The heat wave in Europe has so far claimed the lives of about 1,600 people in Spain and Portugal, according to estimates published Thursday by officials in both countries.
At the beginning of the week, temperatures in England also reached over 40 degrees Celsius (104 Fahrenheit), breaking heat records in the country.
In the last two days, historical heat records have also been smashed in Scotland, with 35.1 degrees Celsius (95 F) and 37.1 (98 F) in Wales. Germany's Hamburg also crossed the 40 degree (104 F) mark for the first time in its history.
Heat waves raise the risk of strokes, harm kidney and heart health, and potentially cause death from heat stroke or dehydration. They are seen as a factor in "excess mortality" – an increase in the annual death rate in a certain nation when compared to the average in earlier years.
The vast heatwave covering swathes of Europe moved steadily eastwards on Thursday, forcing countries including Italy, Poland and Slovenia to issue their highest heatwave alerts as firefighters battled wildfires across the continent.
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Greece, which contained a huge wildfire that raged near Athens for two days and was fanned by high winds, urged Europe to do more to tackle climate change.
"The climate crisis is now evident across Europe, with particular intensity in the wider Mediterranean region. The cocktail of high temperatures, gusty winds and heavy drought inevitably leads to wildfires," government spokesman Giannis Oikonomou said on Thursday.
"Europe must act in a coordinated and rapid manner to reverse the climate crisis," Oikonomou told reporters. "The solution cannot be given at a national level, because the problem is transnational and huge."
Greek firefighters had tackled 390 forest fires in one week, about 50-70 blazes a day, he said. According to the meteorological station in Penteli outside Athens, where the fire broke out on Tuesday, winds reached 113 km per hour (70 mph) at one point.
Fuelled by climate change, wildfires are increasing in frequency and intensity in many countries, spreading smoke that contains noxious gases, chemicals and particulate matter and that can be damaging to health.
Nearly 100 million Americans, or about one-third of the population, were issued warnings against extreme heat in the U.S, as temperatures across 28 sates soared above 38 degrees Celsius (100 degrees Fahrenheit).
Over 43 degrees Celsius (109 F) were reported in Oklahoma City, and 46 degrees in the city of Magnum (114 F).
On Wednesday, heat warnings were also issued for the cities of New York, Philadelphia, and Boston, with temperatures predicted to spike over the weekend.
Public cooling centers were set up in New York due to the extreme heat. In the upcoming days, the city's temperature is predicted to exceed 38 degrees (100 F).
The office of State Governor Kathy Hochul encouraged residents to stay inside for the time being to prevent "dangerous situations that could lead to stress, fever, and disease."
"New Yorkers need to learn the risk factors and symptoms of heat-related illnesses to protect themselves and their loved ones," Hochul said in a statement.
The extreme heatwave is part of a global pattern of rising temperatures, widely attributed by scientists and climatologists to climate change caused by human activity, and the worrying trends in Europe and the U.S. are set to expand further afield, with China forecasted to face a searing heat wave in late August.