Approaches for aggregating data from the regional to the global level: importance for conclusions on Earth system state

A number of initiatives attempt to delimit the safe operating space (SOS) for human pressures on the Earth system, including the Planetary Boundaries framework. In some cases, data describing regional status are spatially aggregated to provide a global assessment. Several aggregation approaches can be observed, and the chosen approach may impact the conclusions. This study systematically reviews approaches of aggregating regional environmental boundaries and their state at the global level and uses a case study to compare them, aiming to highlight assumptions and implications and show how inconsistent approaches affect the accuracy and comparability of global boundary states. In the comprehensive literature review, 25 studies dealing with spatial aggregation of regional occupation of SOS and 43 associated regional boundary records were identified and categorized according to five spatial aggregation approaches and five types of adjustments that apply across approaches. These approaches were further classified as high- and low-risk approaches based on their assumptions and value judgments regarding precautionary levels and accepted regional transgressions. Notably, key publications dealing with multiple environmental boundaries use different aggregation approaches across the boundaries, potentially introducing biases. The application of these approaches to a case study revealed that the choice can influence the resulting aggregated occupation of SOS substantially, impacting conclusions as to whether or not a boundary is exceeded. To mitigate biases and inconsistencies, future estimates of spatially aggregated regional SOS should transparently communicate the assumptions underlying the chosen aggregation approach, address potential inconsistencies across boundaries, and advance our understanding of spatial propagation mechanisms.