the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Climate tipping point interactions and cascades: A review
Nico Wunderling
Yevgeny Aksenov
Stephen Barker
Robbin Bastiaansen
Victor Brovkin
Maura Brunetti
Victor Couplet
Thomas Kleinen
Caroline H. Lear
Johannes Lohmann
Rosa Maria Roman-Cuesta
Sacha Sinet
Didier Swingedouw
Ricarda Winkelmann
Pallavi Anand
Jonathan Barichivich
Sebastian Bathiany
Mara Baudena
John T. Bruun
Christiano M. Chiessi
Helen K. Coxall
David Docquier
Jonathan F. Donges
Swinda K. J. Falkena
Ann Kristin Klose
David Obura
Juan Rocha
Stefanie Rynders
Norman Julius Steinert
Matteo Willeit
Abstract. Climate tipping elements are large-scale subsystems of the Earth that may transgress critical thresholds (tipping points) under ongoing global warming, with substantial impacts on biosphere and human societies. Frequently studied examples of such tipping elements include the Greenland Ice Sheet, the Atlantic Meridional Overturning Circulation, permafrost, monsoon systems, and the Amazon rainforest. While recent scientific efforts have improved our knowledge about individual tipping elements, the interactions between them are less well understood. Also, the potential of individual tipping events to induce additional tipping elsewhere, or stabilize other tipping elements is largely unknown. Here, we map out the current state of the literature on the interactions between climate tipping elements and review the influences between them. To do so, we gathered evidence from model simulations, observations and conceptual understanding, as well as archetypal examples of paleoclimate reconstructions where multi-component or spatially propagating transitions were potentially at play. Lastly, we identify crucial knowledge gaps in tipping element interactions and outline how future research could address those gaps.
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Nico Wunderling et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1576', Steven Lade, 30 Aug 2023
This article is a thorough, well-written and timely review of interactions between climate tipping points.
Section 2 is the key part of the paper. I am not sufficiently familiar with recent research on tipping point interactions to evaluate the statements made there. However all sound broadly plausible. I focus my attention instead on the other Sections.
Abstract
- A review paper should deliver findings or "insights". No insights are present in the abstract. It is all about the research gap and data sources. What are the key results from this paper? That tipping cascades are likely/unlikely to occur within the next couple of centuries?That interactions are mostly stabilising / destabilising? Rather than saying that you "identify crucial knowledge gaps", what are examples of the most urgent of those gaps?
Section 1 introduces the study and makes some key definitions. These definitions could be tightened. For example:
- Lines 23-24 need work. A non-linear response (of output to input) is far from sufficient for a tipping point (e.g. the response could be x^2 rather than x). It is not clear what reorganisation means in this context. The reference cited is Armstrong McKay et al. but this is not the definition that those authors use.
- What exactly do “nonlinear behaviour” at line 27 and "nonlinear component" at line 49 mean? The relationship between driver and response is nonlinear? There is behaviour that matches that from nonlinear differential equations?
- Line 28: ”we also consider elements that can show nonlinear behavior without being tipping elements on their own.” In Fig 1, all elements are listed as either “tipping elements” or “speculative tipping elements” (which is not defined, but I presume means could be confirmed as a tipping element with more data). There is no category “nonlinear but non-tipping element” (to use the authors’ language). Which are the "elements that can show nonlinear behavior without being tipping elements on their own”?
- Line 63 says that Arctic sea ice has almost a linear response to CO2 forcing, but then in line 81 say that “sea ice switches” are a “nonlinear element”. Is this the same sea ice phenomenon?
Section 2.8
- Line 383: The finding by Wang et al. that "a tipping point cascade with large temperature feedbacks over the next couple of centuries remains unlikely" is cited uncritically. However, many interactions in Table 1 / Fig 2 occur on centuries or less. How do you reconcile these two viewpoints? Are there likely to be cascades within the next couple centuries that have significant impacts on climate and/or people?
Sections 3 and 4
- These sections are interesting but introduced poorly. What purpose does introducing these examples serve? I see it as mainly establishing that tipping cascades are plausible, but this point is not made.
- In what sense is the speculated cascade in section 4 an “archetypal example” (see title of Section 4)? It's a speculation; in my opinion certainly not an archetype.
