The World Meteorological Organization (WMO) is warning of an increased risk of deaths as Europe awaits the peak of a punishing heatwave, while wildfires scorched swathes of the northern hemisphere forcing evacuations.
An estimated 61,000 people may have died in heatwaves last year in Europe alone.
Health authorities have sounded alarms 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.
Europe, the world's fastest-warming continent, is bracing for its hottest-ever temperature on Italy's islands of Sicily and Sardinia, where a high of 48C has been forecast by the European Space Agency.
Near Athens, emergency services were battling wildfires in Kouvaras and the resorts of Lagonissi, Anavyssos and Saronida.
Several homes were burned in the area, according to footage from public broadcaster ERT.
A forest fire flared in strong winds by the popular beach town of Loutraki, where the mayor said holiday camps had been evacuated, with 1,200 people moved to safety.
"The extreme weather ... is having a major impact on human health, ecosystems, economies, agriculture, energy and water supplies," said World Meteorological Organization Secretary-General Petteri Taalas.
"This underlines the increasing urgency of cutting greenhouse gas emissions as quickly and as deeply as possible."
In Europe, Italians were warned to prepare for "the most intense heatwave of the summer and also one of the most intense of all time" as temperatures hit a near-record 39C in Rome yesterday.
It was already the world's hottest June on record, according to the EU weather monitoring service, and July looks set to break records as well.
Spain enjoyed little reprieve, with temperatures of 44.7C reported yesterday in the southern town of Jaen.
In Cyprus, where temperatures are expected to remain above 40C through Thursday, a 90-year-old man died as a result of heatstroke and three other seniors were hospitalised, health officials said.
Parts of Asia have baked in record temperatures, triggering torrential rain.
China reported a new high for mid-July in the northwest of the country, where temperatures reached 52.2C in the Xinjiang region's village of Sanbao, breaking the previous high of 50.6C set six years ago.
Heatstroke alerts had been issued in 32 of Japan's 47 prefectures, mainly in central and southwestern regions.
At least 60 people were treated for heatstroke, media reported, including 51 taken to hospital in Tokyo.
A quarter of a million people were evacuated in southern China and Vietnam before a major typhoon roared ashore late yesterday, bringing fierce winds and rain and forcing the cancellation of hundreds of flights and trains.
US climate envoy John Kerry held talks with Chinese officials in Beijing, as the world's two largest polluters revive stalled diplomacy on reducing planet-warming emissions.
Speaking at Beijing's Great Hall of the People with China's top diplomat Wang Yi, Mr Kerry underlined the need for "global leadership" on climate issues.
In western and southern US states, which are used to high temperatures, more than 80 million people were under advisories as a "widespread and oppressive" heatwave roasted the region.
California's Death Valley, often among the hottest places on Earth, reached a near-record 52C on Sunday afternoon.
In Arizona, state capital Phoenix tied its record of 18 consecutive days above 43C, as temperatures hit 45C early Monday afternoon.
The US National Weather Service predicts similar highs at least through Sunday, while warning of overnight lows remaining dangerously elevated, above 32C.
In southern California, several wildfires have ignited over the past few days in rural areas east of Los Angeles.
The biggest, named the Rabbit Fire, had burned nearly 8,000 acres and was 35% contained yesterday morning, according to authorities.
In neighbouring Canada, 882 wildfires were active, including 579 considered out of control, authorities said.
Smoke from the fires has descended on the United States again, prompting air quality alerts across much of the northeast.
World must prepare for more intense heatwaves: UN
The world should prepare to face increasingly intense heatwaves, the United Nations warned on Tuesday.
"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 UN's World Meteorological Organization (WMO), told reporters in Geneva.
Heatwaves are amongst the deadliest natural hazards, with hundreds of thousands of people dying from preventable heat-related causes each year, Mr Nairn said.
Heat is a rapidly-growing health risk, due to burgeoning urbanisation, an increase in high temperature extremes, and demographic changes in countries with ageing populations.
Mr Nairn said that the number of drawn-out and simultaneous heatwaves in the Northern Hemisphere had swelled six-fold since the 1980s.
"This trend shows no signs of decreasing," he said.
"So, we're in for a bit of a ride, I'm afraid, and they will have quite serious impacts on human health and livelihoods."
