The number of extremely hot days every year when the temperature reaches 50C has doubled since the 1980s, a global BBC analysis has found.
They also now happen in more areas of the world than before, presenting unprecedented challenges to human health and to how we live.
The total number of days above 50C (122F) has increased in each decade since 1980. On average, between 1980 and 2009, temperatures passed 50C about 14 days a year.
The number rose to 26 days a year between 2010 and 2019.
In the same period, temperatures of 45C and above occurred on average an extra two weeks a year.
“The increase can be 100% attributed to the burning of fossil fuels,” says Dr Friederike Otto, associate director of the Environmental Change Institute at the University of Oxford.
As the whole world warms, extreme temperatures become more likely.
High heat can be deadly for humans and nature, and cause major problems to buildings, roads and power systems.
Temperatures of 50C happen predominantly in the Middle East and Gulf regions.
And after record-breaking temperatures of 48.8C in Italy and 49.6C in Canada this summer, scientists have warned that days over 50C will happen elsewhere unless we cut fossil fuel emissions.
“We need to act quickly. The faster we cut our emissions, the better off we’ll all be,” says Dr Sihan Li, a climate researcher at the School of Geography and the Environment at the University of Oxford.
“With continued emissions and lack of action, not only will these extreme heat events become more severe and more frequent, but emergency response and recovery will become more challenging,” warns Dr Li.
The BBC analysis also found that in the most recent decade, maximum temperatures increased by 0.5C compared with the long-term average from 1980 to 2009.
But these increases have not been felt equally around the world: Eastern Europe, southern Africa and Brazil saw some maximum temperatures rise by more than 1C, and parts of the Arctic and Middle East recorded increases of more than 2C.
Scientists are calling for urgent action from world leaders at a UN summit in Glasgow in November, where governments will be asked to commit to new emissions cuts in order to limit global temperature rises.