Untangling the fragmented landscape of extreme heat services and warning systems

Extreme heat warning systems are expanding globally, yet remain conceptually fragmented and operationally diverse. With a myriad of heat indices in use and limited guidance on their purpose or performance, countries risk adopting ineffective systems misaligned with local risks and decision-making needs. This Perspective traces the roots of this fragmentation across disciplinary, operational, and institutional lines, showing how differing approaches from health, meteorology, and climate science have led to incompatible definitions and thresholds. We then propose a clear typology of heat indices, aligned with WMO guidance: (1) temperature indicators, (2) thermal indices, and (3) heatwave intensity indices. The typology clarifies what each type measures, where it performs best, and the trade-offs involved, helping systems move toward greater transparency, coherence, and fit-for-purpose. Each type offers distinct strengths, and many countries will benefit from layered approaches that combine them. Moving toward intensity-based approaches represents a conceptual shift, from identifying hot days to quantifying the severity of heatwaves. By aligning early warning systems with this understanding, countries can improve coordination, reduce health and societal impacts, and accelerate progress under global frameworks such as the UN’s Early Warnings for All initiative.