Furthermore, the researchers note that without major reductions in emissions of greenhouse gases like CO2, up to three in four people will face the threat of dying from heat by 2100.
Also, tests showed that around 30 percent of humanity already encounters such a super hot heat wave at least at one point in the year. Even if emissions are considerably reduced, the study predicted that Seoul would still have 18 days of deadly heat by 2100.
"People are talking about the future when it comes to climate change, but what we found from this paper is that this is already happening ... and this is obviously going to get a lot worse", Camilo Mora, lead author of the study and a geography professor at the University of Hawaii at Manoa, said in a statement, as quoted by Reuters.
"Of course, the heart works hard sending blood to the skin to cool down, and thus for many people with heart problems or the elderly, their hearts just give up on them".
The grim assessment would also expose 73.9 per cent of the world's population to climates beyond what the human body can withstand by the end of the century. These included events like the 2003 heatwave that led to almost 5,000 deaths in Paris and the 2010 heatwave that killed over 10,000 people in Moscow.
Heatwaves have also killed people more recently. Right now, temperatures in Pakistan and India have reached a scorching 128 degrees.
Some of the cities that have experienced lethal heat- waves included New York, Washington, Los Angeles, Chicago, Toronto, London, Beijing, Tokyo, Sydney and Sao Paulo.
Those facing the greatest risk live in the wet tropics, where only slight increases in average temperatures or humidity can result in deaths.
In any case, the situation will not improve, and in the worst case scenario, deadly hot days will burn most of Vietnam for between 200 and 250 days a year by 2100.
So while an increase in deadly heat waves may be inevitable, the research underscores the importance of working to reduce greenhouse gas emissions and minimize the future impact of climate change as much as possible, Mora said.
"The combination of an optimum body core temperature (that is, 37 degrees Celsius) and that an object can not dissipate heat to an environment with equal or higher temperature", Professor Mora said. As temperatures rise, the body reacts by sweating to try and cool down. If our internal temperature rises to 104 degrees Fahrenheit (40 degrees Celsius), all-important cellular machinery starts to break down in response. Body temperatures above 104 degrees are extremely unsafe and require immediate medical attention.
"Heatwaves pose a considerable risk to human life because hot weather, aggravated with high humidity, can raise body temperature, leading to life threatening conditions", Mora said.
RG: Who is most at risk? Hundreds more smaller heat waves around the world have caused fatalities in the past three decades.
"Increasing inequality leads to increased deaths from heat extremes", said Keller, according to National Geographic.
"There are multiple ways to die during a heatwave".
The picture is complicated by the fact that people in lower-latitude countries may well be better able to adapt to higher temperatures due to physiological acclimatisation and behavioural adaptation. This means temperature increase is deemed as only mild compared to other countries and regions. Surface temperature measurements have shown that the Earth has warmed around 1.8 degrees Fahrenheit (1 degree Celsius) since preindustrial times, but these additional degrees aren't evenly distributed.