Global warming heat stress could make life intolerable: study

Tuesday, 4 May, 2010
Bob Beale

Heat stress would make life intolerable for humans in most currently inhabited regions on Earth if the worst-case global warming scenario were to happen, according to a new study.

In an often overlooked consequence of climate change, the study looks at potential global temperature increases over the next three centuries and finds that even modest increases would subject many communities to unprecedented levels of heat stress.

Severe warming would be intolerable because it would push most humans and other mammals beyond the point at which their bodies could eliminate heat through sweating and other biological mechanisms.  Though very unlikely this century, that could happen in the following century, the study says.

"Heat stress is already a leading cause of fatalities from natural phenomena," say Professor Steven Sherwood, of the UNSW Climate Change Research Centre, and Associate Professor Matthew Huber of Purdue University, in an article in the journal Proceedings of the National Academy of Sciences.

The study notes that many people believe humans "will simply adapt" to warmer climates by altering their lifestyles and work productivity, "reasoning that humans already tolerate a very wide range of climates today.  But when measured in terms of peak heat stress-including humidity-this turns out to be untrue."

Taking account of humidity is the key to understanding why the study suggests heat stress poses such a widespread threat.

A resting human body generates about 100 Watts of metabolic heat, which must be carried away to avoid over-heating in the body core. But it can be carried away only if the body is warmer than the surrounding wet-bulb air temperature (measured by covering a standard thermometer bulb with a wetted cloth and fully ventilating it).

Humans maintain a core body temperature near 37 C, with a surface skin temperature of up to 35 C. The researchers note that sustained skin temperatures above 35 C can lead to the core body temperature becoming elevated, known as hyperthermia. To avoid dangerously elevated body temperature requires a wet-bulb temperature of 34 C or lower. Skin temperatures of 37-38 C would result in lethal core temperatures of about 42-43C, even for acclimatised and fit individuals. So a prolonged wet-bulb temperature above 35 C would be intolerable after only a matter of hours.

A rise of 4 C in wet-bulb temperatures would subject half the world's population to unprecedented heat stress - equivalent to the maximum reached in only a few places today, the paper says.

A rise of 7 C on global average air temperatures would push wet-bulb temperatures above 35 C in some locations and a shift of 11 C would take the most common peak wet-bulb temperature in the inhabited regions of Earth to 35 C.

The most common maximum wet-bulb temperature today is 26-27 C, with the maximum reaching only about 30 C. The peak potential heat stress is therefore surprisingly similar across many regions on Earth. Even though deserts have the hottest temperatures, for example, the relative humidity there is so low that the maximum wet-bulb temperature is no higher than in the tropics.

The authors note that the depletion of available fossil fuels might lead to global warming of 12 C or more, although this is a worst-case outcome, and would only happen well after 2100.  At the corresponding wet-bulb temperatures, most currently inhabited regions of the earth would be subject to intolerable heat and humidity.

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UNSW Faculty of Science media liaison - Bob Beale 0411 705 435