Increasing Aerosol Direct Effect Despite Declining Global Emissions in MPI-ESM1.2

Hermant, Antoine; Huusko, Linnea; Mauritsen, Thorsten

doi:https://doi.org/10.22541/essoar.170158317.78990757/v1

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https://doi.org/10.22541/essoar.170158317.78990757/v1

Preprints

31 Jan 2024

| 31 Jan 2024

Increasing Aerosol Direct Effect Despite Declining Global Emissions in MPI-ESM1.2

Antoine Hermant, Linnea Huusko, and Thorsten Mauritsen

Abstract. Anthropogenic aerosol particles partially mask global warming driven by greenhouse gases, both directly by reflecting sunlight back to space and indirectly by increasing cloud reflectivity. In recent decades, the emissions of anthropogenic aerosols have declined globally, and at the same time shifted from the North American and European regions to foremost Southeast Asia. Using simulations with the Max Planck Institute Earth System Model version 1.2 (MPI-ESM1.2) we find that the direct effect of aerosols has continued to increase, despite declining emissions. Concurrently, the indirect effect has diminished in approximate proportion to emissions. In this model, the enhanced efficiency of aerosol radiative forcing to emissions is associated with less cloud masking, longer atmospheric residence time, and differences in aerosol optical properties.

Received: 24 Jan 2024 – Discussion started: 31 Jan 2024

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Journal article(s) based on this preprint

25 Sep 2024

Increasing aerosol direct effect despite declining global emissions in MPI-ESM1.2

Antoine Hermant, Linnea Huusko, and Thorsten Mauritsen

Atmos. Chem. Phys., 24, 10707–10715, https://doi.org/10.5194/acp-24-10707-2024,https://doi.org/10.5194/acp-24-10707-2024, 2024

Short summary

Antoine Hermant, Linnea Huusko, and Thorsten Mauritsen

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-224', Anonymous Referee #1, 20 Feb 2024
My understanding of the main objective of this manuscript is to explain why the global aerosol direct radiative forcing can exhibit a different trend than aerosol emissions in some climate models. The authors use the MPI-ESM1.2 model to quantify the historical direct, indirect and clear-sky aerosol forcings, and they find that the efficiency of aerosol direct radiative forcing is enhanced in the low-latitude regions such as the South and East Asia, where cloud masking is less and aerosol residence time is longer, compared to mid-latitude regions like Europe and North America. The work is interesting and falls within the journal’s scope, but there are major concerns on the methodology employed in the current manuscript.
The modeling approach to estimating the historical aerosol direct and indirect forcing from emissions used in MPI-ESM1.2 for this study is different from many CMIP6 models that represent aerosol effects through more explicit aerosol-cloud-radiation processes. This raises the concern of accuracy and consistency issues of the present study. The emissions of major anthropogenic species such as sulfur compounds and black carbon have been changing differently since 1980s across major source regions. However, the prescribed aerosol optical properties in the MPI-ESM1.2 simulations are based on measurements for 2005. I don’t think the year-2005 aerosol properties are representative of the past few decades, especially for this research focused on the time evolution of aerosol forcing. Given that the key conclusion on the impact of changes in the aerosol residence time between mid-latitude and lower-latitude emission regions, I wonder whether the assumptions and model representation of aerosol optical properties reflect such changes. How large is the uncertainty in aerosol forcing comparing to the magnitude of direct and indirect forcing?

The magnitude of present-day aerosol direct radiative forcing is larger than the CMIP model mean (Bellouin et al., 2020) and in the AR6 assessment. Is it because only sulfur aerosol is considered in the MPI-ESM1.2 model estimates? What other anthropogenic aerosol components (or precursor gases) are considered in the simulations? Figure 1 highlights the global anthropogenic SO2 emissions trend, but according to Hoesly et al. (2018), many other important components (e.g., BC, OC, NH3, NOx) had global increasing trends and very different regional trends in the past few decades. BC and OC are particularly important in aerosol direct forcing over South and East Asia, where this study emphasizes an increase in the sulfur emissions and direct forcing. Please explicitly evaluate the impact of BC and OC changes on the direct forcing trends.

