been completely achieved, costing 6 billion euros in delays and causing 11.6 million tonnes of excess emissions. Operations improvements Non- emissions Besides carbon dioxide, aviation produces nitrogen oxides (), particulates, unburned hydrocarbons (UHC) and contrails. Flight routes can be optimized: modelling , and 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%. A 2000 feet (~600 m) lower cruise altitude than the optimal altitude has a % lower radiative forcing, while a 2000 feet higher cruise altitude % higher radiative forcing. Nitrogen oxides () As designers work to reduce emissions from jet engines, they fell by over 40% between 1997 and 2003. Cruising at a lower altitude could reduce -caused radiative forcing from 5 mW/m2 to ~3 mW/m2. 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. 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. 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 emissions by 4%. 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. As 2% of flights are responsible for 80% of contrail radiative forcing, changing a flight altitude by to avoid high humidity for 1.7% of flights would reduce contrail formation by 59%. Reducing air travel 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