Two decades of operating a heat early warning system: lessons from the Netherlands (2007–2025)

Introduction: The 2003 European heatwave marked a turning point in the development of heat early warning systems (HEWS), yet little is known about how these systems have evolved once implemented or about the rationale of underlying operational choices. Methods: This study examines the evolution of the Dutch heat warning system from its introduction in 2007 through 2025, operated by the Royal Netherlands Meteorological Institute (KNMI), focusing on changes in warning criteria, operational procedures, and their role in triggering the national heat-health action plan (HHAP). We analysed 18 internal evaluations of major heat events, policy documents, and institutional reflections from system developers and operators to trace the climatic, operational, and institutional drivers of system change. To assess future relevance, we applied the KNMI’23 climate scenarios to evaluate how current warning criteria perform under projected conditions for 2050 and 2100. Results: Results show a transition from a single trigger for the national heat-health action plan, toward a tiered warning system that increasingly integrates expert judgment and societal impact considerations alongside meteorological thresholds, with a fully impact-based Code Red. A key revision in 2021 removed minimum temperature requirements, leaving maximum temperature as the primary trigger. In 2024, the first evaluation of the national HHAP by health authorities, including epidemiological evidence, provides a basis for further development of the warning criteria. Conclusion: The Dutch case highlights how HEWS function as adaptive systems that must continuously balance operational simplicity, impact relevance, and future climate pressures. We conclude by situating these findings within ongoing research and developments, including the development of heat warnings for the tropical island of Bonaire (also under KNMI’s mandate), and introduction of the wet-bulb globe temperature as a complement to traditional heat warnings. These insights are relevant for advancing heat early warning systems in Europe and beyond.