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 Australian National University (ANU) and the University of Sydney.
A study, led by Professor Sarah Perkins-Kirkpatrick, 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.
"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,'' said Professor Perkins-Kirkpatrick, of the ANU and the ARC Centre of Excellence for the Weather of the 21st Century.
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 yet dry conditions are found to be just as deadly as hot and humid conditions.''
Top dry-bulb temperatures in the studied cities during the events ranged from 41.53 degrees in Mount Isa to 46.73 degrees in Phoenix; the top wet-bulb temperatures ranged from 24.32 degrees in Phoenix to 30.85 degrees in Larkana.
During the heatwaves over the Middle East, Southern Asia, Central America, Australia and India/Pakistan, people aged 65-years and over faced an extremely high risk of physiological heat stress unless they were able to access shade or cooling.
"The vulnerability of humans to extreme conditions will become more frequent as global warming continues,'' said Professor Ollie Jay, Director of the Heat and Health Research Centre at the University of Sydney.
"Adaptation 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.
"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.''
The paper, Deadly heat stress conditions are already occurring , is published in Nature Communications .
A summary of the historical heat events analysed is contained in Table 1 of the paper.
