Italy put 23 cities on red alert as temperatures touched 46° Celsius on Wednesday, one of the global hotspots as a wave of extreme heat, wildfires and flooding wreak havoc from the US to China.

An intensifying heatwave has hit southern Europe during the peak summer tourist season, breaking records including in Rome and bringing warnings about an increased risk of deaths and heart attacks.

Wildfires burned for a third day west of the Greek capital Athens, with air-water bombers resuming operations at first light and firefighters working throughout the night to keep flames away from coastal refineries.

Fanned by erratic winds, the fires have gutted dozens of homes, prompted hundreds of people to flee and blanketed the area in thick smoke. Temperatures could climb to 43° C on Thursday, forecasters said.

In China, which this week was hosting US climate envoy John Kerry for talks, tourists defied the heat to visit a giant thermometer showing surface temperatures of 80° C.

In Beijing, which set a new record as temperatures remained above 35° C for the 28th day in a row, Kerry expressed hope that cooperation to combat global warming could redefine troubled ties between the two superpowers.

A global pattern of heatwaves that have scorched parts of Europe, Asia and the US this week has thrown that challenge into sharp relief.

Temperatures remained high across much of Italy on Wednesday, with 45-46° C expected on the Mediterranean island of Sardinia and some inland areas of Sicily likely to see temperatures in the mid-40s.

The health ministry said it would activate an information hotline and teams of mobile health workers visited the elderly in Rome. “These people are afraid they won’t make it, they are afraid they can’t go out,” said Claudio Consoli, a doctor and director of a health unit. Joggers took to the streets of the capital early in the morning in an effort to beat the heat.

While the heatwave appears to be subsiding in Spain, residents in Greece were left surveying the wreckage of their homes after the wildfires.

“Everything burned, everything. I will throw it all, it’s all waste. The boiler burned, it’s done, it melted,” said Abbram Paroutsidis, 65.

Spain warned of the risk of wildfires in most of the country though residents were allowed to return to their homes in La Palma island where a blaze that raged for five days was stabilised in one sector, although it remained active elsewhere.

Scientists have long warned that climate change, caused by greenhouse gas emissions mainly from burning fossil fuels, will make heatwaves more frequent. They say governments need to drastically reduce emissions to prevent climate catastrophe.