Interacting tipping elements increase risk of  climate domino effects under global warming

Wunderling, Nico; Donges, Jonathan F.; Kurths, Jürgen; Winkelmann, Ricarda

doi:https://doi.org/10.5194/esd-12-601-2021

Articles | Volume 12, issue 2

https://doi.org/10.5194/esd-12-601-2021

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

https://doi.org/10.5194/esd-12-601-2021

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

Articles | Volume 12, issue 2

Research article

|

03 Jun 2021

Research article |

| 03 Jun 2021

Interacting tipping elements increase risk of climate domino effects under global warming

Nico Wunderling, Jonathan F. Donges, Jürgen Kurths, and Ricarda Winkelmann

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Final revised paper (published on 03 Jun 2021)
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Preprint (discussion started on 03 Apr 2020)
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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: 'The authors' findings may be too conceptual to tell something on a real climate.', Anonymous Referee #1, 06 Apr 2020
- AC1: 'Reviewer Response', Nico Wunderling, 14 Jun 2020
RC2: 'Review report', Anonymous Referee #2, 20 Apr 2020
- AC2: 'Reviewer Response', Nico Wunderling, 14 Jun 2020
EC1: 'Conclusion of the discussion phase', Michel Crucifix, 25 Jun 2020

Peer-review completion

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

ED: Reconsider after major revisions (25 Jun 2020) by Michel Crucifix

AR by Nico Wunderling on behalf of the Authors (08 Aug 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Aug 2020) by Michel Crucifix

RR by Anonymous Referee #2 (28 Aug 2020)

Suggestions for revision or reasons for rejection

In response to my earlier review, the authors have added the cascading analysis without ENSO
(in the SM) and have added more argumentation on why ENSO is considered, and to support
the coupling of the different tipping elements. The paper has improved, but I think it still needs
a round of revision to make it suitable for publication in Earth System Dynamics, following
the comments below.

Major:

1. The justification of modelling ENSO through an equation (1) is not adequate. In the
deterministic case, the ENSO transition is between a fixed point and a limit cyle. In the
limit cycle, El Nino's occur but also La Nina's and although the mean state is slightly
skewed towards the warm phase, the effect of the oscillation on the mean state (and hence on the
coupling to the other tipping elements) is rather small. The permanent El Nino state
in the Pliocene is much debated as many GCMs just show El Nino variability during
this period, and it is not relevant for the present climate change context as the
geometry of the Pacific was different (open Panama gateway). As the results for
the case without ENSO (Figs. S3-S5) are not essentially diffferent (apart from the Amazon
Forest behavior in Fig. S5), the paper would be much better when the results without ENSO would
appear in the main text and the results with ENSO are only discussed in the last section
of the paper, mentioning the caveat that one cannot justify (1) for ENSO.

2. In section 2, it should at least be argued very well (i) why each of the tipping
elements considered can be represented by an equation (1), e.g. based on conceptual
models, and (ii) that there are indications from models and/or observations of
coupling between the tipping elements with a plausible physical description. Issue
(i) is mentioned in lines 116-124, but the references for the MIS are rather
sparse (e.g. papers Weertman, Schoof) so this needs to be extended. Issue (ii) is
dealt with in lines 167-176 and Table 2, but this by far insufficient and (apart
from the ENSO-AMOC connection) without any references to model/observation
results. One cannot simply refer only to Kriegler et al., as that assessment was very
rough (Fig. 2 in their paper) and more than 10 years old.

Minor:

1. l123-124: A collapse in the AMOC only affects the position of the
Hopf bifurcation. So clarify what is meant here.

2. l187-188: Current GCMs (in particular with high resolution ocean
models) can adequately resolve nonlinear behavior in
ENSO.

3. The x-axis label in Fig. 2 is not readable (at least in my .pdf file). Please
adapt.

4. In Fig. 3, there is a small transition in ENSO which is also reflected
in the WAIS response. However, it stays within the baseline regime for ENSO.
What is this small transition and what is causing it?

