WASHINGTON • Nearly one-third of the global population suffers deadly levels of heat for at least 20 days during the year, new research suggests, and by year 2100, thanks to climate change, this number could climb above 70 per cent.

The new study, published on Monday in the journal Nature Climate Change, underscores the growing threat that rising temperatures pose to public health.

Certain parts of the world like tropical regions, where temperatures are already high for much of the year, will be harder hit than others, the researchers noted.

The research focuses specifically on heat and humidity conditions known to increase the risk of human mortality - when temperatures climb above the average human body temperature of 37 deg C, but can also include cooler conditions with higher levels of humidity.

"The way in which the body cools down is by sweating - the evaporation of that sweat cools you down. But when it's humid, that sweat doesn't evaporate, so the heat that the body generates, instead of going away, it stays in your body," said the study's lead author Camilo Mora, a geography expert at the University of Hawaii.

The researchers examined more than 900 published papers documenting cases of extreme heat and excess mortality between 1980 and 2014, and altogether identified 783 individual events in 164 cities around the world. By looking at historical climate data, the researchers determined that about 13 per cent of all the world's land area - home to about 30 per cent of the total human population - had faced these deadly conditions for 20 or more days during the year 2000. And this number is only expected to grow.

Meanwhile, the World Meteorological Organisation on Tuesday predicted that heatwaves as a result of global climate change will become more intense, more frequent and longer, leading to an increasing number of hot days in the future.

Large parts of the world, the Middle East, North Africa and the United States, saw record-breaking temperatures last month and this month as heatwaves are unusually early this year. Data from the US and European research and monitoring institutes suggest this year's average global surface temperatures over land and sea have been the second-highest on record until the end of last month, even though there has been so far no El Nino event, which has a warming impact.

In Europe, temperatures of around 40 deg C have contributed to a wildfire in Portugal over the weekend, killing at least 64 people.

WASHINGTON POST, XINHUA