CO<sub>2</sub>-equivalence metrics for surface albedo change based on the radiative forcing concept: a critical review

Bright, Ryan M.; Lund, Marianne T.

doi:https://doi.org/10.5194/acp-21-9887-2021

Articles | Volume 21, issue 12

https://doi.org/10.5194/acp-21-9887-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-21-9887-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 21, issue 12

Review article

|

01 Jul 2021

Review article |

| 01 Jul 2021

CO₂-equivalence metrics for surface albedo change based on the radiative forcing concept: a critical review

Ryan M. Bright and Marianne T. Lund

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Final revised paper (published on 01 Jul 2021)
Preprint (discussion started on 10 Dec 2020)

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Status: closed

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

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RC1: 'Review of Bright & Lund', Anonymous Referee #1, 12 Jan 2021
RC2: 'Review of Bright and Lund, ACPD', Anonymous Referee #2, 26 Feb 2021
AC1: 'Comment on acp-2020-1109', Ryan Bright, 30 Mar 2021

Peer-review completion

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

AR by Ryan Bright on behalf of the Authors (30 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (26 Apr 2021) by Philip Stier

RR by Anonymous Referee #2 (10 May 2021)

Suggestions for revision or reasons for rejection

Thanks to the authors for addressing all the reviewer comments. I have a few additional comments, which relate to new material which has been introduced at the suggestion of the reviewers. Subject to these new comments being addressed, I am happy for the manuscript to be accepted.

Figure 6:
I still think this figure is a bit confusing. The main text around line 353 says we have a new example where we harvest a broadleaf forest and plant an evergreen forest. This gives you the profile of RF in the solid blue line in fig 6. That is clear. However, the introduction of the red curves in the text is less clear. The dashed red is the sum of the RF over time after a 1kg pulse emissions of CO2. What exactly is the solid red line, i.e. what change in albedo is causing it? Can you explain how the two red curves relate to each other / why you are including them?

Section 6:
The equation for GWP* in Lee et al is the version from Cain et al (2019), but Lee et al say that (what they call) alpha is assumed to be zero for their case. Where you apply GWP* in fig 7, as you have a full time series, I think you can assume that alpha is not zero. You could then use the full equation which accounts for the average RF over the period Delta-t. As this accounts for the slower climate response to past changes to RF, perhaps GWP* will have better agreement to Delta T in fig 7b. The equation in Cain et al tried to improve on the Allen et al 2018 equation to have a better agreement with temperature, so it may do so in your example and I think it’s worth testing. If that isn’t possible, then I think you need to say that you haven’t used the extra term in Cain et al (and why) and discuss whether you think it would improve the agreement with temperature (or not). You may also want to then amend your discussion around line 581 related to GWP*.

Regarding the choice of time horizon in GWP* - the authors of GWP* use H=100 years and say that:

‘In defining CO2-e and CO2-e* emissions, we use 𝐻=100 years following established practice. Results under GWP* are insensitive to this provided 𝐻 is much greater than the lifetime of the SLCP because the absolute GWP of an SLCP becomes a constant at these timescales, while the AGWP𝐻 of the reference gas, CO2, increases linearly with 𝐻—see ref. 3 and Fig. 8.29 of ref. 14 Hence the 𝐻-dependence cancels out in the calculation of CO2-e* for both SCLP emissions and radiative forcing. In contrast, GWP-based CO2-e values for SLCPs scale approximately with 1/𝐻, making the nominal relative importance of SLCPs and cumulative pollutants acutely sensitive to this choice of time-horizon.’ (Allen et al 2018)

So I don’t think that GWP* uses a subjective choice of time horizon like standard GWP100 does, as you have said around line 447, and suggest that you discuss what is in the paragraph I have quoted instead.

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (19 May 2021) by Philip Stier

AR by Ryan Bright on behalf of the Authors (28 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (03 Jun 2021) by Philip Stier

AR by Ryan Bright on behalf of the Authors (03 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (04 Jun 2021) by Philip Stier

Short summary

Humans affect the reflective properties (albedo) of Earth's surface and the amount of solar energy that it absorbs, in turn affecting climate. In recent years, a variety of climate metrics have been applied to characterize albedo perturbations in terms of their CO₂-equivalent effects, despite the lack of scientific consensus surrounding the methods behind them. We review these metrics, evaluate their (de)merits, provide guidance for future application, and suggest avenues for future research.

CO2-equivalence metrics for surface albedo change based on the radiative forcing concept: a critical review

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CO₂-equivalence metrics for surface albedo change based on the radiative forcing concept: a critical review