Section 6
- Section 6 is very good at outlining possible directions for future research. I would like to see more discussion of the implications for non-researchers. Does this study show policymakers should be worried about tipping point interactions?
Comments on methods
- The paper lacks description of the literature review method. How do you know you haven’t missed key literature on interactions? The method doesn’t necessarily have to be a systematic review — it may be possible to argue that snowball is also acceptable.
- Section 1 mentions that any "linkage between tipping elements" is called a tipping interaction, with one element tipping causing another element tipping the "extreme case". Section 6 acknowledges that tipping interactions may be non-stationary, that is, the interaction may change between the cases described above. Section 3, however, mixes these cases. For example, GIS->AMOC describes effects of any GIS melting on the AMOC. While 2.2.2 describes the effects of AMOC tipping. I suggest to either make the choice to not distinguish this difference more clear earlier on, or to include in e.g. Table 1 a summary of what type of interaction (e.g. pre or post tipping) was used in each assessment.
Minor Comments
- Line 194 - so winter Arctic sea ice could be considered a tipping element even though summer is not? If so it would be helpful to state more directly.
- I don't understand the title of section 2.2.1. Do you perhaps mean "differentiating" or "distinguishing"?
Citation: https://doi.org/10.5194/egusphere-2023-1576-RC1 -
RC2: 'Comment on egusphere-2023-1576', Anonymous Referee #2, 31 Aug 2023
This is a nice, generally well and clearly written review that concentrates on interactions between tipping elements and the potential for cascades or stabilisations. Section 2 and table 1 are particularly useful. Some of the language is verging towards hyperbole in places however and could be more precise and scientific, particularly in section 1.
I have a few comments and questions listed below:
Line 21: ‘These processes are at the heart of tipping behavior in the climate system and were found in numerous subsystems of the climate system.’ Is this actually true? There are plenty of hypothesized tipping points in simple models and some GCMs but in the actual climate system? This sentence needs further qualification, it currently reads as if there is tipping all over the real world.
Line 70: ‘While most TEs that have 70 been proposed so far are clearly regional (with some being large scale), there are significant knowledge gaps with respect to their tipping probability, impact estimates, time scales, as well as their interactions.’ Why are there significant knowledge gaps? It would be good to outline the main reasons for this after this sentence. This would be very useful information for the reader. Is the main reason the lack of evidence of tipping in observations?
Line 358: ‘Insofar the hydrological cycle due to Permafrost changes may have far-reaching impacts.’ Sentence does not read well. Change to something like ‘Permafrost changes may impact the hydrological cycle with far-reaching impacts.’
Line 360: Title could be misleading. Suggest change to ‘Interactions between multiple tipping elements and planetary scale cascades.’ or something similar (global scale cascades etc)
Title of section 3: I suggest you exchange ‘Archetypal’ with ‘Possible’ or just remove archetypal – Archetypal suggests that these are well accepted examples which, later in 3.1 are acknowledged to only be ‘possible’.
Section 3.1: While the example is interesting from an Earth history point of view, is it really relevant to the subject of the review i.e. abrupt changes causing other abrupt changes? I’m fine with the inclusion of this material but this feels more like ‘discussion/outlook’ content
Section 3.2, line 444: …’ with temperature increase in Greenland by 10-14◦C 445 over a few years; Andersen et al. (2004)’. Over a few years?! Is this true? That is incredibly fast. I read the citation (Andersen, 2004) with this statement and found no evidence of this claim. Please look at this again.
Same thing again, title of section 4: Remove ‘Archetypal’
What is the difference between a tipping element and a nonlinear climate component? It would be good to define their use somewhere in the manuscript, ideally in the introduction.
Line 547: ‘Thus, (4) there is a potential cascading risk of large carbon releases to the ocean and atmosphere due to the coastal collapse. At the same time, coastal ecosystems would be impacted through increased nutrients and other terrigenous matter fluxes as well as local communities and economies (fisheries and infrastructure collapse).’ I think more careful qualification is needed here. This is essentially a regional (Arctic) effect rather than a global one?. I suggest adding this qualification i.e. : ‘Thus, (4) there is a potential cascading risk of large carbon releases to the Arctic ocean due to the coastal collapse. At the same time, Arctic coastal ecosystems would be impacted through increased nutrients and other terrigenous matter fluxes as well as local communities and economies (fisheries and infrastructure collapse).’
