitudes, increasing CO₂ emissions by 4%.^[101] Contrail radiative forcing could be minimized by schedules: night flights cause 60-80% of the forcing for only 25% of the air traffic, while winter flights contribute half of the forcing for only 22% of the air traffic.^[102] As 2% of flights are responsible for 80% of contrail radiative forcing, changing a flight altitude by 2,000 ft (610 m) to avoid high humidity for 1.7% of flights would reduce contrail formation by 59%.^[103]

National carbon budgets[]

In UK, transportation replaced power generation as the largest emissions source. This includes aviation's 4% contribution. This is expected to expand until 2050 and passenger demand may need to be reduced.^[104] For the UK Committee on Climate Change (CCC), the UK target of an 80% reduction from 1990 to 2050 was still achievable from 2019, but the committee suggests that the Paris Agreement should tighten its emission targets.^[104] Their position is that emissions in problematic sectors, like aviation, should be offset by greenhouse gas removal, carbon capture and storage and reforestation.^[104]

In December 2020 the UK Climate Change Committee said that: "Mitigation options considered include demand management, improvements in aircraft efficiency (including use of hybrid electric aircraft), and use of sustainable aviation fuels (biofuels, biowaste to jet and synthetic jet fuels) to displace fossil jet fuel."^[105] The UK will include international aviation and shipping in their carbon budgets and hopes other countries will too.^[106]

Carbon offsetting[]

Money generated by carbon offsets from airlines often go to fund green-energy projects such as wind farms.

A carbon offset is a means of compensating aviation emissions by saving enough carbon or absorbing carbon back into plants through photosynthesis (for example, by planting trees through reforestation or afforestation) to balance the carbon emitted by a particular action.

Consumer option

Some airlines offer carbon offsets to passengers to cover the emissions created by their flight, invested in green technology such as renewable energy and research into future technology. Airlines offering carbon offsets include British Airways,^[107] Continental Airlines,^[108][109] easyJet,;^[110] and also Air Canada, Air New Zealand, Delta Air Lines, Emirates Airlines, Gulf Air, Jetstar, Lufthansa, Qantas, United Airlines and Virgin Australia.^[111] Consumers can also purchase offsets on the individual market. There are certification standards for these,^[112] including the Gold Standard^[113] and the Green-e.^[114]

Airline offsets[]

Some airlines have been carbon-neutral like Costa Rican Nature Air,^[115] or claim to be, like Canadian Harbour Air Seaplanes.^[116] Long-haul low-cost venture aims to be carbon neutral.^[117]

In 2019, Air France announced it would offset CO₂ emissions on its 450 daily domestic flights, that carry 57,000 passengers, from January 2020, through certified projects. The company will also offer its customers the option to voluntarily compensate for all their flights and aims to reduce its emissions by 50% per pax/km by 2030, compared to 2005.^[118]

Starting in November 2019, UK budget carrier EasyJet decided to offset carbon emissions for all its flights, through investments in atmospheric carbon reduction projects. It claims to be the first major operator to be carbon neutral, at a cost of £25 million for its 2019-20 financial year. Its CO₂ emissions were 77g per passenger in its 2018-19 financial year, down from 78.4g the previous year.^[119]

From January 2020, British Airways began offsetting its 75 daily domestic flights emissions through carbon-reduction project investments. The airline seeks to become carbon neutral by 2050 with fuel-efficient aircraft, sustainable fuels and operational changes. Passengers flying overseas can offset their flights for £1 to Madrid in economy or £15 to New York in business-class.^[120]

US low-cost carrier JetBlue planned to use offsets for its emissions from domestic flights starting in July 2020, the first major US airline to do so. It also plans to use sustainable aviation fuel made from waste by Finnish refiner Neste starting in mid-2020.^[121] In August 2020, JetBlue became entirely carbon-neutral for its U.S. domestic flights, using efficiency improvements and carbon offsets.^[122] Delta Air Lines pledged to do the same within ten years.^[123]

To become carbon neutral by 2050, United Airlines invests to build in the US the largest carbon capture and storage facility through the company 1PointFive, jointly owned by Occidental Petroleum and , with Carbon Engineering technology, aiming for nearly 10% offsets.^[124]

Electric aircraft[]

The Velis Electro was the first type certificated electric aircraft on 10 June 2020.

