Mitigation of aviation's environmental impact

Currently, air traffic corridors that aircraft are forced to follow place unnecessary detours on an aircraft's route forcing higher fuel burn and an increase in emissions. An improved Air Traffic Management System with more direct routes and optimized cruising altitudes would allow airlines to reduce their emissions by up to 18%.[7]

In the European Union, a Single European Sky has been proposed for the last 15 years so that there are no overlapping airspace restrictions between countries in the EU and so reduce emissions. As yet, the Single European Sky is still only a plan but progress has been made. If the Single European Sky had been created 15 years ago, 12 million tons of CO₂ could have been saved.[7]

In the EU, aviation will be including the European Emission Trading Scheme from 2012 onwards. The scheme places a cap on the emissions an aircraft operator can emit and forces the operator to either lower emissions through more efficient technology or to buy "Carbon Credits" from other companies who have produced fewer emissions than their cap. It is thought that this will reduce aviation's net environmental impact.[4]

In the 2010s and 2020s, several governments have imposed restrictions and even prohibitions on short-distance aviation, stimulating or pressuring travellers to opt for more environmentally friendly means of transportation, especially trains.[8]

•  Austria: As part of its COVID-19 crisis support programme for Austrian Airlines in June 2020, the conservative–green coalition government introduce a special tax of 30 euros on airline tickets for flights spanning less than 350 kilometres (an unprecedented environmental measure within the EU), while airline connections that covered distances that could be travelled within three hours by train were henceforth prohibited.[9]
•  France: On 3 June 2019, French MPs proposed to prohibit airline connections covering distances that could be travelled within 2.5 hours by train.[10] French Finance Minister Bruno Le Maire stated in April 2020 and repeated in May 2020 that negotiations between the government and Air France–KLM on such a 2.5 hour short-distance ban were underway.[11] On 9 June 2020, as part of its COVID-19 crisis support programme for France's aviation sector, Le Maire confirmed that 2.5 hour short-distance flights would be prohibited,[8][9] while Air France–KLM's domestic flights would be reduced by 40%.[8]
•  Netherlands: In June 2013, Dutch MP Liesbeth van Tongeren (GreenLeft, previously Greenpeace Netherlands director) proposed to prohibit domestic flights in the Netherlands with the argument that they are needlessly inefficient, polluting and expensive, but Environment Secretary Wilma Mansveld (Labour Party) said such a ban would violate EU regulations that allow airlines to fly domestically.[12][13] In March 2019, the House of Representatives of the Netherlands voted to prohibit commercial flights between Amsterdam Airport Schiphol and Brussels Airport (Zaventem).[14] This distance of about 150 kilometres was covered by five return flights a day, most of them feeder flights: passengers from Brussels go to Amsterdam to embark on a long-distance flight from there, or vice versa.[15] However, Infrastructure Minister Cora van Nieuwenhuizen (VVD) stated that such a ban was contrary to the European Commission's free market regulations and was thus not implemented.[14] Although almost all Dutch parliamentary parties agreed that train travel should replace short-distance aviation,[14] there were also some practical problems to be solved before trains could become a viable alternative, such buying a combined train/plane ticket,[14] the lack of a direct Thalys connection from Amsterdam Central and Paris-North to Brussels Airport (forcing passengers to switch trains in Brussels-South),[15] and the fact that the Benelux train (which does directly connect Schiphol and Zaventem) takes over 2 hours (mostly due to the lack of a high-speed rail between Antwerp and Brussels).[15]

One way to reduce feeder flights is to make it easier for passengers to buy a combined train/plane ticket.[14] As of March 2019, Lufthansa and Air France already offer this possibility:[16]

• A passenger who books a trip from Cologne to Madrid via Lufthansa is routed to Frankfurt with a Deutsche Bahn train ticket, from where they will fly to Madrid with Lufthansa.[16]
• A passenger who books a trip from London to Strasbourg with Air France only flies as far as Paris, where they transfer to a TGV train to Strasbourg.[16]

In March 2019, Dutch Infrastructure Minister Cora van Nieuwenhuizen stated the government was working on a combined train/plane ticket, so that 'a Mexican wanting to go to Brussels via Amsterdam could book their journey from Mexico via NS International or Thalys in one go,' making an Amsterdam–Brussels flight unnecessary.[14]

Financial measures can be taken to discourage passengers and airlines from flying in general, and to seek other means of transportation (passengers) or develop more fuel-efficient or less-polluting ways of flying (airlines). The primary means of taxation are:

• Air passenger taxes, paid by passengers to the government for environmental reasons; may be variable by distance and includes domestic flights
  • Departure taxes, paid by passengers leaving the country to the government (sometimes also applies outside of aviation)
• Jet fuel taxes, paid by airline companies to the government for the jet fuel (kerosene) they burn
  • For the European Union, see Kerosene tax
  • For the United States, see Fuel taxes in the United State § Aviation fuel taxes

It's also possible to influence business practices and consumer behaviour by cutting subsidies for unsustainable aviation, or by subsidising the development of more sustainable alternatives both in aviation and other means of transportation wherever aviation can be replaced, such as railway systems.

The Boeing 787 Dreamliner promises to provide 20% lower fuel burn than current-generation aircraft.

