A historic heatwave that has gripped western Europe for the past week is now migrating eastwards, prompting Paris to impose emergency restrictions on alcohol sales as the French capital braces for temperatures exceeding 42°C. The Ile-de-France region has activated its heatwave red alert, the highest level, for the first time since the deadly 2003 event that claimed an estimated 15,000 lives across France. This decision includes a ban on the sale of alcohol in supermarkets and grocery stores between 2 PM and 6 PM, as well as restrictions on open-air containers, to prevent dehydration and alcohol-related accidents, which spike during extreme heat periods.
The meteorological office Météo-France reports that the current heatwave is driven by a stationary high-pressure system over the Bay of Biscay, funnelling hot air from the Sahara. This system is forecast to drift north-eastward over the next 48 hours, with the UK Met Office issuing an amber warning for parts of southern England and the Midlands, where temperatures could reach 38°C by midweek. The physical mechanism is unambiguous: a combination of a strong Azores High and a jet stream positioned far north has created a heat dome that acts as a planetary-scale lid, trapping solar radiation and preventing convection.
The immediate concern for public health is the wet-bulb temperature threshold. At 42°C with 20% humidity, the wet-bulb temperature remains below the survivability limit of 35°C, but the elderly, the young, and those with pre-existing conditions are at acute risk. Paris is activating 1,500 cooling centres and extending public pool hours.
The restrictions on alcohol are a pragmatic step: alcohol acts as a diuretic, accelerating fluid loss and impairing the body’s thermoregulatory capacity. The heatwave is part of a larger pattern. The planet has now recorded 13 consecutive months of record-breaking average global temperatures.
The likelihood of such extreme events has increased by a factor of at least ten since the pre-industrial era, directly attributable to anthropogenic greenhouse gas emissions. The current event is fuelled by ocean heat content anomalies in the North Atlantic, which are at levels 1.5°C above the 1991-2020 baseline.
This heatwave is not an anomaly; it is a repeat of a phenomenon we have documented since the 2003 event. The physical reality is that the atmosphere can hold approximately 7% more water vapour per degree Celsius of warming, which means heatwaves are not just hotter but also more humid, pushing the wet-bulb temperature closer to lethal limits. As the heatwave shifts east, the UK is activating its own emergency protocols.
Network Rail has imposed speed restrictions on trains to prevent track buckling, and the NHS is preparing for an influx of heat-related admissions. The UK Health Security Agency has issued a Level 3 heat-health alert, urging vulnerable groups to stay indoors. The economic cost of such extreme weather is often understated: labour productivity losses alone for the UK during a 3-day heatwave are estimated at £600 million.
The key number to watch is the wet-bulb temperature. The current forecast for London on Wednesday shows a maximum of 38°C with humidity around 30%, yielding a wet-bulb temperature of approximately 26°C. While not immediately life-threatening for healthy adults, it is well above the threshold where outdoor manual labour becomes hazardous.
The restrictions in Paris are a necessary adaptation, but they treat the symptom, not the cause. The cause is a destabilised climate system. Every fraction of a degree of warming increases the probability of these events.
The question is not if the UK will see similar restrictions, but when.
