a b s t r a c tAviation emissions contribute to the radiative forcing (RF) of climate. Of importance are emissions of carbon dioxide (CO 2 ), nitrogen oxides (NO x ), aerosols and their precursors (soot and sulphate), and increased cloudiness in the form of persistent linear contrails and induced-cirrus cloudiness. The recent Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) quantified aviation's RF contribution for 2005 based upon 2000 operations data. Aviation has grown strongly over the past years, despite world-changing events in the early 2000s; the average annual passenger traffic growth rate was 5.3% yr À1 between 2000 and 2007, resulting in an increase of passenger traffic of 38%. Presented here are updated values of aviation RF for 2005 based upon new operations data that show an increase in traffic of 22.5%, fuel use of 8.4% and total aviation RF of 14% (excluding induced-cirrus enhancement) over the period 2000-2005. The lack of physical process models and adequate observational data for aviationinduced cirrus effects limit confidence in quantifying their RF contribution. Total aviation RF (excluding induced cirrus) in 2005 was w55 mW m À2 (23-87 mW m À2 , 90% likelihood range), which was 3.5% (range 1.3-10%, 90% likelihood range) of total anthropogenic forcing. Including estimates for aviation-induced cirrus RF increases the total aviation RF in 2005-78 mW m À2 (38-139 mW m À2 , 90% likelihood range), which represents 4.9% of total anthropogenic forcing (2-14%, 90% likelihood range). Future scenarios of aviation emissions for 2050 that are consistent with IPCC SRES A1 and B2 scenario assumptions have been presented that show an increase of fuel usage by factors of 2.7-3.9 over 2000. Simplified calculations of total aviation RF in 2050 indicate increases by factors of 3.0-4.0 over the 2000 value, representing 4-4.7% of total RF (excluding induced cirrus). An examination of a range of future technological options shows that substantive reductions in aviation fuel usage are possible only with the introduction of radical technologies. Incorporation of aviation into an emissions trading system offers the potential for overall (i.e., beyond the aviation sector) CO 2 emissions reductions. Proposals exist for introduction of such a system at a European level, but no agreement has been reached at a global level.

Global aviation operations contribute to anthropogenic climate change via a complex set of processes that lead to a net surface warming. Of importance are aviation emissions of carbon dioxide (CO 2 ), nitrogen oxides (NO x ), water vapor, soot and sulfate aerosols, and increased cloudiness due to contrail formation. Aviation grew strongly over the past decades (1960–2018) in terms of activity, with revenue passenger kilometers increasing from 109 to 8269 billion km yr −1 , and in terms of climate change impacts, with CO 2 emissions increasing by a factor of 6.8–1034 Tg CO 2 yr −1 . Over the period 2013–2018, the growth rates in both terms show a marked increase. Here, we present a new comprehensive and quantitative approach for evaluating aviation climate forcing terms. Both radiative forcing (RF) and effective radiative forcing (ERF) terms and their sums are calculated for the years 2000–2018. Contrail cirrus, consisting of linear contrails and the cirrus cloudiness arising from them, yields the largest positive net (warming) ERF term followed by CO 2 and NO x emissions. The formation and emission of sulfate aerosol yields a negative (cooling) term. The mean contrail cirrus ERF/RF ratio of 0.42 indicates that contrail cirrus is less effective in surface warming than other terms. For 2018 the net aviation ERF is +100.9 mW (mW) m −2 (5–95% likelihood range of (55, 145)) with major contributions from contrail cirrus (57.4 mW m −2 ), CO 2 (34.3 mW m −2 ), and NO x (17.5 mW m −2 ). Non-CO 2 terms sum to yield a net positive (warming) ERF that accounts for more than half (66%) of the aviation net ERF in 2018. Using normalization to aviation fuel use, the contribution of global aviation in 2011 was calculated to be 3.5 (4.0, 3.4) % of the net anthropogenic ERF of 2290 (1130, 3330) mW m −2 . Uncertainty distributions (5%, 95%) show that non-CO 2 forcing terms contribute about 8 times more than CO 2 to the uncertainty in the aviation net ERF in 2018. The best estimates of the ERFs from aviation aerosol-cloud interactions for soot and sulfate remain undetermined. CO 2 -warming-equivalent emissions based on global warming potentials (GWP* method) indicate that aviation emissions are currently warming the climate at approximately three times the rate of that associated with aviation CO 2 emissions alone. CO 2 and NO x aviation emissions and cloud effects remain a continued focus of anthropogenic climate change research and policy discussions.

