Health authorities have sounded the alarm from North America to Europe and Asia, urging people to stay hydrated and shelter from the burning sun, in a stark reminder of the effects of global warming.
"These events will continue to grow in intensity, and the world needs to prepare for more intense heatwaves," John Nairn, a senior extreme heat advisor at the U.N.'s World Meteorological Organization, WMO, told reporters.
Heatwaves are among the deadliest natural hazards, with hundreds of thousands of people dying from preventable heat-related causes each year.
Nairn warned the health risk was growing rapidly, amid burgeoning urbanization, higher temperature extremes and aging populations.
In the short term, he said the recently-declared El Nino — a warming climate pattern that occurs every two to seven years — "is only expected to amplify the occurrence and intensity of extreme heat events."
But regardless of El Nino, the trend is clear, Nairn said, pointing out that the number of simultaneous heatwaves in the Northern Hemisphere had swelled six-fold since the 1980s.
"This trend shows no signs of decreasing," he said, warning of heatwaves' "quite serious impacts on human health and livelihoods."
Europe, the world's fastest-warming continent, was bracing for the peak of the current heatwave to hit Italy's Sicily and Sardinia islands, amid forecasts of a high of 48 degrees Celsius (118 degrees Fahrenheit).
The WMO said it was monitoring to see if the current European temperature record of 48.8C recorded on Sicily in 2021 might be smashed.
Even more concerning than maximum day temperatures was the high overnight minimum temperatures, Nairn said.
"Repeated high night-time temperatures are particularly dangerous for human health, because the body is unable to recover from sustained heat," he said.
"This leads to increased cases of heart attacks and death."
There is currently no clear definition of what constitutes a heatwave, but the WMO said it was in the process of developing an overarching categorization of heatwave intensity, in a bid to "standardize impact forecasts and warnings worldwide."
Experts say human-induced climate change is exacerbating heatwaves, bringing higher temperatures and also slowing down and "parking" hot weather systems over locations for longer periods of time.
Asked what should be done to counter this, Nairn said the message was "simple:" "Stop carbon fuels; just electrify everything."