Environmental impact of aviation

Factors s from aviation emissions, estimated in 2020 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. 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. 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. 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). This ratio depends on how aviation activity grows. If the growth is exponential then the ratio is constant. But if the growth stops, the ratio will go down because the in the atmosphere due to aviation will continue to go up, whereas the other effects will stagnate. Again, it must be remembered that the effect of accumulates from year to year, unlike the effect of contrails and cirrus clouds. 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. 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. 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 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. 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. This prediction is supported by data showing that incidents of severe turbulence increased by 55% between 1979 and 2020, attributed to changes in wind velocity at high altitudes. 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 ==