The discussion of aerosol SSA got me confused, as there is not sufficient information on the other aerosol species and why biomass aerosol is relevant to the focus of anthropogenic forcing of this study. I also wonder whether the SSA is set to a spatiotemporally uniform value in the simulations, which comes back to my major concern on the methodology.

Although I don’t expect the MPI-ESM1.2 results to be similar to other CMIP6 models, it would be nice to see how comparable the aerosol forcing estimates of this study to other models or observational analysis.

Reference:
Hoesly, R. M., Smith, S. J., Feng, L., Klimont, Z., Janssens-Maenhout, G., Pitkanen, T., Seibert, J. J., Vu, L., Andres, R. J., Bolt, R. M., Bond, T. C., Dawidowski, L., Kholod, N., Kurokawa, J.-I., Li, M., Liu, L., Lu, Z., Moura, M. C. P., O'Rourke, P. R., and Zhang, Q.: Historical (1750–2014) anthropogenic emissions of reactive gases and aerosols from the Community Emissions Data System (CEDS), Geosci. Model Dev., 11, 369–408, https://doi.org/10.5194/gmd-11-369-2018, 2018.
Citation: https://doi.org/10.5194/egusphere-2024-224-RC1
- AC3: 'Reply on RC1', Antoine Hermant, 12 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-224/egusphere-2024-224-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-224-AC3
RC2:
'Review of Hermant et al.', Anonymous Referee #2, 03 Mar 2024
This short study uses the MPI-ESM climate model to suggest that aerosol direct radiative forcing has not followed the same decreasing trend as aerosol emissions. The authors explain that decoupling by an increase in aerosol radiative forcing efficiency (radiative forcing per unit emitted mass or per unit aerosol optical depth) due to a regional shift in aerosol distributions.
It is nice to see the possibility of a change in aerosol direct radiative forcing efficiency from changes in emission regions. That possibility has been mentioned in the past, including in the conclusion of the Bellouin et al. (2020) review paper cited by the authors, but I am not aware of a publication dedicated to the subject. The other results discussed by the authors (reduction in aerosol direct radiative forcing due to cloudiness, aerosol optical depth is more correlated than emissions with direct radiative forcing) have been discussed several times, especially in AeroCom papers, but they provide the context needed to explain the main result.
However, the study needs to go deeper when explaining the reasons for the change in radiative forcing efficiency, as I comment below. For this reason, I recommend additional analyses to clarify the drivers of aerosol direct radiative forcing efficiency in MPI-ESM.
Main comments:
The headline result that aerosol direct forcing is decoupled from aerosol emissions because of a change in radiative forcing efficiency is in many ways “built-in” the MAC-v2SP aerosol prescription used in MPI-ESM. The shapes and slopes of the curves shown on Figure 2 are a consequence of the choices made in MAC-v2SP in terms of aerosol plume properties and relative change in cloud droplet number. So the sensitivity to those choices needs to be explored more critically, specifically:
There is a disconnect in the study between SSA and direct radiative forcing efficiencies that I do not understand. In Figure 2 and 4, why is direct radiative forcing efficiency in South Asia so strong? South Asia aerosols are known to be strongly absorbing, so I would have expected their direct radiative forcing to be less negative for a given AOD compared to regions dominated with more scattering aerosols. Moreover, the suggestion in section 3.5 that SSA does not significantly drive direct radiative forcing efficiency seems to go against AeroCom findings (e.g., the large differences in normalised radiative forcing shown between different aerosol types in the Tables of Myhre et al. (2013) https://doi.org/10.5194/acp-13-1853-2013 ) and I do not understand why. Perhaps MACv2-SP aerosols are too dominated by sulfate?

The link between residence times (different removal rates depending on region) discussed in Section 3.4 and MAC-v2SP is unclear. MAC-v2SP is a combination of global aerosol modelling outputs and satellite retrievals, so it may implicitly account for different residence times, but that would not influence radiative forcing efficiencies, since MACv2-SP simply scales plumes up and down with emission rates. Unless that Section 3.4 discussion is of a speculative nature?

