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.

The Paris Agreement has opened debate on whether limiting warming to 21 1.5°C is compatible with current emission pledges and warming of about 22 0.9°C from the mid-19 th -century to the present decade. We show that limiting 23 cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 24 warming to less than 0.6°C in 66% of Earth System Model members of the 25 CMIP5 ensemble with no mitigation of other climate drivers, increasing to 26 240GtC with ambitious non-CO2 mitigation. We combine a simple climate-27 carbon-cycle model with estimated ranges for key climate system properties 28 from the IPCC 5 th Assessment Report. Assuming emissions peak and decline 29 to below current levels by 2030 and continue thereafter on a much steeper 30 decline, historically unprecedented but consistent with a standard ambitious 31 mitigation scenario (RCP2.6), gives a likely range of peak warming of 1.2-32 2.0°C above the mid-19 th -century. If CO2 emissions are continuously adjusted 33 over time to limit 2100 warming to 1.5°C , with ambitious non-CO2 mitigation, 34 net future cumulative CO2 emissions are unlikely to prove less than 250 GtC 35 and unlikely greater than 540GtC. Hence limiting warming to 1.5°C is not yet a 36 geophysical impossibility, but likely requires delivery on strengthened pledges 37 for 2030 followed by challengingly deep and rapid mitigation. Strengthening 38 near-term emissions reductions would hedge against a high climate response 39 or subsequent reduction-rates proving economically, technically or politically 40 unfeasible. 41 42 Main text: 43The aim of Paris Agreement is "holding the increase in global average 44 temperature to well below 2°C above pre-industrial levels and pursuing efforts 45 to limit the temperature increase to 1.5°C " 1 . The Parties also undertook to 46 achieve this goal by reducing net emissions "to achieve a balance between 47 anthropogenic sources and removals by sinks of greenhouse gases in the 48 second half of this century", and hence implicitly not by geo-engineering 49 Long-term anthropogenic warming is determined primarily by cumulative 64 emissions of CO2 7-10 : the IPCC 5 th Assessment Report (IPCC-AR5) found that 65 cumulative CO2 emissions from 1870 had to remain below 615GtC for total 66 anthropogenic warming to remain below 1.5°C in more than 66% of members 67 of the CMIP5 ensemble of Earth System Models (ESMs) 11 (see Fig. 1a). 68Accounting for the 545GtC that had been emitted by the end of 2014 12 , this 69 would indicate a remaining budget from 2015 of less than 7 years' current 70 emissions, while current commitments under the Nationally Determined 71Contributions (NDCs) indicate 2030 emissions close to current levels 13 . 72 73The scenarios and simulations on which these carbon budgets were based, 74 however, were designed to assess futures in absence of CO2 mitigation, not 75 the very ambitious mitigation scenarios and correspondingly small amounts of 76 additional warming above present that are here of interest. cumulative carbon emissions (5...

Abstract. Simple climate models can be valuable if they are able to replicate aspects of complex fully coupled earth system models. Larger ensembles can be produced, enabling a probabilistic view of future climate change. A simple emissions-based climate model, FAIR, is presented, which calculates atmospheric concentrations of greenhouse gases and effective radiative forcing (ERF) from greenhouse gases, aerosols, ozone and other agents. Model runs are constrained to observed temperature change from 1880 to 2016 and produce a range of future projections under the Representative Concentration Pathway (RCP) scenarios. The constrained estimates of equilibrium climate sensitivity (ECS), transient climate response (TCR) and transient climate response to cumulative CO 2 emissions (TCRE) are 2.86 (2.01 to 4.22) K, 1.53 (1.05 to 2.41) K and 1.40 (0.96 to 2.23) K (1000 GtC) −1 (median and 5-95 % credible intervals). These are in good agreement with the likely Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) range, noting that AR5 estimates were derived from a combination of climate models, observations and expert judgement. The ranges of future projections of temperature and ranges of estimates of ECS, TCR and TCRE are somewhat sensitive to the prior distributions of ECS/TCR parameters but less sensitive to the ERF from a doubling of CO 2 or the observational temperature dataset used to constrain the ensemble. Taking these sensitivities into account, there is no evidence to suggest that the median and credible range of observationally constrained TCR or ECS differ from climate model-derived estimates. The range of temperature projections under RCP8.5 for 2081-2100 in the constrained FAIR model ensemble is lower than the emissions-based estimate reported in AR5 by half a degree, owing to differences in forcing assumptions and ECS/TCR distributions.