Incremental improvements of 2030 targets insufficient to achieve the Paris Agreement goals

Geiges, Andreas; Nauels, Alexander; Parra, Paola Yanguas; Andrijevic, Marina; Hare, William; Pfleiderer, Peter; Schaeffer, Michiel; Schleussner, Carl-Friedrich

doi:https://doi.org/10.5194/esd-11-697-2020

Articles | Volume 11, issue 3

https://doi.org/10.5194/esd-11-697-2020

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

https://doi.org/10.5194/esd-11-697-2020

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

Articles | Volume 11, issue 3

Research article

| Highlight paper

|

03 Aug 2020

Research article | Highlight paper |

| 03 Aug 2020

Incremental improvements of 2030 targets insufficient to achieve the Paris Agreement goals

Andreas Geiges, Alexander Nauels, Paola Yanguas Parra, Marina Andrijevic, William Hare, Peter Pfleiderer, Michiel Schaeffer, and Carl-Friedrich Schleussner

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Final revised paper (published on 03 Aug 2020)
Supplement to the final revised paper
Preprint (discussion started on 20 Sep 2019)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Stong submission but one significant oversight', Anonymous Referee #1, 22 Dec 2019
- AC1: 'Author response', Andreas Geiges, 09 Mar 2020
RC2: 'Very good work, but much to improve', Anonymous Referee #2, 28 Jan 2020
- AC2: 'Author response', Andreas Geiges, 09 Mar 2020

Peer-review completion

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

ED: Reconsider after major revisions (18 Mar 2020) by Ning Zeng

AR by Andreas Geiges on behalf of the Authors (25 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (14 Apr 2020) by Ning Zeng

RR by Anonymous Referee #1 (01 May 2020)

Suggestions for revision or reasons for rejection

(1) General comments/suggestions

This revised paper begins by translating the Nationally Determined Contributions from the Paris Agreement into emissions scenarios, also creating several mitigation emissions scenarios based on the NDC emissions scenarios. Then, using the simple climate model MAGICC6, the paper creates projections of global surface temperature and examines a sampling of consequences to that warming climate. The revision is in response to two initial referee reviews, both of which appreciating the study’s design, the results presented, and the paper’s readability, while both also having requests for major revisions.

Referee 1 expressed concern over the use of MAGICC6 as representative warming in the future, and referee 2 offered varied criticism on the choices and depth of climate consequences examined and how the data was presented. The authors resisted many of these concerns, although a large amount of new prose and a number of figure alterations now appear in response most (but not all) of referee 2’s criticisms in the revised paper.

Some further discussion concerning the authors’ responses to referee 1 are presented below. This issue between the authors and referee 1 may not be able to be resolved. Even if that is the case, some notable revisions would still be suggested before this paper would be ready for acceptance. While one sentence was added to section 2.3 of the paper in response to referee 1, there is enough material in AR1.01 and AR1.02 that an entire paragraph could and should be added to section 2.3 instead so the authors can solidly assure future readers of their confidence in MAGICC6. In a similar fashion, a sentence or two based on material from AR2.04 should be added to section 2.4.2 to further justify that this quantification of extreme warmth (TXx) is a robust measurement.

(2) Detailed responses

There seems to be differing interpretations of Figure 4 of Schwarber et al. 2019. While the authors do note that MAGICC6 (orange line) falls within the range of CMIP5 results (grey lines), the figure caption points out that it is the bold grey lines that show CMIP5 models whose ECS values (range 2.5-3.5°C) are similar to the ECS (3°C) driving the MAGICC6 run. Of these bold grey lines, all but one fall below the orange line of MAGICC6. While Schwarber et al. do state, as the authors quote, that “the comprehensive SCMs can generally replicate the long-term results of general circulation models”, and the SCM results do fall within the overall range GCM results, that “general” replication fails somewhat when more specifics are considered and the SCMs still fall within the upper half of GCM results.

