Changes in the aerosol direct radiative forcing from 2001 to 2015: observational constraints and regional mechanisms

Paulot, Fabien; Paynter, David; Ginoux, Paul; Naik, Vaishali; Horowitz, Larry W.

doi:https://doi.org/10.5194/acp-18-13265-2018

Articles | Volume 18, issue 17

https://doi.org/10.5194/acp-18-13265-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-18-13265-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 17

Research article

|

14 Sep 2018

Research article |

| 14 Sep 2018

Changes in the aerosol direct radiative forcing from 2001 to 2015: observational constraints and regional mechanisms

Fabien Paulot, David Paynter, Paul Ginoux, Vaishali Naik, and Larry W. Horowitz

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Final revised paper (published on 14 Sep 2018)
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Preprint (discussion started on 13 Feb 2018)
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Interactive discussion

Status: closed

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

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RC1: 'review of paper', Anonymous Referee #1, 10 Mar 2018
RC2: 'Review of "Changes in the aerosol direct radiative forcing from 2001 to 2015: observational constraints and regional mechanisms"', Anonymous Referee #2, 16 Apr 2018
AC1: 'Reply', Fabien Paulot, 21 May 2018

Peer-review completion

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

AR by Fabien Paulot on behalf of the Authors (21 May 2018) Manuscript

ED: Referee Nomination & Report Request started (12 Jun 2018) by Anne Perring

RR by Anonymous Referee #1 (17 Jun 2018)

Suggestions for revision or reasons for rejection

I think the direction of the paper is potentially highly scientifically productive in terms of using observations and the GFdL model to explain changes in the (direct) radiative forcing over the time period of the MISR and MODIS time periods. Using this information in the differences between model and data could in principal then be used to provide a more accurate estimate of the anthropogenic forcing (since pre-industrial times). This version of the paper also improves on the previous version by removing much of the jargon and acronyms that plagued the previous version. However, there is still much work to do on this draft before it can be published. My main concerns are 1) the paper is still very challenging to read, possibly because it is too complicated and hence glosses over much of the technical details on how the results are generated and 2) there is no error analysis in the paper, despite substantive use of the word estimate and uncertainty.

My suggestion would be to divide this paper into two with the first paper only looking at the changes in AOD and evaluating how the GFDL model and emissions compares to and explains the observations. The second paper could then be used to project the AOD and its uncertainties into the different radiative forcing components and its variations and uncertainties which in turn would provide more confidence in the changes in forcing since 1850. Certainly, the forcing calculation since 1850 does not yet fit within the scope of this paper due to the lack of discussion on errors and model/data differences and likely issues in the forcing calculations in the tropics.

Regardless of the direction the authors want to take with this paper, there needs to be a description of the errors in the AOD comparisons and how these project onto the attribution of the AOD changes. Currently, the approach appears to be “the AOD’s and its trends look about the same, therefore they agree”, however without any description of the errors it is impossible to tell if this conclusion is robust. There is also no discussion on how these errors as well as errors in the interferences (e.g. albedo and its changes) may affect the projection of AOD variability to forcing variability. There is also a substantial issue with the model in the tropics and its unclear how this structural error projects into decadal and centennial variations in the forcing.

Specific Questions / Comments:

Section 2.2.How do you account for differences in AOD observed by MISR and MODIS?

Line 120 Equation 1: Please rename these acronyms (rsutcsaf, rustics)… Its not obvious to a reader that these should be related to “outgoing clear-sky shortwave radiation”, especially as a reader attempts to keep track of these (and other acronyms) throughout the paper

Line 145: What is this?: EBAFR hereafter (R:raw)? Which estimate are you referring too?

Line 195: What is a dec-1, decade-1?

Why is there a section on aerosol optical depth (from MISR and MODIS) and then another on observations from CERES?

Line 145 and supplemental, there is insufficient information on how you estimate the change in outgoing radiation after accepting for changes in albedo. Also, the variations over the oceans do not appear to follow any type of outflow pattern; are these related to changes in clouds?

The SYN product appears to be much cleaner relative to the EBAF products (variations over the ocean could be due to outflow of aerosols as opposed to cloud variability). Why not use this product for your evaluation?

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RR by Anonymous Referee #2 (19 Jun 2018)

ED: Reconsider after major revisions (28 Jun 2018) by Anne Perring

AR by Fabien Paulot on behalf of the Authors (20 Jul 2018) Manuscript

ED: Referee Nomination & Report Request started (02 Aug 2018) by Anne Perring

RR by Anonymous Referee #1 (14 Aug 2018)

ED: Publish subject to minor revisions (review by editor) (25 Aug 2018) by Anne Perring

Dear authors,

I appreciate your substantial revisions of this paper. I have received additional feedback from one of your previous reviewers and looked the paper over myself and I will be happy to publish the paper after a few small revisions.

First, please consider the referee's comment regarding your abstract. I generally agree that the abstract could represent the findings of the paper more simply and clearly. For example, I actually found your existing conclusions to be a clearer statement of the main points of the manuscript.

Second, a few small corrections and comments.

1. On line 116, I don't understand the origin of the rsutcs and rsutcaf acronyms. Please either use acronyms reflecting the descriptive terms in your text or, if those are common acronyms in the subfield, please state what they stand for. Not knowing what they stand for makes it difficult to follow discussion later when the come back up.

2. In section 2.2.2, I think it might help non-modelers and people not well versed in radiative transfer calculations to explicitly discuss the differences between the EBAF and SYN products. You do describe the SYN product but I have a hard time evaluating what agreement or disagreement with one or the other might mean since I don't have a great sense for how they relate to each other.

3. Line 156, recommend rewording "(MATCH) that" to "(MATCH), which"

4.Line 165, recommend rewording "environmental dataset" to "an environmental dataset"

5. I found lines 404-405 confusing. DRF for both 2001 and 2015 is already relative to 1850 correct? So is this sentence simply saying that DRF due to BC decreased by 75% from 2001 to 2015? The following sentence regarding sulfate is clearer. Please reword.

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AR by Fabien Paulot on behalf of the Authors (30 Aug 2018) Author's response Manuscript

ED: Publish as is (31 Aug 2018) by Anne Perring

AR by Fabien Paulot on behalf of the Authors (31 Aug 2018)

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Fabien Paulot on behalf of the Authors (13 Sep 2018) Author's adjustment Manuscript

EA: Adjustments approved (13 Sep 2018) by Anne Perring

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Short summary

Observations show that the sunlight reflected to space by particles has decreased over the US and Europe, increased over India, and not changed over China from 2001 to 2015. These changes are attributed to different types of particles, namely sulfate over the US and Europe, and black carbon, sulfate, and nitrate over China and India. Our results suggest that the recent shift in human emissions from the US and Europe to Asia has altered their impact on the Earth's outgoing energy.