Belgium opens debate on redefining heatwaves as hotter summers test old thresholds

Belgium’s Royal Meteorological Institute (IRM) acknowledged on 13 August that the country may need to reconsider how it defines heatwaves as summers grow hotter, while stressing that no new national definition has yet been approved. The discussion matters immediately because the familiar label — five consecutive days of at least 25°C at Uccle, including three reaching 30°C — is a climatological measure, not by itself a complete assessment of danger to people across Belgium.

For residents, employers and public services, that distinction can determine how a hot spell is understood, even though official health warnings already use separate criteria. The IRM says its national climatic definition is based on observations at its reference station in Uccle, in the Brussels-Capital Region. A heatwave can therefore be recorded for the national climate series even when conditions differ substantially in coastal West Flanders, central Brussels, the Campine or the Ardennes — and intense local heat may occur without the Uccle sequence qualifying.

The pressure for a broader approach follows an exceptional early episode. According to the IRM’s June 2026 climate assessment, Uccle experienced an official 12-day heatwave from 17 to 28 June, the 51st recorded since measurements began in 1892. Seven days reached at least 30°C, while the highest minimum temperature, 24.1°C, set a heatwave record. June’s second half averaged 24.7°C, far above its 17.4°C normal, and Ophoven in Limburg reached 39.4°C on 26 June.

These figures sharpen the central question: should a definition designed to preserve a consistent climate record also describe present-day health risk? Keeping fixed thresholds allows scientists to compare like with like across decades. The IRM’s 2020 climate report found that Belgian heatwaves had already become more numerous since 1981, by about 0.3 events per decade, while tending to last two days longer and become more intense per decade. Changing the historical yardstick could complicate that valuable comparison.

Public-health specialists and policymakers are asking a different question. Federal Climate Minister Jean-Luc Crucke told the Chamber that Belgium should modernise heat-alert criteria to include night-time temperatures, humidity, duration and observed health effects. He distinguished that work from the climatic definition and said the federal government, the regions and communities were pursuing an interfederal plan. The existing National Ozone and Heat Plan already separates vigilance, warning and alarm phases; the federated entities implement most preventive measures, while a federal Risk Management Group can coordinate an alarm.

The health evidence explains the urgency. Sciensano recorded 1,460 excess deaths during and immediately after the August 2020 heat episode, most among people over 65, although it cautioned that specific causes of death were not yet available. High night-time temperatures are particularly important because bodies and overheated buildings receive little chance to cool. People living alone, young children, chronically ill patients, outdoor workers and residents of poorly insulated homes face disproportionate risks.

France offers a contrasting model rather than a ready-made solution. Météo-France defines canicule using locally calibrated daytime and night-time thresholds sustained for at least three days, reflecting health risk at departmental level. Belgium’s single Uccle benchmark is better suited to a national climatological series, but less sensitive to regional exposure and urban heat islands. Europe’s wider trajectory supports reviewing warning systems: the Copernicus Climate Change Service and World Meteorological Organization describe Europe as the fastest-warming continent and report a long-term rise in heat-stress days.

The sensible outcome may therefore be two complementary instruments: a stable IRM definition for historical climate monitoring and more geographically precise, health-based alerts for operational decisions. Crucke has said Belgium’s heatwave response will be evaluated and that work on an interfederal plan will continue. What remains unknown is whether the IRM will alter its climatic threshold, whether authorities will instead revise only warning criteria, and when residents will see a unified system that clearly distinguishes a statistical heatwave from a dangerous local one.