Reaching 1.5 and 2.0&thinsp;°C global surface temperature targets using stratospheric aerosol geoengineering

Tilmes, Simone; MacMartin, Douglas G.; Lenaerts, Jan T. M.; van Kampenhout, Leo; Muntjewerf, Laura; Xia, Lili; Harrison, Cheryl S.; Krumhardt, Kristen M.; Mills, Michael J.; Kravitz, Ben; Robock, Alan

doi:https://doi.org/10.5194/esd-11-579-2020

Articles | Volume 11, issue 3

https://doi.org/10.5194/esd-11-579-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/esd-11-579-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 11, issue 3

Research article

|

14 Jul 2020

Research article |

| 14 Jul 2020

Reaching 1.5 and 2.0 °C global surface temperature targets using stratospheric aerosol geoengineering

Simone Tilmes, Douglas G. MacMartin, Jan T. M. Lenaerts, Leo van Kampenhout, Laura Muntjewerf, Lili Xia, Cheryl S. Harrison, Kristen M. Krumhardt, Michael J. Mills, Ben Kravitz, and Alan Robock

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Final revised paper (published on 14 Jul 2020)
Preprint (discussion started on 03 Dec 2019)

Interactive discussion

Status: closed

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

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RC1: 'Review of esd-2019-76', Anonymous Referee #1, 04 Jan 2020
- AC1: 'Authors response to Referee 1', Simone Tilmes, 05 Feb 2020
RC2: 'Review of 'Reaching 1.5 °C and 2.0 °C global surface temperature targets using stratospheric aerosol geoengineering'', Anonymous Referee #2, 07 Jan 2020
- AC2: 'Authors response to Referee 2', Simone Tilmes, 07 Feb 2020

Peer-review completion

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

ED: Reconsider after major revisions (22 Feb 2020) by Valerio Lucarini

AR by Simone Tilmes on behalf of the Authors (13 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Apr 2020) by Valerio Lucarini

RR by Anonymous Referee #1 (08 Apr 2020)

Suggestions for revision or reasons for rejection

The authors have provided a reasoned reply to my original set of comments, and I find that the overall message is now much clearer than in the previous version of the paper. I also appreciated the addition of some technical details, such as in Appendix A1. However, there are still a number of inaccuracies and minor issues that I would suggest the authors to address before the paper may be published in ESD.

1. In the abstract, the authors state that: ”The combination of using an overshoot scenario as a baseline that limits the needed amount of SAG applications and the use of a feedback algorithm to reach predefined temperature targets in model experiments is expected to reduce some of the earlier identified side effects of SAG.” This is a somewhat ambiguous statement, as it places on the same level the feedback algorithm and the choice of scenario. However, the feedback algorithm is an implementation method, while the choice of scenario is a somewhat arbitrary assumption on our future social, political and economic development. I think it is important to clearly distinguish between the two, and concisely motivate in the abstract the relevance of using an overshoot scenario. To me, this relevance is not that it limits the needed amount of SAG applications – in that case the most logical scenario to use would be SSP1-1.9 – but rather that it is conducive to investigating a peak-shaving scenario, which is a potentially effective proposal for how to deal with the ongoing rapid climate change.

2. In the abstract, the authors write: ”Others, including global precipitation changes and the recovery of the Antarctic ozone hole, depend strongly on the amount of SAG application”. But is it not the case that they depend: “on the amount and details of SAG application”?

3. Related to (1) the above, in the introduction I would discuss the relation between the overshoot scenario and peak shaving in the paragraph starting on l. 55, instead of the one starting on l. 68. Else the reader in the latter paragraph doesn’t immediately understand the logic of using an OS scenario and how this combines “two main objectives that have only been addressed separately in previous studies”.

4. l. 137 I don’t quite understand this sentence: “While different ensemble members may reach the target temperature at different times, they still have to be setup to reach the same temperature targets and may start at the same time.” What do the authors mean by “start at the same time”? Do they refer to the SAG intervention? Please rephrase.

5. l. 144 Is there a specific reason for picking 180° W or is longitude largely irrelevant and just an arbitrary choice? Either way, please specify this in the text.

6. l. 150 The authors mention the SSP-8.5 scenario here, but only describe the relevant experiment in the paragraph starting on l. 161. It may be more logical to invert the order of the two and first describe all experiments conducted here and then describe the feedback controller and temperature gradient constraints.

7. l. 333 Note that this only holds in the case of a perfectly Gaussian distribution. A more robust approach would be using percentiles of the single-gridpoint distributions.

8. Sect. 4.6/Fig. 10b The offset in the starting point of the Antarctic SMB is quite striking. I would suggest adding a comment concerning the fact that the GrIS is much closer to the historical range in 2030 than the AIS.