Electric aircraft operations do not produce any emissions and electricity can be generated by renewable energy. Lithium-ion batteries including packaging and accessories gives a 160 Wh/kg energy density while aviation fuel gives 12,500 Wh/kg.^[125] As electric machines and converters are more efficient, their shaft power available is closer to 145 Wh/kg of battery while a gas turbine gives 6,545 Wh/kg of fuel: a 45:1 ratio.^[126] For Collins Aerospace, this 1:50 ratio forbids electric propulsion for long-range aircraft.^[127] By November 2019, the German Aerospace Center estimated large electric planes could be available by 2040.^[128] Large, long-haul aircraft are unlikely to become electric before 2070 or within the 21st century, whilst smaller aircraft can be electrified.^[129] As of May 2020, the largest electric airplane was a modified Cessna 208B Caravan.

For the UK's Committee on Climate Change (CCC), huge technology shifts are uncertain, but consultancy Roland Berger points to 80 new electric aircraft programmes in 2016–2018, all-electric for the smaller two-thirds and hybrid for larger aircraft, with forecast commercial service dates in the early 2030s on short-haul routes like London to Paris, with all-electric aircraft not expected before 2045.^[104] Berger predicts a 24% CO₂ share for aviation by 2050 if fuel efficiency improves by 1% per year and if there are no electric or hybrid aircraft, dropping to 3–6% if 10-year-old aircraft are replaced by electric or hybrid aircraft due to regulatory constraints, starting in 2030, to reach 70% of the 2050 fleet.^[104] This would greatly reduce the value of the existing fleet of aircraft, however.^[104] Limits to the supply of battery cells could hamper their aviation adoption, as they compete with other industries like electric vehicles. Lithium-ion batteries have proven fragile and fire-prone and their capacity deteriorates with age. However, alternatives are being pursued, such as sodium-ion batteries.^[104]

See also[]

References[]

External links[]

Institutional

• "Aviation Emissions, Impacts & Mitigation: A Primer" (PDF). FAA Office of Environment and Energy. January 2015.
• "Strategic Research & Innovation Agenda" (PDF). Advisory Council for Aviation Research and Innovation in Europe. 2017.
• "European Aviation Environmental Report" (PDF). EASA. 2019.
• "Environmental Report". ICAO. 2019.

Concerns

• "airportwatch.org.uk". AirportWatch. oppose any expansion of aviation and airports likely to damage the human or natural environment, and to promote an aviation policy for the UK which is in full accordance with the principles of sustainable development

Industry

• "Aviation: Benefits Beyond Borders". Air Transport Action Group. information on the many industry measures underway to limit the impact of aviation on the environment
• "sustainableaviation.co.uk". Sustainable Aviation. collective approach of UK aviation to tackling the challenge of ensuring a sustainable future
• "The aviation sector's climate action framework" (PDF). Air Transport Action Group. November 2015.

Research

• "Aviation Sustainability Center". Washington State University and the Massachusetts Institute of Technology.
• "Laboratory for Aviation and the Environment". Massachusetts Institute of Technology.
• "Partnership for Air Transportation Noise and Emissions Reduction". Massachusetts Institute of Technology.
• "Sustainable Sky Institute". Sustainable Sky Institute.
• Stefan Gössling (5 February 2015). "publications". Lund University — Department of Service Management and Service Studies.^[which?]

Studies

• The Heinrich Böll Foundation and the Airbus Group (May 2016). "Aloft - An Inflight Review" (PDF).
• Antoine Gelain (10 August 2016). "Opinion: The Uncomfortable Truth About Aviation Emissions". Aviation Week & Space Technology.
• "Tracking report: Aviation". International Energy Agency. June 2020.
• Hannah Ritchie (22 October 2020). "Climate change and flying: what share of global CO2 emissions come from aviation?". Our World in Data.