The evolution of the Dutch heat early warning system (2007 - 2025)

Heat early warning systems (HEWS) are critical tools for anticipating hazardous heat and enabling timely response. While some of the earliest systems were developed over two decades ago, little is known about how they have evolved in practice. This study examines the evolution of the Dutch heat warning system from 2007 to 2025 and evaluates the future performance of current criteria. We compiled archival records of heat warnings and Heat-Health Action Plan (HHAP) activations at the provincial level to analyse historical frequency, duration, and spatial coverage. In addition, 18 key documents, including seven post-event evaluations, were analysed to identify operational challenges and drivers of change. Current warning criteria were applied to KNMI’23 climate scenarios to assess future warning frequency, duration, intensity, and season length. Between 2007 and 2025, KNMI issued 21 Code Yellow (5.7 days/year) and 5 Code Orange (0.8 days/year) heat warnings; no Code Red has been issued. The system evolved from a single HHAP trigger into a tiered warning system, while the HHAP remained linked to lower warning levels, creating misalignment between warnings and actions. Criteria remained primarily climatological due to limited epidemiological evidence. Combined Tmax-Tmin criteria introduced in 2015 proved operationally challenging and were removed in 2021. Projections indicate a substantial increase in heat warnings, from ~2 per year currently to 3.9-7 by 2050 and up to 29 by 2100. This case study shows how HEWS are shaped by response systems, operational experience, and changing climate risks, with implications for improving their design and governance worldwide.