Other comments:
Lines 21-23: Note that Forster et al. (2021), cited just a few sentences earlier, prefers the aerosol-radiation and aerosol-cloud terminology instead of direct and indirect.

Line 29: Quaas et al. (2022) https://doi.org/10.5194/acp-22-12221-2022, and especially their Section 5, seems a very relevant reference here, and elsewhere in the paper as well.

Line 73: “as a two-sided version”. What does that mean?

Line 103: How are those “Tg of SO2 equivalent” calculated?

Line 128: “absorption prevails in the presence of clouds” – only if the aerosols are above clouds.

Line 151-152: Do you use the latest CEDS emissions for the MACv2-SP plume scaling, dated 21 April 2021? There have been significant changes in aerosol emission trends, especially over China, that would affect the results presented here.
Citation: https://doi.org/10.5194/egusphere-2024-224-RC2
- AC1: 'Reply on RC2', Antoine Hermant, 12 Apr 2024
  
  Publisher’s note: the content of this comment was removed on 12 April 2024 since the comment was posted by mistake.
  
  Citation: https://doi.org/10.5194/egusphere-2024-224-AC1
- AC2: 'CORRECT reply on RC2', Antoine Hermant, 12 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-224/egusphere-2024-224-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-224-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-224', Anonymous Referee #1, 20 Feb 2024
My understanding of the main objective of this manuscript is to explain why the global aerosol direct radiative forcing can exhibit a different trend than aerosol emissions in some climate models. The authors use the MPI-ESM1.2 model to quantify the historical direct, indirect and clear-sky aerosol forcings, and they find that the efficiency of aerosol direct radiative forcing is enhanced in the low-latitude regions such as the South and East Asia, where cloud masking is less and aerosol residence time is longer, compared to mid-latitude regions like Europe and North America. The work is interesting and falls within the journal’s scope, but there are major concerns on the methodology employed in the current manuscript.
The modeling approach to estimating the historical aerosol direct and indirect forcing from emissions used in MPI-ESM1.2 for this study is different from many CMIP6 models that represent aerosol effects through more explicit aerosol-cloud-radiation processes. This raises the concern of accuracy and consistency issues of the present study. The emissions of major anthropogenic species such as sulfur compounds and black carbon have been changing differently since 1980s across major source regions. However, the prescribed aerosol optical properties in the MPI-ESM1.2 simulations are based on measurements for 2005. I don’t think the year-2005 aerosol properties are representative of the past few decades, especially for this research focused on the time evolution of aerosol forcing. Given that the key conclusion on the impact of changes in the aerosol residence time between mid-latitude and lower-latitude emission regions, I wonder whether the assumptions and model representation of aerosol optical properties reflect such changes. How large is the uncertainty in aerosol forcing comparing to the magnitude of direct and indirect forcing?

The magnitude of present-day aerosol direct radiative forcing is larger than the CMIP model mean (Bellouin et al., 2020) and in the AR6 assessment. Is it because only sulfur aerosol is considered in the MPI-ESM1.2 model estimates? What other anthropogenic aerosol components (or precursor gases) are considered in the simulations? Figure 1 highlights the global anthropogenic SO2 emissions trend, but according to Hoesly et al. (2018), many other important components (e.g., BC, OC, NH3, NOx) had global increasing trends and very different regional trends in the past few decades. BC and OC are particularly important in aerosol direct forcing over South and East Asia, where this study emphasizes an increase in the sulfur emissions and direct forcing. Please explicitly evaluate the impact of BC and OC changes on the direct forcing trends.

The discussion of aerosol SSA got me confused, as there is not sufficient information on the other aerosol species and why biomass aerosol is relevant to the focus of anthropogenic forcing of this study. I also wonder whether the SSA is set to a spatiotemporally uniform value in the simulations, which comes back to my major concern on the methodology.

Although I don’t expect the MPI-ESM1.2 results to be similar to other CMIP6 models, it would be nice to see how comparable the aerosol forcing estimates of this study to other models or observational analysis.

