Quantifying Planetary Boundaries interaction strengths with expert knowledge elicitation

The concept of planetary boundaries (PBs) was developed to set biophysical limits to human perturbations and to maintain the Earth System at its current steady-state. Research has focused on further updating and improving the PBs, while utilizing them as the conditional basis for sustainable development. A limitation of the current approaches, and focus of our work, is that the PBs closely related to food production are assessed individually without considering their interactions and feedbacks. These PBs include surface water use, land-system change, biogeochemical flows, and biosphere integrity. Such an omission could potentially overestimate the margin for food production within PBs on a local scale, which could have negative implications for sustainable food supply.  

Here, we aim to quantify these interactions with the ultimate goal of estimating a more realistic safe operating space (SOS) for future food production. We build on earlier literature review-based work that identified and quantified many PBs interactions on global scale but was unable to identify and quantify some of the interactions that are important to food production. Thus, we move a step forward by using expert knowledge elicitation to quantify the PBs interactions important to food production at local scale and to qualitatively map the mediating biophysical mechanisms. Expert knowledge elicitation suits the study well since it can fill knowledge gaps when quantitative data is scarce. In this work, we identified the missing links and expanded our knowledge on existing PBs interactions. Following recent work on updating PBs definitions, we divided the biosphere integrity PB into land, freshwater, and ocean components and the surface water PB into blue and green water components. These divisions accommodate for the differences among the Earth System functions. Where needed, we developed new interim control variables to enable quantifying the interaction strengths.

The expert knowledge elicitation was conducted remotely following the IDEA elicitation protocol and utilizing a custom-made web application. A total of 37 experts, in various fields of Earth sciences, completed the process and we received input for all 42 interactions, ranging from 5 to 19 responses each, with a median response rate of 9. We collected both quantitative and qualitative data on interaction strengths, tipping points, and mediating mechanisms, which are aggregated and used in synergy to better describe complex Earth System processes. In addition, we aim to highlight the most important interactions in an effort to prioritize them based on their role in the Earth System and existing knowledge gaps.