Aircraft engines produce gases, noise, and particulates from fossil fuel combustion, 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. Since 2021, the IATA members plan net-zero carbon emissions by 2050, followed by the ICAO in 2022. 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