the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Nov 2025
Join the dots on planetary boundary interactions
Abstract. Planetary boundaries (PBs) are complexly interlinked, as the transgression of either one can worsen the status of others. Such cascading processes can accelerate Earth system destabilization and shrink humanity's safe manoeuvring space. To demonstrate the crucial need to maintain multiple PBs, we unravel interactions between the three PBs for freshwater change, climate change and land-system change (representative of key biosphere–atmosphere feedbacks), and how they are linked via PB control and response variables. Thereby we exemplify how transgressions of these PBs are driven both directly by human activities and indirectly by biophysically or anthropogenically mediated effects of other PBs' transgressions. As we also highlight, measures to maintain a single PB – such as large-scale terrestrial carbon dioxide removal aimed at lowering pressure on the climate change PB – can unintentionally become a force of transgression of other PBs, creating new impacts. To identify fallacies and uncontrolled feedbacks that may put Earth system stability at further risk, we propose a systematic model-based assessment of interacting impacts of PB transgressions and of measures to maintain multiple PBs simultaneously.
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Status: open (until 29 Dec 2025)
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RC1: 'PB framework fixed or not?', Ruud van der Ent, 08 Dec 2025
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AC1: 'Reply on RC1', Dieter Gerten, 17 Dec 2025
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Thank you for your thoughts on this manuscript, which we will consider in a revised manuscript, certainly leading to an improvement in terms of its scope and content.
Yes, right, a prime focus of the Perspective paper (though not the sole one) is to suggest development of a quantitative framework for analyzing PB interactions and transgression impacts. Thanks for your related comment that we thereby may “reverse engineering the whole thing” – which however is not what we want to suggest, as we agree that the PB framework is not “perfect” and that the definitions and positioning of the PB values are not set in stone. Actually – as you encourage – the new knowledge generated from our proposed Earth system simulations and data analyses could demonstrate that the definitions of control variables may indeed require improvements and that the thresholds (boundary values, lower and upper end of uncertainty zone) may have to be adjusted. We will add a section in the revised manuscript to make this clear, highlighting it as one additional goal of the suggested comprehensive simulations.
In this context, in response to your last comment, we will also emphasize more than in the current manuscript that a multi-model approach (such as by using ISIMIP data) is highly recommended, as model results might differ significantly due to model-structural differences.
Finally, we will implement your suggestions for improving the Figure and the Table, and how they relate to each other.
Citation: https://doi.org/10.5194/egusphere-2025-4761-AC1 -
RC3: 'Reply on AC1', Ruud van der Ent, 17 Dec 2025
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Dear authors,
Thanks for your thoughts. It seems we are on the same page. I look forward to the revised version.
Kind regards,
Ruud van der Ent
Citation: https://doi.org/10.5194/egusphere-2025-4761-RC3
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RC3: 'Reply on AC1', Ruud van der Ent, 17 Dec 2025
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AC1: 'Reply on RC1', Dieter Gerten, 17 Dec 2025
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RC2: 'Comment on egusphere-2025-4761', Anonymous Referee #2, 14 Dec 2025
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- General comments
This Perspective proposes a new area of research activity devoted to exploring the interaction between the transgression of different Planetary Boundaries (PBs). As an exemplar, the paper considers the interaction between PBs associated with changes in freshwater, climate and land-use. This proposed Perspective takes an “Earth System” approach which is fully consistent with the remit of ESD.
However, in my view, this Perspective lacks the normal characteristics we should expect from a good Perspective: clarity of presentation, and a clear new direction for exploration. Regarding clarity, the paper is written in a rather wordy and intellectual way, and that makes it rather difficult to get to what the authors are proposing. The paper is also beset with technical terms and acronyms that exacerbate the lack of clarity (see ‘Specific Comments’ below).
Regarding direction, the paper seems to evenatually recommend that CMIP Earth System Model runs should be analysed to determine how differant scenarios (and mitigation strategies) result in different and interacting conseqeunces for PBs. However, there is no detail given on what this would entail (a new Model Intercomparison Project, or even new model runs with new scenarios?), and also no recognition that ESMs already simulate interacting impacts of water, land-use and climate (even though they don’t assume PBs to do that).
In truth, the PB approach is just one way to view changes to the Earth System, but the Perspective takes PBs as given (rather than recognising them as subjective expert judgements).
- Specific comments
The second paragraph (beginning “Against this backdrop..”) introduces various technical terms without clear definition (e.g. “control variable”, “biophysically mediated”, “reactive human-mediated”). I advise the authors to try to write introductory text in much more plan language. If some technical terms are unavoidable, these need to be simply defined upfront.
Arguably unnecessary additional terms are also introduced throughout (e.g. “blue” apparently means “streamflow”, and “green” apparently means “soil moisture”).
There are unnecessary acronyms too (e.g. PBcc would be easier to read if it was just written as the Climate Change PB, and likewise for PBLU and PBFC).
There is a lot of text used in describing offline approaches (e.g. LPJmL or ISIMIP), only to propose “an agenda for using Earth System Models”.
- Technical Corrections
There are relatively few technical details in this proposed Perspective, but that it a major limitation. It is easy to propose an ESM Model Intercomparsion Project, but much harder to design and run such a thing. What should be done? Analysis of existing CMIP runs, or new model runs? If new runs, what scenarios and interventions are to be considered
- Overall
In my view, this draft Perspective falls short on clarity and a clear new direction. Regrettably, I therefore cannot recommend publication in its current from.
Citation: https://doi.org/10.5194/egusphere-2025-4761-RC2
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- 1
This manuscript by Gerten et al. represents an interesting perspective on 3 intertwined planetary boundaries (PBs): freshwater, climate and land-system change. Below I provide some thoughts, and I hope the authors take the opportunity of the format of this journal to provide a quick response to allow for some more interactive discussion.
General comments
My take-home message from this manuscript is that the authors advocate for a model- and data-driven framework to analyse, monitor and predict the interactions, feedback and impacts of changing/transgressing (the control variables of) the PBs. I, agree with this message, and it would be good to take the PB interactions beyond expert elicitation and/or literature studies and a 'PB interactions and impacts simulator' would be fantastic. However, does this not mean that we would be reverse engineering the whole thing? It seems that the implicit assumption behind this manuscript is that the PB framework is ‘perfect’ and we just need to understand and model it better. However, when we have a model-framework in place as a sort of real digital twin of the earth system, then for all we know we might come to the conclusion that the PB framework needs to be partly overhauled or at least slightly adjusted. In fact, this is likely as such a modelling framework would likely yield new insights, which in turn should yield a revision of the PB framework. There would be nothing wrong with that as that is how science advances. However, this is a discussion that I miss in this perspective piece, so I hope the authors could reflect on what they think would happen with the PB framework itself when earth system processes become better understood and modelled following their own recommendations.
Specific comments:
For figure 1, please provide a legend of the arrows instead of explaining them in the caption only.
How exactly should a reader interpret the relationship between Table 1 and Figure 1? Are the arrows in Fig. 1 to be interpreted as response variables or not? It would be great if the authors could make this relationship clear.
The examples mentioned regarding a modelling framework sometimes refer to single models like LPJmL, but wouldn’t a multi-modelling framework be better, and if not, why not? It would be great to have the authors’ perspective.