The body’s ability to cope with extreme heat

Professor Steven Sherwood

Climate Change Research Centre at the University of New South Wales

Extreme heat is a silent killer that is responsible for more deaths than any other disaster: several severe European heat waves in recent decades, including last summer, have claimed tens of thousands of excess deaths.

Death tolls in developing countries are not properly counted and probably much larger.

But worse is coming and we need to prepare for it.

The average surface temperature on Earth is now at its highest level since records began and probably before the last ice age.

Recent heat waves show clear fingerprints of global warming, more so than any other climate change impact such as flood or drought. And global warming will continue at least until we reach net zero.

There is a fundamental limit to the body’s coping ability: it is a fixed goalpost.

Research in 2010 demonstrated that a ‘wet-bulb’ temperature of 35 degrees Celsius or higher would make it impossible for humans to exhaust metabolic heat, due to our fixed core body temperature.

It proposed this was an effective survivability limit.

The wet-bulb temperature measures the ability to cool by evaporation; it equals normal temperature if relative humidity is 100 percent, and otherwise is lower. 35C is extreme — most places on Earth never experience wet bulbs above 30C.

But enough global warming could push heat waves in many areas past 35C. This upended the widely held assumption at the time that humans could adapt to any amount of increased heat, i.e., that the goalposts would move. This goalpost will not.

Wet-bulb temperature is used by meteorologists and climatologists to quantify heat stress. It is a combination of heat and humidity: a high wet bulb can occue in humid places at lower temperatrues, as well as in dry places at extremely high temperatures.

New studies are beginning to chart out the road to 35C.

Extreme heat increases health risk for everyone

One study in the US last year found that young, healthy people exposed to very hot conditions started to enter hyperthermia (inability to regulate core body temperature) well below 35C wet bulb, closer to 32C or less.

This is an important reminder that 35C was a theoretical upper limit, not a practical one.

On the other hand they would undoubtedly have found a higher tolerance had they done the study in India or Brazil, because physiology does adjust to heat over time (up to a point).

The UK has a long way to go before reaching 32C and could acclimatise for a while. Wet-bulb temperatures above 32C appear only very rarely today in coastal areas of the Middle East and for very short periods, but these will gradually spread as warming continues.

The heat will force us to change how we live, for example shifting outdoor summertime activities to nighttime or just eliminating them.

Severe heat may already be putting people off traveling to Europe or other locations in the summer.

Researchers at the University of Sydney are developing a heat warning system, and conducting exposure studies similar to the US one. It looks like we’ll soon have a clearer picture of the direct effects of severe heat on physiology.

It remains challenging to measure or predict extreme heat’s overall cost to the community in terms of health, work and quality of life.

To do this, climate and health researchers need to develop models that factor in human behaviour and adaptation along with physiology, weather, and climate information. We also need to understand what will happen to nature, and seek ways to protect wildlife.

Above all we need to reach net zero carbon emissions as soon as we possibly can to arrest the continuing rise in heat.

Steven Sherwood is a professor and Deputy Director of the Climate Change Research Centre at the University of New South Wales. He also serves on the review board of the journal Science and on the steering committee of the World Climate Research Programme’s Grand Challenge on Clouds, Circulation and Climate Sensitivity.

Professor Sherwood’s research has been funded by the Australian Research Council.