Simulated 2050 aviation radiative forcing from contrails and aerosols

Chen, Chih-Chieh; Gettelman, Andrew

doi:https://doi.org/10.5194/acp-16-7317-2016

Articles | Volume 16, issue 11

https://doi.org/10.5194/acp-16-7317-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-16-7317-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 16, issue 11

Research article

|

13 Jun 2016

Research article |

| 13 Jun 2016

Simulated 2050 aviation radiative forcing from contrails and aerosols

Chih-Chieh Chen and Andrew Gettelman

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Final revised paper (published on 13 Jun 2016)
Preprint (discussion started on 15 Jan 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review', Ulrich Schumann, 30 Jan 2016
- AC1: 'response to Dr. Schumann', Chih-Chieh Chen, 12 May 2016
RC2: 'Review of ‚Simulated 2050 aviation radiative forcing‘ by C.-C. Chen and A. Gettelman', Anonymous Referee #2, 25 Feb 2016
- AC2: 'response to SC2', Chih-Chieh Chen, 12 May 2016
RC3: 'referee report', Anonymous Referee #3, 25 Feb 2016
- AC3: 'response to SC3', Chih-Chieh Chen, 12 May 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Chih-Chieh Chen on behalf of the Authors (12 May 2016) Manuscript

ED: Referee Nomination & Report Request started (17 May 2016) by Christopher Hoyle

RR by Ulrich Schumann (20 May 2016)

Suggestions for revision or reasons for rejection

The authors greatly extended the discussion in this paper. They now mention many potential uncertainties and cite more of the related literature. These changes are to be appreciated.

Still, my skepticism about the suitability of the model for contrail cirrus analysis and critics on previous related publications remain. I miss comparisons with observations (for soot, sulfate and contrails) for present climate, and my questions on the previous publications were not answered.

Nevertheless, these points may be left to future studies.

Hence, acceptance may be considered by the Editor after inclusion of the following points :

1) Parts of the abstract (and parts of the conclusions) still phrase the results as if they would be beyond doubt. To avoid mis-citation, I strongly ask to add cautioning words, at least into the abstract:

As a result, the net
2050 aviation radiative forcing MAY HAVE a cooling effect on the
planet.

and

Aviation black carbon aerosols produce a negligible net forcing
globally in 2006 and 2050 IN THIS MODEL STUDY.

2)
I ask to cite Gierens et al. (1999) and Marquart et al. (2003) already in the introduction, because these studies did not introduce new future aviation emission scenarios but did investigate the Topic of this paper, namely the RF changes from changed contrails for given (!) future aviation emission scenarios (including changes in propulsion efficiency)

3)
When citing previous contrail RF results, please cite the results completely, including the published uncertainty estimates:

Burkhardt and Kärcher (2011) 31 (no range)
Chen and Gettelman (2013): 13 +- 10
Schumann and Graf (2003) 50 (40-80)
Schumann et al. (2015) 60 (40-80)
all in mW/m2.

4)
I still do not understand the basic result of this paper of a 6 to 7 fold increase in RF from 2006 to 2050 (Table 3, global).
The given explanation (“This is due to non-uniform regional increase in air traffic and different sensitivities for contrail radiative forcing in different regions.“) is not substantiated with facts.

Fuel consumption increases less than a factor 4.8 in this study, persistent contrail cover gets smaller, and the increase in RF is maximum over East Asia with a factor <6.1 (computed from Table 3). Hence, a factor of 7 increase remains unexplained. One may leave this for the future, but my personal conclusion is that one cannot exclude a trivial analysis error in these results.

5) Concerning the new estimate of CO2 RF, note: The CO2 RF is NOT proportional to the CO2 emissions at a given time but proportional to a life-time weighted integral over the past CO2 emissions (see e.g. Sausen, R., and Schumann, U.: Estimates of the climate response to aircraft CO2 and NOx-emission scenarios, Climatic Change, 44, 27 - 58, 2000.). Also, please note, that the conclusion ("This will overwhelm the net effect of contrail cirrus...") depends strongly on the contrail cirrus model used. Other published contrail RF studies suggest different conclusions. This (new) part needs to be corrected.

.

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ED: Publish subject to technical corrections (24 May 2016) by Christopher Hoyle

AR by Chih-Chieh Chen on behalf of the Authors (27 May 2016) Author's response Manuscript

Short summary

The impact of aviation emissions through 2050 is simulated by a comprehensive global climate model. Four different future emission scenarios of the same flight tracks are considered. The results reveal that the global radiative forcing of contrail cirrus is positive and can increase by a factor of 7 in 2050 from the 2006 level. The aviation aerosols can produce negative forcing, mainly over the oceans, and increase by a factor of 4 in 2050 from the 2006 level.