The focus on the hiatus period in the second half of AR1.01 is less important – and less certain – than the authors argue. In particular, the provided figure from Tokarska et al. shows very high uncertainty in the estimate of cooling from Pacific variability and moderate uncertainty in the effect of changed forcings, with difficulty in comparing the forcing values' effects due to the different combinations of stratospheric volcanic aerosols, solar variation, and anthropogenic aerosols. All options in that category consider volcanic stratospheric aerosol optical depth as part of their changed forcing, but there is some doubt on the actual strength of SAOD cooling. Chylek et al. 2020 (doi: 10.1029/2020GL087047) provides some evidence that SAOD cooling is lower than previously suggested, which would reduce the effect minor volcanoes have had on the hiatus unless the climatic SAOD response is highly nonlinear. Even if the hiatus was understood with the confidence suggested by the authors, particularly with the forcing corrections, it is worth noting that Tokarska figure also shows how the newer generation of CMIP6 models does an even worse job of matching observed warming than the CMIP5 models did, with a slight positive difference between CMIP6 and CMIP5 over the 2006-2015 time period relative to 1986-2005, a fact closely mirrored in the Discussion section of Tokarska et al. 2020 (doi: 10.1126/sciadv.aaz9549).

On one final side note, both referee 1’s figure 11.25 from AR5 and the authors’ figure from Tokarska et al. 2019 have the drawback that the baselines used in each are relatively recent. Since comparisons to the Paris Agreement goals of 2.0 and 1.5°C are based on the preindustrial baseline, these figures both make the model projections appear closer to observed temperature than they would if all temperature data was instead presented on the preindustrial baseline.

(3) Minor issues & corrections

Line 24: extraneous comma

Line 37: extraneous comma or “and” (hard to tell which)

Line 62-64: slightly awkward compounding of phrases; might need either some readjustment or need to be split into two sentences

Line 68: Footnote 2 is awkwardly worded. Also, the acronym LULUCF appears in the footnote, but the acronym itself is not spelled out until line 75. This likely will not be an issue for most readers, but the ordering seems a bit odd

Line 148: First appearance of the acronym GMT, which is not defined. I assume this means “Global Mean Temperature” and likely should be first defined in section 2.3

Line 187: “1.5C” is missing the degree symbol

Line 191: same as above

Figure 3: The addition of the horizontal guidelines on the new CDFs are relatively clear, much more so than the PDFs in the previous version, but the figure is still hard to read. It was previously mentioned that increasing the size of the regional panels would be “technically difficult”. If the difficulty is fitting the regional panels above their respective regions, it might be easier to move the panels off and around the map itself and use line segments to connect each panel to its region; the map could also be scrapped altogether if the panel labels were the full name of each region instead of abbreviations, especially as those abbreviations are not spelled out elsewhere in the paper.

Line 194: first and only appearance of the acronym SREX, which is not defined

Table 2: Awkward spacing. The first and third rows of the table have the text middle-adjusted, while the second and fourth rows have the first column top-adjusted and the other columns bottom-adjusted.

Overall: The authors have done a moderately good job of addressing the concerns of the initial two referee responses, though a bit more could still be done. Aside from that, this remains a clear, accessible presentation of alternate NDC-based warming scenarios and their impacts.

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ED: Publish subject to minor revisions (review by editor) (01 May 2020) by Ning Zeng

AR by Anna Wenzel on behalf of the Authors (19 May 2020) Author's response

ED: Publish as is (31 May 2020) by Ning Zeng

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Andreas Geiges on behalf of the Authors (14 Jul 2020) Author's adjustment Manuscript

EA: Adjustments approved (17 Jul 2020) by Ning Zeng

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Short summary

Current global mitigation ambition in the National Determined Contributions (NDCs) up to 2030 is insufficient to achieve the 1.5 °C long-term temperature limit. As governments are preparing new and updated NDCs for 2020, we address the question of what level of collective ambition is pivotal regarding the Paris Agreement goals. We provide estimates for global mean temperature increase by 2100 for different incremental NDC update scenarios and illustrate climate impacts under those scenarios.