Aviation accounts for 3.5% of the human-made climate impact; two-thirds of this impact are caused by emissions other than CO₂, according to a new study by researchers in Europe and the US. The study was published in the journal Atmospheric Environment

This new study is based on a thorough review of a decade of research on aviation emissions. It makes an important contribution to the scientific understanding we have of the role of aviation for climate change—an understanding decision-makers and politicians may need on the way to achieving the goals of the Paris Agreement.

Eleven years have passed since the last time a similar study was conducted, and there has been a strong growth in the aviation sector since then, particularly in Asia. In the new study, we have analyzed 20 years of aviation emissions, up until and including 2018.

—Marianne Tronstad Lund, co-author

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, NO_x, 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 NO_x 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. Lee et al.

Tronstad Lund emphasizes that the growth in aviation emissions is only partly offset by technological and operational improvements, such as more efficient engines and better landing routines.

In addition to CO₂, air traffic causes condensation trails and nitrogen oxide (NO_x) emissions from the combustion of the airplane’s fuel. Seen together, the climate impact of these two factors is bigger than that of the sector’s carbon emissions.

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 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), CO₂ (34.3 mW m⁻²), and NO_x (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 NO_x aviation emissions and cloud effects remain a continued focus of anthropogenic climate change research and policy discussions.

—Lee et al.

Resources

• D.S. Lee, D.W. Fahey, A. Skowron, M.R. Allen, U. Burkhardt, Q. Chen, S.J. Doherty, S. Freeman, P.M. Forster, J. Fuglestvedt, A. Gettelman, R.R. De León, L.L. Lim, M.T. Lund, R.J. Millar, B. Owen, J.E. Penner, G. Pitari, M.J. Prather, R. Sausen, L.J. Wilcox (2020) “The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018,” Atmospheric Environment doi: 10.1016/j.atmosenv.2020.117834