Section 5: Are there any examples of cascades of tipping points in reasonably realistic models (EMIC, GCMs) under realistic forcing? These examples would be good to note here for the reader. If they have not yet been found, this would also be good to note in this section.
Citation: https://doi.org/10.5194/egusphere-2023-1576-RC2 -
RC3: 'Comment on egusphere-2023-1576', B. van der Bolt, 06 Sep 2023
This is a well-written, interesting and thorough review of the interactions between tipping elements and the potential for cascades. What was unclear to me, however, was the relation between section 3 and 2. In addition, the purpose of section 4 is not clear at all, and the link between section 5 and the first sections is also not clear. To me, it seems that your article is split up in the following parts: introduction of the relevant concepts, overview of the possible interactions between climate tipping elements, evidence for tipping cascades from paleoclimatic data, and model approaches to study tipping cascades. But how these parts lead up to an overall conclusion or overview, remains unclear. The article could be improved by better explaining the argumentation/story line of the paper and improving the connection between the different sections. In addition, I have some small comments which can be found below.
Please not that I do not have sufficient expertise on the topic of tipping point cascades and all the different climate elements to judge whether the main part of the paper includes all the relevant literature and if the information in that section is correct. I therefore focus in that part of the paper on the readability.
Section 1:
Line 22: the words ‘were found in numerous subsystems of the climate system’ implies to me that they are observed in these subsystems, while the evidence for this is more nuanced. This part would be me accurate if you mention that there are indications for tipping point behaviour in these subsystems, based on model observation and paleoclimate datasets.
Line 47: what do you mean with ‘non-linear components’ and how do they differ from the tipping elements you introduced in the section before?
Lines 50-52: Why do you introduce the different types of mathematical bifurcations in relation to the tipping cascades, and not in section 1.1 when you describe the definition of tipping elements?
Lines 63-64: The readability of this sentence would be improved if you cut it up into two sentences.
Section 2:
Figure 1: The coral reef circles have – I think these are islands – in them, which makes the legenda a bit confusing. The figure would be more clear if you position these circles in a location where there is no island underneath (I do really like the coral reef icon in this figure).
Line 305: ‘decreasing resilience’ in this sentence now seems like one of the causes for a decrease in recovery time, while the decrease of resilience is caused by the changes mentioned earlier in the sentence (warming temperatures… ocean acidification). This sentence could be formulated more clearly to show that the resilience decreases as a consequence of these changes.
Section 3
Title section 3: I suggest you change this title to ‘Possible examples of interactions…”
Line 440: I suggest you change ‘chapter’ to ‘section’ and include an introductory sentence to the other sections of section 3, or remove this one for consistency.
Section 4
The purpose of section 4 is not clear to me. How does it relate to the previous sections and what does it add to the possible examples from the paleoclimate? To me, this example seems more like one of the possible cascades as introduced in section 2, than an archetypical example of nonlinear climate component interactions.
Citation: https://doi.org/10.5194/egusphere-2023-1576-RC3 -
RC4: 'Comment on egusphere-2023-1576', Anonymous Referee #4, 12 Sep 2023
The authors present a valuable review on the emerging aspect of Tipping Points (TPs) of TP interactions and TP cascades. Possible interactions are put into a spatial and temporal context, and the potential stabilizing and destabilizing processes are described. A specific example from the past is presented and a sequence of processes in the atmosphere, ocean and on land is sketched. The authors also offer a present-day example: a tipping point in Arctic ice loss triggers a tipping point in coastal retreat. While the paper represents a valuable resource for scholars to inform themselves about this emerging topic, the considerations are speculative in many instances and important caveats and flags are missing. The authors should thoroughly parse the text for such speculations and hypotheses. A more nuanced text and a more formalized way of addressing uncertainties and limits in scientific understanding and knowledge should be the ambition for a revised version of this paper, before it is acceptable for publication.