Reference:
Hoesly, R. M., Smith, S. J., Feng, L., Klimont, Z., Janssens-Maenhout, G., Pitkanen, T., Seibert, J. J., Vu, L., Andres, R. J., Bolt, R. M., Bond, T. C., Dawidowski, L., Kholod, N., Kurokawa, J.-I., Li, M., Liu, L., Lu, Z., Moura, M. C. P., O'Rourke, P. R., and Zhang, Q.: Historical (1750–2014) anthropogenic emissions of reactive gases and aerosols from the Community Emissions Data System (CEDS), Geosci. Model Dev., 11, 369–408, https://doi.org/10.5194/gmd-11-369-2018, 2018.
Citation: https://doi.org/10.5194/egusphere-2024-224-RC1
- AC3: 'Reply on RC1', Antoine Hermant, 12 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-224/egusphere-2024-224-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-224-AC3
RC2:
'Review of Hermant et al.', Anonymous Referee #2, 03 Mar 2024
This short study uses the MPI-ESM climate model to suggest that aerosol direct radiative forcing has not followed the same decreasing trend as aerosol emissions. The authors explain that decoupling by an increase in aerosol radiative forcing efficiency (radiative forcing per unit emitted mass or per unit aerosol optical depth) due to a regional shift in aerosol distributions.
It is nice to see the possibility of a change in aerosol direct radiative forcing efficiency from changes in emission regions. That possibility has been mentioned in the past, including in the conclusion of the Bellouin et al. (2020) review paper cited by the authors, but I am not aware of a publication dedicated to the subject. The other results discussed by the authors (reduction in aerosol direct radiative forcing due to cloudiness, aerosol optical depth is more correlated than emissions with direct radiative forcing) have been discussed several times, especially in AeroCom papers, but they provide the context needed to explain the main result.
However, the study needs to go deeper when explaining the reasons for the change in radiative forcing efficiency, as I comment below. For this reason, I recommend additional analyses to clarify the drivers of aerosol direct radiative forcing efficiency in MPI-ESM.
Main comments:
The headline result that aerosol direct forcing is decoupled from aerosol emissions because of a change in radiative forcing efficiency is in many ways “built-in” the MAC-v2SP aerosol prescription used in MPI-ESM. The shapes and slopes of the curves shown on Figure 2 are a consequence of the choices made in MAC-v2SP in terms of aerosol plume properties and relative change in cloud droplet number. So the sensitivity to those choices needs to be explored more critically, specifically:
There is a disconnect in the study between SSA and direct radiative forcing efficiencies that I do not understand. In Figure 2 and 4, why is direct radiative forcing efficiency in South Asia so strong? South Asia aerosols are known to be strongly absorbing, so I would have expected their direct radiative forcing to be less negative for a given AOD compared to regions dominated with more scattering aerosols. Moreover, the suggestion in section 3.5 that SSA does not significantly drive direct radiative forcing efficiency seems to go against AeroCom findings (e.g., the large differences in normalised radiative forcing shown between different aerosol types in the Tables of Myhre et al. (2013) https://doi.org/10.5194/acp-13-1853-2013 ) and I do not understand why. Perhaps MACv2-SP aerosols are too dominated by sulfate?

The link between residence times (different removal rates depending on region) discussed in Section 3.4 and MAC-v2SP is unclear. MAC-v2SP is a combination of global aerosol modelling outputs and satellite retrievals, so it may implicitly account for different residence times, but that would not influence radiative forcing efficiencies, since MACv2-SP simply scales plumes up and down with emission rates. Unless that Section 3.4 discussion is of a speculative nature?

Other comments:
Lines 21-23: Note that Forster et al. (2021), cited just a few sentences earlier, prefers the aerosol-radiation and aerosol-cloud terminology instead of direct and indirect.

Line 29: Quaas et al. (2022) https://doi.org/10.5194/acp-22-12221-2022, and especially their Section 5, seems a very relevant reference here, and elsewhere in the paper as well.

Line 73: “as a two-sided version”. What does that mean?

Line 103: How are those “Tg of SO2 equivalent” calculated?

Line 128: “absorption prevails in the presence of clouds” – only if the aerosols are above clouds.