9. Sect. 4.7 When discussing the ozone here, or in the conclusions section, it may be worth briefly mentioning that our view of the recovery of the ozone hole through column-integrated measurements may be somewhat biased when focussing exclusively on the high latitudes (e.g. Ball et al., 2018).

https://www.atmos-chem-phys.net/18/1379/2018/

10. Sect. 5 Perhaps worth mentioning again the ”peak-shaving” concept in the first paragraph as motivation for using the OS scenario?

11. Paragraph starting on l. 423: I find the paragraph to be poorly written and the overall message it carries unclear. Specfically:
l. 423 ” to control for”  “to achieve” or “to follow” or similar
l. 424 “can be done in”  “can be done by”
ll. 424-425 Specify what these improvements are (i.e. reduced climate/bio-geophysical impacts, as opposed to improvements in actually meeting the global mean temperature target).
l. 427 “and are therefore provide a complete picture”  “and therefore provide a more complete picture”
”Both limited applications of SAG and improved climate targets result in reduced climate impacts and risks, and are therefore provide a complete picture for studying impacts on society and ecosystems than much larger scale SAG applications.” By “improved climate targets” do you mean following future scenarios of lower emissions? In any case, I do not think that lower emissions and lower SAG injections “provide a complete picture for studying impacts on society and ecosystems than much larger scale SAG applications”. The completeness of the picture is not related in any way to the level of climate impacts and risks. Indeed, one may have a very complete picture of a high-risk scenario or a very incomplete picture of a low risk scenario.

12. l. 435 “result in small differences in the amount of warming in high latitudes between a 1.5 °C and a 2 °C temperature target”. Is this really the case? For the SH I agree, but for the NH high latitudes the difference between the OS1.5 and OS2.0 cases seems to be in the region of 0.5 °C. Perhaps providing exact numbers would help here.

13. Formula A1: I am a bit puzzled by the notation. Having a summation over index j, without any j dependence in the summed terms would equal to multiplying these terms by k. Also, shouldn’t the LHS of the equation have some dependence on “i”? I admit I am confused by this equation, and the individual terms/indices should be explained more clearly in the text.

Other Remarks

l. 170 ”Geo RCP-85 1.5” make sure this matches the label in Table 1 and its caption.

l. 191 ”could be differences”  “could be due to differences”

l. 198 ”falling”  “fall”

l. 222 ”increasing greenhouse gases”  “increasing greenhouse gas concentrations”

Table 1: Missing °C in the left-most column. Final entry reads “2.”

l. 414 ”and”  ”or”

Sect. 4.4 Since the section only discusses data for SMB, it would be better to add ”surface” in the title.

l. 434 “three temperature targets” The temperature targets are only two, and one is considered for two different baseline scenarios.

l. 447 “illustrate”  “illustrates”

Caption Fig. 1 “methan”  “methane”

Fig. 1a Even though you specify the units in the title it is always good practice to label the axes too.

Fig. 1 In all panels, the units (K) appear twice.

Fig. 3 Caption “result”  “results”

Fig. 5 I struggle to identify the significant regions (to me, all regions in all panels appear to be “shaded in colour”). Also, what test was used to define the significance of deviations from climatology?

Fig. 7 “The shaded area is 1 standard deviation of 450 years pre-industrial control simulation” I am not sure I see the logic in this. Why add the PI SD to the SSP scenarios? If the aim is to provide a SD comparison, a table or some numbers in the text would be more indicated.

Fig. 8 Caption “pannel”  “panel”

Fig. 8 “Hatched regions are areas with changes within 1 standard deviation of 450 years pre-industrial control simulation.” It doesn’t make much sense to show this unless it is discussed/commented upon in the paper.

Fig. 9 See comment (7) above concerning the 95% level.

Fig. 11 Caption: “indicats”  “indicates”

Fig. 11 Caption: “from the one-member simulations.” Don’t all simulations have two members? Do the authors mean “the individual model realisations”?

Figure A1 caption: it would be helpful to include these functional shapes in the description of the algorithm in the Appendix text.

Fig. A2, A3 As above: how is significance computed?

Fig. A4 Would it be possible to add a reference historical range as done in Fig. 10?

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RR by Anonymous Referee #2 (23 Apr 2020)

ED: Publish subject to minor revisions (review by editor) (26 Apr 2020) by Valerio Lucarini

AR by Simone Tilmes on behalf of the Authors (07 May 2020) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (20 May 2020) by Valerio Lucarini

AR by Simone Tilmes on behalf of the Authors (21 May 2020) Author's response Manuscript

ED: Publish as is (03 Jun 2020) by Valerio Lucarini

AR by Simone Tilmes on behalf of the Authors (09 Jun 2020) Author's response Manuscript

Short summary

This paper introduces new geoengineering model experiments as part of a larger model intercomparison effort, using reflective particles to block some of the incoming solar radiation to reach surface temperature targets. Outcomes of these applications are contrasted based on a high greenhouse gas emission pathway and a pathway with strong mitigation and negative emissions after 2040. We compare quantities that matter for societal and ecosystem impacts between the different scenarios.