As global temperatures rise above 1.5 degrees Celsius, extreme heat waves will begin to hit much of the world and healthy young people could die within hours if they don't find a way to cope, a study has warned. Carter Powis of the University of Oxford in the UK said this could lead to mass fatalities in places that are not used to extreme heat and have little air conditioning. On September 8, the relevant results were published in "Science Progress".

"You could have a very extreme heat wave that deviates significantly from the historical norm, exceeds relevant thresholds, and causes mortality rates far greater than expected," Powis said. "We're going to see that, particularly in Europe and North America. , as global temperatures rise by 1.5 degrees Celsius to 2 degrees Celsius, the incidence of heat waves will increase significantly."

Global warming is already causing more intense and more frequent heat waves, resulting in large numbers of deaths. For example, it is estimated that in the summer of 2022, 62,000 people will die from heat exposure across Europe. However, the vast majority of them are elderly people over the age of 65, who may have existing health problems of their own.

Will global warming cause parts of the world to become so hot that even healthy young people die? Matthew Huber and colleagues at Purdue University began investigating this issue in 2010.

According to theory, the limit of human survival is reached when the temperature measured by a thermometer covered with a damp cloth exceeds 35 degrees Celsius. This is the so-called wet bulb temperature. It reflects the fact that humidity affects a person's ability to stay cool through sweating. At this wet-bulb reading, people can no longer naturally control core body temperature, posing a potentially fatal risk if action is not taken to stay cool in other ways.

Currently, the wet-bulb temperature at the Earth's surface rarely exceeds 31 degrees Celsius. Huber's team concluded that only when global warming exceeds 7 degrees Celsius will large areas begin to exceed the wet-bulb temperature limit of 35 degrees Celsius - which is considered extremely unlikely. However, recent research suggests that parts of the tropics could exceed this limit under lower warming scenarios. What's more, in practice, most people cannot survive wet-bulb temperatures approaching 35 degrees Celsius. "35 degrees Celsius has always been an upper limit," Huber said.

Last year, Daniel Vecellio of Pennsylvania State University and colleagues tested 24 healthy young men and women to see how much heat and humidity their bodies could withstand without stopping their core temperature from rising, and how much humidity they could endure. Continued exposure to conditions for several hours may result in death.

The results show that the survival limit is closer to the wet bulb temperature of 31 degrees Celsius. Powis said that because the volunteers were not acclimated to high temperatures and were going about their daily routine during the test, this should be considered a lower limit, with a wet-bulb temperature of 35 degrees Celsius being the upper limit.

"Any temperature in between is dangerous," Powis said. "Different populations have different thresholds at which mortality can rise dramatically."

Using data from weather stations and climate models, Powis and colleagues looked at where in the world this might currently be the case, based on the discovery of a wet-bulb temperature of 31 degrees Celsius, and how it might change as the climate warms.

For example, under a 1°C global warming scenario, only 3% of weather stations in Europe are likely to exceed the 31°C threshold once in 100 years. If global warming reaches 2 degrees Celsius, 25% of weather stations could exceed this threshold. In the United States, under 1°C of global warming, 20% of weather stations are likely to exceed this threshold more than once in 100 years, rising to 28% under 2°C of global warming.

"Sometimes these limits of human survivability are useful to understand the problem, but the reality is that even at 'moderate' temperatures we see A significant health burden for humanity.”

"I would like to emphasize that all heat-related impacts on human health and well-being are preventable." said Raquel Nunes of the University of Warwick in the UK. As heat waves become more frequent, more intense and last longer, Urgent action is needed to prevent more heat-related deaths.

Related paper information:

https://doi.org/10.1126/sciadv.adg9297

(The original title is "Once this temperature threshold is exceeded, heat waves may cause mass fatalities")