Line 151-152: Do you use the latest CEDS emissions for the MACv2-SP plume scaling, dated 21 April 2021? There have been significant changes in aerosol emission trends, especially over China, that would affect the results presented here.
Citation: https://doi.org/10.5194/egusphere-2024-224-RC2
- AC1: 'Reply on RC2', Antoine Hermant, 12 Apr 2024
  
  Publisher’s note: the content of this comment was removed on 12 April 2024 since the comment was posted by mistake.
  
  Citation: https://doi.org/10.5194/egusphere-2024-224-AC1
- AC2: 'CORRECT reply on RC2', Antoine Hermant, 12 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-224/egusphere-2024-224-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-224-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Antoine Hermant on behalf of the Authors (26 Apr 2024) Author's response Author's tracked changes Manuscript

ED: Reconsider after major revisions (08 May 2024) by Kostas Tsigaridis

AR by Antoine Hermant on behalf of the Authors (21 May 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (16 Jun 2024) by Kostas Tsigaridis

RR by Anonymous Referee #1 (28 Jun 2024)

RR by Anonymous Referee #2 (10 Jul 2024)

Suggestions for revision or reasons for rejection

I thank the authors for considering my comments, and especially for clarifying how aerosol absorption is represented in MACv2-SP. I have read section 4 of Stevens et al. (2017), which confirms that changes in radiative efficiency due to changes in aerosol composition in the different regional plumes are not represented in MACV2-SP. That choice was made despite evidence of there having been changes in radiative efficiency due to changes in aerosol composition. Stevens et al. (2017) resolves that contradiction by saying “experiments to judge the magnitude of [aerosol composition] changes are warranted” (last paragraph of their Section 4).

That built-in behaviour of MACv2-SP has clear implications for the present study. That lack of representation of changes driven by aerosol composition may let other drivers emerge, like cloud masking. So this MACv2-SP behaviour must be clearly documented in the paper: in the abstract, in the last paragraph of section 2.1, and in section 3.5, and in the conclusion, which could suggest a similar analysis in more complex aerosol-climate models.

There is also a point where the present paper seems to contradict Stevens et al. (2017). The authors write that in the revised section 2.1 that the plume scale factor applied to the reference year (2005) depends on SO2, NH3, and BC emissions. But Eq. 17 and Table 5 of Stevens et al. (2017) clearly say that only SO2 and NH3 are considered, not BC. The mention of BC in their Table 6 is only informative, although it suggests that accounting for BC would have been a good thing. As written in that paper, “Other factors, which do not correlate with regional NH3 and SO2 emissions” are not represented. So a decorrelation between BC and SO2 emissions (and the subsequent change in radiative forcing efficiency) is not possible in MACv2-SP. I am not sure to what extent that matters for this study – it depends on whether the decorrelation has happened in reality, especially in Southeast Asia. But the description of the method should make clear that BC is not involved in the scaling factors, and the implications of that choice should be discussed in sections 3.5 and the conclusion.

Hide

ED: Publish subject to minor revisions (review by editor) (12 Jul 2024) by Kostas Tsigaridis

AR by Antoine Hermant on behalf of the Authors (26 Jul 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (01 Aug 2024) by Kostas Tsigaridis

AR by Antoine Hermant on behalf of the Authors (06 Aug 2024)

Journal article(s) based on this preprint

25 Sep 2024

Increasing aerosol direct effect despite declining global emissions in MPI-ESM1.2

Antoine Hermant, Linnea Huusko, and Thorsten Mauritsen

Atmos. Chem. Phys., 24, 10707–10715, https://doi.org/10.5194/acp-24-10707-2024,https://doi.org/10.5194/acp-24-10707-2024, 2024

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Antoine Hermant, Linnea Huusko, and Thorsten Mauritsen

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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Short summary

Aerosol particles, from natural and human sources, have a cooling effect on the climate, partially offsetting global warming. They do this through direct (sunlight reflection) and indirect (cloud property alteration) mechanisms. Using a global climate model, we found that despite declining emissions, the direct effect of human-made aerosols has increased, while the indirect effect has decreased, attributed to the shift in emissions from North America and Europe to Southeast Asia.


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