Drought as a Continuum: quantifying global Spatiotemporal Connectivity of Drought Events

Droughts are systemic hazards with far-reaching consequences for food security, economic stability, and the environment. While traditionally characterised by deviations from normal conditions over static spatial areas or point-based time series, droughts are increasingly recognised as dynamic continuous processes with large memory effects that propagate through interlinked hydrological, ecological, and socio-economic systems (i.e. “drought as a continuum”). Despite this conceptual shift, gaps remain in capturing the evolving nature of droughts as they move across space and persist through time. This study presents a novel object-based tracking framework based on a three-dimensional Density-Based Spatial Clustering of Applications with Noise (DBSCAN) for identifying and characterising droughts as explicit spatiotemporal entities at the global scale. The proposed methodology integrates in a novel way the Standardized Precipitation-Evapotranspiration Index (SPEI) at two complementary scales: SPEI-01 to capture rapid onset and SPEI-03 to monitor evolving persistence. The spatiotemporal identification of drought events is achieved through a two-stage clustering process: first, a 2D DBSCAN identifies spatial clusters from instantaneous intensity values; second, these entities are integrated into a 3D DBSCAN framework to establish connectivity across the temporal dimension, defining cohesive drought events globally. Additionally, we introduce a novel Drought Event Index, a composite metric synthesising an event’s duration, pace, extent, and intensity into a single metric that enables direct comparison of drought events across diverse geographical locations and historical periods. Methods are applied to the ERA5 reanalysis dataset for the period 1940-2025. Results indicate a marked increase in the frequency and intensity of drought events in recent decades compared to the period 1950-1990, while accurately identifying the spatiotemporal dynamics of recent significant events around the globe, such as the 2018 and 2022 drought events in Europe, and the unprecedented 2019-2025 multi-year droughts in South America. The proposed methodological framework evaluates dynamics often unaccounted for by static analysis, thus enabling the quantitative assessment of droughts as a continuum at the global scale and across different timescales.