Specific Comments:
1) Table 1 provides a systematic description of the different links between two TPs depicted in Figure 1. Column 3 is key in stating the stabilizing or destabilizing nature of the link. However, the column sometimes also provides an uncertainty qualification, such as “highly uncertain”. This is not done in this table in a systematic way. Here it is recommended that the IPCC approach on uncertainty language is adopted (Mastrandrea, M.D., et al., The IPCC AR5 guidance note on consistent treatment of uncertainties: a common approach across the working groups, Clim. Change, 108, 675-691, 2011.). Where possible, also the concept of risk should be applied (Reisinger et al., The Concept of Risk in the IPCC Sixth Assessment Report: A Summary of Cross-Working Group Discussions. IPCC, 2020). This would increase the value of this review, would make it more consistent with the broader, comprehensive assessments which also address TPs in several places, and provide consistency in this very complex topic.
2) In order to facilitate the implementation of 1) above, it is suggested that the authors add two additional columns in Table 1 before the columns collecting the references: (i) a column stating the uncertainty and the level of confidence, and (ii) a column stating the risk associated with the realization of this particular “cascade step” (from TP1 to TP2).
3) In the present-day example depicted in Fig. 4. the TP 2 “Coastal retreat” is illustrated (more on this below). This is missing from the map of Fig. 1 and the spatio-temporal diagram of Fig. 2. Since this is one of only two examples illustrated in a Figure, it would be important to include this in the two first figures.
4) Whether “coast retreat” is a TP or not is hard to qualify. Currently, the case for a TP is not sufficiently well made or convincing.
5) Line 130: here some basic refs are missing, e.g.: Hu, A.X., et al., Energy balance in a warm world without the ocean conveyor belt and sea ice, Geophys. Res. Lett., 40, 6242-6246, 2013.
6) line 320: increased freshwater flux and AMOC reduction, along with a southward shift of the ITCZ appears many times in this manuscript. Please reduce redundant repetition.
7) lines 338-339: “relationship” is vague. Please clarify.
8) line 348: The high-latitude response of the hydrological cycle to an increase of GHGs is pretty robust (see IPCC, 2021). You presumably mean the regional response based on a catchment area perspective. Any ref for this?
9) line 372-378: this is an uncritical repetition of claims and speculations regarding the “hothouse” as a future possibility, promoted by Steffen et al. in 2018 and since then reiterated in several other papers (e.g. Kemp et al., 2022). The authors should provide the appropriate caveats, or else point to original research – not perspective papers or such – that simulate such effects. Repeating catch words without firm evidence should not feature in a review article and is not useful for the progress in this important topic.
10) line 378: the breakup of stratocumulus decks, proposed by Schneider et al, is an interesting hypothesis valid in very limited areas in the subtropics. Furthermore, the tipping appears to occur at CO2 levels above 1200 ppm which is very high. Whether or not this purported instability, shown in a limited area LES model only, would translate into a large-scale effect is currently a speculation. This review should present this accordingly.
11) Figure 3 illustrates a specific example in the paleoclimate record and panel b) provides some linkages. Some of these linkages are more robust, others are highly uncertain. It would be important to reflect this in, e.g., the line thickness of the arrows. Otherwise, the impression is given that all these links are equally well understood and quantified by, e.g., model simulations.
12) line 614: “We conclude that tipping elements interact …” This appears as a very strong statement of fact. The authors provide no evidence for such robustness and hence a more cautious formulation should be chosen.
13) line 634: “tremendous”. Why should a non-quantitative study based on questions and conversations (expert elicitation) be of “tremendous value”? This is overselling this type of information gathering. Of more value would certainly be targeted simulations across a hierarchy of models, careful parameter and sensitivity studies, and large ensemble simulations. The justification of the “tremendous value” is that this would provide “direct expert input”. Such “direct expert input” should be reflected in the authorship of, e.g., review papers.
14) line 676: This review paper ends by referencing a highly speculative piece which was a perspective paper, provocative and stimulating debate. However, it did not contain quantitative analysis or original research. Therefore, it should be considered an opinion piece and thus be treated in a scientific review accordingly. By citing such work in later scientific journals without the proper qualifiers lends undue support for what originally was a stimulating or provocative idea. Clearly, such work can and should be cited, but the context must be given appropriately.
Citation: https://doi.org/10.5194/egusphere-2023-1576-RC4
Nico Wunderling et al.
Nico Wunderling et al.
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