| 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.
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.