a b s t r a c tAviation alters the composition of the atmosphere globally and can thus drive climate change and ozone depletion. The last major international assessment of these impacts was made by the Intergovernmental Panel on Climate Change (IPCC) in 1999. Here, a comprehensive updated assessment of aviation is provided. Scientific advances since the 1999 assessment have reduced key uncertainties, sharpening the quantitative evaluation, yet the basic conclusions remain the same. The climate impact of aviation is driven by long-term impacts from CO 2 emissions and shorter-term impacts from non-CO 2 emissions and effects, which include the emissions of water vapour, particles and nitrogen oxides (NO x ). The presentday radiative forcing from aviation (2005) is estimated to be 55 mW m À2 (excluding cirrus cloud enhancement), which represents some 3.5% (range 1.3-10%, 90% likelihood range) of current anthropogenic forcing, or 78 mW m À2 including cirrus cloud enhancement, representing 4.9% of current forcing (range 2-14%, 90% likelihood range). According to two SRES-compatible scenarios, future forcings may increase by factors of 3-4 over 2000 levels, in 2050. The effects of aviation emissions of CO 2 on global mean surface temperature last for many hundreds of years (in common with other sources), whilst its non-CO 2 effects on temperature last for decades. Much progress has been made in the last ten years on characterizing emissions, although major uncertainties remain over the nature of particles. Emissions of NO x result in production of ozone, a climate warming gas, and the reduction of ambient methane (a cooling effect) although the overall balance is warming, based upon current understanding. These NO x emissions from current subsonic aviation do not appear to deplete stratospheric ozone. Despite the progress made on modelling aviation's impacts on tropospheric chemistry, there remains a significant spread in model results. The knowledge of aviation's impacts on cloudiness has also improved: a limited number of studies have demonstrated an increase in cirrus cloud attributable to aviation although the magnitude varies: however, these trend analyses may be impacted by satellite artefacts. The effect of aviation particles on clouds (with and without contrails) may give rise to either a positive forcing or a negative forcing: the modelling and the underlying processes are highly uncertain, although the overall effect of contrails and enhanced cloudiness is considered to be a positive forcing and could be substantial, compared with other effects. The debate over quantification of aviation impacts has also progressed towards studying potential mitigation and the technological and atmospheric tradeoffs. Current studies are still relatively immature and more work is required to determine optimal technological development paths, which is an aspect that atmospheric science has much to contribute. In terms of alternative fuels, liquid hydrogen represents a possibility and may reduce some of aviation's impacts on clim...

This study describes the methodology and results for calculating future global aviation emissions of carbon dioxide and oxides of nitrogen from air traffic under four of the IPCC/SRES (Intergovernmental Panel on Climate Change/Special Report on Emissions Scenarios) marker scenarios: A1B, A2, B1, and B2. In addition, a mitigation scenario has been calculated for the B1 scenario, requiring rapid and significant technology development and transition. A global model of aircraft movements and emissions (FAST) was used to calculate fuel use and emissions to 2050 with a further outlook to 2100. The aviation emission scenarios presented are designed to interpret the SRES and have been developed to aid in the quantification of the climate change impacts of aviation. Demand projections are made for each scenario, determined by SRES economic growth factors and the SRES storylines. Technology trends are examined in detail and developed for each scenario providing plausible projections for fuel efficiency and emissions control technology appropriate to the individual SRES storylines. The technology trends that are applied are calculated from bottom-up inventory calculations and industry technology trends and targets. Future emissions of carbon dioxide are projected to grow between 2000 and 2050 by a factor in the range of 2.0 and 3.6 depending on the scenario. Emissions of oxides of nitrogen associated with aviation over the same period are projected to grow by between a factor of 1.2 and 2.7.

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