5. I miss a discussion on the time of transition and the delayed effect one
transition has on the occurrence of another one. Here the ice sheets play a
dominant initiator role simply because their temperature threshold is lowest.
However, it takes a significant amount of time for subsequent meltwater
(and sea level) to affect other tipping elements. Of course, it is not in the
approach followed in the paper but it is relevant to discuss in the last section.

Finally, the software used for the results in the paper should be made
publicly available (e.g. through github) so other reseachers can check
the computations.

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ED: Reconsider after major revisions (14 Sep 2020) by Michel Crucifix

AR by Nico Wunderling on behalf of the Authors (25 Jan 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 Feb 2021) by Michel Crucifix

RR by Anonymous Referee #2 (04 Mar 2021)

Suggestions for revision or reasons for rejection

It is a pleasure to see that the authors have taken the time to
produce a much better paper than the previous versions. Section
2 now contains a much better justification of the methodology and
the coupling between the tipping elements. Also the section 3.4 on
including ENSO is now much better positioned. I still have some
relatively minor comments, and I hope the authors will consider
these to improve the manuscript.

1. l28: It is better to use Global Mean Surface Temperature than
Global Mean Temperature, because otherwise this can be
confused with a volume average, and this is what the authors
use.

2. l39: Specify better where 'this' refers to.

3. l73: the Marine Ice Sheet Instability -> local Marine Ice Sheet instabilities
(these are local instabilities related to the topographic bottom slope)

4. l77: the influence of Greenland melt water on the AMOC is relatively small;
the weakening is mainly due to a changing surface buoyancy forcing. Many
CMIP5/6 models show this decline even when there is no melt water input.

5. l93: Following the introduction, in -> In

6. caption Figure 1, l6: initiates -> initiates cascades

7. l120: c_{i 1,2} -> c_{i}^{1,2} or c_{i}^{\pm}

8. l145: In the Mecking et al. 2016 paper a weak AMOC state is found
over a few hundred years. However, the pattern of the AMOC does not
correspond to a collapsed state, so this is not really evidence of a
multiple equilibrium regime.

9. l140-l151: It is much more convincing to cite papers where explicit
bifurcation diagrams have been computed for global ocean models,
e.g. Huisman et al., JPO, 40, 551, (2010).

10. l150: AMOC -> AMOC is

11. l201-212: The effect of Greenland melt water on the AMOC in the
present-day climate is considered to be weak, so I would recommend
to restrict to the paleoclimate `evidence' here. If one forces a climate
model with realistic melt water fluxes, the response of the AMOC cannot
be distinguished from the intrinsic variability of the AMOC.

12. l233: conducted hosing experiment -> the release of freshwater in the
Southern Ocean.

13. l244: `stabilization' is confusing here. Probably it is meant that the AMOC
amplitude increases.

14. l278, l280, equation (3): it is \Delta T_{limit,i} instead of T_{limit,i}, in
correspondence with Table 1.

15. l321-322: model years -> time
(`model years' always have units)

16. l355: `randomly' is not specific enough. What distribution is used?

17. l381: 'likely due to' is too vague. For such a simple model, this can be precisely determined so please do so.

18. l431: the multiple equilibria view on ENSO has long been abandoned (it is
also not in the Dekker et al. (2018) paper), so please omit it here.

19. l499: start a new paragraph

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ED: Publish subject to minor revisions (review by editor) (05 Mar 2021) by Michel Crucifix

AR by Nico Wunderling on behalf of the Authors (15 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (07 Apr 2021) by Michel Crucifix

AR by Nico Wunderling on behalf of the Authors (12 Apr 2021) Manuscript

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Short summary

In the Earth system, climate tipping elements exist that can undergo qualitative changes in response to environmental perturbations. If triggered, this would result in severe consequences for the biosphere and human societies. We quantify the risk of tipping cascades using a conceptual but fully dynamic network approach. We uncover that the risk of tipping cascades under global warming scenarios is enormous and find that the continental ice sheets are most likely to initiate these failures.