As stated previously, reducing the direct fuel burn of an aircraft is the most obvious and arguably the most economical way of reducing emissions attributable to aviation. Over the years 1977–2007, commercial jet airliners have become 70% more fuel efficient, and have been predicted to be another 25% more fuel-efficient by 2025 according to Giovanni Bisignani.[7]

The next-generation of aircraft, including the Boeing 787 Dreamliner, Airbus A350 and Bombardier CSeries, are 20% more fuel efficient per passenger kilometre than current generation aircraft. This is primarily achieved through more fuel-efficient engines and lighter airframes & supporting structures made of composite materials but is also achieved through more aerodynamic shapes, winglets, a "one-piece" fuselage and more advanced computer systems for optimising routes and loading of the aircraft. [17]

British Airways will be using half a million tonnes of waste annually to create biofuels for commercial use from 2014 onwards.

Biofuels are fuels derived from biomass material such as plants and waste. Plant derived biofuels offer large savings in CO₂ emissions as they absorb Carbon Dioxide and release it as Oxygen when they grow and so in a life-cycle, emissions can be drastically reduced. A number of airlines have operated biofuel test flights including Virgin Atlantic Airways, which flew with one engine operating on a blend of 20% coconut oil and 80% traditional jet fuel, and Continental Airlines which flew with one engine operating on a blend of 44% Jatropha oil, 6% Algae oil and 50% traditional jet fuel. Other airlines to demonstrate biofuels include Air New Zealand and Japan Airlines.[18]

In the Continental Airlines test, the engine running partly on biofuel burned 46 kg less fuel than the conventionally fuelled engine in 1 and a half hours while producing more thrust from the same volume of fuel. Continental Airlines' CEO, Larry Kellner, commented "This is a good step forward, an opportunity to really make a difference to the environment" citing jatropha's 50–80% lower CO₂ emissions as opposed to Jet-A1 in its lifecycle.[18]

From 2014 onwards, British Airways, in co-operation with Solena, is going to turn half a million tonnes of waste annually that would normally go to landfill from the City of London into biofuel to be used in the British Airways fleet. Waste derived biofuel produces up to 95% less pollution in its life-cycle and so therefore this measure will reduce emissions by the equivalent of taking 42,000 cars off the road every year. [19]

Scandinavian Airlines is operating their 737 aircraft at slower cruising speeds to reduce emissions by 7–8%.

Airlines and airports are looking at ways of reducing emissions and fuel burn through the use of improved operating procedures. Two of the more common ones in operation are a single-engine taxi to and from the runway and the use of a Continuous Descent Approach, or CDA, which can reduce emissions significantly during the operations in and around an airport.[20] Scandinavian Airlines (SAS) is now operating its Boeing 737 fleet at a slower cruising speed to help reduce emissions by 7–8%.[21]

Nitrogen oxides have a far stronger impact upon climate change than Carbon Oxides and are produced in small quantities from aircraft engines. Engine designers have worked since the start of the jet age to reduce NO_X emissions and the result is ever reducing levels of nitrogen oxide emissions. For example, between 1997 and 2003, NO_X emissions from jet engines fell by over 40%.[22]

Particulates and smoke were a problem with early jet engines at high power settings but modern engines are designed so that no smoke is produced at any point in the flight.[22]

Contrails formed by high altitude aircraft.

Unburned hydrocarbons (UHC) are products of incomplete combustion of fuel and are produced in greater quantities in engines with low pressure gains in the compressors and/or relatively low temperatures in the combustor. As with particulates, UHC has all but been eliminated in modern jet engines through improved design and technology.[22]

Aircraft flying at high altitude form condensation trails or contrails in the exhaust plume of their engines. While in the Troposphere these have very little climatic impact. However, jet aircraft cruising in the Stratosphere do create an impact from their contrails, although the extent of the damage to the environment is as yet unknown. Contrails can also trigger the formation of high-altitude Cirrus cloud thus creating a greater climatic effect.[22] 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.[23] On days with outbreaks the day/night temperature difference was diminished by about 6F° in the U.S. South and 5F° in the Midwest.[24]

In the three days following the September 11 attacks on the World Trade Center in New York City, when no commercial aircraft flew in the United States, climate scientists measured the daily temperature range over 5000 weather stations across the USA. The results showed a 1 ° Celsius change in the average daily temperature range for those days of the year, thus showing contrails do have a significant impact on climate.[3] Potential ways of reducing the impact of contrails on our climate include reducing the maximum cruising altitude of aircraft so high-altitude contrails can not form. Cruising at lower altitudes would marginally increase flight time and increase fuel consumption by 4%.[25]

Next-Generation engines are not only more fuel-efficient but also tend to be quieter with Pratt & Whitney's PurePower PW1000G fitted to the Bombardier CSeries aircraft being 4 times quieter than aircraft currently in service.[26] Engines can also incorporate serrated edges or 'chevrons' on the back of the nacelle to reduce noise impact as shown in this picture.[22]

A Continuous Descent Approach, or CDA, not only reduces fuel burn but also allows airlines to provide quieter approaches for part of the descent to a runway. As the engines are at close to idle power, less noise emissions are produced and combined with new engine technology, the reductions in noise emissions can be large.[20]

British Airways' carbon offsetting scheme involves paying a fee dependant on aircraft type, class of travel and distance flown and therefore prices vary. Funds generated are currently awarded to three renewable energy projects around the world: Bayin'aobao wind farm in Inner Mongolia, Faxinal dos Guedes hydroelectric power plant in Brazil and Xiaohe hydroelectric power plant in Gansu Province, China.[31]

Continental Airlines' carbon offsetting scheme involves paying a fixed fee of $2 to cancel out emissions through reforestation. Passengers can also choose to pay $50 for offsetting emissions through renewable energy projects.[30]