Preprints
https://doi.org/10.5194/egusphere-2024-347
https://doi.org/10.5194/egusphere-2024-347
22 Feb 2024
 | 22 Feb 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

The importance of an informed choice of CO2-equivalence metrics for contrail avoidance

Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

Abstract. One of the proposed ways to reduce the climate impact of civil aviation is rerouting aircraft to minimise the formation of warming contrails. As this strategy may increase fuel consumption, it would only be beneficial if the climate impact reduction from the avoided contrails exceeds the negative impact of any additional carbon dioxide (CO2) emitted by the rerouted flight. In this study, we calculate the surface temperature response of almost half-a-million flights that crossed the North Atlantic sector in 2019 and compare to the response of hypothetical rerouted flights. The climate impacts of contrails and CO2 are assessed through the perspective of CO2-equivalence metrics, defined here as nine combinations of different definitions and time horizons. We estimate that the total emitted CO2 and the persistent contrails formed will have warmed the climate by 16.9 µK in 2039, 13.5 µK in 2069, and 14.0 µK in 2119. Under a scenario where 1 % additional carbon dioxide is enough to reroute all contrail-forming flights and avoid contrail formation completely, total warming would decrease by 4.6 (−27 %), 2.4 (−18 %), and 1.8 (−13 %) μK in 2039, 2069, and 2119, respectively. In most rerouting cases, the results based on the nine different CO2-equivalence metrics agree that rerouting leads to a climate benefit, assuming that contrails are avoided as predicted. But the size of that benefit is very dependent on the choice of CO2-equivalence metrics, contrail efficacy and CO2 penalty. Sources of uncertainty not considered here could also heavily influence the perceived benefit. In about 10 % of rerouting cases, the climate damage resulting from contrail avoidance indicated by CO2-equivalence metrics integrated over a 100-year time horizon is not predicted by metrics integrated over a 20-year time horizon. This study highlights, using North Atlantic flights as a case study, the implications of the choice of CO2-equivalence metrics for contrail avoidance, but the choice is ultimately political.

Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

Status: open (until 17 Apr 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Review of "The importance of an informed choice of CO2-equivalence metrics for contrail avoidance" by Borella et al., egusphere-2024-347', Anonymous Referee #1, 08 Mar 2024 reply
Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin
Audran Borella, Olivier Boucher, Keith P. Shine, Marc Stettler, Katsumasa Tanaka, Roger Teoh, and Nicolas Bellouin

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Short summary
This work studies how to compare the climate impact of the CO2 emitted and contrails formed by a flight, applied to contrail avoidance strategies that would decrease the climate impact of flights by changing the trajectory of aircraft to avoid persistent contrail formation, at the risk of increasing CO2 emissions. We find that different comparison methods lead to different quantification of the total climate impact of a flight, but lead to similar decisions about rerouting an aircraft or not.