New study indicates non-CO2 impacts of aviation are twice as large as the CO2 alone

A new study trying to elucidate the various non-CO2 impacts of aviation has been published. There is very complicated science about the positive radiative forcing (ie. extra impact on increasing global temperature) of the water vapour, NOx and other gases, and particles emitted from jet engines at altitude. This study concludes that the non-CO2 impacts of “aviation emissions are currently warming the climate at approximately three times the rate of that associated with aviation CO2 emissions alone.” They have looked in detail at the various effects and interactions. There are numerous non-CO2 impacts, some of which cause more radiation to be reflected back out to space, and some cause heat to be trapped, warming the earth. These effects include the contrails, ice cloud changes, sulphate and soot particles from jet engines, water vapour from jet engines, NOx emissions and production of ozone. The effects of contrails and extra cloud formation are perhaps easier to study, and more research is needed on the impacts of soot and sulphate particles.  The confirmation of the large contribution to warming, from the non-CO2 impacts of aviation is important.  The climate impact of aviation, including non-CO2 effects, has to be fully taken into account in how the sector fits into the UK’s climate targets, and reaching “net zero”.  Currently the DfT ignores non-CO2 impacts, though the CCC has recommended that they should be included.



The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018

Academic study, published through Science Direct

Dedication: This paper is dedicated to the memory of Professor Ivar S. A. Isaksen of the University of Oslo, whose scientific excellence, friendship, and mentorship is sorely missed.

Author links

Author links open overlay panelD.S.LeeaD.W.FaheybA.SkowronaM.R.AllencnU.BurkhardtdQ.CheneS.J.DohertyfS.FreemanaP.M.ForstergJ.FuglestvedthA.GettelmaniR.R.De LeónaL.L.LimaM.T.LundhR.J.MillarcoB.OwenaJ.E.PennerjG.PitarilM.J.PratherkR.SausendL.J.Wilcoxm

Received 9 February 2020, Revised 2 July 2020, Accepted 30 July 2020, Available online 3 September 2020.


• Global aviation warms Earth’s surface through both CO2 and net non-CO2 contributions.

• Global aviation contributes a few percent to anthropogenic radiative forcing.

• Non-CO2 impacts comprise about 2/3 of the net radiative forcing.

• Comprehensive and quantitative calculations of aviation effects are presented.

• Data are made available to analyze past, present and future aviation climate forcing.


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 (CO2), nitrogen oxides (NOx), 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 CO2 emissions increasing by a factor of 6.8–1034 Tg CO2 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 CO2 and NOx 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), CO2 (34.3 mW m−2), and NOx (17.5 mW m−2). Non-CO2 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-CO2 forcing terms contribute about 8 times more than CO2 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.

CO2-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 CO2 emissions alone.

CO2 and NOx aviation emissions and cloud effects remain a continued focus of anthropogenic climate change research and policy discussions.

Graphical abstract

Image 1

1. Introduction

Aviation is one of the most important global economic activities in the modern world. Aviation emissions of CO2 and non-CO2 aviation effects result in changes to the climate system (Fig. 1 above).

Both aviation CO2 and the sum of quantified non-CO2 contributions lead to surface warming. The largest contribution to anthropogenic climate change across all economic sectors comes from the increase in CO2 concentration, which is the primary cause of observed global warming in recent decades (IPCC, 2013, 2018).

Aviation contributions involve a range of atmospheric physical processes, including plume dynamics, chemical transformations, microphysics, radiation, and transport. Aggregating these processes to calculate changes in a greenhouse gas component or a cloud radiative effect is a complex challenge for contemporary atmospheric modeling systems.

Given the dependence of aviation on burning fossil fuel, its significant CO2 and non-CO2 effects, and the projected fleet growth, it is vital to understand the scale of aviation’s impact on present-day climate forcing.

Fig 1 is a schematic overview of the processes by which aviation emissions and increased cirrus cloudiness affect the climate system.

Net positive RF (warming) contributions arise from CO2, water vapor, NOx, and soot emissions, and from contrail cirrus (consisting of linear contrails and the cirrus cloudiness arising from them).

Negative RF (cooling) contributions arise from sulfate aerosol production. Net warming from NOx emissions is a sum over warming (short-term ozone increase) and cooling (decreases in methane and stratospheric water vapor, and a long-term decrease in ozone) terms.

Net warming from contrail cirrus is a sum over the day/night cycle. These contributions involve a large number of chemical, microphysical, transport and, radiative processes in the global atmosphere. The quantitative ERF values associated with these processes are shown in Fig. 3 for 2018.

Historically, estimating aviation non-CO2 effects has been particularly challenging. The primary (quantified) non-CO2 effects result from the emissions of NOx, along with water vapor and soot that can result in contrail formation.

Aviation aerosols are small particles composed of soot (black and organic carbon (BC/OC)) and sulfur (S) and nitrogen (N) compounds.

The largest positive (warming) climate forcings adding to that of CO2 are those from contrail cirrus and from NOx-driven changes in the chemical composition of the atmosphere (Lee et al., 2009 (L09)).

L09 estimated that in 2005, aviation CO2 radiative forcing (RF (Wm−2)) was 1.59% of total anthropogenic CO2 RF and that the sum of aviation CO2 and non-CO2 effects contributed about 5% of the overall net anthropogenic forcing.

Understanding of aviation’s impacts on the climate system has improved over the decade since the last comprehensive evaluation (L09), but remains incomplete.

Published studies of aviation contributions to climate change generally focus on one or a few ERF terms. For example, about 20 studies are cited here that quantify the contribution from global NOx emissions.

In contrast, only a few studies have addressed the net RF from global aviation (IPCC, 1999Sausen et al., 2005; L09). A more recent study updated some aviation terms without providing a net RF (Brasseur et al., 2016).

Here, a comprehensive analysis of individual aviation ERFs is undertaken in order to provide an overall ERF for global aviation, along with the associated uncertainties, which is an analysis unavailable elsewhere.

This step updates and improves the analysis of L09. Best estimates of individual aviation ERF terms are derived here for the first time and combined to provide a net ERF for global aviation. Quantifying the terms required new analyses of CO2 and NOx ERFs and recalibration of other individual ERFs accounting for factors not previously applied in a common framework.

…. and it continues. See the whole (64 pages) study at

… it is quite technical ….