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 emissions alone, excluding induced cirrus cloud, with a very low level of scientific understanding. 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). 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. In 2018, represented 34.3 mW/m2 of aviation's effective radiative forcing (ERF, on the surface), with a high confidence level (± 6 mW/m2), 17.5 mW/m2 with a low confidence level (± 14) and contrail cirrus 57.4 mW/m2, also with a low confidence level (± 40). All factors combined represented 43.5 mW/m2 ( that of alone) excluding contrail cirrus and 101 mW/m2 (±45) including them, 3.5% of the anthropogenic ERF of 2290 mW/m2 (± 1100). 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. In 1992, aircraft emissions represented 2% of all man-made emissions, having accumulated a little more than 1% of the total man-made increase over 50 years. 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. Emissions from flights rose by 32% between 2013 and 2018. 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,