The extreme temperatures sweeping the globe this week are the new normal in a world warmed by climate change, the UN weather agency says.
Temperatures went over 50C (122F) in parts of the US and China on Sunday.
The World Meteorological Organisation warned the heatwave in Europe could continue into August.
Millions around the world are under heat advisories as officials warn of danger to life from the hot temperatures.
Night-time in Europe and the US is not expected to bring widespread relief as temperatures stay above 30C in places including Arizona or southern Spain.
Large areas of the world saw up close again on Monday what life is like under extreme temperatures:
* Death Valley in California hit 53.9C (128F ) on Sunday. The hottest-ever temperature reliably recorded on Earth was 56.7C (134F)
* China provisionally broke its record for all-time highest temperature on Sunday when it recorded 52.2C (126F) in its western Xinjiang region, according to the UK Met Office
* Temperatures in southern Spain on Monday peaked at 46C (115F). The heat is expected to intensify in Italy where 46C (115F) is forecast in Sardinia. Eastern Europe is also predicted to get hotter.
Scientists say climate change is making heatwaves longer, more intense and more frequent.
"The extreme weather - an increasingly frequent occurrence in our warming climate - is having a major impact on human health, ecosystems, economies, agriculture, energy and water supplies," said World Meterological Organisation MO Secretary-General Prof Petteri Taalas.
"We have to step up efforts to help society adapt to what is unfortunately becoming the new normal," he added.
It underscores the urgency of cutting greenhouse gas emissions as quickly and as deeply as possible, he suggested.
'We are not in a stable climate'
Leading UK scientist Dr Frederieke Otto, from Imperial College London, told the BBC that "what we are seeing at the moment is exactly what we expect in a world where we are still burning fossil fuels".
Humans are "100% behind" the upward trend in global temperatures, she explains.
The world has already warmed by 1.1C since the Industrial Revolution when humans started burning fossil fuels like coal, oil and gas which releases carbon dioxide into the atmosphere.
Dr Otto says we have no idea what the new normal will look like because "we are not in a stable climate".
We will only find out what the new climate looks like once the world stops burning fossil fuels and switches to green energy, she suggests.
The International Energy Agency has said that there can be no new oil, gas or coal projects if governments are serious about tackling climate change.
Scientists say that Europe in particular is warming faster than many climate models predicted. Using very powerful computers, models can compare a world affected by climate change to one without the impacts of global warming.
"There is a feeling that it's going out of control. We have a lot of work to do to pin down exactly what's happening," Prof Cloke explains.
Countries remain unprepared for the reality of excessive heat, explains Julie Arrighi, interim director of the Red Cross Red Crescent Climate Centre.
"If we are not ready for the heat of today, we are definitely not ready for the heat of tomorrow. The risk is only increasing," she says.
In Greece, 1,200 children were evacuated from holiday camps as wildfires broke out. Flames destroyed homes on the Spanish island La Palma over the weekend.
In Phoenix, Arizona temperatures above 43C (110F) have persisted for 18 days. The city has been giving out water and cooling towels, as well as opening respite centres to help residents cope.
"These heatwaves are frightening.... We know this will be really deadly," Prof Hannah Cloke at the University of Reading, UK, told the BBC.
She said more than 61,000 people were estimated to have died from heat in Europe last year, and this year would be similar. (BBC)