Deadly heatwave events are occurring at temperatures and humidity levels previously thought to be survivable, according to a new paper by a team of international researchers, including from the University of Sydney and the Australian National University (ANU).

The study looked at heatwaves that occurred In Mecca (Saudi Arabia) during 2024, Bangkok (Thailand) during 2024, Phoenix (United States) during 2023, Mount Isa (Australia) during 2019, Larkana (Pakistan) during 2015 and Seville (Spain) in 2003. The events featured climatologically extreme conditions and, with the exception of Australia, were associated with at least 1000 deaths. Non-survivable thresholds were surpassed during all six of the events.

"The vulnerability of humans to extreme conditions will become more frequent as global warming continues,'' said co-author Professor Ollie Jay, Director of the Heat and Health Research Centre in the Faculty of Medicine and Health. 

"Adaption initiatives against extreme heat and humidity are extremely important, including ensuring people have increased access to shade, buildings designed with cooling considerations, fans and Interventions such as skin wetting," said Professor Jay, who also holds an affiliation with the Charles Perkins Centre.

“This is particularly important for vulnerable groups, such as the elderly, pregnant people, young children, those with underlying chronic conditions and those who have difficulty accessing adequate cooling due to economic limitations.’’ 

Lead author from the ANU and the ARC Centre of Excellence for the Weather of the 21st Century, Professor Perkins-Kirkpatrick said: "While many people are rightly concerned about the possible effects of future heatwaves as global warming continues, our research shows that non-survivable conditions are occurring during present-day heat events."

Heat stress limits for human survivability were previously defined using a wet-bulb temperature of 35 degrees Celsius. "Wet-bulb temperature" is the lowest temperature that can be achieved by evaporative cooling with unlimited sweating. However, recently the HEAT-Lim model, co-developed by researchers at the University of Sydney and Arizona State University, demonstrated that after accounting for limitations of human physiology, environmental heat stress thresholds may be cooler and drier than previously thought.

"The events we studied using the HEAT-Lim model were all below 35 degrees wet-bulb temperature and included regular exceedances of deadly thresholds for older people directly exposed to the sun,'' Professor Perkins-Kirkpatrick said.

"Moreover, extremely hot, dry conditions are found to be just as deadly as hot and humid conditions.''